/ Omnidirectional Antenna Pattern P2
,
Fig. 2-Sidelobe-suppression (SLS) operation.
ATCRBS Limitations
The current ATCRBS satisfies operational requirements in most airspace. The system, however, has the following limitations in regions of high traffic and sensor densities.
(1)
Synchronous garbling (described later).
(2)
Azimuth inaccuracy (described later).
(3)
Fruit. Replies received from interroga-tions by neighboring sensors are called fruit. These unwanted replies are not synchronized with the local sensor's interrogations, and are thus received at random times. The presence offruit can interfere with the reception of a wanted reply. As a result, high fruit rates can produce a detectable decrease in per-formance. The use of high pulse-repeti-tion frequencies (PRF) for sliding-window detection contributes to this problem. (Sliding-window detection will be discussed in a following section.)
(4)
Overinterrogation. In a region containing manysensors, a transponderwill receive a high rate of interrogations and SLSs. Consequently, the transponder may be unable to replywhen it receives an inter-rogation from the local sensor. As is the case with fruit, the use of high PRFs aggravates overinterrogation.
(5) Aircraftidentification. In many regions of the world, the limit of 4,096 different Mode-A codes is insufficient.
The Lincoln Laboratory Journal. Volume 2. Number 3 (1989)
Orlando -The Mode S Beacon Radar System
Synchronous Garbling of ATCRBS Replies
Synchronous garbling occurs when two ATCRBS aircraft (shown asA and Bin Fig. 3) are within about 3° in azimuth from an ATCRBS radar and when their slant ranges (Le., their line-of-sight distances from the sensor) differ by less than 1.64 nautical miles (nmi). Under such conditions, the transmitted interrogation elicits replies from both transponders, and the replies overlap at the receiver. The overlap can lead to missing or incorrectly decoded replies, which results in a loss of information on the control-ler's display. The loss persists until the aircraft change their relative positions. Thus the reply overlap canlastfor many scans, hence the name synchronous garble. Note that the altitudes of the two aircraft do not have to be close for garbling to occur.
Azimuth Inaccuracy
Current ATCRBS sensors in the United States use sliding-window detection (Fig. 4), a technique that determines the azimuth of an aircraft by marking the center of the aircraft's run length. (A run length is a series oftranspon-der replies that are observed as the antenna beam of a sensor scans past an aircraft.) A leading-edge detector determines the beginning of a run length by detecting the presence of a minimum of m replies from n reply opportuni-ties. (Following an interrogation, the detector
Aircraft
Azimuth
Characteristics: High Pulse-Repetition Frequency, Susceptible to Azimuth Splits
Fig. 4-Sliding-window beam splitter.
waits a certain time, called the listeninginterval, for a reply to be issued.) A similar algorithm is used for trailing-edge detection. Once the lead-ing and trailing edges have been determined, the aircraft azimuth is calculated as the center of the run length with an offset to account for the bias that the edge detectors introduce.
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