US3787665A

US3787665A - Means for synchronizing remotely positioned timing devices

Info

Publication number: US3787665A
Application number: US00272472A
Authority: US
Inventors: F Watson; J Mohr
Original assignee: McDonnell Douglas Corp
Current assignee: McDonnell Douglas Corp
Priority date: 1972-07-17
Filing date: 1972-07-17
Publication date: 1974-01-22
Anticipated expiration: 1991-01-22

Abstract

Improvements in means for synchronizing remotely positioned timing devices including means to provide multiple error correction factors which are functions of the time error or time difference between the time keeping means in spaced systems or units being synchronized, said means being particularly useful in group synchronization by providing time convergence to a common synchronizing base. The use of multiple error correction factors restricts or limits relative timing errors between systems or units especially when in the process of changing from one reference time base location to another, and it provides means to bring timing devices at remote locations into closer synchronization on a continuing basis. The present means provide correction factors which are directly proportional to the synchronizing error including those cases where the correction versus error proportion is other than in a one-to-one relationship. This is especially advantageous in situations where there is a ground or master station responsibility that is shared on a rotating, shifting or location basis with other stations that are or may be synchronized but on a different time schedule or time base. Partial corrections in such situations can cause convergence of the synchronizing operation on a global or regional basis to a common time synchronization base and on a continuous and uninterrupted basis. The present means are also able to provide time synchronization of remotely located timing devices or units with or without the need to designate specific units or stations, or sets of units or stations, as masters and slaves. This means that the need for master or central accurately synchronized time references or stations which has characterized prior art time synchronized systems is no longer necessary.

Description

United States Patent Watson et al.

[ MEANS FOR SYNCHRONIZING REMOTELY POSITIONED TIMING DEVICES Inventor s: Fred D. Watson, Ballwin; John L.

Mohr, Creve Coeur, both of Mo.

Assignee: McDonnell Douglas Corporation, St.

Louis, Mo.

Filed: July 17, 1972 Appl. No.: 272,472

[52] US. Cl. 235/150.23, 235/92 DM, 343/7.5, 343/112 CA [51] Int. Cl. G061 15/48 [58] Field of Search 235/1052, 150.23, 92 DM;

Primary ExaminerFelix D. Gruber [57] ABSTRACT Improvements in means for synchronizing remotely positioned timing devices including means to provide multiple error correction factors which are functions [451 Jan. 22, 1974 of the time error or time difference between the time keeping means in spaced systems or units being synchronized, said means being particularly useful in group synchronization by providing time convergence to a common synchronizing base. The use of multiple error correction factors restricts or limits relative timing errors between systems or units especially when in the process of changing from one reference time base location to another, and it provides means to bring timing devices at remote locations: into closer synchronization on a continuing basis. The present means provide correction factors which are directly proportional to the synchronizing error including those cases where the correction versus error proportion is other than in a one-to-one relationship. This is especially advantageous in situations where there is a ground or master station responsibility that is shared on a rotating, shift-- ing or location basis with other stations that are or may be synchronized but on a different time schedule or time base. Partial corrections in such situations can cause convergence of the synchronizing operation on a global or regional basis to a common time synchronization base and on a continuous and uninterrupted basis. The present means are also able to provide time synchronizationof remotely located timing devices or units with or without the need to designate specific units or stations, or sets of units or stations, as masters and slaves. This means that the need for master or central accurately synchronized time references or stations which has characterized prior art time synchronized systems is no longer necessary.

14) Claims, 4 Drawing Figures MESSHQE SLOT TIMER FA P FzoPs Em 0E ram? mg mo 1 TRMISM IT 8 op 1 m {45 P555,

MEANS FOR SYNCHRONIZING REMOTELY POSHTKONEED TIMING DEVICES There has been considerable effort to try to develop a collision avoidance system for aircraft and other relatively movable objects which will be reliable, fast acting, and able to provide timely warnings of the presence of dangerous or potentially dangerous situations so that appropriate escape maneuver instructions can be presented to the pilot or pilots in time for them to place their craft on safe flight paths. Many attempts to develop a collision avoidance system which satisfies all of these and other requirements have been made. One of the basic building blocks of the most successful system to date is the provision of means for very accurately synchronizing the time keeping means located in each cooperating vehicle or craft to ground stations which are suitably located and cooperate therewith. One such means for synchronizing time keeping means is disclosed in Perkinson et al. U. S. Letters Patent No. 3,250,896, granted May 10, 1966 and assigned to Applicants assignee. The synchronizing means disclosed in this patent has been and continues to be one of the basic components of a highly accurate collision avoidance system such as the system disclosed in Perkinson et al U. S. Letter Patent No. 3,341,812. However, situations and circumstances have developed even with the accurate synchronizing means disclosed in these patents which have required certain changes and modifications in order to satisfy certain system requirements. One such change is to eliminate the need to maintain all ground stations in exact synchronization with each other.

The present construction overcomes these and other requirements of the prior art and represents an important improvement over the synchronizing means disclosed in Perkinson et al U. S. Letters Patent No. 3,250,896, making it possible using the same or similar equipment to provide corrections that are proportional to but not necessarily total corrections of synchronizing errors. In other words, the improved construction makes possible error correction where the corrections are proportional to the error sensed but is in a relation to the errors that is other than a one-to-one relation ship. This is especially advantageous to do in situations where several different synchronized conditions may exist simultaneously in different locations, and where it may be desirable to use less than the complete error correction in order to correct for a difference between two or more different conditions which may exist at the same time. These partial corrections may also be car ried by aircraft from the area of one ground station to the area of another and in so doing cause a conver gence or coming together of the time keeping means at all locations. it is especially important to be able to do this where the duty cycle of the master or synchronizing unit is shared between several different units on a rotating or shifting basis, and/or where the several different synchronizing units may have different synchronized conditions as aforesaid. The present improved construction therefore provides means that use a partial and in some cases an excessive correction rather than a full correction each time an error is sensed in order to cause all of the various cooperating units which may be uncoordinated or in unsynchronized condition to have their time keeping means converge toward but not necessarily be at exactly the same common time synchronization condition. With the present improved means therefore cooperating units including master and slave units, if both types are employed, continuously have their time keeping means brought closer and closer to a synchronized condition with each other without requiring that all cooperating units be maintained in exact synchronism at all times. As explained, this has certain advantages over a system which re quires that all cooperating units be maintained in exact synchronism. For example, it means that an aircraft equipped with a system that should be maintained in synchronism with other systems can be maintained ,at or close to exact synchronism with all other similar systems, and can operate as a link to help maintain an overall condition of near universal synchronism in which all ground stations as well as cooperating aircraft are very close to but not necessarily exactly synchronized to each other. This also means that an aircraft flying from the control of one master station at one location can be updated to cooperate with another master station at a different location which may have a some what different time base (be slightly out of synchronism with the one master station) without encountering the difficulty of having to completely change its time base in order to be in synchronism with the new station. The same aircraft at the same time can cause or tend to cause a convergence between the time base at the two or more ground stations or other aircraft with which it comes in contact so that they by their cooperation can be brought closer into synchronism with each other.

The synchronizing operations of the ground stations, which may be synchronizers of the subject system, is very similar to the synchronizing operation of the ground stations as described in U. S. Letters Patent No. 3,250,896. In the patented system each ground station within range of an aircraft receives synchronizing signals therefrom, and these signals are transmitted at times t from the airborne units regardless of whether the airborne units are synchronized. Thereafter, in the same message slots assigned to each respective cooperating aircraft return signals are transmitted back to the airborne units to be synchronized from the ground stations. The return signals from the ground station arrive back at the airborne unit at later times that are a function of the timing or the phase difference (AT between the two units involved. The amount of the time difference between the uncorrected preselected times as measured by the airborne stations and the times at which the return signals are actually received in each case is equal to twice the time or phase difference between the time keeping devices at the communicating units involved or ZAT In the prior construction, each airborne unit operates as aforesaid by transmitting at its time t which is the beginning of each occurrence of its message slot, and at the same time commences counting or scaling at some predetermined rate. The counting or scaling can be done by a counting circuit, a voltage ramp generator device such as a capacitor charging circuit or by some other similar constantly increasing (or decreasing) signal generator means which are able to count at some constant rate R The counting rate continues uninterruptedly at the airborne unit until the response signal is received from the ground station, if one is received, at which time the timing rate in the prior art construction is changed in some manner either by counting in an opposite or decreasing direction or by doubling the counting rate. If no reply signal is re ceived from the ground station before the end of the time period at the airborne unit, the counting means in the airborne unit will simply reset automatically to time .the accumulated count equals some predetermined value. In the prior art construction when the counting rate changes upon receipt of a reply signal from the ground station, a new counting rate R is established and this is double the initial counting rate R,, and as aforesaid the new double counting rate continues until a predetermined final count of value C is reached at which time the counter is reset and thisestablishes the new time t,, at the airborne unit. This new rest time corresponds to a correction of T If there is no synchronizing error detected then, T is equal to zero, and the response from the ground station or synchronizer will be received at the airborne unit at time t In the impr'oved construction means are provided for setting the final count C, to any desired value to complement the selection of the new or second counting rate. This means that an exact synchronization will not take .place. The present invention therefore provides means to enable a plurality of cooperating units which rely for their operation on how closely theyare synchronized to each other to be brought ever closer toward a common synchronized condition without requiring exact synchronism for all units, and the convergence toward true or common synchronism with the present means can be adjusted to take place at any desired rate.

It is therefore a principal object of the present invention to provide an improved more universal means for maintaining remotely positioned time keeping means close to a synchronized condition with respect to each other.

Another object is to provide means by which cooperating time dependent units can have their time keeping sal collision avoidance protection foraircraft and other relatively movable ojbects.

Another object is to provide means to continuously update and reduce synchronization errors between spaced time ordered communicating units.

Another object is ,to provide improved means for maintaining the units of a time ordered system close to a synchronized condition.

These and other objects and advantages of the present invention will become apparent after considering the following detailed specification which discloses a preferred embodiment of the subject construction in conjunction with the accompanying. drawings, wherein:

FIG. I is a diagramatic view showing in-flight aircraft and spaced ground stations all equipped with timing devices and related circuits constructed according to the present invention;

FIG. 2 is a block diagram of a system that includes accurate time keeping means and means to synchronize the time keeping means with other similar devices according to the teachings of the present invention;

FIG. 3 is a graph of the time corrections required to bring one timing device into synchronism or partial synchronism with another timing device as a function of synchronization error; and,

FIG. 4 is a graph of time counting rates plotted as a function of time over a fixed time period.

The present invention represents a substantial improvement over known time synchronizing means including the synchronizing means shown and described in Perkinson et al. U. S. Pat. No. 3,250,896. The apparatus and circuitry required for the present construction may be very similar in construction and operation to that disclosed in the said patent. In fact, the circuitry shown in block form in FIG. 2 is substantially similar to the circuit construction shown in FIG. 9 of the said Perkinson et al patent. For convenience, the elements shown in FIG. 2 are numbered to correspond to the elements in FIG. 9 of the patent. The differences between the constructions will be described in detail hereinafter.

Referring to FIG. 1' two aircraftv 10and l2 are shown flying af different locations, andtwo spaced

ground station locations

14 and 16 are also shown. The present means will also be described in connection with a colli- 'sion avoidance system for use on aircraft since this is one of the principal applications for which the system is to be used. It is contemplated, however, that other uses and applications could also be made including any use or application where itis requiredjthat two or more units be maintained in close or exact synchronization for some purpose. In the system covered by the aforementioned Perkinson etal. patent, all of the ground stations are maintained in exact synchronism with each other and aircraft within radio communication of a ground station are synchronized exactly to the synchronization of the ground station with which it is communicating. As a practical matter, however, there are operational difficulties in maintaining exact synchronism between all of the ground stations, and the units in the cooperating aircraft may also have some difficulty maintaining exact synchronism at all times. This is especially true when flying over great distances where there are no ground stations. For these and other reasons it is desirable to provide means to maintain all of the systems in as close to an exact synchronism as possible but without necessarily requiring exact synchronization for all the cooperating units. This obviously means that some limited amount of variation may be present but this is not necessarily undesirable as will be explained, and in fact, this principle is made use of by having all cooperating units make partial corrections of their synchronized condition each time a synchronizing operation takes place in order to cause the synchronized condition of all cooperating units to converge toward, but not necessarily exactly reach, the same synchronized condition. These partial corrections which may occur at many places around the world and at many different stations are designed to cause all of the cooperating stations including the movable or slave stations as well as the ground or master stations to have their time keeping means converge toward a common synchronized condition. In other words, if there are two ground stations spaced a considerable distance. apart of the earths surface andthey are somewhat out of synchronism with each other, then when one of the ground stations communicates with an aircraft corrections will be made which may cause both the aircraft and the ground station to be partially synchronized toward each other and this can bring about almost exact synchronization between them after only a few syncrhonizing operations. However, when the same aircraft leaves the operating area of the first ground station and enters that of a second ground station which may have a somewhat different time schedule or one that is out-ofsynchronism with the first ground station, additional partial corrections will take place between the aircraft and the second ground station and this may cause both to have their time schedules converge toward another in between time schedule. These changes, however, will be in the direction to make the second ground station more nearly in synchronization with the first ground station. The accumulative effect of many aircraft communicating with many ground stations on this basis will cause all of the cooperating units to have their time schedules or time keeping means converge toward the same synchronized condition. The advantages of this kind of convergent operation are great especially when trying to maintain world-wide collision avoidance protection. While some slight difference may exist between the time schedules of the various cooperating units, it has been found that the amount of difference is so small as to be virtually negligible, and does not adversely effect the operation of a collision avoidance system or destroy its reliability.

Referring to FIG. 4 it can be seen that all cooperating units keep time in the same constantly repeating similar time periods each of which is subdivided into a plurality of individual message slots. A counter or voltage ramp device that is capable of keeping time at a constant rate by charging a capacitor or otherwise may be used for this purpose. The slope of the counting or charging rate of the first portion of the line in FIG. 4,'indicated by R,, represents the initial counting rate, and this counting or charging rate will continue from the beginning of the message slot until the end unless a response synchronizing signal is received from the ground or master station or from some other synchronizing unit. If no such signal is received, the initial counting rate (R,) will continue to the end of the message slot, which is represented by the final count or charge C at which time the counting means will be reset to zero which represents time t and the counting means will begin again to count at the same rate R, for the succeeding message slot. If the unit being synchronized receives a synchronizing response during the message slot, the counting rate will be altered in some way such as by being increased to count at a faster rate as indicated by the greater slope of the line portion R and this new counting rate will continue until some final count (or voltage charge on a capacitor) is reached as represented by the count C,,. When this final count is reached the counting means will be reset and commence counting again at time t for the next message slot. In the prior art construction disclosed in Perkinson et al. U. S. Pat. No. 3,250,896, the second counting rate R in some embodiments is arbitrarily selected to be twice or double the initial counting rate R,, and a full out-of-synchronism correction is made each time that a synchronism error is detected in order to bring the unit being synchronized into exact synchronism with the unit to which it is being synchronized. In the case shown in FIG. 4, however, the time t becomes the new end of the message slot and at this time the counter means are reset with the correction being equal to (AT,-), which is the time difference between the new end of the message slot and the original uncorrected end of the same message slot or 251532 tum in; Farina situation in which no correction is required, AT 0, which is another way of say ing that the response from the ground station is re ceived at the unit being synchronized at the time t which is the time that the counting rate changes from rate R, to rate R so that when the final count C, is reached it will be exactly time t,,. The value of the final count C, is determined by the equation:

s C0 (R2 1) o su) I t i) The general equation for the amount of the correction AT which as aforesaid is equal to t t is then derived by the following equations:

From the description of the operation of the synchroniz naei ttqi a iqni can 3 e said that:

As explained above, in the construction disclosed in Perkinson U. S.-Pat. No. 3,250,896, the counting rate R is double the counting rate R, and under this special situation Equation 7 reduces to Equation 8 as follows:

AT 01' G l When the special situation in Equation 8 is used, a full correction equal and opposite. to the error is provided for every synchronism error that is detected. This is not so of the present improved construction wherein the constant term G in equation 7 is expressed by the equation:

wherein the two counting rates can have any relation to each other that is desired. For example, the second counting rate R can be expressed as a multiple of the first counting rate R, by the expression:

R2 MR provides means for adjusting the correction factor G to any desired value from zero to two inclusive with the value of the term M ranging anywhere from I to infin' ity. For example, if R, is expressed as a rate to 7 units and R as a counting rate of 10 units, then,

AT -0.6 AT or G 0.6

Using the same principles, it is also possible to make the second counting rate R less than the'first or initial counting rate R in which case the final counts C, will be proportionately less than the count C which is the count that represents the end of the message slots assuming that no signal is received from the ground or other cooperating station. The correction factor G can thus be set to positive as well as to negative values also ranging between a value of zero and a value of negative infinity, in which case the parameter M will range from 5*v3rreerrteavaae of zerol Thiis, with the present improved construction any ratio of the scaling values or counting rates R and R, can be selected and this adds and wherein the various lines A, B, C and D represent different ratios or relationships between thecounting ratesR and R In the diagram the lines A, C, and D provide corrections that are directly proportional to the error but wherein the proportion is other than in a one-to-one relationship as disclosed in the prior art Perkinson et al patent. Line B, on the other hand, represents the situation wherein the ratio is one-to-one as disclosed in the Perkinson et al. patent.

By using ratios other than a one-to-one ratio only partial corrections are made each time a synchronization operation takes place and this causes the unit being synchronized to be brought closer in synchronism with the unit to which it is being synchronized but not necessarily in exact synchronization therewith. This is highly desirable especially in a multi-unit system which includes a plurality of so-called master units and a plurality of so-called slave units. It is also desirable where all cooperating units are constructed to be synchronized only to each other.

Referring to FIG. 1, each aircraft l0 and 12 carries transmitter means 22, receiver means 24, means for synchronizing its timing device with the standard timing device 20 at the ground station, and each aircraft carries as part of its equipment means for accumulating or counting time in predetermined increments and at a predetermined rate, the counter means being the means 28. Similar counting means 28 are provided at each ground station I4 and 16 and each ground station also includes transmitting and receiving means 36 and 30, respectively. The counting means 28 at the aircraft and at the ground stations are constructed to count at the desired rates required for the present invention.

In FIG. 2 the counting inputs are shown supplied by the multiplier-dividers 136b and 1366 which receive their inputs from precision oscillator I36a. These inputs can also be supplied by separate precision oscillators or by one multiplier-divider circuit and by a direct connection from the precision oscillator to the counter 140. The input rate supplied to the counter message slot timer flip-flops is controlled by AND

gates

138 and 142 in the same manner as the normal and double count rates are controlled in the construction disclosed in the Perkinson et at. U. S. Pat. No. 3,250,896. Other input points to the message slot timer flip-flops 140 are between the 1 and 2 flip-flop stages as in the patented construction, or between other flip-flop stages including between the 2 and 4 and between the 4 and 8 flipflop stages of the circuit 140, depending upon the embodiment and design choices involved in implementing specific desired values for the counting rates R and R These connections provide the desired different counting rates that are required.

One matter that should be borne in mind in considering the present system is that the present so-called convergent system, as explained, operates to bring all cooperating units ever closer into real time synchronism with each other. This will usually-means units on fixed as well as on movable devices. If there are found stations used as masters and airborne stations used as slaves then the masters within their sphere of operation will effect the synchronization of the slaves in communication therewith and the slaves must also have an effect, although possibly a lesser amount, on the masters. This may mean that the masters 'as well as the slave stations will be assigned message slots for synchronization purposes and it may also means the ratio of the correction factor for the masters may be different than for the slave stations. In this case when an airborne station leaves the sphere of one master station and enters the sphere'of another it will have some effect on the condition of the latter ground station into whose sphere it has flown. If this were not the case, no overall convergence toward a common synchronism would take place. By the same token, if convergent synchronism is to be established only between coequal stations then there may be no need to designate some as masters and others as slaves, and in that case'also there is no need to use more than one correction factor ratio. Other situations where the present means can be used are also possible.

Thus there has been shown and described novel means for causing time keeping means at spaced locations to continuously converge toward a common time keeping basis so that all such devices which are constructed to keep time in the same time periods subdivided into individual message slots will be able to operate cooperatively and accurately on real times and on one-way transmissions therebetween. It will be apparent, however, that many changes, modifications, alterations, and other uses and applications of the subject convergent means will become apparent after considering this specification and the accompanying drawings. All such changes, modifications, alterations and other uses and applications which donot depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.

What is claimed is:

- 1. Means for maintaining a condition close to but not necessarily exactly of synchronism at a plurality of spaced units each of which includes means capable of keeping time in the same length repeating time periods, the amount of difference between the time of occurrence of corresponding time periods at communicating units being the out-of synchronism error, means (144) for assigning a different distinctive time period to each cooperating unit as its synchronizing time, means (136a and l36b) included in each unit for establishing an initial counting rate at the beginning of each occurrence of its assigned time period, means (136a, 1360) at each unit for establishing a second counting rate thereat different from the initial counting rate, means (22) at selected ones of said units for transmitting a signal therefrom beginning at the beginning of each oc' currence of the time periods assigned thereto, means (24) at other of said cooperating units within range of said transmitting units for receiving said signals in the same time periods assigned to the said transmitting units and for transmitting reply signals at later times in the same time periods based at least in part on the time of receipt of each such transmitted signals, means (138, 142, 150, 152, 154, 156) at each of said transmitting units for changing the counting rate thereat from the initial counting rate to the second counting rate in response to the receipt in the same time period of the reply signal, the ratio of the initial counting rate before receipt of the reply signal to the second counting rate after receipt of said reply signal being selectable to produce a time error correction that is different from the out-of-synchronism error.

2. The means defined in claim 1 wherein the ratio of the initial counting rate before receipt of the reply signal to the second counting rate after receipt of the reply signal is greater than'one.

3. The means defined in claim 1 wherein the ratio of the initial counting rate before receipt of the reply signal to the second counting rate after receipt of the reply signal is less than one.

4. The means defined in claim 1 wherein all cooperating units include means for shifting the time of occurrence of the repeating time periods thereat in a direction to make the later occurrences thereof occur more nearly simultaneously at all cooperating units.

of remotely positioned devices in a system to converge on a system-wide bases toward a common synchronized condition comprising a plurality of spaced units each of which includes means capable of keeping time in the same length repeating time periods, means dividing the time periods into a plurality if distinct message slots for assignment to respective ones of the cooperating units as its time for transmitting, receiving and synchronizing, means in each unit for establishing an initial time keeping rate commencing at the beginning of each occurrence of its assigned message slot, means at each unit for establishing a second counting rate thereat different from the initial time keeping rate, other means at each unit for receiving signals transmitted by other cooperating units within range thereof and in the message slots assigned to said other units, means at each receiving unit for transmitting a reply signal at a later time in the same message slot based on the time of receipt of the said received signal, means at each unit operable in its assigned message slot and responsive to receipt of the reply signal for changing by shifting the times of occurrence of future occurrences of the said time periods and of the message slots therein based on the time keeping means reaching a predetermined count, said means responsive to the time of receipt of the reply signal including means to change from the initial to the second time keeping rate, the ratio of the difference between the times of occurrence of the message slots at two cooperating units before receipt of said reply signal to the correction. introduced by shifting the times of occurrence thereof after receipt of the reply signal during the same message slot and until the predetermined count is reached being other than in a one-to-one relationship. I

7. The means defined in claim 6 wherein the relationship of the difference between the times of occurrence of the message slots and the correction introduced is greater than one.

8. The means defined in claim 6 wherein the relationship of the difference between the times of occurrence of the message slots and the correction introduced is less than one.

9. The means defined in claim 6wherein each cooperating unit includes means for shifting the times of occurrence of its assigned message slot in a direction to make future occurrences thereof in all cooperating units occur more nearly simultaneously, the accumulative effect of the said means at all cooperating units causing the time keeping means at all said units to converge toward a condition in which all of the cooperating units are synchronized on real time which is a condition in which the occurrences of each message slot occurs simultaneously in all cooperating units.

10. The means defined in claim 6 wherein the relationship of the difference between the times of occurrence of the message slot and the correction-introduced is different in at least two spacedl cooperating units.

UNITED STATES PATENT OFFICE CERTIFICATE OF CGRRECTION Patent No. 3,787,665 Dated January 22, 1974 Inventofls) Fred D. Watson and John L. Mohr It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 26 "Letter" should be "Letters".-

Column 3, line 16, "rest" should be "reset"; line 49, "ojbects" should be "objects".

l 6 u ll n l I II Co umn line 5, AT 321: t should be AT I t t C l 6 V f o umn line 26, C R t R (t t should be II A I C R t R (t t column 6, line 30,

t (C R t R t )/R (C (R R )t )/R should be t (C R t R t )/R2 (Cs (R2 Rl) t /Rz".

Column 7, line 16, after "rate" insert "equal", I

Column 8, line 38, "mes ms" should be "mean"; line 47, "meens" should be "mean".

Signed and sealed this 18th day of June 19711..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM P071050 7 uscoMM-oc 60376-P69 r: u. mam Pmmmc omc 959 0-366-334 v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 7, I v Dated January 22, 1974 Inventor) Fred D. Watson and John L. Mohr It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 26 "Letter" should be "Letters";

I II n I I n I I I Column 6, line 5, AT 32t v t should be AT t t I n n Column 6, line 26, C R t R (t t I should be "c R t R (t 1: mm 6, line 30,

s 1 so o *f- =3 vn (c (R2 R )t )/R should be I I I Q V II v t (C R t R2 t )/R (C (R R1) tso/Rz Column 7, line 16, a fter"ra te" insert "equal".

Column 8, line 38, "means" should be "mean"; line 47, "means" should "mean".

Signed and sealed this 18th day of June 1.971;.

(SEAL) Attest:

EDWARD M.FLETCH R,JR. Attesting Officer C. MARSHALL DANN Commissioner of Patents FORM PO-1050 (10-69) v uscoMM-Dc 60576-P69 Y f: u.s4 dovanuuzrn PRINTING OFFICE: 1969 o3ss-a;u

Claims

1. Means for maintaining a condition close to but not necessarily exactly of synchronism at a plurality of spaced units each of which includes means capable of keeping time in the same length repeating time periods, the amount of difference between the time of occurrence of corresponding time periods at communicating units being the out-of synchronism error, means (144) for assigning a different distinctive time period to each cooperating unit as its synchronizing time, means (136a and 136b) included in each unit for establishing an initial counting rate at the beginning of each occurrence of its assigned time period, means (136a, 136c) at each unit for establishing a second counting rate thereat different from the initial counting rate, means (22) at selected ones of said units for transmitting a signal therefrom beginning at the beginning of each occurrence of the time periods assigned thereto, means (24) at other of said cooperating units within range of said transmitting units for receiving said signals in the same time periods assigned to the said transmitting units and for transmitting reply signals at later times in the same time periods based at least in part on the time of receipt of each such transmItted signals, means (138, 142, 150, 152, 154, 156) at each of said transmitting units for changing the counting rate thereat from the initial counting rate to the second counting rate in response to the receipt in the same time period of the reply signal, the ratio of the initial counting rate before receipt of the reply signal to the second counting rate after receipt of said reply signal being selectable to produce a time error correction that is different from the out-of-synchronism error.

2. The means defined in claim 1 wherein the ratio of the initial counting rate before receipt of the reply signal to the second counting rate after receipt of the reply signal is greater than one.

5. The means defined in claim 1 wherein the ratio of the said counting rates is other than two-to-one.

6. Means to cause time keeping means at a plurality of remotely positioned devices in a system to converge on a system-wide bases toward a common synchronized condition comprising a plurality of spaced units each of which includes means capable of keeping time in the same length repeating time periods, means dividing the time periods into a plurality if distinct message slots for assignment to respective ones of the cooperating units as its time for transmitting, receiving and synchronizing, means in each unit for establishing an initial time keeping rate commencing at the beginning of each occurrence of its assigned message slot, means at each unit for establishing a second counting rate thereat different from the initial time keeping rate, other means at each unit for receiving signals transmitted by other cooperating units within range thereof and in the message slots assigned to said other units, means at each receiving unit for transmitting a reply signal at a later time in the same message slot based on the time of receipt of the said received signal, means at each unit operable in its assigned message slot and responsive to receipt of the reply signal for changing by shifting the times of occurrence of future occurrences of the said time periods and of the message slots therein based on the time keeping means reaching a predetermined count, said means responsive to the time of receipt of the reply signal including means to change from the initial to the second time keeping rate, the ratio of the difference between the times of occurrence of the message slots at two cooperating units before receipt of said reply signal to the correction introduced by shifting the times of occurrence thereof after receipt of the reply signal during the same message slot and until the predetermined count is reached being other than in a one-to-one relationship.

9. The means defined in claim 6 wherein each cooperating unit includes means for shifting the times of occurrence of its assigned message slot in a direction to make future occurrences thereof in all cooperating units occur more nearly simultaneously, the accumulative effect of the said means at all cooperating units causing the time keeping means at all said units to converge toward a condition in which all of the cooperating units are synchronized on real time which is a condition in which the occurrences of each message slot Occurs simultaneously in all cooperating units.

10. The means defined in claim 6 wherein the relationship of the difference between the times of occurrence of the message slot and the correction introduced is different in at least two spaced cooperating units.