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Garbling/Degarbling

Garbling is a fundamental problem in the design of the classical SSR system and the situation is made worse by increased traffic. Aircraft are often closely spaced in range and azimuth but at different heights. Replies from two aircraft will overlap if their range separation is within the equivalent of the 20.3 µs reply length. This is approximately 1.7 Nm. The most serious garbling situations occur when the azimuth separation is very small such that replies from both aircraft are received from all interrogations across the beam. With advanced reply processing techniques and algorithms, it may sometimes be possible to extract all some or all of the replies from the received signal.

Figure 1: Synchronous garbling: the individual pulses of two different replies cannot be separate and it would decodes an incorrect information

Figure 1: Synchronous garbling: the individual pulses of two different replies cannot be separate and it would decodes an incorrect information

Figure 1: Synchronous garbling: the individual pulses of two different replies cannot be separate and it would decodes an incorrect information

At this, in principle, one distinguishes two manners of the overlapping:

Two replies overlaps in time such that its time grids are not congruent, so one speaks about Non-synchronous Garbling. Such answers can separated and one by one be decoded correctly!

But if two or more replies overlaps in time such that its time grids are congruent, so one speaks about Synchronous Garbling.
It cannot to state in the decoding any more, whether this a single impulse belongs to one or the other ones response telegrams. Through this it would come to the decoding of completely new and wrong replies and difficult from the original replies. These replies must therefore be disabled!

F1 C2 F2 SPI

Figure 2: C2-SPI phantom bracket

F1 C2 F2 SPI

Figure 2: C2-SPI phantom bracket

F1 C2 F2 SPI

Figure 2: C2-SPI phantom bracket

Wirings which reduce the effects of the Garbling are called Degarbling Wirings. Bracket detection is usually implemented by a digital delay line in which the presence of the F1-F2 bracket pair is detected by tapping points 20.3 µs apart with some additional tolerance. Unfortunately the position of the SPI-pulse is spaced 20.3 µs after the C2 pulse and if both pulses are present in a reply then C2-SPI phantom bracket will occur:

But in this case this reply may be decoded and displayed! The airplane with which you have a radio link is of special interest. It would be a pity, if it disappears of the screen as long as the operator talks with the pilot.

Replies which are “Closly Spaced” represent a further special case shown in the bottom one example in the following table.

GarblingPulsesto display
non-syn­chronuous
Garbling
ein Antworttelegramm eines zweiten Transponders fällt zeitlich in die Zwischenräume der ersten Antwort beide Antworttelegramme können decodiert werden
synchronuous
Garbling
die Impulse eines Antworttelegrammes eines zweiten Transponders fallen zeitlich genau auf die Impulse der ersten Antwort beide Antworttelegramme dürfen nicht angezeigt werden
C2-SPI phantom
bracket
ein zweites Impulspaar der gleichen Antwort hat ebenfalls den Abstand von 20,3µs die Antwort muss angezeigt werden
“Closly spaced” zwischen dem letzen Impuls der ersten Antwort und dem ersten Impuls der zweiten Antwort ist ebenfalls ein Abstand von 20,3µs die Antworten müssen angezeigt werden

Table 1: kinds of garbling


Degarble Wiring

To recognize and be able to process the described garbling cases, special degarble wirings are used.

delay line DL 1: 20,3 µs
DL 2: 20,3 µs
DL 3: 20,3 µs
video
to the decoder
H=“enable”
C2-SPI phantom bracket
H=“bracket detect”
L=“garbling”

Figure 3: Principle of a degarble wiring

delay line DL 1: 20,3 µs
DL 2: 20,3 µs
DL 3: 20,3 µs
video
to the decoder
H=“enable”
C2-SPI phantom bracket
H=“bracket detect”
L=“garbling”

Figure 3: Principle of a degarble wiring

The needed delay lines with the taps of 1,45 µs (pulse grid!) can be created also as a digital shift register.
The complete process also can be carried out by a processor controlled wiring.

Sequences of operation during a correct replay

  1. Bracket detect
  2. delay of the recognized brackets
  3. check whether there brackets overlaps in time
  4. if no overlapping, then the decoder is enabled.
  

Sequence of operation during a garbling reconnaissance

  1. overlapping detect, then the decoder is not enabled.
  

Sequences of operation during C2-SPI Phantoms

  1. the phantom-bracket from the pulse pair of C2 and SPI is ignored!
  2. the decoder is enabled in this case.