Monopulse beam splitting. The development of monopulse was critical to Mode S. Selective addressing makes a sliding-window detector impractical because of the lack of channel time necessary to conducta selective interrogation of each aircraft 15 or more times.
Single surveillance interrogation andreplyper antenna scan. The use of monopulse, coupled with a more capable data format that provides altitude and the Mode-S address in a single reply, makes routine surveillance with one transaction (Le., one interrogation and reply) per scan possible.
All-call acquisition. An all-call interrogation elicits replies from all Mode-S aircraft that are not being selectively interrogated. By periodi-cally transmitting all-call interrogations, Mode-S sensors can obtain the addresses ofunknown aircraft.
Lockout. Once a Mode-S aircraft is acqUired via an all-call interrogation, the vehicle is in-structed to ignore (Le., to lock out) subsequent all-call interrogations. This lockout option re-duces the probability of synchronously garbled all-call replies.
Error detection and correction. The Mode-S data formats enable an extremely high degree of error detection. The system boasts a rate ofless than one undetected error in 108 messages (6). In addition to detection, error correction is pro-vided on the downlink (7).
Basic Mode-S Surveillance Interrogation and Reply Formats
Figure 7 shows the basicMode-S surveillance
INTERROGATION
All-Call
r
Discrete
REPLY
All-Call
Discrete
Fig. 7-Basic Mode-S surveillance formats.
formats [8, 9], which include the following. All-callinterrogation. This format contains the same Pj, P2 , and Ppulses that ATCRBS uses.
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The additional pulse P4 labels this format as originating from a Mode-S sensor. When an ATCRBS transponder receives a Mode-S all-call interrogation, the transponder cannot detect the Ppulse. It therefore responds with the
4
appropriate Mode-AorMode-C reply, depending on the spacing of the Pj and P3 pulses. On the other hand, a Mode-S transponder will detect the Ppulse and, if it is not in a lockout state,
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respond with an all-call reply. Thus one interro-gation can satisfybothATCRBS and Mode-S all-call requirements. Because of this feature. the Mode-S all-call interrogation format is also re-ferred to as the Mode A/C/S all-call. Note that a Mode-S transponder will never generate an ATCRBS reply to a Mode-S sensor's Mode-A/C/S all-call interrogation. This detail is im-portant since it ensures that a Mode-S aircraft will never be reported as both a Mode-S and an ATCRBS vehicle.
All-callreply. The reply ofa Mode-S transpon-der to a Mode-S all-call interrogation is com-posed largely of the aircraft's Mode-S address. which is used in subsequentselective interroga-tions of the vehicle.
Discrete interrogation. This format contains
The Lincoln Laboratory Journal. Volume 2. Number 3 (J 989)
Orlando -The Mode S Beacon Radar System
the Mode-S address ofthe aircraft for which the interrogation is intended, surveillance informa-tion, and communication-control information.
Discrete reply. The basic surveillance reply to a discrete interrogation contains the aircraft's altitude code and Mode-S address.
Mode-S Elimination ofSynchronous Garble
Selective addressing completely eliminates the problem of synchronous garble. As before, the two aircraftin Fig. 8 are atthe same azimuth and slant range. The sensor has knowledge of the azimuth and range ofeach aircraft from the previous scan, and schedules an interrogation to one oftheaircraft (A in Fig. 8). The sensorthen schedulesan interrogationofthe second aircraft such that both the interrogation and reply for aircraftBwill occur attimes thatdo notinterfere with the reception of the reply from aircraft A. The scheduling technique can be extended to cover three or more aircraft.
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