The Mode S Beacon Radar System(6)

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The Pz pulse is used for SLS of the Mode-A/C and Mode-A/C/S all-call interrogations in the manner described earlier for ATCRBS interrogations.
Mode-S Addressed Interrogation
A Mode-S addressed interrogation (Fig. 11) begins with a two-pulse preamble followed by a data block. The data block is encoded with differential phase-shiftkeying (DPSK) at a rate of 4 Mb/s. A logical 1 is encoded as a 1800 phase shift and a logical 0 as the absence of a phase shift. DPSK was selected because of its resis-tance to interference [10]. All data blocks begin with a sync phase reversal that establishes the timing for the remaining phase-reversal posi-tions. The data block is either 16.25).ls or 30.25 ).ls long, and provides for either 56 bits or 112 bits of data. respectively.
The two-pulse preamble is an important ele-ment in the reduction of mutual interference with ATCRBS. It is important that ATCRBS transponders notrespond to Mode-S interroga-tions. Ifthey did, such responseswould produce ATCRBS replies that would interfere with the reception of Mode-S replies. Moreover, the ATCRBS replies would be synchronized with the Mode-S reply and would therefore lead directly to synchronous garble. Tests with many ATCRBS transponders indicated that there is no practical waveform that is invisible to all
TIle Lincoln Laboratory Journal. Volume 2. Number 3 (1989)
Orlando -The Mode S Beacon Radar System

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Differential Phase-Shift Keying (DPSK) Modulation

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Data Rate: 4 Mb/s

 

Fig. 11-Mode-S interrogation waveform.
ATCRBS transponders. Consequently, the ap-proach that was adopted was to precede the Mode-S data blockwith a preamble consistingof aPj and P2 pulse of equal amplitude. When received by an ATCRBS transponder, these pulses put the transponder into a period of suppression for 35 )1s. The data block is sent dUring this interval and is therefore undetect-able by the ATCRBS transponder. Note that the length of the suppression interval dictated the maximum length of the Mode-S data block.
SLS is not required with a selectively ad-dressed Mode-S interrogation, since the interro-gation is transmitted while the addressed air-craft is in the main beam of the antenna. Also, other Mode-S aircraft in the antenna sidelobe regions that receive the interrogation will not reply, since the interrogation does not contain their Mode-S address.
On the other hand, the Mode-S-only all-call format requires SLS. The P2 pulse cannot be used for SLS because it is used in the Mode-S preamble to prevent ATCRBS replies to Mode-S interrogations. Thus SLS for Mode S is provided by an additional pulse, which is shown as P5 in Fig. 11. This pulse is transmitted in an omnipat-tern at the time of the sync phase reversal. The presence of P5 obliterates the sync phase rever-sal for aircraft in the sidelobes, an action that makes decoding the interrogation impossible.
Note that the Mode-S transponder does not require any SLS circuitry because the suppres-sion occurs through controlled interference of the sync phase reversal. However, since P5 must be transmitted at the same time as the data block, a second transmitter (of a much lower duty cycle than the main transmitter) is needed in the Mode-S sensor.
Mode-S Reply
A Mode-S reply (Fig. 12) begins with a four-pulse preamble followed by a data block. The data block is encoded with pulse-position modulation (PPM) at a rate of 1 Mb/s-the highest rate compatible with a low-cost imple-mentation for the Mode-S transponder. In PPM, alogical 1 isencodedasthepresenceofa0.5-)1s pulse in the first halfof the 1-)1s data interval of Fig. 12. Alogical 0 is encoded as the presence of a 0.5-)1s pulse in the second halfofthe interval. The data block is either 56 )1S or 112 )1s long, thus providing either 56 data bits or twice that amount.
PPM was chosen because it provides bit-interference detection. PPM detects interference ofa data-bit position by comparing the received energy in both halves of the data chip interval. The absence of interference results in received energy in only one half of the interval. Thus
　
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