h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Can anyone explain their differences. Also can a SSR be used as IFF or vice-versa.

Thank You

SBS from Singapore

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Manda

Hi Researcher 200104

From a technical point of view, SSR and IFF are indeed the same. Both systems make use of a transponder which reacts on a 'query' signal by transmitting a 'response' signal. The difference is the type of messages exchanged:

In an SSR, the messages are defined by standards and there's nothing secret about the contents. The query is of the 'Hey there, you're on my radar screen. Please tell me your flight number, flight level and fuel state' type and the reply is along the lines of 'Hi there, I'm flight No. XYZ012 on level 350, with enough fuel for another 30 minutes' type.

In an IFF system, the query and reply messages are encrypted and the query goes along the lines of 'Unknown airborne object, this is urgent: I've got you centred in my crosshairs and I'm trying to decide whether you're a good guy or a bad guy. If you're unable to tell me what the number 424242 translates to when applying our code of the day then this could result in something you might not want to experience. Silence or a wrong answer from your part will be taken to mean that you don't know how to handle this query. In that case, be prepared to bail out.'

But in the case of a SSR used by a typical Airport Air Traffic Control Radar can it be also used as an IFF too or purely civilian role, i.e interrogating civilian aircraft only.

Regards.

SSR works the same wherever you go so that for example, an aircraft built in the USA and operated by a Japanese airline can land safely in Bolivia. The pulse patterns that form a 'question' are the same throughout the lifetime of an SSR box, so manufacturers may hard-code them.

IFF is by its very nature a national/proprietary thing because the secret needs to be kept. You'll need a crypt-o-magic box on the ground and within all your aircraft, plus a mechanism to distribute the code of the day, plus a bunch of trusted people to handle the machinery and procedures. Due to the variable nature of the query/answer messages, IFF equipment needs to be more flexible. This and of course the cryptology involved makes IFF equipment much more expensive to buy and to operate.

I'm not sure but I think IFF and SSR operate on different frequencies. If so, then this would make them incompatible even if the software allowed to use one in place of the other.

An IFF transponder will receive and respond to SSR (ATC) interrogations,just as if it were an SSR transponder, if the pilots wants it to.

SSR and IFF use THE SAME frequencies (1030 MHz up and 1090 MHz down). The differences are in the MODES used. Some of the military ones (1-5) are encrypted - this is IFF or Identification Friend or Foe. Nothing civil is enrypted (Modes A C and S). One radar can easily do civil AND military modes at the same time. Just spend the money for the right 'cards'. Hope this helps.

"The pulse patterns that form a 'question' are the same throughout the lifetime of an SSR box, so manufacturers may hard-code them"

This answer is misleading. There is only one thing normally hard-coded and that is in a Mode S transponder. It is the 24-bit address allocated by the registering authority of that aircraft (e.g. UK CAA) and this is unique to the aircraft. It is Mode S specific and if you dont hace Mode S (and TCAS if fitted) then you wont have an address.

Hi,

Is all SSR and IFF fixed at 1030 MHz. What if it uses 1031.5 MHz for example. Can it still interrogate? Will the transponder reply?