Radar question

[Fly-by-Wife]

Aplogies if this is not the correct forum - perhaps ATC issues would be more appropriate - but it is a technical question!

My understanding is that Primary Radar provides information on range and bearing. SSR provides information on identity and altitude.

In modern ATC systems, is the SSR altitude information integrated with the primary data to provide corrected distances (for altitude) from the primary radar head to the controllers' screens?

Thanks!

FBW

[CJ Driver]

Your initial understanding is not quite correct. Confusingly, the "secondary" radar is often the "primary" source of all information, including range and bearing. In a modern radar with both primary and secondary returns the two are synthesised into a single target by the computers that drive the display. The response from the transponder can often be more accurately seen in both range and bearing than the primary, and therefore it will be the determining input for the position. This is particularly true when there is a lot of ground clutter or weather interfering with the primary returns, and some radar site operate ONLY with secondary returns, because the primary would be unusable.

Having said all that, I don't know the answer to your question as to whether any radar computer tries to correct for slant angle, but I suspect not. In the en-route environment the error is infinitesimal, but I suppose a keen radar designer might try to do something clever for a terminal radar. Note that if the radar is on the airfield, an aircraft on a 10 nm final (3 degree glideslope) will actually appear to be 12 feet further away than it really is.

[Dream Land]

The Primary radar is simply the raw return you receive when the radar hits a reflective object, often you see this with your aircraft weather radar.

[Mad As A Mad Thing]

SSR doesn't only provide identity & altitude information. I also gives range and bearing. I have never seen a radar display that corrects for slant angle, but that's not to say that they aren't in existence.

One interesting aspect to consider is when the secondary data is being combined with primary data where the 2 radars are sited in different locations. In this case there can be a significant different difference in the computed position of the 2 radar returns.

[ChristiaanJ]

Originally Posted by Fly-by-Wife

My understanding is that Primary Radar provides information on range and bearing. SSR provides information on identity and altitude.

No, as already said above.

Primary radar works by sending out radio wave pulses, and receiving the resulting reflections (echos). Those echoes can be from anything.... birds don't carry SSR but can still show up on primary radar!
The snag is, that those reflections are extremely weak... only a tiny bit of the energy of the radar strikes the "target" (plane in our case), and that tiny bit again gets spread out on the way back. It's amazing it works at all....

So people came up with the idea of a 'transponder' (which then became SSR), a small transmitter that would send a 'beep' back whenever it was 'hit' by a radar beam. So now you could rely on the plane 'shouting back' (the transponder reply) instead of the primary radar having to 'listen' for the faint echo.

But the principle stayed the same, the transponder, substituting itself for the echo, would show up at the same range and bearing as the echo.

That should explain why SSR provides information on range and bearing.

It didn't take long for people to figure out that more data could be added to the reply from the aircraft. From the few dashes added in WWII, it went to the various "squawks", set on a control unit in the aircraft, to the full ident, and now altitude. But the original range and bearing are always there.

Just thought I'd scribble this down, in case you wanted a halfway non-tech answer

Cheers

CJ

[Spitoon]

An interesting question about slant range.

Having said all that, I don't know the answer to your question as to whether any radar computer tries to correct for slant angle, but I suspect not. In the en-route environment the error is infinitesimal...

In an en-route environment, as a target goes overhead the radar at, say 30,000 ft, the error would be something approaching 6 miles.

Most SSR display systems can correct for this error. Those which combine primary and secondary sources or multiple sources do some very clever mathematics before ploting the target on the screen. The downside of all of this clever stuff, along with things like track prediction, is that the target displayed to a controller is not necessarily where the aeroplane is!



P.S. - not trying to scaremonger, when determined correctly radar separation standards take such things into account.

[Yellow Sun]

Having said all that, I don't know the answer to your question as to whether any radar computer tries to correct for slant angle

The Calc5, Item 22 in the picture, of the Navigation Bombing System (NBS) fitted to the V-bombers did just that. It was analogue and consisted of a pair of servomotors that moved along tracks set at 90 degrees with a thin metal strip runnning between them. The strip was kept taught by a reel mechanism. The vertical servo moved up and down according to the aircraft height and the horizontal servo moved until the resistance in the metal strip was propotional to the range to the response on the screen. The position of the horizontal servo was then sent as a correction to the display range on the screen. If it could be done all those years ago with the next thing up from clockwork I am sure that a box of "1"s and "0"s can do it better today.

I suspect that some correction is applied to processed ATC radars. Best ask in the ATC section.

YS

[gearpins]

cant contribute tecnically on this subject...but I remember the air peru 757 crash..the a/c's on board Air data system was corrupted..hence the crew asked ATC ...who read back corrupted alt info (they were using SSR)...sorry about the thread drift..