Passive SSR system utilizing P3 and P2 pulses for synchronizing measurements of TOA data

The range from which a collision avoidance system at an Own station can receive SSR interrogations and reply messages from Other stations, is extended by utilizing P2 pulses for timing TOA measurement in the event P1-P3 pulse pairs are unavailable from the scanning main beam or its side lobes. The amplitude of the P2 pulses in the SLS radiation pattern being greater than the P1-P3 side lobes of the main beam over an angular sector of about 60.degree. centered on the main beam can insure reception of P2 pulses at much greater ranges than P3 pulses contained in the main beam side lobes can be reliably received. Using P2 timing, interlaced Mode A and Mode C reply messages contained in a main beam burst reply sequence are separated into two "families" of TOAs, the Mode C (altitude) TOAs always being longer than the Mode A TOAs by 13 .mu.sec. A "true" TOA is obtained by subtracting an appropriate time period from the TOA of each family, from which identity, altitude and range information is readily derived. The system continuously adapts to the best instantaneously available timing pulses, alternating between P3 timing and P2 timing throughout the time it takes for a main beam rotation of the received SSRs, thereby extending the operation area of multiple TOA measurements from multiple SSRs which, in turn, provides added safety and reduced false alarms compared to prior passive collision avoidance systems.

BACKGROUND OF THE INVENTION 
This invention relates generally to passive air traffic control and 
collision warning systems which utilize interrogation signals transmitted 
from a ground SSR station and associated transponder reply messages 
transmitted by vehicles, such as aircraft, that are elicited by the SSR, 
for determining the ranges, azimuth angles, altitude and identity of one 
vehicle relative to another or to the ground station and, more 
particularly, is concerned with improvements to systems of this type, 
specifically the collision avoidance system described in applicants, U.S. 
Pat. No. 4,486,755. 
The system shown in U.S. Pat. No. 4,486,755, the disclosure of which is 
hereby incorporated herein by reference, receives standardized 
interrogation signals, having the waveforms shown in FIG. 1, transmitted 
from a ground station at a frequency of 1030 MHz on a narrow rotating main 
beam and in the side lobes of the main beam. The standardized 
interrogation signal consists of three pulses each 0.8 .mu.sec. wide: a P1 
pulse; a P2 pulse spaced 2.0 .mu.sec. from the P1 pulse; and a P3 pulse 
spaced from P1 by either 8.0 .mu.sec or 21.0 .mu.sec. The P1 and P3 pulses 
are transmitted by the main beam and also, unintentionally, by the main 
beam side lobes which, unless suppressed, may be sufficiently strong to 
interrogate nearby transponders, creating false replies. Referring to FIG. 
2, the rotating directional antenna that has been employed over the past 
several years, many of which likely will still be in service for many 
years to come, produces a scanning beam 10 which is about 2.5.degree. to 
3.0.degree. wide at its 3dB point and slightly wider at its suppression 
control point. 
In most SSRs, a second, static antenna omnidirectionally broadcasts a side 
lobe suppression control pattern 12 containing P2-only pulses or P1-P2 
pulse pairs, wherein the P2 pulse is synchronized with the P1 pulse in the 
main beam, at a significantly higher level than the main beam side lobes, 
the purpose of which is to prevent transponder replies to other than main 
beam interrogation pulses when such main beam pulses exceed the P2 pulse 
suppression signal level by a fixed amount. On some SSR control patterns 
only P2 pulses are transmitted, which combine with the P1 pulses of 
stronger main beam side lobes to create a P1-P2 suppression pair. More 
specifically, the radiated amplitude of P2 at the transponder is (1) equal 
to or greater than the signal amplitude of P1 from the greatest side lobe 
transmission of the antenna radiating P1 (i.e., the rotating main beam 10) 
and (2) at a level lower than 9dB below the radiated amplitude of P1 
within the desired arc of interrogation. When main beam P1 pulse levels 
exceed P2 pulse levels, P3 is no longer suppressed and P1-P3 pulse pairs 
interrogate transponders that are in the main beam. 
A P1-P3 pulse pair with a separation of 8.0 .mu.sec. between P1 and P3 
transmitted on the main beam interrogates the identity (Mode A) of a 
transponder-equipped aircraft, and a separation of 21.0 .mu.sec. between 
P1 and P3 interrogates that aircraft's altitude (Mode C). A series of 
about twenty such P1-P3 pulse pairs, one-half of which typically are Mode 
A interrogations and the other half Mode C interrogations, is received at 
a transponder, within a beam's width, during each 360 degree scan of the 
rotating beam. During the period that the rotating beam is pointing at the 
transponder, that is, the time the beam takes to scan approximately 
4.degree., known as the "beam-dwell" time, the transponder replies in 
accordance with the "un-suppressed" P1-P3 spacings of the interrogation 
message. Interlaced Mode A and Mode C interrogation messages, such as 
ACACAC, or AACAAC, are separated by intervals typically of about 2500 
.mu.sec. but in the range between a minimum of about 2,000 .mu.sec. to a 
maximum of approximately 5,000 .mu.sec. The broad SLS pattern, being 
significantly stronger at all azimuths outside of the main beam skirt 
(approximately 14-16 dB down from the main beam peak), prevents 
interrogation pulse pairs from being received by a transponder unless they 
are in the sector defined by the 3.degree.-4.degree. width of the main 
beam. 
Summarizing, a P1-P3 pulse pair transmitted on the SSR main beam will 
interrogate an airborne transponder, causing it to transmit mode A and 
mode C messages, only if the amplitude of the P1-P3 pulses received at the 
transponder exceeds the amplitude of any received associated P2 pulses. 
Each qualifying transponder within the 360.degree. scanning coverage of 
the SSR main beam transmits in response a reply message on a 1090 MHz 
radio frequency carrier back to the SSR, with a known delay, so that the 
reply message is propagated along the path of the main beam and thus its 
signal strength is increased by beam gain, and received by a 1090 MHz 
receiver at the SSR. Each such 1090 MHz transponder transmission, known as 
a "reply message" and depicted in FIG. 3, includes a pair of framing 
pulses F1 and F2 separated by 20.3 .mu.sec. which define the start and 
stop, respectively, of the message, between which thirteen information 
pulses (twelve of which are currently used) are spaced in increments of 
1.45 .mu.sec. from the first framing pulse, each of which is 0.45 .mu.sec. 
wide and may or may not be present depending upon the content of the 
message transmitted in reply to the 1030 MHz interrogation signal. The 
format of the message contained between framing pulses F1 and F2 is 
similar for any one of 4,096 identity codes transmitted. The absence or 
presence of each of twelve information pulses establishes which code is 
transmitted on 1090 MHz in response to the reception of an interrogating 
P1-P3 pulse pair spaced by 8.0 .mu.sec. 
Similarly, the format of the message contained between the framing pulses 
is the same for any one of the altitude codes, which do not use D.sub.1 
pulses, each of which represents the altitude of the aircraft to within 
.+-.50 feet in 100-foot increments up to a maximum in excess of 125,000 
feet. Thus, the structure of the reply message allows for the possibility 
of 4,096 different code groups, each representing one or more pieces of 
information such as identity or altitude of the responding aircraft. As 
previously mentioned, 1030 MHz P1 and P3 interrogation pulses separated by 
8.0 .mu.sec. when decoded elicit a reply code group transmitted on 1090 
MHz representing identity. Similarly, a P1-P3 spacing of 21.0 .mu.sec. 
elicits a reply code representing the altitude of a given aircraft. As 
assigned by ATC or other authorities such as the military, the identity 
code is set in by the pilot with a cockpit "digit switch", while the 
altitude code is automatically established by a barometric altimeter and 
an associated encoder. The identity code designations consist of four 
digits, each of which lies between 0 and 7, inclusive, and is determined 
by the sum of the pulse subscripts given in FIG. 3. The identity code of 
the aircraft may be 1543, for example, which is represented by the 
presence of A.sub.1 ; (B.sub.1 B.sub.4); C4; and (D.sub.1 D.sub.2) pulses. 
The transponder automatically continuously transmits this identity code in 
response to every received Mode A interrogation regardless of which radar 
is interrogating, the beam width of the interrogating radar, or whether it 
is a civil, military or European radar. 
In a similar manner, in response to interrogation P1-P3 pulses spaced by 
21.0 .mu.sec., the transponder automatically looks at an automatic 
altitude encoder coupled to the aircraft's own barometric altimeter, which 
automatically changes the code with changes in altitude according to a 
pattern prescribed by the U.S. NATIONAL STANDARD FOR THE IFF MARK X (SIF) 
AIR TRAFFIC CONTROL SYSTEM (Oct. 10, 1968), and the reply message 
transmitted by the transponder is changed accordingly. Although the 
altitude information is presented in the same pulse format as the identity 
information, the ground system readily discriminates between Mode A and 
Mode C replies to its interrogation, because the relatively long interval 
between PRPs, and thus between interrogation messages, is such that only 
during a specific period of, say 3,000 .mu.sec., representing a round trip 
of about 250 nautical miles (3000/12 .mu.sec. per NM), following an 
interrogation message wherein the P3 pulse is spaced from P1 by 8.0 
.mu.sec., all aircraft that are within the beam and within 250 NM respond 
with identity codes. Since the ranges of most SSR radars are limited to 
about 200 miles line-of-sight, all targets reply within typically 2500 to 
3000 .mu.sec. During the following PRP, during which, say a Mode C 
interrogation is transmitted by the SSR, all aircraft out to a similar 
predetermined range that are intercepted by the main scanning beam will 
reply only with altitude codes. In this way there is no confusion between 
identity and altitude replies even though both use identical signal 
formats, because each pulse has a different significance. These identity 
and altitude codes are interpreted by an airborne collision warning system 
in the same way as does the ground station so as to provide collision 
warning data on all nearby transponders. 
In the passive threat warning and collision avoidance system described in 
the '755 patent, an Own station receives interrogations from at least one 
and usually multiple SSR's within operating range, not only when the main 
SSR beam is pointing at it but also when Own station is illuminated by 
lower level side lobes of one or more main beams, and capitalizes on time 
of arrival (TOA) data from multiple SSRs to create a small cocoon of 
airspace that represents the approximate range and near exact altitude of 
any nearby transponder-equipped aircraft that may be a threat to Own's 
aircraft. Use of such transponders is mandated in some 240,000 aircraft in 
the United States alone and about 350,000 worldwide. 
During a brief "listen-in" period of about 200 .mu.sec. initiated by Own's 
reception of a P1-P3 decode, Own station receives replies transmitted by 
transponders at Other stations in the general vicinity of Own station in 
response to each interrogation from an SSR. The received replies are 
decoded and using the P3 time of the associated interrogation message 
received by the transponder's 1030 MHz receiver, produce time of arrival 
(TOA) data for all surrounding aircraft and SSR stations within the 
sensitivity range of the Own station's 1030 MHz and 1090 MHz receivers. 
Operation of the '755 system depends on the amplitude of the side lobes of 
the rotating main beam being sufficiently high that a P1-P3 pulse pair 
would be received via the main beam side lobes so long as that receiver 
was within a given operating range of an SSR. Thus, the '755 system 
provides such TOA measurements not only during, but also before and after 
passage of the main beam, so long as P1-P3 pulse pairs can be received; 
the rotating main beam may be pointing in a direction other than at Own 
station and interrogating other transponders. Consequently, it is 
essential to the operation of the '755 system that it receive P1-P3 pulse 
pairs, and the associated 1090 MHz responses, both before and after 
passage of the SSR main beam through Own's station, throughout an angular 
sector of about .+-.30.degree. straddling the main beam's axis. The 
inability to receive P1-P3 pulse pairs in the deep nulls between the many 
such side lobes limited the effectiveness of the system. 
The last decade has witnessed an evolutionary change in the design of 
ground SSR antennas, in particular the antenna system employed in SSR 
systems of the type here under discussion. Several hundred U.S.-based 
SSR's are now or are in the process of being equipped with an improved 
antenna system which is electrically phased so as to create a narrow, main 
scanning beam on which P1-P3 interrogation pulses are transmitted and 
reply messages are received, and which has very low side lobes. The new 
antennas usually do not include the static stand-alone antenna used in the 
earlier system for omnidirectionally broadcasting a P1-P2 side lobe 
suppression pattern, but, instead, employ antenna structure and radiating 
elements integral and rotatable with the rotating main beam-forming 
antenna structure for generating an SLS control pattern. As shown in FIG. 
4, the SLS control pattern of this new system, containing either P1-P2 
pulse pairs or only stand-alone P2 pulses, is generally "egg-shaped" in 
the horizontal plane, or may have a narrow null along the main beam's 
axis. The maximum signal of the SLS pattern, and therefore its maximum 
range of reception, is aligned with the axis of the main beam 16 and 
rotates with it; thus, the maximum signal level and therefore the range of 
the rotating SLS pattern traces an imaginary circle 18 as it rotates with 
the main beam. However, the signal strength is maximum only within a 
sector approximately .+-.40.degree. wide which straddles the rotating main 
beam. The signal level of the SLS control pattern in the direction of the 
main beam typically is about 14 dB to 16 dB down from the peak amplitude 
of the main beam and about 20 dB above the average level of the main beam 
side lobes. The level of the control pattern above the side lobe level 
varies with the angular displacement from the main beam, as much as 30 dB 
at an angle of 180.degree. from the main beam, while averaging 
approximately 20 dB above the main beam side lobes during a rotation 
period. The new SLS pattern exhibits high signal levels, without deep 
nulls, at all azimuths, within the .+-.40.degree. angular sector 
straddling the main beam, outside of which there is some diminution in 
level but still greatly exceeding the level of the main beams side lobes. 
Unfortunately, however, this recent reduction in level of the main beam's 
side lobes turns out to be a disadvantage to the '755 system, the 
operation of which depends on reception of P1-P3 pulse pairs, not only 
those contained in the main beam but also those transmitted in and between 
adjacent side lobes. Consequently, the major reduction in side lobe level 
provided by the improved SSR antenna significantly reduces the operational 
range of the '755 system and, indirectly, the accuracy of its collision 
warnings, by reducing the probability of receiving multiple SSR's at most 
locations. As the population of improved antenna systems becomes larger, 
the useful collision warning range of '755 systems could be reduced. 
Adding to the challenge is the fact that of the approximately 3,000 SSR's 
currently in service throughout the world, some already are using the 
improved antenna system, others are in the process of being updated, and 
others may continue using the "old" system, without change, for many more 
years. It is projected that there will be a "mix" of old and new antenna 
systems for approximately ten to twenty years before the "old" antennas 
are totally phased out. 
Thus, there is a current and compelling need for a passive threat warning 
and collision avoidance system that is adaptable to the radiation 
characteristics of both the "old" and the "new" SSR antenna systems. The 
system should also be operable in geographical areas where only the P2 
pulse is transmitted in the SLS control pattern, as is the case of SSR's 
in England and some other European countries. Some U.S. stations such as 
ASR-9 SSRs may also transmit only P2 pulses on the SLS control pattern. 
Accordingly, the primary object of the present invention is to provide an 
"adaptive" collision avoidance system embodying the principles of the '755 
system and capable of operation with any of the three types of ground 
radar transmission systems described above. 
Another object of the invention is to extend the useful range of such 
system from an SSR thereby to increase the probability that SLS signals 
from two or more SSRs interrogating nearby targets will be received by the 
collision avoidance system and thereby significantly reduce false alarms 
and provide more precise measurement of pseudo-range. 
Additionally, the system must be passive (that is, it should not itself 
transmit for the purpose of detecting a potentially colliding airplane), 
thus avoiding interference on either the 1030 MHz channel or the 1090 MHz 
channel of the standardized SSR system. 
The system should also be relatively simple and inexpensive to manufacture 
so as to be economically feasible for owners of light aircraft such as 
those used in general aviation. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, which will be described in 
association with the collision avoidance system shown in U.S. Pat. No. 
4,486,755, but the principles of which are also applicable to other PSSR 
systems such as that disclosed in U.S. Pat. No. 4,115,771. In the event 
P1-P3 pulse pairs are not available, the system is adapted to use either 
P1-P2 pulse pairs or "stand-alone" P2 pulses, for timing the "listen-in" 
period of an Own station. More specifically, in the event of 
unavailability of P1-P3 pulse pairs, of which P3 is normally used for 
initiation of time of arrival (TOA) measurements, the system automatically 
selects, as a second choice, P1-P2 pulse pairs because of their pulse 
width and unique separation by exactly 2.0 .mu.sec. and selects, as a 
third choice, stand-alone P2 pulses, for timing the "listen-in" period. 
The amplitude of the P2 pulse contained in the SLS radiation pattern of 
both the "old" and the improved SSR antenna system is greater than the 
level of the main beam side lobes over an angular range of at least about 
.+-.40.degree. from the direction of the main beam axis, which insures its 
reception, before and after passage of the main beam through a 
transponder, at ranges much greater than the range at which side lobes of 
the main beam and nulls between them can be reliably received. For 
example, by quadrupling the reception range in the .+-.40.degree. sector, 
which is readily possible, by using P2 signal strength versus the much 
weaker side lobe P3 signal strength and deep nulls between side lobes the 
useful air-to-air collision protection area surrounding an SSR is 
increased by a factor of sixteen. This increase would also apply to any 
adjacent SSR's within Own's operating range, thereby providing large 
overlapping protection areas. 
That P2 time (derived from either the P2 pulse of a P1-P2 pair or a 
"stand-alone" P2 ) can be used for synchronizing TOA measurements is based 
on the different spacings between P2 and P3 pulses in Mode A and the P2-P3 
spacing of Mode C interrogation messages, and the fact that Mode A and 
Mode C reply messages elicited by an interrogating main beam mimic most of 
the interrogation messages and other characteristics of the beam. From 
Own's examination of the 1090 MHz "mimic" patterns, it is possible to use 
P2 time to measure TOA values of reply messages. More particularly, since 
the spacing between P2 and P3 pulses is exactly 6.0 .mu.sec. (8 minus 2) 
for Mode A (identity) and 19.0 .mu.sec. (21 minus 2) for Mode C 
(altitude), if P2 pulse time, instead of P3 pulse time, is used to 
synchronize the start of the "listen-in" period during which the TOA of a 
Mode A or a Mode C reply message is measured, the TOA can be corrected, if 
the interrogation mode is known, to a "synthetic" P3 time. However, the 
time difference between Mode A reply relative to P2 time and a Mode C 
reply relative to P2 time will always be exactly 13.0 .mu.sec. (19 minus 
6). 
This 13.0 .mu.sec. difference in TOA measurements is critical to the 
system's ability to identify and separate Mode A and Mode C replies. The 
typical 1090 MHz "beam burst" of about twenty reply messages received at 
the Own station transponder during each scan of a scanning SSR beam past 
Other contains an approximately equal mix of Mode A and Mode C reply 
messages. Importantly, all Mode A replies will have shorter TOAs, relative 
to P2, than similar TOAs of Mode C replies. Because the "mimic" of the 
beam received on 1090 MHz at Own station will contain the interrogating 
radar's PRP, and the spacing and interlace pattern of the Mode A and Mode 
C interrogation messages, by initiating time of arrival (TOA) measurements 
with a P2 pulse, two "families" of TOA's, both referenced to P2 time, are 
created, one family of essentially equal TOAs for Mode A and another 
family of essentially equal TOAs for Mode C. By virtue of the 13 .mu.sec. 
time differential between the two families of TOAs, and the fact that the 
reply messages are contained in the same "burst", they can be readily 
identified and separated. 
Other objects, features and advantages of the invention will become 
apparent, and its construction and operation better understood, from the 
following detailed description when read in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Briefly referring again to FIG. 4, newer ground SSR stations differ from 
the one depicted in FIG. 2 in the significant respect that they have a 
phased array antenna which creates the interrogation radiation pattern 
shown in plan view in FIG. 4 comprising a narrow clockwise rotating main 
beam 16 and a much wider "egg-shaped" side lobe suppression control 
pattern 14 which is aligned with, and rotates along with, the narrow 
scanning beam. The side lobes of the main beam of the new antennas are 
very low in signal power, which greatly reduces the useful range of 
systems such as that described in the '755 patent that depends for its 
operation on direct reception of P1-P3 pulses contained in the multi-beam 
side lobes. All SSR ground stations, whether equipped with the "old" or 
the newer antenna system, transmit on the main scanning beam at a 
frequency of 1030 MHz, the internationally standardized interrogation 
signals shown in FIG. 1, consisting of three 0.80 .mu.sec. pulses; 
equal-amplitude P1 and P3 pulses separated by a specified interval and a 
lower amplitude P2 control pulse separated from the P1 pulse by 2.0 
.mu.sec. The ATCRBS (SSR) system relies on pulse amplitude comparison 
between pulses P1 and P2 as received by the transponder to prevent 
response to side lobe interrogation, and the Standards therefore specify 
that the radiated amplitude of P2 at the antenna of the transponder shall 
be (1) equal to or greater than the radiated amplitude of P1 from the side 
When main beam P1 pulse levels exceed P2 pulse levels, P3 pulses are no 
longer suppressed and the transponder replies by transmitting on 1090 MHz 
over the desired arc of main beam interrogation. The signal strengths at 
different azimuths outside the main beam's angular sector are such that 
the P2 or P1-P2 combination is always greater than the P1-P3 combination 
and thereby "suppress" any transponder, preventing it from receiving P3 
pulses contained in the side lobes. 
Side lobe suppression control pulses P2, synchronously locked to the timing 
of the main beam P1-P3 pulses, are radiated at the same frequency using 
the same time-shared transmitter, i.e., 1030 MHz, on a control pattern. In 
much of the United States the control pattern is as depicted in FIG. 5, a 
predominant P2 pulse, a P1 pulse 3 dB down from the level of P2, both of 
which dominate a P3 pulse, with the P1-P2 pair dominating the P1-P3 side 
lobe pulse pairs. In Great Britain and in some European countries the 
control signal of FIG. 5 consists of only the P2 pulse which predominates 
any received P1 or P3 pulse outside the main beam and, accordingly, will 
suppress the transponder when a P1 from a strong side lobe combines with 
the control pattern's single P2. 
The relative amplitude levels of the main beam 10 and its side lobes and 
the SLS control pattern of the "old" antenna system shown in FIG. 2 are 
quantitatively depicted in FIG. 5, wherein all levels are indicated in 
terms of dB down from the peak level (0 dB) of the main beam 10 used for 
transmitting interrogation signals on 1030 MHz. The peak signal level of 
the control beam typically is 16 to 18 dB below the peak level of the main 
beam. At any azimuth the relative amplitude of the pulses changes with the 
rotating antenna's instantaneous direction such that the level of pulse P2 
is greater than that of P1 in all directions except in the direction of 
the main beam. Similarly, as shown in FIG. 4, the relative amplitude of 
the pulses change with antenna direction such that the signal strength of 
the P2 pulse is greater than that of any P1-P3 pair in all directions 
except for the narrow angular sector of the main beam. Thus, the "old" and 
the improved antenna systems both produce radiation patterns which contain 
P2 pulses greater in strength than P1-P3 pulses over a wide angular sector 
except in the main interrogate beam which essentially bisects the angular 
sector. Importantly, this insures long range reception of P2 pulses, both 
before and after passage of the main beam through a transponder's 
location, whether radiated by an "old" or a "new" antenna, so as to be 
available for TOA timing purposes in the event normally-used P3 pulses are 
absent. 
Referring now to FIGS. 6A and 6B, a receiver 20 is designed to receive via 
an antenna 22 standard interrogation signals and side lobe suppression 
signals transmitted at the SSR frequency of 1030 MHz from a ground 
station. The antenna may be a highly directive antenna pointed at a 
distant radar in the case of a passive ground radar, or it may be 
omnidirectional for an airborne application; the invention will be 
described in an airborne environment. Because of constraints placed on the 
location of antennas on aircraft, it is typical to use a single antenna 
that both receives at 1030 MHz and transmits at 1090 MHz located on the 
bottom of the aircraft, especially in the case of small general aviation 
aircraft, although sometimes both top and bottom mountings are used with 
two receivers to create a diversity system. The bottom antenna mounting is 
preferred in a low-cost system because the ground antennas are beneath the 
aircraft with the consequence that a bottom mounted antenna receives a 
stronger signal than an antenna mounted on the top of the aircraft. 
The output of receiver 20 is applied to a threshold device 24 arranged to 
pass to a pulse width discriminator 25 any output from receiver 20 
exceeding a predetermined threshold level. The pulse width discriminator 
eliminates all pulses except those having a pulse width which satisfies 
the 0.80 .mu.sec. width and other specifications of standard interrogation 
pulses. The qualifying pulses are passed to a P1-P3decoder 26 designed to 
provide an output on line 26A when an identity (Mode A) interrogation is 
received, or an output on line 26B when an altitude (Mode C) interrogation 
is received. These outputs are applied as information inputs to a switch 
28. 
The P1-P3 decoder 26 also provides an output on line A which represents the 
P3 pulse of each received and decoded interrogation containing P1 and P3 
pulses spaced by either 8.0 .mu.sec. or 21.0 .mu.sec., establishing that 
the receiver is being interrogated by the main beam of an SSR or that the 
SSR is sufficiently close to Own that the signal strength of the main beam 
side lobes is high enough for P1-P3 pulse pairs to be received 
consistently. All P3 pulses provided on line A by decoding of received 
P1-P3 pulse pairs are passed with first priority by a priority selector 
device 32 for use as the synchronizing signal for measuring TOA's. 
The 0.8 .mu.sec. pulses passed by pulse width discriminator 25 are applied 
to a P1-P2 decode 27 which provides an output on line B when the received 
interrogation passed by threshold device 24 and pulse width discriminator 
25 contains two 0.80 .mu.sec. wide pulses spaced, between their leading 
edges, by 2.0 .mu.sec., which establishes that the system is receiving 
P1-P2 pulse pairs conveyed by the SLS wide beam control pattern of an SSR 
within the extended operating range of such stations. The amplitude of P1 
compared to P2 can differ, but a pulse pair decode can occur so long as 
both are above threshold within a typically 50 db dynamic range. An output 
on line B is assigned second priority by priority selector device 32 and 
is coupled to a box 82 labeled "P1-P2" for utilization in a manner to be 
explained later. 
The interrogation pulses passed by threshold device 24 and pulse width 
discriminator 25 are also applied to a pulse storage device 36 which is 
repeatedly opened and closed by a reset line 38 at 50.0 .mu.sec. (.+-.25 
.mu.sec.)intervals to provide an approximately 50 .mu.sec. "window" during 
which the applied signal is examined to determine whether any other pulse 
or pulses similar to a single P2 pulse have appeared either before or 
after it. Thus, absent P1-P3 pairs, P1-P2 pairs, or "stray" pulses within 
the 50 .mu.sec. window, a "stand-alone" P2 pulse can be detected. The 
output of this "window", which will be either an 0.8 .mu.sec. wide pulse 
or multiple similar pulses,, is applied in parallel to a processor which 
includes a unit 40 that examines the stored data and passes only one 
single 0.8 .mu.sec. wide pulse, if present, and to a unit 42 which 
examines the same stored data and detects if there are other 0.8 .mu.sec. 
wide pulses contained within the 50 .mu.sec. "window" period. If unit 42 
determines that multiple pulses are not present, a "kill data" bus 
incorporated therein is not activated and precludes an output from unit 
42; when the "kill data" bus is not activated a single pulse detected by 
unit 40 is passed to one input of a unit 44 labeled "Single P2" and 
provides an output on line C, establishing that only a single P2 pulse 
exists within the 50 .mu.sec. window. If, on the other hand, unit 42 
detects additional pulses, the "kill data" from unit 40 applied to a 
second input of gate 44 will kill or terminate such pulses including a 
single P2 pulse detected by unit 40. 
The just-described process is available again immediately after either the 
"killing" of multiple 0.80 .mu.sec. pulses or the passing of a stand-alone 
P2 pulse. Since, on average, 1600 P2 pulses can occur in a single rotation 
of the main beam (4.0 seconds/rotation.times.400 (PRP)), the killing of a 
few stand-alone P2 pulses that may result from mutual interference of 
multiple SSRs is generally insignificant in low and medium densities of 
SSR stations and the system will operate without them. For example, if the 
P2 pulse is killed because of the presence of multiple pulses in the 
window, which is wide enough to include P1-P3 messages, and this occurs 
say four times during the beam rotation period of four seconds, sixteen 
out of 1600 stand-alone P2 pulses would be lost, but the same sixteen 
would not necessarily be lost during the next beam rotation. However, on 
such occasions the P1-P3 pairs or P1-P2 pairs would be used to create the 
"P 3 time" via priority selector 32. Because the beam rotations of 
multiple adjacent SSRs and, accordingly, their interferences, are random, 
and the system is flexible enough to utilize data from two or three beam 
rotations, of two or three SSRs, the loss of such P2 pulses on an single 
rotation is insignificant. In a multiple SSR environment the priority can 
change continuously during a period corresponding to average beam rotation 
times. The single P2 pulses produced by this process are coupled to a box 
84 labeled "P2 Only" for utilization in a manner to be explained. 
The P2 pulses derived from either box 82 or box 84 are applied as one input 
to a switch 110, and P3 pulses, that is, the priority #1 output of 
priority selector 32, are applied as a second input to the switch. Switch 
110 is so designed that in the absence of P3 timing pulses from selector 
32 it feeds P2 pulses via line 112 to a line 68. If, on the other hand, 
priority #1 P3 timing is available from priority selector 32, switch 110 
instead applies the P3 pulses via line 112 to line 68 for utilization in a 
manner to be described. 
Reviewing the operation of the SSR in the just-described context, and 
assuming another example of a single SSR received at Own transmitting only 
P2 pulses (stand-alone, absent P3 absent P1) on its SLS control pattern 
and a main beam rotation period of five seconds, for about 99% of the time 
after the main beam carrying P1-P3 pulses passes the receiver, P2 pulses 
will be present at the PRP of the SSR, which, for example, might be 400 
pulses per second, thereby producing about 2000 "stand-alone" P2 pulses 
nearly all of which pass through the 50-.mu.sec. window. For the single 
SSR example, during the relatively long period of about 4.8 million 
.mu.sec. for each rotation of the main beam in an English or European 
radar, the "stand-alone" P2 pulses will be separated by about 2500 
.mu.sec., with the consequence that there will be a probability of only 
about 50/2500, or 2% of possible "stray" P2 pulses being present in the 50 
.mu.sec.-wide window and passed as a true P2 pulse to priority selector 
32. Because P2-only pulses are somewhat less reliable than a true P1-P2 
pair for synchronizing the start of "listen-in" time, they are assigned a 
lower priority, priority #3, by priority selector 32. Should a maximum 
main beam side lobe conveying P1 pulses combine with a P2 pulse, priority 
selector 32 determine that P1-P2 priority be used. Thus, the priority 
selector adapts to available signals continuously throughout the time it 
takes for a main beam rotation. 
The interrogation code priority selector 32 is so arranged that if adequate 
P1-P3 data is available over a given period of time, for example a few 
interrogation periods, then that data is used as the timing signal for 
measuring TOAs, just as in the system described in U.S. Pat. No. 
4,486,755. However, if the SSR beam is pointing elsewhere than toward Own 
station, and its side lobes are at such a low level that adequate P1-P3 
data is not available at Own's location, then, if employed in the United 
States (with P1-P2 pairs on the control pattern), decoded P1-P2 data will 
usually be available over a wide angular sector surrounding and moving 
with the main scanning beam. The United States has a population of several 
hundred SSR's that radiate precisely separated (2.0 .mu.sec) pairs of P1 
and P2 pulses on the SLS control pattern. If adequate P1-P2 data should be 
unavailable, as, for example, in systems employed in England or other 
European countries (and occasionally some United States SSRs) wherein only 
P2 pulses are transmitted on the SLS control pattern, the lower priority 
single P2 pulse data is utilized. 
Thus, if priority #1 data is available for a "listen-in" period of 200 
.mu.sec., priority selector 32 insures that priority #2 and priority #3 
data is not used. If priority #1 data is missing, but priority #2 data is 
available, the latter is utilized for that "listen-in" period in 
preference to priority #3 data. After each "listen-in" period the priority 
selector 32 selects in descending order the best, most likely and most 
useful received data at that instant. Recalling that the 200 .mu.sec. 
"listen-in" period is only 10% or less of the average SSR PRP of say, 2500 
.mu.sec., following the close of the listen-in period there remains a 
period of about 2300 .mu.sec. within which other radar interrogations can 
be interleaved. This creates many synchronizing timing signals for TOA 
measurements from which the best are selected by priority selector 32, 
thereby maximizing the operating range of the equipment while maintaining 
a high degree of integrity. The selections are automatic, which maximizes 
the data from an SSR throughout its rotation period and avoids loss of TOA 
data when deep nulls between main beam side lobes are pointing at the Own 
aircraft. For example, the present invention should extend the useful 
range from an SSR to an Own's airborne collision warning system 
constructed in accordance with U.S. Pat. No. 4,486,755 by as much as four 
or five times, say, from 20 miles to as much as 80-100 miles from an SSR. 
This can increase useful coverage area of the herein described system 
relative to each SSR's location by as much as twenty-five times, thereby 
enhancing the desired overlap of multiple SSRs. 
To repeat, when P1-P3 pulse pairs are not available, P1-P2 pulse pairs, if 
available, are preferred over P2 -only pulses. However, if P1-P3 pairs and 
P1-P2 pairs are both absent, P-2 only pulses can, as will be explained 
presently, be used to initiate a 200 .mu.sec. "listen-in" period for TOAs. 
In addition to being "adaptive" to the "old" and "new" antenna systems, 
the just-described decoding/priority selection process is "adaptive" in 
the sense that it is able continuously to examine received 1030 MHz 
signals to determine every few hundred microseconds (e.g. 200-300 
.mu.sec.) which of the three possible priorities are available and which 
of them should be utilized at that instant to time the start of the 
"listen-in" period. If, for example, interrogations are received from two 
SSRs, one of which has P1-P2 pulse pairs on the SLS control pattern and 
the other has only P2 control pulses, and P1-P3 pulse pairs are received 
from neither, the system can extract useful information from both, 
concurrently, as they interrogate other aircraft in the vicinity of Own. 
If too many SSRs are present, overload control 119 can limit SSR data to 
say the closest four or five SSRs. 
Information contained in reply messages transmitted by Other aircraft is 
received at the Own station during each scan of the scanning beam of an 
SSR located within operating range. A 1090 MHz receiver 60, provided with 
an antenna 61 preferably mounted on the top of the aircraft and designed 
to receive standard transponder reply signals, is connected to a reply 
decoder 64 via a threshold device 62 designed to pass any output from 
receiver 60 exceeding a given threshold level, which level may be 
controlled by a sensitivity time control generator 66. STC generator 66 is 
controlled by timing pulses on line 68, which may be P3 or P2 pulses 
depending on the output of switch 110, to initially provide a relatively 
high threshold level, and then reduce the level over a period of, say 5 
.mu.sec., thereafter maintaining the lower level so as to receive weaker 
replies until the next P3 or P2 pulse occurs. If the equipment is embodied 
in a passive ground radar (PSSR), the receiver antenna 64 may be the 
highly directive, switched beam antenna system described in applicant 
Litchford's U.S. patent application Ser. No. 07/813,137, filed Dec. 23, 
1991, pointed directly at the responding aircraft; if embodied in an 
airborne system, antenna 61 must be omnidirectional and preferably is 
mounted on top of the aircraft. Receiver 60 may be similar to usual 
transponder receivers but receiver 20 is about 20 dB more sensitive so as 
to be capable of operating at, typically, -91 dBm sensitivity. 
A "listen-in" gate generator 70 is connected to line 68 and arranged to 
produce a gate signal of about 200 .mu.sec. duration following each P3 or 
P2 pulse applied to line 68. The gate signal on line 70a enables the reply 
decoder 64, which in the absence of the gate signal is disabled. When 
enabled, for about 200 .mu.sec., decoder 64 produces an output on lines 72 
and 74 which represents either the identity or the altitude information 
contained in the current reply message, As shown in FIG. 5, each message 
contains an initial framing pulse F1 and a second framing pulse F2 which 
follows F1 by 20.3 .mu.sec., the interval between them containing thirteen 
sub-intervals, of which twelve are currently used, in each of which a 
pulse may or may not be present, providing for possibility of 4096 
different codes, each code representing one or more pieces of information. 
Since all messages are elicited by interrogation messages alternating 
between Mode A and Mode C in synchronism with P3 pulses carried by the 
scanning SSR beam, and both Mode A and Mode C replies utilize the same 
twelve sub-intervals for carrying information, it is essentially 
impossible to determine at Own's receiving station, using P2 timing 
without more data, whether a given message is a response to a Mode A or to 
a Mode C interrogation. 
For example, a "burst" of about twenty reply messages are received by 
receiver 60 from an Other aircraft during each 360.degree. scan of such 
Other aircraft by the scanning SSR beams. Each 1090 MHz burst, which 
represents 50 to 100 milliseconds of main beam dwell time depending upon 
the type of radar (i.e. whether it is a rapidly rotating airport radar or 
a slowly rotating en route radar), will produce on line 74 for application 
to a beam burst signal processor 76 a mix of pulse messages representing 
identity and altitude of each of the aircraft surrounding the Own station 
out to the maximum reception range of Own's 1090 MHz receiver (say, 30-40 
miles). Processor 76 is enabled by a P2 pulse on line 86, the source of 
which will be described presently. Each data burst on line 74 is a "mimic" 
(or imitation) of the SSR beam (or beams) that are interrogating the 
airspace surrounding Own's transponder and all nearby transponders. The 
mimic characteristics include exact, unique SSR spacing of interrogation 
messages (often known as the SSR/PRP characteristics). Since all SSRs are 
on the same RF channel, they are effectively finger printed or identified 
by each having a unique PRP "signature" for interrogating; the period may 
be fixed or it may be staggered. The mimic also creates replicas of the 
Mode A and Mode C main beam interrogation patterns, such as AACAAC, 
ACACAC, etc., the exact beam rotation period for any SSR that is being 
received, the exact measured value of TOAs for Mode A replies and the 
exact value of TOA measured for Mode C replies when the P2 pulse is used 
as the start of the listen-in window in place of the P3 pulse start time 
used in the '755 system. 
For the sake of simplicity it will be assumed that the main beam's width 
interrogates any transponder within its range of coverage, out to 100 
miles and throughout an azimuth of 360.degree., and will elicit twenty 
replies: ten Mode A reply messages and ten Mode C reply messages. 
Utilizing the above-discussed observation that because of exact 
standardized spacing of the P1, P2 and P3 pulses, the measured TOA at Own 
of Other's identity (Mode A) reply messages, using P2 time, is exactly and 
always 13 .mu.sec. shorter than the measured TOA of reply messages from 
the same Other transponder station elicited by the main beam's Mode C 
interrogation. When activated by a P2 pulse on line 86 (indicating a lack 
of P3 data), beam burst signal processor 76, using techniques extensively 
described in the literature, organizes the "burst" into two "histograms", 
a "short TOA" histogram of identity code pulses and a "long TOA" histogram 
of altitude code pulses. The "short" and "long" TOA histograms are applied 
to units 78 and 80, respectively, which correlate the F1-F2 pulses of a 
respective histogram relative to P2 time to determine the time of arrival 
(TOA). Because in this example there are ten short TOAs, by creating a 
histogram the processor 76 shows that the ten agree with each other 
closely enough (in TOA and code content) to belong to the same "family" 
and therefore to a single Other transponder station, and thus amenable to 
autocorrelation. Similarly, the ten "long TOAs" within the time duration 
of the beam burst are in sufficiently close agreement to belong to the 
same "family" and also capable of being autocorrelated. A typical 1090 MHz 
burst of twenty reply messages, spaced 2500 .mu.sec. apart, has a duration 
of 20.times.2500 .mu.sec.=50 milliseconds. The following a P2 pulse any 
associated 20.3 .mu.sec. message will be received. 
The "short" and "long" TOAs are individually correlated, using P2 pulses 
derived from either priority #2 data (box 82) or priority #3 data (box 
84), and applied via line 86 to both of correlators 78 and 80. While each 
of the TOAs may be of any specific length, depending upon Other station's 
location relative to that of Own's station, because they are referenced to 
the P2 pulse and processor 76 is activated by the P2 pulse, the "short" 
and "long" TOA values in the burst of messages always must differ in 
length from each other by 13 .mu.sec. Although a TOA value may be anywhere 
in the range from 0.1 .mu.sec. to 200 .mu.sec., for purposes of the 
discussion to follow, the "short TOA" will be arbitrarily assumed to be 46 
.mu.sec. and the "long TOA" will then, of necessity, be 59 .mu.sec. These 
values are indicated in blocks 88 and 90, and a comparator 92, shown 
connected between these blocks establishes that the TOA value represented 
by block 88 is shorter by 13 .mu.sec. than the TOA value represented by 
block 90. It will be understood that this 13 .mu.sec. difference would 
also be satisfied if, for example, the "short" TOA were 51 .mu.sec. and 
the "long" TOA were 64 .mu.sec. 
The correlated "short" and "long" TOAs which may, for example, contain a 
message composed of A.sub.1, B.sub.2, C.sub.4 and D.sub.1 pulses 
distributed between framing pulses F.sub.1 and F.sub.2, are applied to 
similar correlators 94 and 96, respectively, which autocorrelate the code 
information reply pulse-by-reply pulse. Since 4096 different Other's 
identity "messages" are possible, when for example two or more successive 
code patterns agree exactly, the probability of an erroneous output from 
code correlators 94 and 96 would be less than one in sixteen million 
(i.e., (4.times.10.sup.3).times.(4.times.10.sup.3). Thus, the system 
provides the same enormous discrimination between "false" and "true" codes 
as that available in current SSR systems. 
A "true" TOA of 40 .mu.sec. for the "short" TOAs is obtained by subtracting 
(depicted by block 98) 6 .mu.sec. (i.e., the P2-P3 spacing in Other's Mode 
A) from the 46 .mu.sec. TOA represented by the autocorrelated code output 
of correlator 94, the same as if P1-P3 pulse pairs had been received and 
the TOA timed with respect to the P3 pulse instead of P2. Thus, the 
described correlations automatically identify the family of "short" TOAs 
as Mode A identification codes, which is outputted from block 100. 
It is important to recognize that the TOA and the code structure really can 
never be separated; since the TOA is referenced to the F1-F2 framing 
pulses, specifically, F2 timing relative to the P3 timing (P3 actual or 
P2-corrected time), the information contained between F1 and F2 is always 
"locked-in". The code information may occasionally be garbled, but it is 
nevertheless locked to TOA and cannot be separated. One or two garbled 
messages, if present in a burst, are ignored as they do not correlate with 
the several others. 
A "true" value of 40 .mu.sec. for the "long" TOAs is obtained by 
subtracting 19 .mu.sec. (Other's P2-P3 Mode C interrogation spacing) from 
the 59 .mu.sec. TOA represented by the autocorrelated output of code 
correlator 96, as depicted by block 102. Because of the above-described 
separation of families of "long" TOAs (representing Mode C) from families 
of "short" TOAs (representing Mode A), the "true" TOAs depicted by blocks 
100 and 104 are both 40 .mu.sec. Other's ten Mode A replies on 1090 MHz 
agree exactly with Other's ten Mode C replies by the described arithmetic 
corrections. Consequently, twenty consistent TOA measurements are 
available at Own from reply messages received from Other which can be 
separated to provide an altitude code and an identity code of Other each 
associated with the same TOA, as depicted by blocks 100A and 104A. This 
enables the described diverse data to be combined, as represented by block 
106, timed to "P2-time" but corrected after the beam burst to P3 time, 
just as if it were timed from two P1-P3 decodes of 8.0 .mu.sec. and 21.0 
.mu.sec. Arbitrarily assuming a Mode A code of 1253, and a Mode C code 
representing an altitude of 3000 feet, by the above process it is 
determined from twenty replies from this aircraft (i.e., No. 1253) that 
the true TOA is 40 .mu.sec. and true altitude is 3000 feet. 
Thus, the just-described system produces: an output on line 105 
representing altitude information contained in the current reply message; 
an output on line 107 representing identity information contained in the 
current reply message; and an output on line 109 representing the distance 
between Own and Other. The output on line 105 is applied to an altitude 
comparator 111 having as a second input data provided by an encoding 
altimeter 113 representing Own's altitude encoded in a similar format. 
Comparator 111 produces an output representing the difference between 
Own's and Other's altitudes when a Mode C reply occurs. The output of 
comparator 111 is an information input to switch circuit 28. 
Briefly reviewing the priority selection process, if priority #1 P3 timing 
is available from priority selector 32, it is fed via line 114 to a gate 
generator 116, the function of which will be described presently, and via 
switch 110 and line 112 to line 68. In the absence of P3 pulses from 
selector 32, P2 pulses produced at the output of either box 82 or 84 are 
applied to line 86 and are also coupled via switch 110 and line 112 to 
line 68. Line 68 is also connected to an overload control circuit 119 
arranged to control the threshold level of device 24, as in a standard 
ATCRBS transponder. It will be understood that any one or all three of the 
signals outputted by priority selector 32 can be operating concurrently 
depending upon the number and relative positions of multiple 1030 MHz SSRs 
within the surrounding environment, and the strength of the signals that 
are present. The extended ranges afforded by P2 timing increases the 
likelihood that at least two SSRs will be received, thereby creating 
throughout a much greater airspace higher accuracy and much fewer false 
alarms than the pseudo-range methods described in U.S. Pat. No. 4,486,755. 
The output of electronic switch 110, with P1-P3 pulse pairs always taking 
precedence if available, can similarly be fed to appropriate connection 
points of the PSSR system described in U.S. Pat. No. 4,115,771. 
Returning now to the description of the collision avoidance system, a clock 
generator and counter 118 is arranged to be reset by each P3 pulse 
appearing on line 68, and to apply the current count, which may be a 
numerical representation of the number of microseconds elapsed since the 
last preceding P3 pulse was applied to counter 118. Each F2 pulse applied 
to a gate 120 transfers the current count to line 122. The output of gate 
120 on line 122 represents the differential time of arrival TOA of a 
received interrogation and the corresponding received reply from a 
transponder at an Other station. Clock generator and counter 118 is not 
enabled by P2 pulses on line 68. 
Whenever P3 is present on line 68, reply decoder 64 produces an output on 
line 72 representing either the identity or the altitude information 
contained in the current reply message. This output is applied to 
comparator 111 and to switch circuit 28; comparator 111 produces an output 
representing the difference between Own's and Other's altitude when a Mode 
C reply occurs. The output of comparator 111 in response to a Mode A reply 
will be spurious. In either case the output of comparator 111 is an 
information input to switch 28. 
Multiple line 72 is connected to supply all decoded outputs timed to P3, 
both altitude and identity, from decoder 64 as information inputs to 
switch circuit 28. When a P1-P3 identity interrogation is received, line 
26a is energized to actuate switch 28 to pass the identity message to 
switch output line 124. The output of comparator 111 at this time is 
discarded. When a P1-P3 altitude interrogation message is received, 
decoder 26 energizes line 26b, thereby activating switch circuit 28 to 
apply the output of comparator 111 to line 124, discarding the input from 
line 72. 
Lines 122 and 124 are connected to a reply storage device 126, which may 
comprise a plurality of digital registers arranged in known manner to 
store associatively the TOA and identity or differential altitude 
information corresponding to approximately twenty successive reply 
messages. Preferably, the differential altitude information is stored 
associatively with the identity and differential time of arrival data. The 
information contained in each new reply message displaces the oldest such 
stored information, so the storage device 126 maintains a running account 
of identification and associated TOA and differential altitude 
information. 
A comparator 128, when enabled by P3 via gate generator 116, compares the 
associated entries in storage device 126 with each other to select those 
nearly identical entries that appear currently in the reply storage device 
126. When such a match occurs the respective entry is transferred to a 
selector device 130. The gate generator 116, which is similar to the 
"listen-in" gate generator 70, is arranged to enable the comparator 128 
for a period, beginning at the end of the listen-in gate, of sufficient 
duration for completion of the operation of comparator 128. 
The output of comparator 128 may and generally will, include several 
entries containing the same identity information but substantially 
different TOA information. The selector 130 rejects all such entries 
except the one containing the largest TOA, which it transfers, together 
with the associated identity and differential altitude information, to a 
selected reply storage device 132. Storage device 132 is similar to device 
126, but retains its entries for a period somewhat longer than the longest 
radar beam rotation period to be expected, say fifteen seconds. If during 
that time a new entry with a larger TOA value is presented, the new larger 
value of TOA is substituted for the old, smaller value associated with 
that particular identity. 
It will have been recognized that the processing performed by reply storage 
device 126 and comparator 128 is functionally equivalent to that 
accomplished in the processing of "short" and "long" TOAs 78 and 80 when 
beam burst signal processor 76 is enabled by the presence of a P2 pulse on 
line 86. As just mentioned, when P1-P3 pulse pairs are available, reply 
storage 126 device stores approximately twenty successive reply messages, 
which is the equivalent of storing a total beam burst, just as is done by 
the signal processor 76. In other words, when P2 is present on line 68, 
signal processor 76 is activated and all information contained in about 
twenty successive reply messages, which takes about 50 milliseconds 
(50,000 .mu.sec.) to complete, is processed relative to P2 time. When P3 
is present on line 68, signal processor 76 is disabled and the 
approximately twenty successive reply messages are, instead, stored in 
reply storage device 126 and compared in comparator to select those nearly 
identical entries that appear currently in the reply storage device 126. 
This being the case, whenever P2 pulses are being used for timing, the 
identity and TOA information present on lines 107 and 109, respectively, 
and the differential altitude information present on line 115, are all 
applied to selector 130, instead of information supplied from comparator 
128 when P3 timing is available. As before, selector 130 selects 
information entries containing the largest TOA, which it transfers, 
together with the associated identity and differential amplitude 
information, to selected reply storage device 132. 
The storage device 132 is connected to a threat detector 134 designed to 
transfer, following a delay of 15 seconds, any entry containing a 
differential altitude of less than a given valve, such as 3000 feet, and a 
TOA of a given valve, such as less than 36 .mu.sec., to a display logic 
device 136. At the same time, detector 134 provides an output on line 138 
to start an alarm timer circuit 140 which may be similar to "listen-in" 
gate generator 70, but designed to provide an output also lasting about 15 
seconds. The output of timer 140 actuates an alarm device 142. 
The display logic device 136 converts the output of detector 134 to a form 
suitable for display on an identity indicator 144, a differential altitude 
indicator 146 and a pseudo-range indicator 148. The pseudo-range 
indication is a display of the differential TOA in terms of distance, 
i.e., one-half the distance radiation travels during the time TOA. This is 
what is meant by pseudo-range and corresponds to the actual range to a 
degree that depends upon the positional relationship between Own and Other 
stations and the SSR. The pseudo range is never greater than the actual 
range. When Own and Other stations are both interrogated by a number of 
SSR's, the likelihood of which is enhanced by the present invention, the 
largest value of the pseudo-range associated with a particular Other may 
closely approximate the actual range of said Other. 
Referring to FIG. 8, which is a plan or map-like representation showing the 
locations of an Own, an Other and two SSRs, line 201 represents the 
distance from SSR-1 to Own, line 202 represents the distance from SSR-1 to 
Other, and line 203 represents the range between Own and Other. The 
differential time of arrival T1 in this case is the difference between the 
sum of the travel times over paths 202 and 203 and the travel time over 
path 201, generally expressed in microseconds. Any particular time T1 
defines an ellipse such as 204, which is a locus of Other's position, 
i.e., time T1 signifies only that Other is at some unspecified point on 
ellipse 204. 
It will be seen in FIG. 8 that lines 201 and 202 are approximately parallel 
and thus T1 is very nearly twice the propagation delay along line 203, the 
true range between Own and Other. Thus (cT1)2, referred to herein as the 
pseudo range associated with SSR-1, is essentially equal to the true 
range, where c is the propagation velocity. 
Line 205 represents the distance from SSR-2 to Other. In this case the 
differential time of arrival T2 defines ellipse 207 as a locus of Other's 
position. Owing to the positional relationship between Own, Other and 
SSR-2, the pseudo range associated with SSR-2, that is (cT2)2, cT2 is 
considerably less than the true range, and may be shown to be a little 
more than one-half the true range. Regardless of the relative position of 
Own and any Other station and any SSR, the pseudo range can never be 
greater than the true range and generally will be somewhat less. 
Therefore, in a multiple SSR environment the largest determined pseudo 
range to a particular Other is always selected by selector 130 as the 
value most nearly equal to the true range. Thus, the larger TOA associated 
with SSR-1 is selected as representative of "pseudo-range" of Other from 
Own. 
When an Other station is much closer to the SSR than Own station the pseudo 
range may become a small fraction of the true range and, if the Other is 
within the differential altitude limits, may initiate a threat detection 
when in fact no threat exists. Such false threats are minimized by the 
action of the STC generator 66 of FIG. 6A controlling the threshold device 
62 to reject relatively weak replies received within a few microseconds 
after reception of an interrogation. 
While the principles of the invention have been described with the aid of a 
block diagram of a currently preferred embodiment, it will now occur to 
those skilled in the art that the invention can be modified in numerous 
ways without departing from the spirit of the invention. It is the 
intention, therefore, that the invention not be limited except as defined 
by the appended claims.