GB2059709A - Marine transponder system - Google Patents

Abstract

Each interrogator 1 has a scanning radar system 3, 4, 5 and a radio transceiver 9 connected to an omni-directional radio aerial 10 and each responder has a radar transreceiver 11 connected to an omni-directional radar aerial 12 and a radio transceiver 20 connected to an omni-directional radio aerial 21 all of the radio transceivers in the system operate at the same frequency. At each interrogator a radio frequency call sign is transmitted as its scanning radar beam sweeps across a selected responder and at the responder means responsive to the receipt (11R) both of radar energy from the scanning radar beam and the radio call sign (20R) controls the transmission (11T) at radar frequency of an identifying pulse code for receipt by the interrogator. At the interrogator the identifying pulse code is decoded and causes the radio transmitter equipment 9T at the interrogator to transmit an interrogating radio signal based upon the decoded identifying pulse code. At the responder receipt (20R) of the interrogating radio signal, causes the radio transmitter 20T thereof to transmit, for reception by the radio receiver 9R of the interrogator, data relating to the responder station. <IMAGE>

Description

SPECIFICATION Improvements in or relating to transponder systems This invention relates to transponder systems that is to say systems in which an interrogator station is able to interrogate a responder station in order to elicit data therefrom.

The invention is particularly applicable to marine transponder systems.

An object of the present invention is to provide improved transponder systems and in particular improved marine transponder systems.

According to this invention a transponder system comprises at least one interrogator station and at least one responder station, the or each interrogator station having a scanning radar system and a radio transmitter and receiver equipment which latter is connected to an omnidirectional radio aerial system and the or each responder station having a radar receiver connected to an omni-directional radar aerial system and radio transmitter and receiver equipment connected to an omni-directional radio aerial system and wherein at said interrogator station means are provided for transmitting a radio call sign via its omni-directional radio aerial system as its scanning radar beam sweeps across a selected target, at said responder station means responsive to the receipt both of radar energy from said scanning radar beam and said radio call sign transmitted by said interrogator station are provided to control the transmission via said omni-directional radar aerial system of said transponder station of an identifying pulse code at radar frequency for receipt by said radar system of said interrogator station, means are provided at said interrogator station for decoding said last mentioned identifying pulse code and causing the radio transmitter equipment of said interrogator station to transmit an interrogating radio signal based upon said decoded last mentioned identifying pulse code and means are provided at said responder station for causing the radio transmitter at said responder station to transmit for reception by the radio receiving equipment at said interrogator station data relating to said responder station.

Preferably said transponder system is a marine transponder system, in which case said interrogator station or stations and said responder station or stations may be either shore-based or ship-borne.

In each case an interrogator station may be combined with a responder station so that, for example, if a plurality of ships carry a combined interrogator and transponder station a shore based combined interrogator and responder station may interrogate any of said ships or any of said ships may interrogate another one of said ships or said shore-based station.

Preferably all of the radio transmitter and receiver equipments in the system operate on a single dedicated channel i.e. at the same frequency for both transmission and reception, preferably a UHF frequency.

At a responder station means may be provided responsive to the receipt of radar energy from the scanning beam transmitted by the radar system of an interrogator station for responding automatically with a pulse of energy transmitted via the omni-directional radar aerial system of the responder station at a radar frequency within the bandwidth of a radar receiver at said interrogator station and preferably at the same radar frequency as that of the radar energy of the scanning beam of said interrogator station. Thus, where, as will normally be the case, the radar system of the transponder station includes a PPI or other radar display unit targets interrogated by the scanning radar beam will exhibit a flag if that target is equipped for co-operation in the system.

Said radar system or systems may be such as to operate at S-band but preferably said radar system or systems are such as to operate at X-band.

Preferably the radar system at the or each interrogator station includes a PPI radar display and preferably the means provided at said interrogator station for transmitting a radio call sign via its omni-directional radio aerial system comprises electro-optical means for deriving a light input from a selected target displayed on said PPI display and means responsive to noise speckling effect as said scanning radar beam approaches said target for causing said last mentioned radio call sign to be transmitted.

Preferably said electro-optical means comprises a light probe which may be aligned with the display target by an operator via an optical sight, said light probe including a photoelectric diode arranged to receive defocused light from the region of the target on the PPI display whereby to provide an electrical output control signal in response to noise speckling effect as aforementioned said control signal being utilised to control the transmission of said radio call sign via the omni-directional radio aerial system of said interrogator station.

Preferably said light probe includes an energising control button which may be operated by an operator when he is satisfied that the light probe is satisfactorily in register with said selected target on said PPI display and which button he may hold activated until said selected target next brightens up as the scanning radar beam sweeps thereacross.

Preferably said light probe includes a second photoelectric diode which is arranged to receive focused light from said selected target on said PP display screen which is provided to read said identifying pulse code transmitted at radar frequency via the omni-directional radar aerial system of said responder station, the output of said second photo-electric diode being connected to said decoding means.

Preferably within said light probe are provided a semi-reflective outer ring and a semireflective inner disc the former arranged to direct defocused light from the region of a selected target on the PPI display to said first mentioned photo-electric diode and the latter being arranged to direct focused light from said selected target on the PPI display onto said second photo-electric diode.

Preferably between said outer ring and inner disc and the end of said light probe placed in operation on the screen of said PPI display is a focusing system provided to compensate for variations in the distance between the phosphor on the PPI screen and the aforementioned outer ring and inner disc.

Preferably the said end of said light probe comprises a resilient mount arranged to engage the faceplate of a PPI display tube.

Preferably in each case the omni-directional radio aerial system comprises a whip antenna and preferably said omni-directional radar aerial system comprises a slotted waveguide as known per se.

Preferably the means provided at said interrogator station for transmitting said interrogating radio signal based upon the identifying pulse code received at radar frequency from said responder station is such that said interrogating radio signal is transmitted in a time slot defined from the instant of radar transmission and preferably at the corresponding responder station the transmission of said data relating thereto is initiated if said interrogating radio signal is recognised as being based upon the identifying pulse code transmitted at radar frequency by said responder station and if said interrogating radio signal is received within an appropriate time slot defined by the reception of radar energy by the radar receiver of said responder station as the scanning radar beam of the interrogator station sweeps thereacross.

Preferably the arrangement for transmitting said data by said responder station is such that said data is transmitted in pulse code form with parity bits and including an identifying pulse code portion which may be compared at said interrogator station with said decoded identifying pulse code received at radar frequency by said interrogator station whereby the validity of the data received may be checked.

Preferably at said interrogator station means are provided for checking that the said identifying pulse code portion in said data received is compatible with said decoded identifying pulse code received at radar frequency, that parity is correct and that said data is received in an appropriate time slot.

Preferably said identifying pulse code transmitted by a responder station at radar frequency represents part of its normally allocated radio call sign, preferably the last digits thereof, and said identifying pulse code portion in the data transmitted by said responder station includes said normally allocated radio call sign in full.

Typically said data transmitted by a responder station where said responder station is ship borne comprises the full radio call sign of the ship, the registration letters of the ship, the type of vessel, the ship's compass heading the ship's helm setting (e.g. midships, port, starboard), the ship's speed and any emergency navigational information.

At the interrogator station means may be provided for displaying said received data in the form of a television type video display unit or in the form of a dot matrix display. In the last mentioned case particularly, said means for displaying said data may be contained in a housing of said light probe.

Preferably the housing of said light probe includes a bulb indicator arranged to be activated when said checking means at said interrogator station determines as aforesaid that the data received from said responder station is valid.

The invention is further described with reference to the accompanying drawings in which, Figure 1 is a block schematic diagram illustrating at 1 an interrogator station and at 2 a responder station forming part of a transponder system in accordance with the present invention, Figure 2 illustrates one form of combined interrogator and responder station which may be used in a system in accordance with the present invention, Figures 3.a., 3.b. and 3.c. schematically illustrate a light probe which may be used at an interrogator station in a system in accordance with the present invention, and Figures 4.a., 4.b. and 5 are explanatory diagrams.

Referring to Fig. 1 this illustrates, to the left as viewed, an interrogator station 1, and to the right as viewed, a responder station 2. For present purposes the interrogator station 1 is taken to be shore based whilst the responder station 2 is taken to be ship borne. In general, however, interrogator stations and responder stations in a system in accordance with the present invention may be shore based or ship-borne and each shore base and ship in the system may have both interrogator and responder facilities.

The interrogator station 1 consists of an X-band marine radar set of which the scanner, incorporating a rotating pencil beam antenna, is represented at 3, the transmit/receive unit is represented at 4 and the PPI display is represented at 5.

A light probe arrangement 6, described later in greater detail with reference to Figs. 3.a., 3.b.

and 3.c. is provided for use in selecting targets to be interrogated appearing on the PPI display 5. A control unit 7 provides logic, timing, interface and control functions whilst a display unit 8 is provided to display information received via a UHF transmitter/receiver 9 from the transponder station 2 as a result of interrogation. In this embodiment information display unit 8 is a separate video display unit but in other embodiments the information display unit is a dot matrix array character display unit in some cases incorporated in the body of the light probe 6.

The UHF transmitter/receiver has one dedicated channel, that is to say one channel of fixed frequency, and an omni-directional whip antenna 10. The transmitter portion is referenced 9T and the receiver portion is referenced 9R.

The responder station consists of an X-band responder 11 having an omni-directional slotted waveguide antenna 1 2 which responds to the carrier wave of the radar signal from the interrogator station 1 as the beam from the rotating antenna of scanner 3 sweeps across the aerial 1 2. The responder 11 consists of a receiver section 1 1 R and a transmitter section 11 T, with the transmitter section 1 1 T arranged to respond at the same frequency as is received by the receiver section 11 R. This is represented by the loop connection 1 3. The receiver section 1 R is gated as known per se in order to prevent interference from its own separate marine radar set (if fitted).The marine radar set is not shown but the gating control connection is represented at 14. The responder station 2 also has a control unit 1 5 providing for logic, timing, interface and control functions and derives preset data on input leads 1 6 and 1 7 and variable data on leads 1 8 and 1 9. The preset data applied to leads 1 6 and 1 7 consists of the ship's call sign on lead 1 6 and the vessel type and registration letters on lead 1 7. The variable data consists of navigational data (speed etc) on lead 1 8 and emergency data on lead 1 9. A UHF transmitter/receiver 20 with an omni-directional whip aerial 21 is provided to communicate the data received on leads 16, 17, 1 8 and 1 9 to the transmitter receiver 9 of the interrogator station 1 when controlled to do so by control unit 1 5. The transmitter portion of transmitter/receiver 20 is referenced 20T and the receiver portion is referenced 20R.

When no information is required of any target shown on the PPI display 5 the light probe 6 is laid to one side and the radar set of the interrogator station 1 is operated normally.

Once per scan of the scanner 3 a train of pulses at radar p.r.f. (typically fourteen pulses per bandwidth) will be received at the X-band receiver 11 R of the responder station 2. Upon the receipt of this, transmitter 1 1T of the responder 11 is energised so that for 2.5 seconds each fifteen seconds the responder 11 replies with a similar train of pulses each a few microseconds long at the frequency of the radar carrier of the interrogator station 1. This carrier frequency may, of course, be anywhere in the allotted band (9320-9500 MHz).

When the responder transmitter 1 1T of the responder station 2 is transmitting (this will be approximately one scan in every six by the scanner 3 under the control of timers within control unit 15) a line a few tenths of a mile long (equivalent to a few microseconds in duration) is displayed on the PPI display 5 radially outwards from the corresponding target echo and, of course, on the displays of all other X-band radars within range. In this particular example no data is transmitted by the transmitter 1 1 T but if desired the transponder 11 could be arranged to reply with a simple morse letter code giving limited information on the class of vessel e.g.

VLCC with limited manoeuvrability, tug with tow, etc.

At the interrogator station the response received from responder station 11 indicates that the target in question carries the responder equipment and can therefore be interrogated for further information when required as herein after described. Under the quiescent conditions so far described the transmitter/receiver 9 of the interrogator station 1 is not used and at the target responder station 2 the radio receiver 20R listens but transmitter 20T does not transmit. If such transmissions by a responder station as described above are likely to be confused with the responses of navigational aid beacons (racons), these may be omitted or suppressed at the responder station.

When a radar operator at the interrogator station requires further information concerning a target on the screen of the PPI display 5 whose occasional response "flag" indicates that it carries a responder the operator places the light probe 6 over the selected target echo on the display. When the operator has obtained correct register on the target echo of interest he presses an interrogator" button on the probe holding the probe in register on the target echo until the echo next brightens up as the radar beam from the scanner 3 passes across it within 2.5 seconds.

The light probe 6 includes a photo-diode which detects noise speckling on the radar display as the scan approaches the echo and via the control unit 7 switches on the transmitter portion 9T of the transmitter/receiver 9 a few tens of milliseconds before the echo actually brightens up. This causes a transmission over the omni-directional aerial 10 of a calling sign for approximately 50 milliseconds which is received by all targets within range which are carrying transponder equipments and thus a transmitter/receiver such as 20 of responder station 2. All transmitter/receiver radio equipments such as 9 at an interrogator station and 20 at a responder station operate at the same fixed frequency (dedicated channel).

The result is that targets on the same bearing from the interrogator station to the desired target responder station receive the interrogated radio transmission at the same time as the normal radar transmissions from the scanner 3 of the interrogator station 1. Targets outside of a five degree co-incidence arc do not receive radio and radar signals at the same time. Targets within this arc which do receive both radio and radar signals together (as determined by coincidence circuits within control unit 15) are arranged to transmit via their X-band transmitters corresponding to transmitter 1 1T a coded reply in response to each radar pulse received. Each target is allocated one of a number (e.g. 32) of display codes based on for example the last digits of its radio call sign.

The coded abbreviated call sign received via the radar set at the interrogator station is displayed on the PPI display 5 in the form of a dash-dot code line stretching outwards from the displayed target echo. This signifies to the operator that that selected target has responded. As will be appreciated however the character lengths may be too short for visual reading of the abbreviated call sign by the operator.

A photodiode in the light probe 6 reads the abbreviated call sign as a succession of brightenups with micro-second spacings. In general, although there may be two or three targets on the same bearing they will usually carry different codes and it is therefore possible uniquely to address a particular target on a bearing/code basis.

The transmitter portion 9T of the transmitter/receiver 9 now transmits a radio call carrying the abbreviated call sign of the desired target. This call occupies a defined time slot (say 50 to 1 50 milliseconds) measured from the instant of radar transmission. This radio transmission is acknowledged only by targets which (a) have the correct call sign and (b) which were illuminated by radar the correct number of milliseconds previously (i.e. when the interrogator station radar scanner 3 was bearing on the desired target). To ensure this selection, control unit 1 5 incorporates gates controlled by a call sign responsive circuit and a timing circuit set by the reception of the radar beam by receiver 11 R.

The desired target having identified itself now responds. Assuming the responder station is that illustrated at 2 the transmitter 20T under the control of control unit 1 5 transmits a UHF radio response which is digitally coded with identification and navigation data. In the example at present being described the identification consists of the full radio call sign (appearing on data input lead 16) the registration letters and the type of vessel (appearing on data input lead 17) the compass heading, helm setting and speed (appearing on lead 18) and emergency data such as "not under command" appearing on lead 20.

This radio transmission by transmitter 20T is controlled by control unit 1 5 to occur after cessation of the call received by receiver 20R in a defined time cell measured from the radar interrogation. The digital reply signal transmitted by transmitter 20T includes parity check bits.

At the interrogation station the receiver 9R is energised during the appropriate time cell under the control of control unit 7. The receipt of a radio signal during this time cell with valid parity check and full radio call sign which is compatible with the abbreviated call sign read from the radar display by the light probe 6 is regarded as indicating a valid response from the desired target. When control unit 7 determines that the response is a valid one a lamp carried by the light probe 6 is caused to flash to indicate this and, after decoding, the information is displayed by the information display unit 8. As will be appreciated however, if an operator requires further information from the target he may contact the target by radio telephone utilising its call sign and so ensuring that the ensuing conversation is with the correct target.

It is useful to consider the types of interference to which the system described above may be subject. In principle there are two types of interference (a) unwanted interrogations of targets and (b) unwanted responses at an interrogator.

With a dedicated radio channel for data communication one may assume that the interference arises only from other ships using the system. Radar interference can arise from any ship except that on which the responder is mounted since here the responder 11 is linked'with the ship's radar via link 14 as has already been described in order to prevent the responder being triggered by radar transmissions originating from the same ship and to prevent microwave responses from displaying on the radar.

If each radar in a system has an antenna beam width of approximately 1" the beam will bear upon the target antenna 12 for 1/360th of the time (e.g. 7 milliseconds each 2.5. seconds). Each interrogation from a radar occupies the responder microwave receiver 1 R and transmitter 1 1 T for approximately 100 microseconds and typically the radar recurrence interval is 500 microseconds (2,000 pulses per second) so that up to five radars may bear on the target antenna 12 simultaneously without excessive loss of responses.

Assuming:thirty radar operating ships within range with non-correlated scan rates and pulsed repetition frequencies the chance of serious interference is of the order of 1 1 30 > c x - > c X 100% = 1.67%.

360 5 If several radars happen to bear on the same target at one particular time and cause interference, variations in their scan rate make it most unlikely that the interference will persist in subsequent scans. Experience in viewing responses from navigation-aid racons, where the parameters are similar, confirms that interference of this sort is unlikely to be serious in service.

The radio interrogations by interrogation transmitter 9T each occupy typically 6" of the interrogating radar scan (total 100 milliseconds). During this period ships other than the desired target will display the lengthened code rather than the short response normally transmitted during periods of quiescent operation i.e. when no interrogation is initiated. If each of thirty ships is equipped with an interrogator such as 1 and makes one interrogation per minute (which in itself is a high figure) the probability of any ship receiving the lengthened response instead of the correct short response is 1 1 30 > c X x - 100% = 1%.

60 50 Such a probability is considered to be acceptable.

Considering interrogator station 1, responses transmitted by a target in response to other ships' radars will not be displayed unless its own radar is bearing on the target. At this time, responses to other ships can be displayed at ranges which are random because the various radar transmissions are not synchronized, and will therefore not often lie within the range bracket covered by the light probe. In addition it would be necessary for the interference to lie near the interrogators own radar frequency for it to be received and displayed.

For a ship using p.r.f. 2000pps, (maximum non-ambiguous range 40 miles) with probe bracket 0.25NM x 5" beamwidth 1", scan time 2.5s, receiver bandwidth 10MHz, total radar permissible band 180MHz, 30 ships in system, the probability of an interference response being picked up by the light probe is of the order of 0.25 5 10 1 x ----- x ----- x ---- x 2000 x 30 X 100% = 0.2% 40 360 180 360 The first four terms represent the proportion of range, angle, beamwidth and time that the radar receiver is open; the next three represent the number of interference pulses generated per second.

The effective radio beam width of the first interrogation will be approx 5 ; that is, any targets lying in a 5" arc centred on the bearing of the desired target will receive radio and radar interrogations in the proper time sequence. The relatively broad arc of 5" arises from the rather long time (50ms) to make a radio call in the limited available channel bandwidth.

If there are 30 targets within range, uniformly distributed over the sea area, the probability of two targets being in the 5" arc is 5 > c x 30 X 100% = 42%.

360 In practice the probability will be higher as traffic will often be concentrated on certain tracks, constrained, for example, by traffic separation schemes, and this is the reason why a second interrogation in which the target is readdressed with an abbreviated version of its own call sign is provided for in the system described above.

If each target is randomly allocated one of, say, 32 call signs and there are as many as 3 targets in the 5" arc there is approximately a 10% probability of one of the unwanted targets having the same call sign as the wanted target. In this case a garbled response may be received which will usually fail to meet the parity check requirements, with no information displayed.

The operator can initiate further interrogations. Eventually the unwanted target should go out of range or move off the bearing of the wanted target and information would then be displayed.

Referring to Fig. 2 this shows in block schematic form an interrogator/responder station which in effect is a single station incorporating the interrogator 1 and responder 2 of Fig. 1. In Fig. 2 parts corresponding to these shown in Fig. 1 bear the same reference numerals.

It is believed that from the previous description Fig. 2 will be found self-explanatory.

Referring to Fig. 3 the light probe is represented in side elevation at A and in plan at B. At C is represented a section in elevation of the optical system within the optical body 21 in Fig. 3.a.

and at D an operator's view into the eyepiece 23.

The main body 22 carries not only the optical body 21 with an eyepiece 23, a focusing ring 24 and a resilient rubber mount 25, but also an electrical bulb indicator 26 which, as labelled, indicates when a response transmitted by transmitter 20T is a valid one, as has already been described. The "press to interrogate" button already mentioned is represented at 27. A cable 28 connects the light probe to the control unit 7.

As illustrated a dot matrix display 29 is embodied in the main body 22 of the light probe.

This is illustrated for purpose of explanation and, if used, replaces the information display unit 8 of Fig. 1 or Fig. 2. As shown the information displayed is Call sign - GABC Class - VLCC Speed/Engine - 1 6 knots-full ahead Heading/Helm -- 345"M (M indicating "midships helm") Emergency Within the optical body 21 is the optical system shown at C. This consists of the aformentioned eyepiece 23 which focuses onto the face plate 30 of the tube of the PPI display 5. Dashed line 31 represents the phosphor on the inside surface of the face plate 30. In the optical path between the eyepiece 23 and the face plate 30 is a semi-silvered outer ring 32 and a semi-silvered inner disc 33.Semi-silvered ring 32 directs defocused light via a lens 34 onto a photocell 35 which provides an electrical output signal for use by the control unit 7, from either the response code read by the light probe 6 or from noise speckling as has already been described with reference to Fig. 1. The semi-silvered inner disc 33 directed focused light via lens 36 onto a further photocell 37 which provides an output whenever the target echo is painted.

The view seen by an observer's eye positioned at 38 is as shown at D. The eyepiece is provided with a cross-hairs sight 39 on which a target echo 40 is focused and centred. A coded response from the target 40 (the target call sign) is represented at 41.

In addition to the elements shown in Fig. 3.c. other elements may be included, in particular a focusing system situated between the semi-silvered mirrors 32 and 33 and controlled by the focusing ring 24 shown in Fig. 3.a. in order to allow for variations in the distance between a phosphor such as 31 and the mirrors 32, 33 (due to differences in the thickness of PPI tube face plates for example).

The purpose of the rubber mount 25 shown in Fig. 3.a. is not only to exclude extraneous light but also to assist in the maintenance of register when a ship is in a seaway.

Referring to Fig. 4 this illustrates graphically the operation of the system when in a quiescent state i.e. with no interrogator station interrogating a responder station. Line (i) of the timing diagram illustrates the switching on and off of the responder/receiver 20R; line (ii) illustrates the responder antenna 1 2 illuminated by the radar scanner 3 of the interrogator station; line (iii) illustrates the radar pulses transmitted within one of the pulses in line (ii); line (iv) illustrates the radar pulses received, with a receiver threshold 42; line (v) represents the responses transmitted by the transmitter 1 T of a responder 2, and line (vi) shows responses as received at the interrogator station 1. In lines (iv), (v) and (vi) the time delays 43, 44 and 45 represents the effects of range.

A typical display on the PPI display 5 at this time is shown at Fig. 4.b. where 46 represents the coastline, 47 represents the operators own ship, 48 represents a target echo from a ship which includes responder equipment, 49 represents the response one scan in every six provided by target 48 and 50 represents other targets within the range of the PPI system.

Referring to Fig. 5 this represents a timing diagram relating to an interrogation of the responder 2, of Fig. 1 by the interrogator 1. Line (a) is the time scale line (b) shows the scanner bearing with the target on 90 line (c) shows the noise due to speckling at the photocell 35 (the "defocused" photocell) of the light probe 6, line (d) shows the echo output at the "focused" photocell 37 of light probe 6, line (e) shows an X-band transmission received by the receiver 11 R of the responder 2, line (f) shows the UHF radio signal received at the receiver 20R of the responder 2 received as a result of the first interrogation by transmitter 9T of interrogator 1, line (g) represents the train of X-band responses (each being the abbreviated call sign) transmitted by responder transmitter 11T, line (h) represents the UHF signal received by receiver 20R as a result of the second interrogation and line (j) represents the UHF response transmitted by transmitter 20T and containing the required navigational and other data relating to the target.

Claims (30)

1. A transponder system comprising at least one interrogator station and at least one responder station, the or each interrogator station having a scanning radar system and a radio transmitter and receiver equipment which latter is connected to an omni-directional radio aerial system and the or each responder station having a radar receiver connected to an omnidirectional radar aerial system and radio transmitter and receiver equipment connected to an omni-directional radio aerial system and wherein at said interrogator station means are provided for transmitting a radio call sign via its omni-directional radio aerial system as its scanning radar beam sweeps across a selected target, at said responder station means responsive to the receipt both of radar energy from said scanning radar beam and said radio call sign transmitted by said interrogator station are provided to control the transmission via said omni-directional radar aerial system of said transponder station of an identifying pulse code at radar frequency for receipt by said radar system of said interrogator station, means are provided at said interrogator station for decoding said last mentioned identifying pulse code and causing the radio transmitter equipment of said interrogator station to transmit an interrogating radio signal based upon said decoded last mentioned identifying pulse code and means are provided at said responder station for causing the radio transmitter at said responder station to transmit for reception by the radio receiving equipment at said interrogator station data relating to said responder station.

2. A system as claimed in claim 1 and embodied as a marine transponder system in which said interrogator station or stations and said responder station or stations are either shore-based or ship-borne.

3. A system as claimed in claim 1 or 2 and wherein in each case an interrogator station is combined with a responder station.

4. A system as claimed in any of the above claims and wherein all of the radio transmitter and receiver equipments in the system operate on a single dedicated channel.

5. A system as claimed in claim 4 and wherein all of the radio transmitter and receiver equipments in a system operate at the same UHF frequency.

6. A system as claimed in any of the above claims and wherein at a responder station means are provided responsive to the receipt of radar energy from the scanning beam transmitted by the radar system of an interrogator station for responding automatically with a pulse of energy transmitted via the omni-directional radar aerial system of the responder station at a radar frequency within the bandwidth of a radar receiver at said interrogator station.

7. A system as claimed in claim 6 and wherein said last mentioned radar frequency is the same radar frequency as that of the radar energy of the scanning beam of said interrogator station.

8. A system as claimed in claim 6 or 7 and wherein the radar system of the transponder station includes a PPI or other radar display unit whereby targets interrogated by the scanning radar beam exhibit a flag if that target is equipped for co-operation in the system.

9. A system as claimed in any of the above claims 6 to 8 and wherein said radar system or systems are such as to operate at S-band.

1 0. A system as claimed in any of the above claims 6 to 8 and wherein said radar system or systems are such as to operate at X-band.

11. A system as claimed in any of the above claims wherein the radar system at the or each interrogator station includes a PPI radar display and wherein the means provided at said interrogator station for transmitting a radio call sign via its omni-directional radio aerial system comprises electro-optical means for deriving a light input from a selected target displayed on said PPI display and means responsive to noise speckling effect as said scanning radar beam approaches said target for causing said last mentioned radio call sign to be transmitted.

1 2. A system as claimed in claim 11 and wherein said electro-optical means comprises a light probe which may be aligned with the display target by an operator via an optical sight, said light probe including a photoelectric diode arranged to receive defocused light from the region of the target on the PPI display whereby to provide an electrical output control signal in response to noise speckling effect as aforementioned said control signal being utilised to control the transmission of said radio call sign via the omni-directional radio aerial system of said interrogator station.

1 3. A system as claimed in claim 1 2 and wherein said light probe includes an energising control button which may be operated by an operator when he is satisfied that the light probe is satisfactorily in register with said selected target on said PPI display and which button he may hold activated until said selected target next brightens up as the scanning radar beam sweeps thereacross.

1 4. A system as claimed in claim 1 3 and wherein said light probe includes a second photoelectric diode which is arranged to receive focused light from said selected target on said PPI display screen which is provided to read said identifying pulse code transmitted at radar frequency via the omni-directional radar aerial system of said responder station, the output of said second photo-electric diode being connected to said decoding means.

15. A system as claimed in claim 14 and wherein within said light probe are provided a semi-reflective outer ring and a semi-reflective inner disc the former arranged to direct defocused light from the region of a selected target on the PPI display to said first mentioned photoelectric diode and the latter being arranged to direct focused light from said selected target on the PPI display onto said second photo-electric diode.

1 6. A system as claimed in claim 1 5 and wherein between said outer ring and inner disc and the end of said light probe placed in operation on the screen of said PPI display is a focusing system provided to compensate for variations in the distance between the phosphor on the PPI screen and the aforementioned outer ring and inner disc.

1 7. A system as claimed in any of the above claims 1 2 to 1 6 and wherein the said end of said light probe comprises a resilient mount arranged to engage the faceplate of a PPI display tube.

1 8. A system as claimed in any of the above claims and wherein in each case the omnidirectional radio aerial system comprises a whip antenna.

1 9. A system as claimed in any of the above claims and wherein said omni-directional radar aerial system comprises a slotted waveguide.

20. A system as claimed in any of the above claims and wherein the means provided at said interrogator station for transmitting said interrogating radio signal based upon the identifying pulse code received at radar frequency from said responder station is such that said interrogating radio signal is transmitted in a time slot defined from the instant of radar transmission.

21. A system as claimed in claim 20 and wherein at the corresponding responder station the transmission of said data relating thereto is initiated if said interrogating radio signal is recognised as being based upon the identifying pulse code transmitted at radar frequency by said responder station and if said interrogating radio signal is received within an appropriate time slot defined by the reception of radar energy by the radar receiver of said responder station as the scanning radar beam of the interrogator station sweeps thereacross.

22. A system as claimed in any of the above claims and wherein the arrangement for transmitting said data by said responder station is such that said data is transmitted in pulse code form with parity bits and including an identifying pulse code portion which may be compared at said interrogator station with said decoded identifying pulse code received at radar frequency by said interrogator station whereby the validity of the data received may be checked.

23. A system as claimed in claim 22 and wherein at said interrogator station means are provided for checking that the said identifying pulse code portion in said data received is compatible with said decoded identifying pulse code received at radar frequency, that parity is correct and that said data is received in an appropriate time slot.

24. A system as claimed in any of the above claims and wherein said identifying pulse code transmitted by a responder station at radar frequency represents part of its normally allocated radio call sign, preferably the last digits thereof, and said identifying pulse code portion in the data transmitted by said responder station includes said normally allocated radio call sign in full.

25. A system as claimed in any of the above claims and wherein said data transmitted by a responder station where said responder station is ship borne comprises the full radio call sign of the ship, the registration letters of the ship, the type of vessel, the ship's compass heading the ship's helm setting (e.g. midships, port, starboard), the ship's speed and any emergency navigational information.

26. A system as claimed in any of the above claims and wherein at the interrogator station means are provided for displaying said received data in the form of a television type video display unit.

27. A system as claimed in any of the above claims 1 to 25 and wherein at the interrogator station means are provided for displaying said received data in the form of a dot matrix display.

28. A system as claimed in any of the above claims 12 to 27 and wherein means for displaying said data are contained in a housing of said light probe.

29. A system as claimed in claim 23 as dependent upon any of claims 12 to 1 7 or a system as claimed in any of claims 24 to 28 as dependent upon claim 23 and any of claims 1 2 to 1 7 and wherein the housing of said light probe includes a bulb indicator arranged to be activated when said checking means at said interrogator station determines as aforesaid that the data received from said responder station is valid.

30. A transponder system substantially as herein described with reference to the accompanying drawings.