Garbling
The term garbling is used in secondary radar technology for a special type of interference in reception. It is an overlap of responses from two or more transponders , the simultaneous reception of which can falsify the transmitted transponder code . The German equivalent of key confusion is rarely used in everyday language.
Technical background
The response from a transponder consists of two to 15 pulses, the position of which between the two outer frame pulses is a coded response to the interrogation of a ground station. The length of a response telegram in the Mk Xa system is 20.3 µs. In terms of wave propagation, this corresponds to a distance of 3 km. If there are two or more flight targets with a radial distance of less than 3 km in the detection range of the directional antenna , the response telegrams sometimes run into one another.
There are basically two types of overlap:
- a) Nonsynchronous Garbling;
- b) Synchronous garbling.
If two answers are superimposed in such a way that their time rasters do not coincide, one speaks of non-synchronous garbling. Such responses can be correctly decoded separately and individually. However, if two answers are superimposed so that they have a common time grid, it is a synchronous garbling.
It can no longer be determined in the decoding whether a single pulse belongs to one or the other or even to both response telegrams. This would lead to the decoding of completely new and incorrect answers that no longer have anything to do with the original answers. Thus, the ambiguous answers must be blocked.
Suppression of wrong answers
This process of separating incorrect or ambiguous answers from correct answers is called "degarbling". The problem with degarbling is the detection of decodable and non-decodable variants of the garbling. Finally, the frame can appear a second time even with a correct response with an existing C 2 pulse (see response telegram ). This special case is called a sham carbling . The following table shows the possible cases of garbling and their decoding options.
| Garbling | Impulse image | Decoding |
|---|---|---|
| non-synchronous garbling |
|
possible |
| synchronous garbling |
|
not possible |
| Mock carbling |
|
possible |
| "Closly spaced" |
|
possible |
Degarbling circuit
In order to recognize the garbling cases described and to be able to process them accordingly, special degarbling circuits are used. The technical process in these circuits is a storage of the received pulse pattern and the comparison with the frame pulses.
The required delay lines with taps of 1.45 µs (cycle of the response pulse pattern) can also be implemented as shift registers. The entire process can also run in a processor-controlled circuit or as a program.
Functional sequence
- Detection of frame pulses by delay line DL1
- delay the recognized frame pulses through delay line DL2
- check whether frame pulses run into each other (through delay line DL3 and the following OR gate)
- no: so release of the decoding
Animation of the process in a degarbing circuit
| normal answer | synchronous garbling | Mock carbling |
If it is a synchronous garbling, the answer is not forwarded in the OR gate. If a sham carbling occurs, precisely this cycle (at 15.95 µs) is simply not used.