SSR Questions

[c-bert]

Gents,

Hope someone can help me with the following:

I have heard that NATS control en-route primarily with the use of SSR. Can azimuth and range be derived from these and if so, how?

Thanks in advance.

[Chilli Monster]

Simply put - no different to normal primary radar. It's displayed on the screen in conjunction with the primary.

The only difference with the principle is you emit an interrogation pulse rather than a beam of energy, you get back a response from the transponder rather than reflected energy. Calculating the time to response and return, plus where the aerial was pointing during the process, gives you the return (the lable) on the display - no different to the principle behind primary radar.

No doubt you'll get a far more technical explanation shortly.

[Tarq57]

Not entirely sure what you want to know, but I'm going to assume you have a basic understanding of radar.
Can't speak at all about NATS, but SSR does give azimuth and range. It does this in exactly the same way as primary radar, the difference being that it will only receive signals generated from a transponder. (Thus all that lovely weather and terrain clutter I now miss so much is not displayed.) (Well, maybe not the terrain clutter.)
In its basic form it presents a radar "blip". (The data information, altitude, callsign, speed etc, is an extra.) And the direction and distance from the radar station is presented, quite accurately, in plan form.
One of the advantages of SSR over primary radar, apart from the non-display of clutter, is that, since the received signal is a powered signal, ie: transmitted from the aircraft, rather than an echo of the sent signal, the transmitter doesn't have to be anywhere near as powerful, nor the receiver anywhere near as sensitive, for the same range of coverage.
Hope that helps.

[c-bert]

Many thanks guys. Most informative.

[MrApproach]

.......and then there is multilateration, which instead of have a rotating data link ie the SSR antenna, positions little bar fridges all over the countryside and measures time of receipt of the reply from the transponder. Hence three of more will triangulate the target. Very useful where a rotating antenna cannot work very well like the Innsbruck area or at low level over the North Sea oil platforms.

[Minesapint]

MLAT is coming to a field near you, probably in the midlands! Then there is ADS-B.

[MrApproach]

MLAT v ADS-B

I believe that all of the MLAT "boxes" contain ADS-B receivers because they use it when it is available to cross check the MLAT triangulation. Hence buy MLAT, use it on Mode A/C/S transponders and when ADS-B is widespread turn off the MLAT. Better still, leave it on to cross check the ADS-B, after all until Galileo we are getting GPS at the whim of the USAF.

[BDiONU]

Originally Posted by MrApproach

Hence buy MLAT, use it on Mode A/C/S transponders and when ADS-B is widespread turn off the MLAT. Better still, leave it on to cross check the ADS-B

Definitely leave it on otherwise we have no ground based and ground checked/calibrated independant system. You'd be reliant on the airborne equipment alone and reliant on a single aircrafts system to be working absolutely correctly, at all times, to ensure seperation.

BD

[Minesapint]

Sounds like a strategy to me! Mode S supported by MLAT, then full ADS-B & (maybe, when the force is strong) ASAS then - who knows or cares!