Evidence of transponders being turned off (and can primary radar see altitude?)

Peter wrote:

Note also that the < 1500ft crowd will be invisible to much civilian primary radar so ATC won’t have useful data on non-txp % there anyway.

This is interesting. So you are suggesting someone transitting under the Gatwick 1,500 feet inner ring quite legally who strays up a hundred feet and is non TXP will not attract attention?

Part of the reason for the question was (on the other thread) exactly this – going through Gatwick last week at 2,800 on a clearance there was another aircraft under the inner ring on my TA which was showing no altitude and from the comms the controller appeared to also have no idea of the height of the aircraft.

I have numerous enounters with gliders over the years while on a traffic service but never been passed any height information – is it available when a service is being provided and disregarded for obvious reasons I wonder, or is the information not available? I was also asked once by Garwick to go and ID a ballon that was well within CAS but I suspect Gatwick had no idea vertically or horizontally the extent of the CAS bust, but were aware it could be a problem from pilot reports.

So you are suggesting someone transitting under the Gatwick 1,500 feet inner ring quite legally who strays up a hundred feet and is non TXP will not attract attention?

Yes.

Primary radar gets no height data. Maybe there is some phased array stuff coming in now but as of some years ago it was 100% rotating dishes with absolutely no azimuth data.

You can get altitude with “dish” radar; this is how it was done historically in the military sector

So, I used to be a director of a radar company until a few years ago, which is to say that I’m not completely up to date. However, primary is called primary for a reason – it’s the primary and that’s because it requires no cooperation from the target. It therefore sees everything that simply reflects enough signal (passively). This is what is displayed on the screen in front of the controller. Primary radar doesn’t normally have elevation but the elevation derived from secondary radar can be superimposed over the primary display provided the traffic density is low enough. This (lack of elevation for primary radar) is one of the arguments for transponder mandatory zones – it ensures the elevation information is available for something flying e.g. under the overhanging shelf of a CTA like at Stansted. There are technologies that can derive the elevation information passively but they are not in widespread use (AFAIK).

Peter wrote:

Yes.

Primary radar gets no height data. Maybe there is some phased array stuff coming in now but as of some years ago it was 100% rotating dishes with absolutely no azimuth data.

You can get altitude with “dish” radar; this is how it was done historically in the military sector

That was indeed my understanding, but now in view of some of the other posts I wonder.

Raiz wrote:

Primary radar doesn’t normally have elevation but the elevation derived from secondary radar can be superimposed over the primary display

but presumably only of value if the aircraft is transponding? Without it will not be seen on secondary?

Peter wrote:

Primary radar gets no height data.

In my past (waivered) flying with no transponder in a US mandatory Mode C area, it was typical for towers with radar feed to say “I have you on primary” and then ask me to report altitude.

Ibra wrote:

https://www.radartutorial.eu/19.kartei/03.atc/karte028.en.html

This one says its precision of altitude is ±3000ft. That’s useless to “detect airspace infringements”.

Ultranomad wrote:

There are about 160 civil primary radars in Europe plus some military ones. (…) the resolution would be 1-2° in azimuth, about the same in elevation

1° is about 100ft/nmi, meaning for each nmi one is away from the radar antenna, the altitude imprecision is 100ft. So at 10nmi, it is 1000ft… Doesn’t look that useful to “detect airspace infringements”.

lionel wrote:

This one says its precision of altitude is ±3000ft. That’s useless to “detect airspace infringements”.

Let’s be optimistic and say more tolerances on CAS busts otherwise you will get flagged on GND setting inside LTMA

Fuji_Abound wrote:

but presumably only of value if the aircraft is transponding? Without it will not be seen on secondary?

Correct, if the aircraft isn’t transponding, there will be no superposition of altitude information, only a “blob” on the screen at the range and azimuth. If you get a traffic service from a LARS you often get stuff like “traffic in your 2 o’clock, right to left, 2 miles, no altitude information”. That’s all they can do when they get a primary return from a target that is not transponding or transponding in mode A.