This occurs when overlapping replies, from two aircrafts that are closer than 2 Nm in slant range, are received.
Figure 13: Synchronous garbling
Refer to Figure 13
. In the case of synchronous garbling, the reply pulses occupy legitimate positions that cause the reply to either be incorrectly decoded or dropped.
Figure 14: Non-synchronous garbling
Refer to Figure 14
. In the case of non-synchronous garbling, the reply pulses occupy inter pulse group positions. This typically may allow the first reply to be correctly decoded but not the second.
Using stochastic acquisition to minimize garbling
Stochastic (probabilistic) acquisition is a technique used during the all-call period to acquire closely spaced (in slant range) targets entering coverage. Refer to Figure 4
. Mode S only all-call interrogations (UF11) can be sent with a probability of reply weighting built into them. The 4 bit PR field contains the probability of reply of 100 percent, 50 percent, 25 percent, 12.5 percent or 6.25 percent.
For example, two aircraft designated X & Y, closely spaced in slant range but at different heights, receive an all-call interrogation with a PR of 50 percent. The possible outcomes are stated in Cases 1, 2, 3 and 4.
All-Call interrogation with PR = 50 percent is transmitted. Aircraft X and Aircraft Y receive interrogation. Aircraft X and Aircraft Y both reply (both processed 50 percent probability and decided to reply). The replies overlap in time at the interrogator receiver and the de-garbling processes were unable to decode them, so both replies were lost.
Figure 15: Case 1
Case 2: All-Call interrogation with PR = 50 percent is transmitted. Aircraft X and aircraft Y receive interrogation. Aircraft X decides on a ‘No Reply’ (50 percent) and aircraft Y Replies. Aircraft Y is then selectively interrogated and locked out.
Figure 16: Case 2
Case 3: All-Call interrogation with PR = 50 percent is transmitted. Aircraft Y is locked out and ignores the interrogation. Aircraft X decides on a ‘No Reply’ (50 percent). No replies sent.
Figure 17: Case 3
Case 4: All-Call interrogation with PR = 50 percent is transmitted. Aircraft Y is locked out and ignores the interrogation. Aircraft X decides to Reply (50 percent). Aircraft X is then selectively interrogated and locked out. Both targets are now locked out to the ground interrogator.
Figure 18: Case 4
As both targets X & Y could be closely spaced in slant range for several interrogator antenna revolutions, it is possible that neither target would be acquired without stochastic probabilities of reply, as the replies may have garbled, being overlapped in time.
The use of the Mode S lockout and stochastic acquisition techniques provide immunity to garbling and reduce RF pollution. Other advantages Mode S provides over Mode A/C are:
* Selective (addressed interrogations) based on 24-bit aircraft addresses
* Relief from Mode A code shortage (when Aircraft Identification is used)
* Additional information (i.e. identity and pressure altitude in 25 ft increments) plus DAP’s.
* Data error detection
* 112 bit GICB data link
About the author:
is the Assistant Director - Avionics Business Unit, Aeroflex Test Solutions.
He has been with Aeroflex for 16 years and has held positions in Product Development, Service Management & Sales Management. Prior to Aeroflex, he was with Hunting Aviation for 18 years in several
positions, including Service Management, Avionics & GSE Sales. He
can be reached at: email@example.com
See related links:
Ultrawideband radar system design
Europe develops space safety radar
Principles of modern radar (Part 1 of 3)
Principles of modern radar (Part 2 of 3)
Principles of modern radar (Part 3 of 3)
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