Transponder Codes
 

Hello, why can a transponder not use digits 8 and 9? What is the techincal explanation for this?

I would say it has something to do with binairy codes.
I'll leave the exact explanation to the experts on this piece of kit.

C

The 'squawk' is made up of 13 binary digits. 12 normals make up the 4096 possible squawks and 1 for the SPI or ident.

CF

The original code was written in Octal (number base 8 using the figures 0-7) I believe so that is why there only this number of codes. Probably wrong tho...:sad::sad:

Yes it is octal. Each octal digit (0 to 7) represents 3 binary bits so 4 digits make the 12-bit binary number CF mentioned.

Thanks chaps, I was having trouble sleeping but I sure aint now.



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ps only joking really

Mind you, I've heard replies from pilot's mistakenly told, by a flustered controller to squawk a code with 8 in it, along the lines... "Err the box only seem to go up to seven, I'll snag it on landing"

SSR sends out a pattern of up to twelve pulses in reply to interrogation, that are read as a binary code e.g Code 7031 would be 111 000 011 001... a bit like a barcode?

As seen on a T-shirt:
There are 10 types of people in the world: Those who understand binary, and those who don't...

Here's one for the tecchies ...

The VC10s I used to fly in the 1970s (or was it the early 747s, I can't remember now) had selectable transponder digits (!) which all ran from 0 to 8 (yes eight) inclusive. By experimenting, we found that setting an 8 seemed to produce the same result as if a 0 had been set.

I asked many people at the time why/how the numbers could run beyond 7 as the system was octal but was never offered a plausible explantion.

Maybe someone out there may now have an answer ... ?

JD

my question is: why don't SID numbers go above 6?

But they do - for example the SFD 8P out of EGKK ...



JD
;)

possibly because six is more than enough to cover most different departure directions?

An IFF interrogator sends out two pulses modulated on a 1.03 GHz carrier. To interrogate Mode 3/A, the pulses are 8 microseconds apart. To interrogate Mode C, the pulses are 21 microseconds apart.

The aircraft transponder replies on 1.09 GHz with 12 pulses giving the requested information, with each pulse providing a "0" or "1" bit giving a total of 4,096 codes.

3/A and C modes are common to both civil and military aircraft, but the military also has other modes reserved for its own use. In Mode 1, two pulses are sent by the interrogator 3 microseconds apart, and the IFF transponder responds with an 8-bit response enabling 64 codes that indciate the type of mission . In Mode 2, the two pulses are sent 5 microseconds apart, and the aircraft provides a 12 bit reply, returning an airframe assigned code.

Mode 4 is similar to Mode 3/A, but the challenge and response are encrypted.

Mode S is more recent and a bit more complicated.

Mode 3 was the precursor to 3A and was limited to 2 digits/64 codes. Some jets still had them fitted well into the 80s. I remember the Italian F-104s visiting Leuchars and ScATCC complaining they wouldn´t squawk the last 2 digits.

There also used to be a military emergency button on the old aircraft IFF boxes which, when pressed, transmitted the currently set Mode 3 code 4 times giving a 4 bar indication on the tube but still allowing the controlling unit and position to be identified. Only visible at the older ADGE sites however....

SID No's change according to the changes. A no.1 will be the first SID assigned, if it changes due to noise etc. it will become a no.2 it will be highly unlikely that a SID will change more than 6 times, however it may happen. The no's are all to do with the amount of times the SID changes.

I SOOO want that t-shirt!!

James

qcode

Not sure where in the World you are refering to (although I see your location is given as the UK?)... what you say may have been the numbering convention used at one site, but in my experience I have worked at airfields where we have had SID 1, SID 2 and SID 3 current at the same time?

Alkthough, maybe we should have numbered them in binary?

Courtenay, check your PM's.