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Old 02-12-2019, 10:10   #46
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Re: How to tell when someones radar is left on?

Originally Posted by a64pilot View Post
They can if they are close to the traffic, but with a transponder it increases their range greatly.
Which is exactly what we want on our boats right, not only to be seen, but to be a large bright target when far away.

They can see cars depending on how they have their Doppler notch adjusted, a Doppler notch is of course based on Doppler shift and is a way to filter out all stationary targets, or targets below a set speed to eliminate ground clutter.
You can use the Doppler notch to defeat threat Radar by making sure your closing speed is lower than its Doppler notch, a fixed wing can do that by flying a circular route spiraling in towards the Radar.

Of course a standard pulsed Radar can tell the speed of an object by knowing precisely the frequency transmitted, then the difference in frequency of the return signal will equate to a speed, due to Doppler shift.
The Longbow Radar on the Apache would often display highway traffic as aircraft due to their speed, no ground vehicle on the battlefield cruises at 80 MPH.

What I have yet to figure out is how a FMCW Radar which sweeps frequency up and down handles Doppler shift. You see it knows range by knowing the exact time the frequency that is received was transmitted, since the speed of light is a constant it can compute distance.
But a moving target will change return frequency and would therefore interfere with range estimation.
I assume it just ignores Doppler shift assuming nothing is going very fast on the water?

I’ve called B&G tech support and I don’t think they know, or at least the guy I talked to didn’t.
So when my RADAR on the boat is picking up planes, which it has done in the past, is that my RADAR receiving the return or some kind of active return made my a plane transponder? I presume transponders on respond to a particular frequency.

I can tell you one thing though, it doesn't half cause a little head scratching when your guard zone alarms start going off and you can't see anything in the water for miles around.
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Old 02-12-2019, 10:18   #47
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Re: How to tell when someones radar is left on?

Just noticed on watch the other night .... I can SEE my radar working!

Dark night, no lights, and once every few seconds the radome flashes. Clearly there is something inside flashing a light sufficiently bright to be seen through the dome, not synchronized with rotation because the light changes with each flash, different parts of the dome light up.

Now that I know to look for it I see it whenever it’s sufficiently dark.

Raymarine. Maybe that explains it.
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Old 02-12-2019, 10:23   #48
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How to tell when someones radar is left on?

If your picking up an aircraft with your Marine Radar it’s surely a primary return, off the top of my head I don’t know the frequency of ATC Radar, but even then a transponder has to be “triggered” if you will I believe. I know for sure that the IFF or identify friend or foe mode does, it wouldn’t do you much good if the bad guy’s could make your transponder respond.

Now I don’t know if it takes a special signal to interrogate a civilian transponder or if they just transmit whenever they are hit by Radar, but either way I’d bet it won’t respond to Marine Radar, and even if it did the Marine Radar wouldn’t see the response

Even if your on the same band there can still apparently still be differences, for instance traffic Radar will not trip the Radar Jammer on an AH-64, and even though the Longbow Radar operates on the Ka band it doesn’t interfere with nor is interfered by Ka band police Radar.
There are actually apparently types of Radar that takes a left or right spiraled antenna to detect too
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Old 04-12-2019, 03:10   #49
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Re: How to tell when someones radar is left on?

"IFF" or secondary surveillance RADAR runs on (rusty memory) about 1050MHz, quite different (and deliberately so) from most primary RADARs mode 1,2,3(a,b,c) are goverened by international agreements and are only supposed to reply to an appropriate valid interrogation (from memory, certain sized pulses, on the right freq, a certain timing apart) and will only reply on the right freq - so they cannot reply to a primary RADAR (weather airborne, ground based or maritime), only to a SSR interrogator . Mode 4 (western military) and the commie equivalents also involve cryptographic proscesses in their interrogations and responses.

OTOH, there are other transponders fitted to civvy aircraft that might reply to a "nar enough is good enough" signal.... for instance TCAS will respond (or activate or whatever it does, never worked on that one myself but did get taught about segments of it on my IFF course..) to almost any signal on the IFF/SSR band
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Old 04-12-2019, 04:33   #50
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Re: How to tell when someones radar is left on?

Radar and IFF are total different from each other.

The Interrogator, land based, transmits a signal challenge to the aircraft's transponder and receives a signal back which is processed by maybe the TPX-42 system and finally sent to the radar scope in FSK (frequency shift key) format. One frequency for a 1 and a higher frequency for a 0.

This video is called secondary video.

If a plane turns off it's transponder you will receive and display nothing on the scope. (except raw video if the radar is on and the plane is close enough usually within 60 nm)

A raw radar return is the bounce back of the transmitted signal that was transmitted by that radar which is receiving the bounce back which is received during transmitter dead time on a pulse radar before the next transmit pulse.

Secondary video is processed etc during dead time also. There is a separate IFF antenna on the radar antenna for IFF.

Mode C is for altitude. Usually the selector on the scope (Air Traffic Control, ATC) will be set to mode 3C.

Civilian and military aircraft IFF is the same during peacetime operations which allows some military based airports to also accept civilian airliners and visa versa. Example MCAS Yuma, AZ. Civilian airliners, UPS, DHL, etc also land on Naval Station Norfolk's runway the same used by the F18's, E-2's, C2's, and so on.

We had FAA Rep's and sometime Customs at the RATCF (Radar Air Traffic Control facility) on base with us in Yuma, AZ and I'm sure it's the same at Naval Station Norfolk these days
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Old 04-12-2019, 05:03   #51
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Re: How to tell when someones radar is left on?

The Echomax Active XS Radar Target Enhancer (and detector) on the other hand actually recognizes an X or S band marine radar transmitted signal and enhances it for a more amplified display while also alerting the vessel that was just "painted" by radar with either a light (green for X band, Yellow for S) or an audio alarm
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