Question about doppler radar

[Laser9]

The manual says that two things are filtered out. Things with no ground speed, such as mountains and a notching aircraft, and things with no relative speed, such as an aircraft moving away from you at the same speed as you. My question, is why is it like this? Do these both create no doppler shift? If so, why? Since a mountain is not moving, it should create no doppler shift, so it is filtered, then why is an aircraft moving away from you at your same speed filtered out? It is moving away; shouldn't it create a doppler shift? Otherwise, why does a mountain not create a doppler shift? The mountain is still moving towards you. My question is, why do these both create no doppler shift? I would thing one would create no doppler shift and the other would create a doppler shift.

[Despayre]

I can't speak to the mountain (but don't worry, I'm sure someone will be along in a moment), but as to the fleeing plane, Doppler works by sending out a radar wave, as the distance between the planes increases or decreases, the time it takes for the returning wave to arrive is either increased, or decreased. Some fancy math then tells the plane the range (from the time it takes for the return) and the velocity variance (from the slightly different return time of the next wave/s). And actually, now that I think about it, the mountain radar returns would equate to a stationary object once the velocity of the plane is removed, and stationary objects, probably aren't flying.

[WarthogOsl]

I'm not sure it really says that, or perhaps it's badly worded.  If you look at the diagram of the DDD with an target at 7 various aspects and rates of closure, only the one flying at 90 degrees to the radar is invisible.

[wwWolfcom]

1. things with no relative speed(closure rate = 0)
relative speed between you and target is 0, this also means doppler effect is 0.
Doppler radars are based on this doppler effect.

0 doppler effect returns are most likely noise and could not distinguish if it is returns from target or noise/error etc.

So those 0 doppler shift signals are flittered out by Zero Doppler Filter(±100kts 0 closure speed) and this cannot be turned off at all.
Simply, Zero Doppler is blind spot of PD radars.

2. things with no ground speed(or closure rate = my speed)
These things make Doppler effect(unless Radar is not moving) unlike Q1.

Returns at Clousure rate = my speed are usually returned signal of 'Main Lobe' that reflected from ground, which hinder normal detection for look down situation. These are called Main Lobe Clutter(MLC)

(Image from FlyandWire, AWG-9 WCS Advanced – Part II)

For look down situation, MLC is undesirable, MLC Filter will filter out those returns(±133kts 0 relative speed or closure speed=own ground speed)

Notching is basically using this filtering.
If target fly perpendicular to radar, it's closure speed of target will be same as ground clutters.
From radar's view, it can not distinguish if it is returns from ground or target. Since amount of doppler shift is same as ground clutter(MLC)

thus, filtered out by MLC Filter

But if we are looking up sky, there will be no Ground clutter.
That's why MLC filter is automatically turned off when looking up 3', or even RIO can manually turn off MLC filter.



TLDR;
Q1 is basically 'blind spot' of Doppler radars, no Doppler effects
Q2 is filter for Quality of Life and this can be turned off by RIO, but has Doppler effects


Some artcles by FlyandWire(Karon), which might help you understanding AWG-9
AWG-9 WCS Advanced – Part I – FlyAndWire
AWG-9 WCS Advanced – Part II – FlyAndWire
Detail Data Display (DDD) in Pulse Doppler mode – FlyAndWire

and Another forum article.

 

Edited 10 hours ago by wwWolfcom

[Ivandrov]

To supplement, here's a screenshot of what your TID looks like without the MLC filter.

Lots of garbage on the TID from the ground clutter. There is doppler shift happening from these ground objects, but since we know ground objects generally don't move that fast towards or away from around your own airspeed unlike aircraft, the MLC filters these contacts for you to clean things up, with some drawbacks as mentioned.

A good RIO can use the MLC filter off even over ground and just tell you to ignore the TID while they find what they are looking for on the DDD.

Also, to clarify, "Doppler Shift" refers to the shift in frequency of the radar wave depending on relative closure of the object making the return.

Targets moving towards you effectively causes the radar wave to bunch up, increasing the frequency. Targets moving away lengthen the radar wave decreasing frequency. "Zero Doppler Shift" also means zero closure. I.E. you are following a plane that has the same airspeed as you do.

Zero doppler filter is actually pretty useful as well, if you are flying in a loose combat formation with friendlies, they won't be added as tracks and added to the TWS firing order on accident.

Edited 14 hours ago by Ivandrov