CA1296794C - Advanced instrument landing system - Google Patents
Advanced instrument landing systemInfo
- Publication number
- CA1296794C CA1296794C CA000537206A CA537206A CA1296794C CA 1296794 C CA1296794 C CA 1296794C CA 000537206 A CA000537206 A CA 000537206A CA 537206 A CA537206 A CA 537206A CA 1296794 C CA1296794 C CA 1296794C
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- CA
- Canada
- Prior art keywords
- vehicle
- guidance
- range
- path
- guiding
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S11/00—Systems for determining distance or velocity not using reflection or reradiation
- G01S11/02—Systems for determining distance or velocity not using reflection or reradiation using radio waves
- G01S11/08—Systems for determining distance or velocity not using reflection or reradiation using radio waves using synchronised clocks
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
- G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
- G01S1/08—Systems for determining direction or position line
- G01S1/10—Systems for determining direction or position line using amplitude comparison of signals transmitted sequentially from antennas or antenna systems having differently-oriented overlapping directivity characteristics, e.g. equi-signal A-N type
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/14—Receivers specially adapted for specific applications
- G01S19/15—Aircraft landing systems
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- Engineering & Computer Science (AREA)
- Remote Sensing (AREA)
- Radar, Positioning & Navigation (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Computer Networks & Wireless Communication (AREA)
- Aviation & Aerospace Engineering (AREA)
- Position Fixing By Use Of Radio Waves (AREA)
- Traffic Control Systems (AREA)
- Forklifts And Lifting Vehicles (AREA)
- Fish Paste Products (AREA)
- Spray Control Apparatus (AREA)
Abstract
ADVANCED INSTRUMENT LANDING SYSTEM
Abstract A system for landing an aircraft is described, using a ground installation and an airborne installation which are synchronized together using GPS system time.
Specifically, the ground installation includes a ground transmitter (32) which radiates the sequence of signals (A,B,C,D) which provide precision guidance information to the aircraft, an aircraft installation which includes a radio receiver (8) and a processor (15) to receive a process the transmitted guidance signals and to provide indications (18,20) to aid the pilot in landing the aircraft, a GPS receiver in the air (68) and on the ground (60) for producing signals representative of GPS system time, and a channel selector (72) in the aircraft for actuating the processor to synchronize its operation with the ground installation transmitter. Range information is provided by measuring the time interval between the transmission of a reference at the ground and its receipt in the air.
Abstract A system for landing an aircraft is described, using a ground installation and an airborne installation which are synchronized together using GPS system time.
Specifically, the ground installation includes a ground transmitter (32) which radiates the sequence of signals (A,B,C,D) which provide precision guidance information to the aircraft, an aircraft installation which includes a radio receiver (8) and a processor (15) to receive a process the transmitted guidance signals and to provide indications (18,20) to aid the pilot in landing the aircraft, a GPS receiver in the air (68) and on the ground (60) for producing signals representative of GPS system time, and a channel selector (72) in the aircraft for actuating the processor to synchronize its operation with the ground installation transmitter. Range information is provided by measuring the time interval between the transmission of a reference at the ground and its receipt in the air.
Description
ADVANCED INSTR~ENT LANDING SYSTE~
Technical Field -This invention relates to an advanced instrument landing system, and more particularly relates to 05 improvements in a type of landing system in which all ground installations seauentially radiate guidance pulses on the sa~e frequency, the improvement permitting the ground installation of such a same-frequency landing system to radiate at uniquely assigned times in an established clock sYstem and to be uniquely identified by an approaching aircraft that has both knowledqe of such uniquely assigned times and access to the established clock system time. The improvement also provides, to the aircraft, a precision range to the q~ound station.
Backqround of the Invention In cases where a ground installation of a single fre~uency landing system is remotely located from ot~er ~imilar installations, there is no need for the ground :~ station ~o radiate at an assigned time, and the approaching aircraft has no difficulty in identifying the ground installation. However, in impacted geoqeaphic locations, ~here there are multiple similar landinq installations located relatively closely together, it is ,. ~k PATENT
Technical Field -This invention relates to an advanced instrument landing system, and more particularly relates to 05 improvements in a type of landing system in which all ground installations seauentially radiate guidance pulses on the sa~e frequency, the improvement permitting the ground installation of such a same-frequency landing system to radiate at uniquely assigned times in an established clock sYstem and to be uniquely identified by an approaching aircraft that has both knowledqe of such uniquely assigned times and access to the established clock system time. The improvement also provides, to the aircraft, a precision range to the q~ound station.
Backqround of the Invention In cases where a ground installation of a single fre~uency landing system is remotely located from ot~er ~imilar installations, there is no need for the ground :~ station ~o radiate at an assigned time, and the approaching aircraft has no difficulty in identifying the ground installation. However, in impacted geoqeaphic locations, ~here there are multiple similar landinq installations located relatively closely together, it is ,. ~k PATENT
-2- ~96~ B02348 necessary to provide means for uniquely identifyinq at least one ~uch same-frequencY installation to the exclusion of others in the vicinity.
In conventional landing systems, such as the 05 conventional Instrument Landing System tILS) and the FAA
Microwave Landing System (MLS)~ uniaue identification an~
signal exchanges between approaching aircraft and a particular ground installation are established by uniquely assigning different freauencies out of a band of frequencies to each of the various installations, and tuning the airborne units to the frequency of the selected installation. The FAA MLS system has 200 separate ~eequency channels assigned for its use in the band of 5000 to 5250 MHz. The ILS system has some 40 channels in paired bands allocat~d to its use in the vicinity of 100 ~ and 300 MHz. Therefore, an adequate number of separately ; indentifiable channels for a sinqle frequency landing system can be inferred as being between 40 and 200 channels. In ~y U.S. patent 4,429,312 entitled "Independent Landing Monitoring Systemn, a different tYpe of identification of a same-freauencY landing installation is discussed in which some of the signals transmitted to ~he aircraft are pulse encoded to identify that installation.
That system is generally satisfactory when the aircraft is in a remote area isolated from other ground stations and when ~he aircraft has a weather radar to interrogate the ground installation, a decoding circuit added to the radar together, and an appropriate code selector switch for station selection in the cockpit.
However, not all aircraft bave weather radars to ; interrogate the ground installation. In addition, where there are several airfields in close geographic proximity, or where there are several landing installa~ions of this type at tbe same airpo~t, the same-frequency signals PATENT
_3- B02348 from all such landing systems can arrive at the aircraft simultaneously and hence they cannot be adequately separated for unique range tracking identification and guidance generation purposes. This is basically the same 05 problem that plagues the conventional Air Traffic Control Radar Beacon System (ATCRBS) used by the FAA for air traffic control purposes, it is called ~garblingn. The weather radar technique of my U.S. patent 4,429,312, with associated identifying codes, is thus very suitable for use at isolated remote sites, such as offshore oil ri~s, but not sui~able for areas with many same-freauency landing systems in close proximity. The problem comes basically from the fact that these syste~s, and the airborne radars all use a common frequencv. Thus, there is no way to trigger one particular installation uniquel~
for positive identification purposes. There is, therefore, always the risk of undesirably triqqering a nearby installation, with the result that confusing responses to the aircraft from b~th locations will be synchronously received in that aircraft.
In addition to a method of uniquely identifyinq a particular same frequency ground station, a very desirable characteristic for a landing system is the capability of providinq range information. Range data has at least three major uses:
1. a means for alerting the pilot of his proximity to touchdown;
2. a means for automatically reducinq the ~ain of the landinq installation as the aircraft ranqe to touchdown diminishes in order to maintain loop stability (often referred to as ~course softening'`): and 7~
PATENT
_4_ B~2348 3. a means for usinq the elevational angular data provided by the landing system to determine altitude above ~he runway during the approach.
.' :
In the conventional ILS sys~em, range to 05 touchdown is generally provided by marker beacons on the ground at establi~hed distances from touchdown. These beacons radiate vertical fan shapped-beams throuqh which the aPproaching aircraft passes~ The ranqe information thus acauired in the aircraft is used for pilot alertinq and for "course softening" purposes.
In FAA MLS and conventional ILS practice, an alternative and more accueate measurement of ranqe is provided by conventional TACAN/DME interrogators which are carried by almost all aircraft. The airborne TACAN/D~E
equipment interrogates a D~E beacon that is co-locat~ed with the MLS or ILS ground ins~allation and receives therefrom a direct measurement of range using usual D~E
techniques.
For some landing apPlications, a very precise measurement of range is required, and for t~is purpose, a Precision DME (usually referred to as PDME) is employed.
The PDME is similar to the conventional DME, but uses faster eise ti~e pulses to obtain higher precision. This PDME system imposes on aircraft, which have to use it in order to obtain a re~uired very precise measurement of range, the additional burden of having installeld on board appropriate PDME airborne equipment. Another technique for obtaining precision range in a landing system is provided by the teaching of my U.S. pa~en~ 4,42~,312.
Ran~e is measured in this disclosure by having the weather radar interrogate the landing system qround installation and trigger the transmission of pulsed angular guidance signals. These pulsed replies are synchronous with the ~_2~;t7~
PATENT
_5_ B02348 weather radar interrogations and are range ~racked in a conventional manner to provide precision range in the aircraft. Range measurements of higher precision can be ob~ained by the use of ast rise time pulses.
05 Both of the above described methods for identifyinq ground station installations (i.e., freauency selection or pulse group encoding) require additional equipment and adjustable cockpit controls for either tuning to the freauencv of the ground installation, or for selecting the decodement of the signals radiated from that ground station. In addition, measurement of ~an~e, by means of marker beacons or DME equiPment~ reauires the installation of appropriate marker beacons or D~E beacons at the landing system ground installation. The measurement Oe very precise range requires the addition of speciali2ed PDME eauipment, both air and ground. ~hile the use of the weather radar to provide precision range ~as taught in my U.S. patent 4,429,312) eliminates the need for added PD~E
eauipment, not all aircraft carry a weather radar. ~hus, all conventional landing sYstems have tended to require added airborne equipment, or cockpit controls, or both, in order to achieve unique communication with and range to a selected ground installation.
In addition to the two techniaues discussed above for obtaining range (i.e., the aircraft passing over marker beacons and the measurement of the time elapsed between an airctaft's transmission of an in~errogation and the aircraft's reception of a reply from a transponder located at ~he landing system), there is the clocked station technique. That technique may be practiced using high precision clocks and low precision clocks. For example, equipment in one participating uni~, such as a ground or airborne station, ~ransmits a signal at a known time in an established very precise cloc~ system.
PATENT
-6- - ~02348 Equipment in a second unit, such as an aircraft, measures the time of reception of that transmittled siqnal in ~he same established cloc~ system; by knowing the time at ~hich the signal was transmitted, the propa4ation time 05 between the t~o stations and thus the distance can be computed. One known use of this clocked ranging method is the United States Air Force AN/APN-169, Station Xeepinq Equipment (SKEt.
Another means for establishing a co~mon clock time, is for each Participant to carry low-cost clocks of nominal stability and to periodically synchronize those clocks to a common time reference. Such synchronization of low-cost clocks may be established by an initial conventional two-way ranqin~ process that determines the ranges between participants and thereafter uses measured range data, by an exchanqe between participants of relative clock times. Tnus, the low-cost clocks of each participant are synchronized to a clock in one selected aircraft out of all participating aircraft. This synchroniza~ion process is then repeated at periodic intervals, which intervals occur frequently enough to maintain the common time base to adequate accuracy. A
variation of this method of synchronizing all clocks to a clock in a selected unitJis to synchronize all clocks to an "average value~ of all the clock times that exist when the clock synchronization process is initiated.
Therefore, this "local" synchronization process requires a precision ranging and a data exchange cr communications system, including trans~itters an~
30 eeceivers in each Participating unit. A requirement for clock synchronization equipment, in all aircraft, is undesirable in manY applications, (i.e., cost, wei4ht and complexity.
6~
PATENT
_7_ Bn2348 One advantage of using a common clock system i~ that identity may be established by the use of ~time slotting". In this time slotting concept, each of the participants is assigned a specific clock time at which to 05 radiate, which time repeats at specified intervals. For example, a specific participant, such as No. 3, mi~ht radiate on the t~ird second of every minute. Associa~ed with this radiation at a specific time is a subsequent time interval or time slot, durinq which no other participant can radiate. This use of an established clock time and an associated time slot, by the participant to which it is assigned, permits reception of that transmission by other participants to be used to establish the identity of the sender of that tcansmission (i.e., anY
transmission ~eceived during that time interval must be from the participant assi~ned to transmit in that time slot). This use of an assiqned time slot or time period to pro~ide a protected identity system can be viewed as being similar to the use of a distinctive frequency for ~0 identity, which frequency cannot be used by another ; station in a specific geographic area.
W~ile the use of established and precise clock time, with precision of the order of a fraction of a microsecond, can provide both precision ranging and unique identitY, current use of such a common and precise clock time is limited by the attendant cos~ and complexity of very stable clocks, such as atomic cloc~s, or by the cost and complexity of the synchronizin~ equipment ~i.e., communications system, etc.) required for lower cost 3~ cloc~s.
Considered broadly, a landing system does not inherently require the use of multiple different ~ frequencies since operation at all installation si~es is ; usually performed on a single fre~uency. Single-frequency - ~. .. :
i'7~3~
PATENT
operation is an advantage because, if the actual landing guidance system can always operate on the same f requency for different sites, great simplification in ter~s of airborne equipment complexity and cost is possible. For 05 example, the airborne receiver can be a fixed-frequency device.
;
Therefore, it sho~ld be appreciated that a landinq system is needed which would provide both station selection and ranging data in a fixed frequency landing system, while using only airborne equipment whic~ is installed in an IFR ~Instrument Flight Rules) aircraft.
Moreover, such a system should be simple and low in cost.
With the eventual addition of NAVSTAR or Global Positining System ~GPS) Navigation Sets on all but the smallest aircraft, it also would be ver~ desirable to find a means to use GPS to provide channelization (i.e., identity) and range data for single frequency landinq systems. This is especially true since such fixed freauency landing svstems are, inherently, lower cost and are found in more - 20 locations throughout the world. Thus, there is a need for an advanced instrument landing system.
Summary of the Invention In accordance with one embodiment of the present invention, a landing system is provided comprising a ground installation and an airborne installation wherein each installation is provided with a GPS receiver that produces a trigger signal representa~ive o~ GPS system time, a ground installation which radiates a sequence of transmissions which include precision guidance information to assist the aircraft in landing, and an aircraft installation having a receiver and a processor for - transforming the precision guidance si~nals to provide the pilot of the aircraft with indications for guiding the landing of the aircraft. ~ore specifically, the trigger PATENT
signal from the GPS receiver at the ground installation is further processed to provide, at a specific time assigned to a specific ground installation, a second triqger or actuation signal. The ground installation, in response to 05 the actuation signal, radiates the aircraft guidance signals. In the aircraft installation, the triqger signal from the airborne GPS receiver is further processed to provide, at a specific time ~i.e., corresponding to the selection of a specific ground station from which it is desired to receive landinq guidance, and a time interval or slot durins which signals can be usefullY received from such selected 9round stations), a signal for actuating the processor to permit such processor to generate landing guidance information. This technique of using GPS clock time, which clock time is available at no incremental cost to a GPS receiver, thus provides positive identity of a selected low-cost single frequency landing system ground station.
In one specific embodiment of the invention, the sequence of signals radiated from the qround installation includes a ranging reference signal. These ran~ing reference signals, and associated guidance pulses, will be synchronously repetitive as received in the aircraft, with respect to the GPS clock time triqger signals generated in the aircraft. The time at which the rangin~ reference siqnal is received in the aircraft can thus be measured by conventional synchronous range tracking circuits, and the distance between the aircraft and the ground installation can thus be precisely calculated. Thus, GPS clock time 3Q provides precision range data to a selected low-cost single frequency ground station.
~ ith the advent of GPS, precisely synchronized airborne clocks will become commonplace in airbocne vehicles. Thus, availability of such universal ~ime makes 6~
PATE~T
-ln- B02348 the consideration of apPlying such clock techniques on a wider scale very appropriate. Other advantages and features of the present invention will be come readily apparen~ from the detailed description of the invention, 05 the embodiments presented, the accompanying drawings, and the claims.
., Brief Description of the Drawinq FIG. 1 is a block diagram of the advanced landing system that is the s~bject of the present invention.
Detailed Desciption While this invention is susceptible of embodiment in many different forms, there is shown in the drawing and will herein be described in detail, one specific embodiment, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated.
A preferred embodiment of the presen~ advanced landing system will ~e illustrated and described with reference to the landing system of my U.S. patent 4,429,312. It should be understood, however, that this invention provides techniques which are applicable to many different landing systems, and therefore it is not limited ~o improvements to the precision landing sys~em of the type shown and described in that patent.
As shown in ~hat patent, and described in Columns 8 and 9 thereof, the patented system provides a ground based precision landing guidance installation which radiates localizer and glideslope guidance beams from separate antennas which are directed along the approach path toward 7~
PATENT
a landing aircraft, and which are received in the aircraft and processed to recover signals which provide landing indications to the pilot. In the system shown in that patent, the ground based system can ei~her be trigqered to 05 generate replies in synchronization to some reference si~nal, as for example in synchronization to siqnals received from a weather radar, or can be free running and simply received and used by an approaching aircraft. The former type of sYstem is the system to which the present improvements are directed and therefore the free running mo~e of ope~ation of the landing system will not be furthec discussed.
Turning to FIG. 1, the landing system there illustrated includes a ground installation and an airborne installation. The ground installation includes, for lateral aircraft guidance, two directive antennas 21 and 22 having precision guidance localizer antenna beam patterns 23 and 24, with cross-sections marked B and C.
The ground installation also includes a non-directive antenna 5 deliverin~ an omni-pattern ~5. These antennas 5, 21 and 22 are connected by a switch 26 and cable 27 to a radar transmitter 32. ~iming and switching circuitry 29 controls the switch 26 and initiates the outputs of the transmitter 32.
The directive antenna patterns of two paired glideslope antennas 33 and 34 are aligned and partially overlapped respectively above and below a predetermined glideslope tusually 3 deg~ees), so that for aircraf~
approaching precisely along the glideslope, the signal intensities received in the aircraft from these paired an~ennas 33 and 34 will be equal.
For vertical glidelsope guidance, the ground installation furt~er includes two directive antennas 33 ~2~
PATENT
and 34 for radiating paired precision glideslope guidance beams 33a and 34a, with cross-sections marked D and E.
These two antennas 33 and 34 are likewise connected to the transmitter 32 through the antenna switcn 26. The paired 05 beam patterns of the antennas 23 and 24 for lateral guidance overlap so that they provide equal intensity signals along the extended centerline C'L of the runway.
Thus, if the signal intensities of both antennas are eq~al, as received in the airborne vehicle, it must be laterally located over the centerline of the runway.
For an on-course approach, all four guidance signal intensities received in the aircraft will be equal.
However, deviation above or below, or to the riqht or left of the desired approach course, wiill cause an unbalance in the paired signals received at the receiver, indicating to the pilot the direction in which the aircraft has deviated from the desired course. This operation is thoroughly described in my U.S. patent 4,429,312.
~' The airborne installation of the system according to ~hat paten~ is shown in the right in FIG. 1, and includes a radar receiver 8 and an antenna 3. The eadar receiver 8 is connected to a range gate and navigation processor 15 which provides range data to a range readout 18 and to a course deviation indicator 20 connected thereto.
A transmission from the ground installation is initiated by sending a trigger signal T2 to the timing and switching circuitry 29. ~his ~ransmission includes a sequence of multiple successively delivered signals.
First, the timing and switching circuitry 29 delivers, through the omni antenna 5, a coded pulse group reference signal A from the ~ransmitter 32. The strength of the reference signal is used to set ~he qain of the aircraft receiver 8 so as to keep the airborne receiver operating . . ~
~9~7~4 PATENT
within a linear portion of its response charac~eeistic.
After a fixed delay, determined by the timing a~d switching circuitry 29, the switch 26 then steps sequentially to connect the transmitter 32 in turn to each 05 of the four directive antennas 21, 22, 33 and 34 to deli~er transmissions, includin~ right and left paired locali~er pulses, and up and down paired glidesloPe pulses. These Pulses are delivered one at a ti~e with suitable delays between them. Ad~ustable attenuators 44 serve to balance the antenna drives so that the guidance signals are all of equal amplitude when the aircraft is exactly on course for landing, as explained in my U.S.
patent 4,429,312. The sequence of these four quidance signals is predetermined and fixed so that the aircraft can identify the signals by their order in the sequence.
The pulses radiated in these precision guidance beams B, C, D and E in FIG. 1, plus the reference signal group A from the omni antenna, are received at the airborne antenna 3, and delivered by the receiver 8 to a processor 15 in the aircraft. The processor 15 is programmed to use the reference signal A to determine range and to display it at the range readout 18, and to use the precision landing signals B, C, D and E to create and deliver to the course deviation indicator 20 output signals which show the position of the aircraft ~ith respect to the desired approach path.
The equipment used to uniquely identify the ~round installation of FIG. 1 will now be described. The ground ; installation of FIG. 1 is provided with a GPS
receiver/computer 50, a GPS antenna 62 connected to the receiver, and a Time Slot Selector 64. The GPS
receiver/computer 60 provides precision qeographic position using the GPS or NAVSTAR Satellite system 66.
The ground installation need only be provided with a 7~34 PATENT
receiver suitable for providing an output signal Tl representative of GPS system time. The availability of such receivers is becominq all the more commonplace. A
relatively current description of available equipment is 05 provided in the November 4, 1985 edi~ion of Aviation ~eek S~ce Technolo~y, "Global Positioning Develops As Civil Navigation System~, page 58. The Time Slot Selector 64 uses the GPS system time output signal Tl to develop a trigger signal T2 for the timing and switching means 29.
Each ground installation would have a unique time slot or channel assi~ned so that it's transmissions are differentiated from those of surrounding or nearby ground stations. This trigger signal T2 is delivered by the Time 510t Selector 64 at speci~ic GPS clock times assigned to that specific ~round station for purposes of uniq~ely identifying that ground station. The number of times per second that the trigger signal T2 must be qenerated depends on the rate at which guidance signals are re~uired by the aircraft in order to have adequate guidance loop stability. A nominal value is twenty times a second. The timin~ and switching circuitry 29 sets the switch 26 to the correct position, provides delays, and drives the ~ransmitter 32 to deliver the omni encoded reference signal A followed by the two sets of paired directive signals B and C, and D and E.
Turning now to the aircraft installation, just as in the case of the ground installationr an aircraft need only be provided with a simple GPS receiver/computer which provides an output signal Tl representative of GPS system time. This receiver 68 is also provided with a suitable antenna 70 and the output of ~he receiver is connec~ed to a time slot selector 72. Prefer~bly, the Time Slot Selector 72 is tunable to whatever channel the pilot desires in order to receive a selected gro~nd installation whicn, in accordance with the drawin~, is the channel - ~l2~i7~
PATENT
-lS- B02348 corresponding to the ground installation of FIG. 1. Once the Time Slot Selector 64 is tuned to the ground station of interest, the synchronous guidance signals returned from the ground installation throu~h the aircraft antenna 05 3 and aircraft receiver 8 become isolated by the time slot gating process and thus become identifiable as returns of interest to that aircraft, as distinguished from same-freauency synchronous landing guidance sign~ls from other ground installations in the vicinity transmitting at the same fre~uency.
Since the time at which the selected qround installation transmitter 32 is triggered into operation is known, and since the time that it takes for the synchronous transmitted signals to be received in the aircraft can be measured, the ranqe gate and navigation processor 15 can easily calculate the ranqe between the aircraft and the ground installation; This ranqe may be displayed on a diqital readout 18 in the cockpit of the aircraft. If the GPS receiVer/comPuter installed in the aircraft is a "full computer", an output signal PA can be obtained which is representative of the aircraft's geographic position relative to the surface of ~he earth.
Since the ground installation of interest is fixed on the earth, the position of the ground installation PG can be used, together with the aircraft position signal PA, to obtain a direct readout of the range 18' between the aircraft and the ground installation. This readout may also be used as a cross reference or check on the range readout 18 obtained by measuring the time between the transmission and the receipt of signals at tbe aircraft.
It may also be used with altitude information to determine glideslope position and, hence, as an instrument cross reference check. Furthermore, it may be used wi~h glideslope information to cross-check the aircraft's 3S altimeter.
67~
PATENT
-16- 8023~18 The ~iming accuracy of the GPS clock time ~rigger signal Tl is limited by the GPS circular error probability ; ~CEP). Conventional P-Code and C/A code CEP's are expected to be 10 to 50 meters. Differential P-Code and 05 differential C/A CEP accuracy is typically from 2 to 6 meters. Since the GPS CEP and hence GE'S clock errors due to unknown propagation delays at both the ground and airborne installations should be the same, assuming the use of similar GPS constellations, the range obtained by measuring the diffe~ence or interval between the GPS clock time at which landing guidance signals are trans~itted from the ground and the GPS clock time at which they are received in the air ~i.e., clocked ran~e measure~ent) would have these unknown e~rors eliminate~. Therefore, the clocked range measurement accuracy should coincide with differential GPS position accuracy (i.e., better than conventional). In other words, we have the surprisinq result that the use of clocked GPS time for channelization results in a range-to-touchdown measurement approaching ; 20 that of differential GPS, while using only a conventional GPS receiver.
Those skilled in the art will appeeciate the fact that excellent range measurement accuracy is of importance for the landing operation, particularly for those applications where centerline guidance is desired for an offset g~ound beacon installation. This would be especially useful in military applications. Centerline guidance may be achieved by the use of the ranqe readout 18 and course deviation indicator 20 signals.
The number of stations that can be uniquely and usefully identified by this use of GPS clock time and associated time slotting technique may be deteFmined by considering the following:
PATENT
(1) No two ground stations should radiate close enough together in time such that signals from one ~round station can possibly arri~e at an aircraft and be detectable and, hence, potentially generate siqnals falselY usable to 05 generate landing quidance in the time slot assigned to another ground station. To preclude this occurrance, it is necessary to first establish the distance from a ground station at which an aircraft can usabl~ detect signals fro~ that ground station. Assuming this distance is 80 miles, then no two ground stations in such proximity can, in qeneral, radiate guidance signals closer toqether in time than the noted 80 miles, multiplied by the 6 microseconds per mile speed of radio frequency proPagatiOn, or approximately 500 microsecon~s. If this precaution is not taken, then sigOals from both qround stations could possihlY arrive at one aircra~t in the J P~ ~e~ O~i~ ~ time slot G2 Aurinq which the / /~ processor is activated to qenerate landinq guidance, and hence, either cause garbling or pro~ide quidance data directin~ the aircraft to the wronq ground station.
(2) A further consideration in this regard is that each ground station should, preferably, radiate 20 quidance pulse groups per second to maintain quidance loop stability. This then means that each ground station must have allocated to it, each second, 20 x 500 microseconds, or 10,000 microseconds, durinq which no other qround station in the noted 80 mile proximity can radiate. This 10,000 microseconds, or 0.0l seconds total time interval, must therefore be alloca~ed to each ground station per second for this typical illustration. This means that only 100 uniquely identifiable ground stations can be located in the noted 80 mile proximity. As a practical mat~er, therefore, one can note that at least some 100 unique sets of identifying radiatin~ times can be assigned for uniquely identifying any one of 100 same frequency ground stations within an 80 mile radius area. This PATENT
-18- ~02348 number is more than adequate since, as noted previously, the conventional ILS system has only 40 fre~uency channels assigned for unique identity purposes.
Finally, the airborne navigation processor 15 may be 05 provided with a relatively narrow range gate for tracking all ground installation response signals, including the omni signals A and the paired directive signals B and C, and D and E from the ground installation. The directive signals would be processed to give precision guidance to the pilot, using the visual course deviation indica~or display 20. Infrequently, however, other same-frequency signals from the selecte~ landing installation or other landin~ installations in the vicinity (such as generated by a system covered by my Patent 4,429,312), may fall within the range gate. The effect of these same-frequency signals will be minor, if averaged with the desired signals from the selected ground installation, since they occur relatively infrequently and are not synch~onous to GPS time~ This minor effect may be further minimized by storing the values of all received signals that fall within the range gate in a computer memory, and by using, for guidance purposes only, those stored signals that fall within prescribed limits of a running average of all ; siqnals. This is termed ~wild-point" editing.
This invention is not to be limited to the embodiments shown and described, because changes may be made within the scope of ~he following claims. ~or example, the technique of ~y ~.S. Patent 4,429,312 may be used simultaneously with the technique described in this patent application. This simultaneous use may be of particular advantage in making a transition to GPS ~i.e., the ground installation could radiate both on a clock basis and in response to weather radar interrogations or free run. Thus, it should be understood that no :
~2~67~34 PATENT
limitation with respect to the specif ic apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims, all such ~odifications as fall within the scope of the : 05 claims.
, . . .
In conventional landing systems, such as the 05 conventional Instrument Landing System tILS) and the FAA
Microwave Landing System (MLS)~ uniaue identification an~
signal exchanges between approaching aircraft and a particular ground installation are established by uniquely assigning different freauencies out of a band of frequencies to each of the various installations, and tuning the airborne units to the frequency of the selected installation. The FAA MLS system has 200 separate ~eequency channels assigned for its use in the band of 5000 to 5250 MHz. The ILS system has some 40 channels in paired bands allocat~d to its use in the vicinity of 100 ~ and 300 MHz. Therefore, an adequate number of separately ; indentifiable channels for a sinqle frequency landing system can be inferred as being between 40 and 200 channels. In ~y U.S. patent 4,429,312 entitled "Independent Landing Monitoring Systemn, a different tYpe of identification of a same-freauencY landing installation is discussed in which some of the signals transmitted to ~he aircraft are pulse encoded to identify that installation.
That system is generally satisfactory when the aircraft is in a remote area isolated from other ground stations and when ~he aircraft has a weather radar to interrogate the ground installation, a decoding circuit added to the radar together, and an appropriate code selector switch for station selection in the cockpit.
However, not all aircraft bave weather radars to ; interrogate the ground installation. In addition, where there are several airfields in close geographic proximity, or where there are several landing installa~ions of this type at tbe same airpo~t, the same-frequency signals PATENT
_3- B02348 from all such landing systems can arrive at the aircraft simultaneously and hence they cannot be adequately separated for unique range tracking identification and guidance generation purposes. This is basically the same 05 problem that plagues the conventional Air Traffic Control Radar Beacon System (ATCRBS) used by the FAA for air traffic control purposes, it is called ~garblingn. The weather radar technique of my U.S. patent 4,429,312, with associated identifying codes, is thus very suitable for use at isolated remote sites, such as offshore oil ri~s, but not sui~able for areas with many same-freauency landing systems in close proximity. The problem comes basically from the fact that these syste~s, and the airborne radars all use a common frequencv. Thus, there is no way to trigger one particular installation uniquel~
for positive identification purposes. There is, therefore, always the risk of undesirably triqqering a nearby installation, with the result that confusing responses to the aircraft from b~th locations will be synchronously received in that aircraft.
In addition to a method of uniquely identifyinq a particular same frequency ground station, a very desirable characteristic for a landing system is the capability of providinq range information. Range data has at least three major uses:
1. a means for alerting the pilot of his proximity to touchdown;
2. a means for automatically reducinq the ~ain of the landinq installation as the aircraft ranqe to touchdown diminishes in order to maintain loop stability (often referred to as ~course softening'`): and 7~
PATENT
_4_ B~2348 3. a means for usinq the elevational angular data provided by the landing system to determine altitude above ~he runway during the approach.
.' :
In the conventional ILS sys~em, range to 05 touchdown is generally provided by marker beacons on the ground at establi~hed distances from touchdown. These beacons radiate vertical fan shapped-beams throuqh which the aPproaching aircraft passes~ The ranqe information thus acauired in the aircraft is used for pilot alertinq and for "course softening" purposes.
In FAA MLS and conventional ILS practice, an alternative and more accueate measurement of ranqe is provided by conventional TACAN/DME interrogators which are carried by almost all aircraft. The airborne TACAN/D~E
equipment interrogates a D~E beacon that is co-locat~ed with the MLS or ILS ground ins~allation and receives therefrom a direct measurement of range using usual D~E
techniques.
For some landing apPlications, a very precise measurement of range is required, and for t~is purpose, a Precision DME (usually referred to as PDME) is employed.
The PDME is similar to the conventional DME, but uses faster eise ti~e pulses to obtain higher precision. This PDME system imposes on aircraft, which have to use it in order to obtain a re~uired very precise measurement of range, the additional burden of having installeld on board appropriate PDME airborne equipment. Another technique for obtaining precision range in a landing system is provided by the teaching of my U.S. pa~en~ 4,42~,312.
Ran~e is measured in this disclosure by having the weather radar interrogate the landing system qround installation and trigger the transmission of pulsed angular guidance signals. These pulsed replies are synchronous with the ~_2~;t7~
PATENT
_5_ B02348 weather radar interrogations and are range ~racked in a conventional manner to provide precision range in the aircraft. Range measurements of higher precision can be ob~ained by the use of ast rise time pulses.
05 Both of the above described methods for identifyinq ground station installations (i.e., freauency selection or pulse group encoding) require additional equipment and adjustable cockpit controls for either tuning to the freauencv of the ground installation, or for selecting the decodement of the signals radiated from that ground station. In addition, measurement of ~an~e, by means of marker beacons or DME equiPment~ reauires the installation of appropriate marker beacons or D~E beacons at the landing system ground installation. The measurement Oe very precise range requires the addition of speciali2ed PDME eauipment, both air and ground. ~hile the use of the weather radar to provide precision range ~as taught in my U.S. patent 4,429,312) eliminates the need for added PD~E
eauipment, not all aircraft carry a weather radar. ~hus, all conventional landing sYstems have tended to require added airborne equipment, or cockpit controls, or both, in order to achieve unique communication with and range to a selected ground installation.
In addition to the two techniaues discussed above for obtaining range (i.e., the aircraft passing over marker beacons and the measurement of the time elapsed between an airctaft's transmission of an in~errogation and the aircraft's reception of a reply from a transponder located at ~he landing system), there is the clocked station technique. That technique may be practiced using high precision clocks and low precision clocks. For example, equipment in one participating uni~, such as a ground or airborne station, ~ransmits a signal at a known time in an established very precise cloc~ system.
PATENT
-6- - ~02348 Equipment in a second unit, such as an aircraft, measures the time of reception of that transmittled siqnal in ~he same established cloc~ system; by knowing the time at ~hich the signal was transmitted, the propa4ation time 05 between the t~o stations and thus the distance can be computed. One known use of this clocked ranging method is the United States Air Force AN/APN-169, Station Xeepinq Equipment (SKEt.
Another means for establishing a co~mon clock time, is for each Participant to carry low-cost clocks of nominal stability and to periodically synchronize those clocks to a common time reference. Such synchronization of low-cost clocks may be established by an initial conventional two-way ranqin~ process that determines the ranges between participants and thereafter uses measured range data, by an exchanqe between participants of relative clock times. Tnus, the low-cost clocks of each participant are synchronized to a clock in one selected aircraft out of all participating aircraft. This synchroniza~ion process is then repeated at periodic intervals, which intervals occur frequently enough to maintain the common time base to adequate accuracy. A
variation of this method of synchronizing all clocks to a clock in a selected unitJis to synchronize all clocks to an "average value~ of all the clock times that exist when the clock synchronization process is initiated.
Therefore, this "local" synchronization process requires a precision ranging and a data exchange cr communications system, including trans~itters an~
30 eeceivers in each Participating unit. A requirement for clock synchronization equipment, in all aircraft, is undesirable in manY applications, (i.e., cost, wei4ht and complexity.
6~
PATENT
_7_ Bn2348 One advantage of using a common clock system i~ that identity may be established by the use of ~time slotting". In this time slotting concept, each of the participants is assigned a specific clock time at which to 05 radiate, which time repeats at specified intervals. For example, a specific participant, such as No. 3, mi~ht radiate on the t~ird second of every minute. Associa~ed with this radiation at a specific time is a subsequent time interval or time slot, durinq which no other participant can radiate. This use of an established clock time and an associated time slot, by the participant to which it is assigned, permits reception of that transmission by other participants to be used to establish the identity of the sender of that tcansmission (i.e., anY
transmission ~eceived during that time interval must be from the participant assi~ned to transmit in that time slot). This use of an assiqned time slot or time period to pro~ide a protected identity system can be viewed as being similar to the use of a distinctive frequency for ~0 identity, which frequency cannot be used by another ; station in a specific geographic area.
W~ile the use of established and precise clock time, with precision of the order of a fraction of a microsecond, can provide both precision ranging and unique identitY, current use of such a common and precise clock time is limited by the attendant cos~ and complexity of very stable clocks, such as atomic cloc~s, or by the cost and complexity of the synchronizin~ equipment ~i.e., communications system, etc.) required for lower cost 3~ cloc~s.
Considered broadly, a landing system does not inherently require the use of multiple different ~ frequencies since operation at all installation si~es is ; usually performed on a single fre~uency. Single-frequency - ~. .. :
i'7~3~
PATENT
operation is an advantage because, if the actual landing guidance system can always operate on the same f requency for different sites, great simplification in ter~s of airborne equipment complexity and cost is possible. For 05 example, the airborne receiver can be a fixed-frequency device.
;
Therefore, it sho~ld be appreciated that a landinq system is needed which would provide both station selection and ranging data in a fixed frequency landing system, while using only airborne equipment whic~ is installed in an IFR ~Instrument Flight Rules) aircraft.
Moreover, such a system should be simple and low in cost.
With the eventual addition of NAVSTAR or Global Positining System ~GPS) Navigation Sets on all but the smallest aircraft, it also would be ver~ desirable to find a means to use GPS to provide channelization (i.e., identity) and range data for single frequency landinq systems. This is especially true since such fixed freauency landing svstems are, inherently, lower cost and are found in more - 20 locations throughout the world. Thus, there is a need for an advanced instrument landing system.
Summary of the Invention In accordance with one embodiment of the present invention, a landing system is provided comprising a ground installation and an airborne installation wherein each installation is provided with a GPS receiver that produces a trigger signal representa~ive o~ GPS system time, a ground installation which radiates a sequence of transmissions which include precision guidance information to assist the aircraft in landing, and an aircraft installation having a receiver and a processor for - transforming the precision guidance si~nals to provide the pilot of the aircraft with indications for guiding the landing of the aircraft. ~ore specifically, the trigger PATENT
signal from the GPS receiver at the ground installation is further processed to provide, at a specific time assigned to a specific ground installation, a second triqger or actuation signal. The ground installation, in response to 05 the actuation signal, radiates the aircraft guidance signals. In the aircraft installation, the triqger signal from the airborne GPS receiver is further processed to provide, at a specific time ~i.e., corresponding to the selection of a specific ground station from which it is desired to receive landinq guidance, and a time interval or slot durins which signals can be usefullY received from such selected 9round stations), a signal for actuating the processor to permit such processor to generate landing guidance information. This technique of using GPS clock time, which clock time is available at no incremental cost to a GPS receiver, thus provides positive identity of a selected low-cost single frequency landing system ground station.
In one specific embodiment of the invention, the sequence of signals radiated from the qround installation includes a ranging reference signal. These ran~ing reference signals, and associated guidance pulses, will be synchronously repetitive as received in the aircraft, with respect to the GPS clock time triqger signals generated in the aircraft. The time at which the rangin~ reference siqnal is received in the aircraft can thus be measured by conventional synchronous range tracking circuits, and the distance between the aircraft and the ground installation can thus be precisely calculated. Thus, GPS clock time 3Q provides precision range data to a selected low-cost single frequency ground station.
~ ith the advent of GPS, precisely synchronized airborne clocks will become commonplace in airbocne vehicles. Thus, availability of such universal ~ime makes 6~
PATE~T
-ln- B02348 the consideration of apPlying such clock techniques on a wider scale very appropriate. Other advantages and features of the present invention will be come readily apparen~ from the detailed description of the invention, 05 the embodiments presented, the accompanying drawings, and the claims.
., Brief Description of the Drawinq FIG. 1 is a block diagram of the advanced landing system that is the s~bject of the present invention.
Detailed Desciption While this invention is susceptible of embodiment in many different forms, there is shown in the drawing and will herein be described in detail, one specific embodiment, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated.
A preferred embodiment of the presen~ advanced landing system will ~e illustrated and described with reference to the landing system of my U.S. patent 4,429,312. It should be understood, however, that this invention provides techniques which are applicable to many different landing systems, and therefore it is not limited ~o improvements to the precision landing sys~em of the type shown and described in that patent.
As shown in ~hat patent, and described in Columns 8 and 9 thereof, the patented system provides a ground based precision landing guidance installation which radiates localizer and glideslope guidance beams from separate antennas which are directed along the approach path toward 7~
PATENT
a landing aircraft, and which are received in the aircraft and processed to recover signals which provide landing indications to the pilot. In the system shown in that patent, the ground based system can ei~her be trigqered to 05 generate replies in synchronization to some reference si~nal, as for example in synchronization to siqnals received from a weather radar, or can be free running and simply received and used by an approaching aircraft. The former type of sYstem is the system to which the present improvements are directed and therefore the free running mo~e of ope~ation of the landing system will not be furthec discussed.
Turning to FIG. 1, the landing system there illustrated includes a ground installation and an airborne installation. The ground installation includes, for lateral aircraft guidance, two directive antennas 21 and 22 having precision guidance localizer antenna beam patterns 23 and 24, with cross-sections marked B and C.
The ground installation also includes a non-directive antenna 5 deliverin~ an omni-pattern ~5. These antennas 5, 21 and 22 are connected by a switch 26 and cable 27 to a radar transmitter 32. ~iming and switching circuitry 29 controls the switch 26 and initiates the outputs of the transmitter 32.
The directive antenna patterns of two paired glideslope antennas 33 and 34 are aligned and partially overlapped respectively above and below a predetermined glideslope tusually 3 deg~ees), so that for aircraf~
approaching precisely along the glideslope, the signal intensities received in the aircraft from these paired an~ennas 33 and 34 will be equal.
For vertical glidelsope guidance, the ground installation furt~er includes two directive antennas 33 ~2~
PATENT
and 34 for radiating paired precision glideslope guidance beams 33a and 34a, with cross-sections marked D and E.
These two antennas 33 and 34 are likewise connected to the transmitter 32 through the antenna switcn 26. The paired 05 beam patterns of the antennas 23 and 24 for lateral guidance overlap so that they provide equal intensity signals along the extended centerline C'L of the runway.
Thus, if the signal intensities of both antennas are eq~al, as received in the airborne vehicle, it must be laterally located over the centerline of the runway.
For an on-course approach, all four guidance signal intensities received in the aircraft will be equal.
However, deviation above or below, or to the riqht or left of the desired approach course, wiill cause an unbalance in the paired signals received at the receiver, indicating to the pilot the direction in which the aircraft has deviated from the desired course. This operation is thoroughly described in my U.S. patent 4,429,312.
~' The airborne installation of the system according to ~hat paten~ is shown in the right in FIG. 1, and includes a radar receiver 8 and an antenna 3. The eadar receiver 8 is connected to a range gate and navigation processor 15 which provides range data to a range readout 18 and to a course deviation indicator 20 connected thereto.
A transmission from the ground installation is initiated by sending a trigger signal T2 to the timing and switching circuitry 29. ~his ~ransmission includes a sequence of multiple successively delivered signals.
First, the timing and switching circuitry 29 delivers, through the omni antenna 5, a coded pulse group reference signal A from the ~ransmitter 32. The strength of the reference signal is used to set ~he qain of the aircraft receiver 8 so as to keep the airborne receiver operating . . ~
~9~7~4 PATENT
within a linear portion of its response charac~eeistic.
After a fixed delay, determined by the timing a~d switching circuitry 29, the switch 26 then steps sequentially to connect the transmitter 32 in turn to each 05 of the four directive antennas 21, 22, 33 and 34 to deli~er transmissions, includin~ right and left paired locali~er pulses, and up and down paired glidesloPe pulses. These Pulses are delivered one at a ti~e with suitable delays between them. Ad~ustable attenuators 44 serve to balance the antenna drives so that the guidance signals are all of equal amplitude when the aircraft is exactly on course for landing, as explained in my U.S.
patent 4,429,312. The sequence of these four quidance signals is predetermined and fixed so that the aircraft can identify the signals by their order in the sequence.
The pulses radiated in these precision guidance beams B, C, D and E in FIG. 1, plus the reference signal group A from the omni antenna, are received at the airborne antenna 3, and delivered by the receiver 8 to a processor 15 in the aircraft. The processor 15 is programmed to use the reference signal A to determine range and to display it at the range readout 18, and to use the precision landing signals B, C, D and E to create and deliver to the course deviation indicator 20 output signals which show the position of the aircraft ~ith respect to the desired approach path.
The equipment used to uniquely identify the ~round installation of FIG. 1 will now be described. The ground ; installation of FIG. 1 is provided with a GPS
receiver/computer 50, a GPS antenna 62 connected to the receiver, and a Time Slot Selector 64. The GPS
receiver/computer 60 provides precision qeographic position using the GPS or NAVSTAR Satellite system 66.
The ground installation need only be provided with a 7~34 PATENT
receiver suitable for providing an output signal Tl representative of GPS system time. The availability of such receivers is becominq all the more commonplace. A
relatively current description of available equipment is 05 provided in the November 4, 1985 edi~ion of Aviation ~eek S~ce Technolo~y, "Global Positioning Develops As Civil Navigation System~, page 58. The Time Slot Selector 64 uses the GPS system time output signal Tl to develop a trigger signal T2 for the timing and switching means 29.
Each ground installation would have a unique time slot or channel assi~ned so that it's transmissions are differentiated from those of surrounding or nearby ground stations. This trigger signal T2 is delivered by the Time 510t Selector 64 at speci~ic GPS clock times assigned to that specific ~round station for purposes of uniq~ely identifying that ground station. The number of times per second that the trigger signal T2 must be qenerated depends on the rate at which guidance signals are re~uired by the aircraft in order to have adequate guidance loop stability. A nominal value is twenty times a second. The timin~ and switching circuitry 29 sets the switch 26 to the correct position, provides delays, and drives the ~ransmitter 32 to deliver the omni encoded reference signal A followed by the two sets of paired directive signals B and C, and D and E.
Turning now to the aircraft installation, just as in the case of the ground installationr an aircraft need only be provided with a simple GPS receiver/computer which provides an output signal Tl representative of GPS system time. This receiver 68 is also provided with a suitable antenna 70 and the output of ~he receiver is connec~ed to a time slot selector 72. Prefer~bly, the Time Slot Selector 72 is tunable to whatever channel the pilot desires in order to receive a selected gro~nd installation whicn, in accordance with the drawin~, is the channel - ~l2~i7~
PATENT
-lS- B02348 corresponding to the ground installation of FIG. 1. Once the Time Slot Selector 64 is tuned to the ground station of interest, the synchronous guidance signals returned from the ground installation throu~h the aircraft antenna 05 3 and aircraft receiver 8 become isolated by the time slot gating process and thus become identifiable as returns of interest to that aircraft, as distinguished from same-freauency synchronous landing guidance sign~ls from other ground installations in the vicinity transmitting at the same fre~uency.
Since the time at which the selected qround installation transmitter 32 is triggered into operation is known, and since the time that it takes for the synchronous transmitted signals to be received in the aircraft can be measured, the ranqe gate and navigation processor 15 can easily calculate the ranqe between the aircraft and the ground installation; This ranqe may be displayed on a diqital readout 18 in the cockpit of the aircraft. If the GPS receiVer/comPuter installed in the aircraft is a "full computer", an output signal PA can be obtained which is representative of the aircraft's geographic position relative to the surface of ~he earth.
Since the ground installation of interest is fixed on the earth, the position of the ground installation PG can be used, together with the aircraft position signal PA, to obtain a direct readout of the range 18' between the aircraft and the ground installation. This readout may also be used as a cross reference or check on the range readout 18 obtained by measuring the time between the transmission and the receipt of signals at tbe aircraft.
It may also be used with altitude information to determine glideslope position and, hence, as an instrument cross reference check. Furthermore, it may be used wi~h glideslope information to cross-check the aircraft's 3S altimeter.
67~
PATENT
-16- 8023~18 The ~iming accuracy of the GPS clock time ~rigger signal Tl is limited by the GPS circular error probability ; ~CEP). Conventional P-Code and C/A code CEP's are expected to be 10 to 50 meters. Differential P-Code and 05 differential C/A CEP accuracy is typically from 2 to 6 meters. Since the GPS CEP and hence GE'S clock errors due to unknown propagation delays at both the ground and airborne installations should be the same, assuming the use of similar GPS constellations, the range obtained by measuring the diffe~ence or interval between the GPS clock time at which landing guidance signals are trans~itted from the ground and the GPS clock time at which they are received in the air ~i.e., clocked ran~e measure~ent) would have these unknown e~rors eliminate~. Therefore, the clocked range measurement accuracy should coincide with differential GPS position accuracy (i.e., better than conventional). In other words, we have the surprisinq result that the use of clocked GPS time for channelization results in a range-to-touchdown measurement approaching ; 20 that of differential GPS, while using only a conventional GPS receiver.
Those skilled in the art will appeeciate the fact that excellent range measurement accuracy is of importance for the landing operation, particularly for those applications where centerline guidance is desired for an offset g~ound beacon installation. This would be especially useful in military applications. Centerline guidance may be achieved by the use of the ranqe readout 18 and course deviation indicator 20 signals.
The number of stations that can be uniquely and usefully identified by this use of GPS clock time and associated time slotting technique may be deteFmined by considering the following:
PATENT
(1) No two ground stations should radiate close enough together in time such that signals from one ~round station can possibly arri~e at an aircraft and be detectable and, hence, potentially generate siqnals falselY usable to 05 generate landing quidance in the time slot assigned to another ground station. To preclude this occurrance, it is necessary to first establish the distance from a ground station at which an aircraft can usabl~ detect signals fro~ that ground station. Assuming this distance is 80 miles, then no two ground stations in such proximity can, in qeneral, radiate guidance signals closer toqether in time than the noted 80 miles, multiplied by the 6 microseconds per mile speed of radio frequency proPagatiOn, or approximately 500 microsecon~s. If this precaution is not taken, then sigOals from both qround stations could possihlY arrive at one aircra~t in the J P~ ~e~ O~i~ ~ time slot G2 Aurinq which the / /~ processor is activated to qenerate landinq guidance, and hence, either cause garbling or pro~ide quidance data directin~ the aircraft to the wronq ground station.
(2) A further consideration in this regard is that each ground station should, preferably, radiate 20 quidance pulse groups per second to maintain quidance loop stability. This then means that each ground station must have allocated to it, each second, 20 x 500 microseconds, or 10,000 microseconds, durinq which no other qround station in the noted 80 mile proximity can radiate. This 10,000 microseconds, or 0.0l seconds total time interval, must therefore be alloca~ed to each ground station per second for this typical illustration. This means that only 100 uniquely identifiable ground stations can be located in the noted 80 mile proximity. As a practical mat~er, therefore, one can note that at least some 100 unique sets of identifying radiatin~ times can be assigned for uniquely identifying any one of 100 same frequency ground stations within an 80 mile radius area. This PATENT
-18- ~02348 number is more than adequate since, as noted previously, the conventional ILS system has only 40 fre~uency channels assigned for unique identity purposes.
Finally, the airborne navigation processor 15 may be 05 provided with a relatively narrow range gate for tracking all ground installation response signals, including the omni signals A and the paired directive signals B and C, and D and E from the ground installation. The directive signals would be processed to give precision guidance to the pilot, using the visual course deviation indica~or display 20. Infrequently, however, other same-frequency signals from the selecte~ landing installation or other landin~ installations in the vicinity (such as generated by a system covered by my Patent 4,429,312), may fall within the range gate. The effect of these same-frequency signals will be minor, if averaged with the desired signals from the selected ground installation, since they occur relatively infrequently and are not synch~onous to GPS time~ This minor effect may be further minimized by storing the values of all received signals that fall within the range gate in a computer memory, and by using, for guidance purposes only, those stored signals that fall within prescribed limits of a running average of all ; siqnals. This is termed ~wild-point" editing.
This invention is not to be limited to the embodiments shown and described, because changes may be made within the scope of ~he following claims. ~or example, the technique of ~y ~.S. Patent 4,429,312 may be used simultaneously with the technique described in this patent application. This simultaneous use may be of particular advantage in making a transition to GPS ~i.e., the ground installation could radiate both on a clock basis and in response to weather radar interrogations or free run. Thus, it should be understood that no :
~2~67~34 PATENT
limitation with respect to the specif ic apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims, all such ~odifications as fall within the scope of the : 05 claims.
, . . .
Claims (24)
1. A system for guiding a vehicle along a path utilizing a plurality of satellites each of which transmits signals representing the position of the satellite and precise time in a common time reference, comprising:
a guidance station located at a fixed position on the earth including:
means for receiving said position and precision timing signals to provide a precision, guidance station clock signal in said common time reference;
means responsive to said precision, guidance station clock signal for transmitting, within a time period assigned to said guidance station, guidance signals with respect to said guidance path to direct a vehicle along said path;
said vehicle including:
means for receiving said position and precision timing signals to provide a precision, vehicle clock signal in said common time reference, said precision, vehicle clock signal being in synchronization with said precision, guidance station clock signal;
means for receiving said guidance signals; and means coupled to said guidance signal receiving means and responsive to said vehicle clock signal for processing guidance signals received only in said time period associated with said guidance station to guide said vehicle along said path.
a guidance station located at a fixed position on the earth including:
means for receiving said position and precision timing signals to provide a precision, guidance station clock signal in said common time reference;
means responsive to said precision, guidance station clock signal for transmitting, within a time period assigned to said guidance station, guidance signals with respect to said guidance path to direct a vehicle along said path;
said vehicle including:
means for receiving said position and precision timing signals to provide a precision, vehicle clock signal in said common time reference, said precision, vehicle clock signal being in synchronization with said precision, guidance station clock signal;
means for receiving said guidance signals; and means coupled to said guidance signal receiving means and responsive to said vehicle clock signal for processing guidance signals received only in said time period associated with said guidance station to guide said vehicle along said path.
2. A system for guiding a vehicle along a path as recited in claim 1 wherein said transmitting means transmits a pair of overlapping precision guidance signal beams symmetrically disposed about said path.
3. A system for guiding a vehicle along a path as recited in claim 2 wherein said transmitting means transmits said signal beams in a predetermined sequence.
4. A system for guiding a vehicle along a path as recited in claim 1 including a plurality of said guidance stations wherein the transmitting means associated with each of said guidance stations transmits said guidance signals at the same frequency.
5. A system for guiding a vehicle along a path as recited in claim 4 wherein each of said transmitting means has an operating range and each guidance station within the operating range of one or more other guidance stations has an assigned time period that is nonoverlapping with the time periods associated with the other of said guidance stations.
6. A system for guiding a vehicle along a path as recited in claim 1 wherein said transmitting means has an operating range and said assigned time period is sufficiently long to allow a guidance signal transmitted within said assigned time period to be received within said time period by a vehicle within said operating range.
7. A system for guiding a vehicle along a path as recited in claim 1 wherein said vehicle includes:
a range gate for tracking received guidance signals;
means for storing guidance signals falling within said range gate;
means for averaging said stored guidance signals- and means for determining whether a stored guidance signal falls within predetermined limits of said average, wherein only guidance signals falling within said limits are used to guide said vehicle along said path.
a range gate for tracking received guidance signals;
means for storing guidance signals falling within said range gate;
means for averaging said stored guidance signals- and means for determining whether a stored guidance signal falls within predetermined limits of said average, wherein only guidance signals falling within said limits are used to guide said vehicle along said path.
8. A system for guiding a vehicle along a path as recited in claim 1 wherein said guidance station has a fixed position on the surface of the earth and said vehicle means for receiving said position and precision timing signals includes means for determining said vehicle's geographic position relative to the surface of the earth and further including means for storing said fixed position of said guidance station; and means responsive to said vehicle's position and to said guidance station's position for determining the range between said vehicle and said guidance station.
9. A system for guiding a vehicle along a path utilizing a plurality of satellites each of which transmits signals representing the position of the satellite and precise time in a common time reference comprising:
first means disposed at a location on the surface of the earth for receiving said position and precision timing signals to provide a first clock signal in said common time reference;
means disposed at a location on the surface of the earth responsive to said first clock signal for transmitting within a time period assigned to said transmitting means guidance signals with respect to said guidance path to direct a vehicle along a path and a reference signal, said reference signal being transmitted at a precise time with respect to said first clock signal and within said time period;
second means disposed in said vehicle for receiving said position and precision timing signals to provide a second clock signal in said common time reference, said second clock signal being in synchronization with said first clock signal;
means disposed in said vehicle for receiving said guidance and reference signals; and means coupled to said guidance and reference signal receiving means and to said second means for processing guidance signals received only in said time period assigned to said transmitting means to guide said vehicle along said path, said processing means including:
means for determining the time of receipt of said reference signal in said common time reference; and one way range determining means for determining the range of said vehicle from said transmitting means based upon the difference between said precise time at which said reference signal is transmitted and said time of receipt of said reference signal.
first means disposed at a location on the surface of the earth for receiving said position and precision timing signals to provide a first clock signal in said common time reference;
means disposed at a location on the surface of the earth responsive to said first clock signal for transmitting within a time period assigned to said transmitting means guidance signals with respect to said guidance path to direct a vehicle along a path and a reference signal, said reference signal being transmitted at a precise time with respect to said first clock signal and within said time period;
second means disposed in said vehicle for receiving said position and precision timing signals to provide a second clock signal in said common time reference, said second clock signal being in synchronization with said first clock signal;
means disposed in said vehicle for receiving said guidance and reference signals; and means coupled to said guidance and reference signal receiving means and to said second means for processing guidance signals received only in said time period assigned to said transmitting means to guide said vehicle along said path, said processing means including:
means for determining the time of receipt of said reference signal in said common time reference; and one way range determining means for determining the range of said vehicle from said transmitting means based upon the difference between said precise time at which said reference signal is transmitted and said time of receipt of said reference signal.
10. A system for guiding a vehicle along a path as recited in claim 9 wherein said transmitting means includes an omni-directional antenna for radiating said reference signal.
11. A system for guiding a vehicle along a path as recited in claim 9 wherein said reference signal is encoded to identify said guidance station.
12. A system for guiding a vehicle along a path as recited in claim 9 wherein said guidance station has a fixed position on the surface of the earth and said second means disposed in said vehicle for receiving said position and precision timing signals includes means for determining said vehicle's geographic position relative to the surface of the earth and further including means for storing said fixed position of said guidance station; and range determining means responsive to said vehicle's position and to said guidance station's position for determining the range between said vehicle and said guidance station.
13. A system for guiding a vehicle along a path as recited in claim 12 including means for comparing the vehicle's geographic position relative to the surface of the earth as determined by said second means to the range between said vehicle and said guidance station as determined by said range determination means to check the accuracy thereof.
14. A system for guiding a vehicle utilizing a plurality of satellites each of which transmits signals representing the position of the satellite and precise time in a common time reference, comprising:
first means disposed at a location on the surface of the earth for receiving said position and precision timing signals to provide a first clock signal in said common time reference;
means disposed at a location on the surface of the earth responsive to said first clock signal for transmitting a range signal at a precise time with respect to said first clock signal and with a time period assigned to said transmitting means, second means disposed in said vehicle for receiving said position and precision timing signals to provide a second clock signal in said common time reference, said second clock signal being in synchronization with said first clock signal;
means disposed in said vehicle for receiving said range signal; and one way range means coupled to said range signal receiving means and to said second means for processing a range signal received within said time period to determine range from the difference between said precise time of transmission and the time of receipt of said range signal.
first means disposed at a location on the surface of the earth for receiving said position and precision timing signals to provide a first clock signal in said common time reference;
means disposed at a location on the surface of the earth responsive to said first clock signal for transmitting a range signal at a precise time with respect to said first clock signal and with a time period assigned to said transmitting means, second means disposed in said vehicle for receiving said position and precision timing signals to provide a second clock signal in said common time reference, said second clock signal being in synchronization with said first clock signal;
means disposed in said vehicle for receiving said range signal; and one way range means coupled to said range signal receiving means and to said second means for processing a range signal received within said time period to determine range from the difference between said precise time of transmission and the time of receipt of said range signal.
15. A system for guiding a vehicle as recited in claim 14 wherein said transmitting means further transmits a pair of overlapping precision guidance signal beams symmetrically disposed about a path within said time period and said processing means includes means for processing guidance signal beams received by said receiving means to guide said vehicle along said path.
16. A system for guiding a vehicle as recited in claim 15 wherein said transmitting means transmits said signal beams in a predetermined sequence.
17. A system for guiding a vehicle as recited in claim 14 wherein said first means and transmitting means are located at a ground station.
18. A system for guiding a vehicle as recited in claim 17 including a plurality of ground stations wherein the transmitting means associated with each of said ground stations transmits said guidance signals at the same frequency.
19. A system for guiding a vehicle along a path as recited in claim 18 wherein each of said transmitting means has an operating range and each ground station within the operating range of one or more other ground stations has an associated time period that is nonoverlapping with the time periods associated with the other of said guidance stations.
20. A system for guiding a vehicle as recited in claim 14 wherein said transmitting means has an operating range and said time period is sufficiently long to allow a range signal transmitted within said time period to be received within said time period by a vehicle within said operating range.
21. A system for guiding a vehicle as recited in claim 14 wherein said transmitting means includes an omnidirectional antenna for radiating said range signal.
22. A system for guiding a vehicle as recited in claim 14 wherein said range signal is encoded to identify said guidance station.
23. A system for guiding a vehicle as recited in claim 14 wherein said guidance station having a fixed position relative to the surface of the earth and said vehicle means for receiving said position and precision timing signals includes means for determining said vehicle's geographic position relative to the surface of the earth and further including means for storing said fixed position of said guidance station; and range determining means responsive to said vehicle's position and to said guidance station's position for determining the range between said vehicle and said guidance station.
24. A system for guiding a vehicle along a path as recited in claim 23 including means for comparing the vehicle's geographic position relative to the surface of the earth as determined by said second means to the range between said vehicle and said guidance station as determined by said range determination means to check the accuracy thereof.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86366286A | 1986-05-15 | 1986-05-15 | |
| US863,662 | 1986-05-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1296794C true CA1296794C (en) | 1992-03-03 |
Family
ID=25341527
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000537206A Expired - Fee Related CA1296794C (en) | 1986-05-15 | 1987-05-15 | Advanced instrument landing system |
Country Status (6)
| Country | Link |
|---|---|
| CN (1) | CN1009964B (en) |
| AU (1) | AU600740B2 (en) |
| CA (1) | CA1296794C (en) |
| IL (1) | IL82496A0 (en) |
| WO (1) | WO1987007030A1 (en) |
| ZA (1) | ZA873358B (en) |
Families Citing this family (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4635064A (en) * | 1985-04-04 | 1987-01-06 | Sundstrand Data Control, Inc. | Microwave landing system |
| US4680587A (en) * | 1985-08-14 | 1987-07-14 | Sundstrand Data Control, Inc. | Instrument landing system |
| FR2611399B1 (en) * | 1987-02-27 | 1994-06-17 | Lmt Radio Professionelle | LANDING ASSISTANCE SYSTEM USING NAVIGATION SATELLITES |
| FR2626677B1 (en) * | 1988-02-01 | 1990-06-22 | Thomson Csf | RADIONAVIGATION SYSTEM |
| US5311194A (en) * | 1992-09-15 | 1994-05-10 | Navsys Corporation | GPS precision approach and landing system for aircraft |
| MX9604303A (en) * | 1994-03-25 | 1997-12-31 | Qualcomm Inc | A position determination method for use with analog cellular system. |
| CN101927834B (en) * | 2010-08-19 | 2012-09-05 | 中国航空工业第六一八研究所 | Automatic landing guide signal management method for airplane with three redundancies |
| CN104406605B (en) * | 2014-10-13 | 2017-06-06 | 中国电子科技集团公司第十研究所 | Airborne many navigation sources integrated navigation analogue systems |
| CN104332073A (en) * | 2014-10-27 | 2015-02-04 | 重庆布伦坦茨航空技术进出口有限公司 | Smart air traffic control system |
| CN108725819A (en) * | 2017-04-14 | 2018-11-02 | 刘明成 | Coordinate type aircraft carrier ship-board aircraft landing airmanship |
| CN113465603B (en) * | 2021-05-31 | 2023-05-16 | 西南电子技术研究所（中国电子科技集团公司第十研究所） | Takang navigation automatic channel selection method |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3434140A (en) * | 1966-10-26 | 1969-03-18 | John P Chisholm | Matrix navigation system |
| US3412399A (en) * | 1966-12-29 | 1968-11-19 | John P. Chisholm | Mobile unit ranging system |
| US3566404A (en) * | 1968-12-16 | 1971-02-23 | Trw Inc | Vehicle collision avoidance system |
| US3634862A (en) * | 1969-05-19 | 1972-01-11 | Westinghouse Electric Corp | Precision approach and landing system |
| US4445118A (en) * | 1981-05-22 | 1984-04-24 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Navigation system and method |
| US4429312A (en) * | 1981-07-24 | 1984-01-31 | Chisholm John P | Independent landing monitoring system |
| US4680587A (en) * | 1985-08-14 | 1987-07-14 | Sundstrand Data Control, Inc. | Instrument landing system |
-
1986
- 1986-05-11 ZA ZA873358A patent/ZA873358B/en unknown
-
1987
- 1987-05-12 IL IL82496A patent/IL82496A0/en unknown
- 1987-05-13 WO PCT/US1987/001087 patent/WO1987007030A1/en unknown
- 1987-05-13 AU AU76943/87A patent/AU600740B2/en not_active Ceased
- 1987-05-15 CA CA000537206A patent/CA1296794C/en not_active Expired - Fee Related
- 1987-05-15 CN CN87104303A patent/CN1009964B/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| WO1987007030A1 (en) | 1987-11-19 |
| AU7694387A (en) | 1987-12-01 |
| CN87104303A (en) | 1988-03-02 |
| CN1009964B (en) | 1990-10-10 |
| IL82496A0 (en) | 1987-11-30 |
| ZA873358B (en) | 1987-11-03 |
| AU600740B2 (en) | 1990-08-23 |
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| MKLA | Lapsed |