The Mode S Beacon Radar System(3)

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To ensure accuracy, a sliding-window detec-tor requires a relatively small interval between successive replies. Typically, a PRF of approxi-mately 400 interrogations/s is used, which produces the 15 or more replies required for sliding-window beam splitting. A disadvantage of such a high PRF, however, is that it can interfere with the operation of neighboring sensors.
Another disadvantage of a sliding-window beam splitter is a susceptibility to azimuth splits, which occurwhen interference (e.g., from fruit) or blockage from a physical obstruction, such as a building, causes a loss of data in the centerofthe replyrun length. The loss results in the false declaration of a trailing edge followed by a leading edge, which leads to the erroneous declaration that there are two aircraft instead of one. To make matters worse, neither of the two target reports contains the correct azimuth of the one aircraft.
The Monopulse Technique
To address the many limitations ofATCRBS, Mode-S sensors use the monopulse method [3] in which only one reply is reqUired to determine an aircraft's azimuth. Monopulse azimuth de-termination requires an antenna with two types of beam patterns (Fig. 5):

(1)  
Sum beam (labeled Lin Fig. 5[a].) A sum beam is equivalent to the single main r beam of a nonmonopulse antenna.

(2)  
Difference beam (labeled,1 in Fig. 5[a].) A differencebeamiscomposed oftwo lobes with a null at the antenna boresight.

Areply received from a target that is an angle eoffboresight produces different signal ampli-tudes from the receivers associated with the sum and difference beams. The monopulse processor uses these amplitudes to calculate a return signal that is a function of ,1/L, Le., the ratio of the signal amplitudes in the difference and sum channels. The ,1/L value is then used to obtain efrom a graph of ,1/L versus e(Fig. 5[b]). (The graph was derived with calibration of the sensor against a fixed transponder located near the sensor.)
Thus the use ofmonopulse makes it possible to estimate the azimuth for each reply. This capability prevents azimuth splits.

ATCRBS Monopulse Azimuth Determination
Monopulse makes surveillance of ATCRBS aircraft at very low interrogation rates possible. In theory. monopulse surveillance can be per-

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formed with as little as one Mode-A and one Mode-C reply opportunity per scan. In practice. however. additional replies are needed to ensure correct Mode-A and Mode-C code reception and to suppress false alarms. The Mode-S sensor interrogates at a rate sufficient to elicit two replies for each ATCRBS mode within the ATCRBS antenna's 3-dB beamwidth of 2.4°. This capability leads to a PRF of approximately 100 pulses/so which is about one-quarter ofthe current interrogation rate ofATCRBS.
Monopulse Degarbling of ATCRBS Replies
A second benefit of monopulse is that it
Orlando -The Mode S Beacon Radar System

enables the degarbling of ATCRBS replies [4). Figure 6 shows two aircraft (labeled A and B) thatare simultaneouslyinthe mainbeamofand near the same slant range from a radar. The received signal data show an interleaved mix of code pulses from the two aircraft. Referencing the monopulse data enables the pulses to be identified easily.
In the example of Fig. 6. the pulses do not overlap; hence they could have been sorted into the correct replies if the pulse-timing data were used. However. in instances of pulse overlap that cannot be resolved by timing alone. mono-pulse degarbling can sort the pulses.

Mode-S Surveillance
The principal features ofMode-S surveillance [5) are as follows.
Selective addressing. Mode-S signal formats enable the selective interrogation of individual Mode-S transponders. More than 16 million

 

Reply
Reply B
n rLJl

Fig. 6-Monopulse degarbling ofATeRBS replies.
The Lincoln Laboratory Journal. Volume 2. Number 3 (1989)
addresses are available, enough for each aircraft in the world to have its own unique address.
　
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