Primaray Radar Question, Which heli part is most reflective?
Joined: Oct 2003
Posts: 1,852
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From: Canada
For what it's worth, I was flying a Hughes 300 with doors on and no transponder in the Toronto (YYZ) control zone. The contoller could not identify me on radar even though I made a few turns at his request. (He let me continue anyway as I was well away from any reported traffic.)
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
Guys, radar is like light, and most metal surfaces are mirror-bright to it. Like light, radar bounces back most efficiently from a corner made of 3 plates that all meet at right angles, like a corner of a box. (if you look carefully at the red reflector material on cars, you can see all the little corner pyramids cast into the plastic. Those corners reflect light directly back at its source. If you look into a perfect 90 degree corner of mirrors, you see your eye, tracking perfectly as you move!
It is hard for a helicopter to not have that shape somewhere, so most medium/small helicopters have big signatures. Remember that the windshields are transparent to radar, as are most composites (if not lined with mesh for lightning protection), so the radar is looking into your machine thru the windows and it is probably seeing lots of box angles in the cabin.
Look at some ads for buoys and radar reflectors for boats:
http://www.zenilite.co.jp/english/6_1.HTM
http://www.pelangi.co.uk/pdf/s09/PA9-18.pdf
Note that in both cases, small corner reflectors (about .5 meters on a side) have signatures like 50 to 250 square meters of metal. In other words, if the radar can see a box angle of 500 mm on a side, you look like a 747 to that radar.
ATC has purposely reduced their radar sensitivity to primary skin paint, because of the widespread use of transponders, and the flakyness of primary returns by comparison, so they might not see any skin paint very well.
It is hard for a helicopter to not have that shape somewhere, so most medium/small helicopters have big signatures. Remember that the windshields are transparent to radar, as are most composites (if not lined with mesh for lightning protection), so the radar is looking into your machine thru the windows and it is probably seeing lots of box angles in the cabin.
Look at some ads for buoys and radar reflectors for boats:
http://www.zenilite.co.jp/english/6_1.HTM
http://www.pelangi.co.uk/pdf/s09/PA9-18.pdf
Note that in both cases, small corner reflectors (about .5 meters on a side) have signatures like 50 to 250 square meters of metal. In other words, if the radar can see a box angle of 500 mm on a side, you look like a 747 to that radar.
ATC has purposely reduced their radar sensitivity to primary skin paint, because of the widespread use of transponders, and the flakyness of primary returns by comparison, so they might not see any skin paint very well.
Joined: Dec 2001
Posts: 943
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From: Gold Coast, Queensland
During my radar controller days in Sydney in the 70s, all the radar returns of aircraft were of a fixed size. For example a Cesna 150 had the same size blip as a 747 and so the return was no indication of size.
Weather and aircraft radars show size because that is what you are interested in whereas ATC just need to know where you are.
Proably all changed since my time!!
Weather and aircraft radars show size because that is what you are interested in whereas ATC just need to know where you are.
Proably all changed since my time!!
Joined: Jul 2002
Posts: 230
Likes: 0
From: Murica.
During one of my flights as a CFII my student took a LONG time setting up an ILS approach, and we were not yet talking to ATC....on of the dreaded pop-ups!!
Anyways, I was on the controls and decided to do some OGE hovering at 2100' while my student did his bit, he got on the horn to ATC and they gave us a squawk code. They could not find us on the radar although we told them exactly where we were. Still hovering I figured we would try to move, and as I passed about 30 KIAS he got radar contact.
Was that all just coincidence, or can we go stealth just by stopping...some kind of doppler effect?
300CB, no doors
Anyways, I was on the controls and decided to do some OGE hovering at 2100' while my student did his bit, he got on the horn to ATC and they gave us a squawk code. They could not find us on the radar although we told them exactly where we were. Still hovering I figured we would try to move, and as I passed about 30 KIAS he got radar contact.
Was that all just coincidence, or can we go stealth just by stopping...some kind of doppler effect?
300CB, no doors
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
Posts: 10,959
Likes: 1,814
From: EGDC
Many air to ground/air radars (as in fast jets) use pulsed doppler radars that use the change in speed of the return pulse compared to the sent pulse to identify moving targets (high threat). These radars often have a lower limit set to take out relatively slow ground targets like traffic; if the limit is 40 kts for example and you are doing 30 kts your return will be processed out. If you are moving faster but at a constant distance from the radar head then you will not be seen as you have no velocity relative to the direction of travel of the FJ radar - this is known as the ZD notch (Zero Doppler).
I don't know if ATC use pulsed doppler radars nowadays but if they do then similar rules would apply.
I don't know if ATC use pulsed doppler radars nowadays but if they do then similar rules would apply.
Joined: Mar 2001
Posts: 65
Likes: 3
From: UK
A typical ATC radar will be using a low Pulse Repetition Frequency (i.e. number of pulses sent and recieved is low) which gives less ambiguous range resolution but is very poor at assessing Doppler shifts. This means that speed info / separation from clutter becomes harder as a consequence. This is what has happened in TIMTS case - the radar has filtered out his chopper as the radar cannot seperate his minimal doppler shift from the ground clutter at such low speeds. As he speeds up, his doppler frequency becomes detectable and his blip appears on screen.
Fighters have to trade these capabilities off against one another - as with any engineering, it's a botch!
Fighters have to trade these capabilities off against one another - as with any engineering, it's a botch!
Joined: Jan 2000
Posts: 438
Likes: 2
From: Derby
I used to work on a Naval surface to air missile system. This used a pair of pulse-doppler radars. Talking to the radar guys, I remember one of them saying that with Helicopters, they'd get high power returns indicating both low speed (fuselage) and high speed (rotor) emanating from the same area. Internally, the radar signal processor would reconfigure itself slightly to allow for the 'split personality' of the signal (and maybe the pilot !!)
As an aside, during tracking & firing trials, there was a camera atop the tracker mount. In the picture produced by this camera was a little white box indicating the strongest radar return, i.e. where the radar was actually 'looking'. It was interesting to watch this square jumping around on the target as it came towards us. It pretty much jumped from rivet to rivet.
As an aside, during tracking & firing trials, there was a camera atop the tracker mount. In the picture produced by this camera was a little white box indicating the strongest radar return, i.e. where the radar was actually 'looking'. It was interesting to watch this square jumping around on the target as it came towards us. It pretty much jumped from rivet to rivet.
