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 June 8 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 June 8 by Ivandrov

[Panny]

On 6/8/2025 at 7:16 AM, wwWolfcom said:

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.

 

And this is a nice simple guide alongside!

https://flyandwire.com/2022/04/12/the-basics-the-detailed-data-display-in-pulse-and-pulse-doppler-search/

[Dragon1-1]

On 6/8/2025 at 4:28 PM, Ivandrov said:

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.

Worth noting, that's not the main reason for the zero closure filter. It's there so you're not getting returns from your own radome. That is also why it can't be turned off, the radar would be quite useless without it. All it'd see would be parts of your own aircraft, which quite obviously have zero closure (if they don't, you have bigger problems, so to speak ).

Edited June 12 by Dragon1-1

[Ivandrov]

7 hours ago, Dragon1-1 said:

Worth noting, that's not the main reason for the zero closure filter. It's there so you're not getting returns from your own radome. That is also why it can't be turned off, the radar would be quite useless without it. All it'd see would be parts of your own aircraft, which quite obviously have zero closure (if they don't, you have bigger problems, so to speak ).

It's not the reason for it, no. It's a useful aspect of it.

[Dragon1-1]

A "useful aspect" that makes the radar able to see anything but the inside of your own nosecone. What is the main reason for zero doppler filter, then, in your opinion? Yes, the radome is mostly transparent to radar, but not perfectly, and it's close, so the return is fairly strong. 

BTW, in most proper formations (and all you'd want to be in when in combat), you'd never see the rest of the division on radar, except in trail, where you often want to have the lead on the scope (though if flying radar trail, you want to lock onto him anyway).

[Ivandrov]

On 6/12/2025 at 7:11 PM, Dragon1-1 said:

A "useful aspect" that makes the radar able to see anything but the inside of your own nosecone. What is the main reason for zero doppler filter, then, in your opinion? Yes, the radome is mostly transparent to radar, but not perfectly, and it's close, so the return is fairly strong. 

BTW, in most proper formations (and all you'd want to be in when in combat), you'd never see the rest of the division on radar, except in trail, where you often want to have the lead on the scope (though if flying radar trail, you want to lock onto him anyway).

I was agreeing with you. 

The useful aspect I was referring to was the combat formation, where you won't have the other members show up on your TID.

In combat, it happens often enough. I will notice the AGC trace pick them up with no contacts forming.

Edited June 15 by Ivandrov

[WarthogOsl]

Is PD any more prone to seeing the inside of the radome versus pulse radar?

[Ivandrov]

7 hours ago, WarthogOsl said:

Is PD any more prone to seeing the inside of the radome versus pulse radar?

Don't see why it would be, the practical difference is processing technique.

Edited June 16 by Ivandrov

[WarthogOsl]

16 hours ago, Ivandrov said:

Don't see why it would be, the practical difference is processing technique.

Yeah, I don't get why you'd need the zero doppler filter for that reason then.

[Karon]

1 hour ago, WarthogOsl said:

Yeah, I don't get why you'd need the zero doppler filter for that reason then.

Apologies, I am probably misunderstanding here. Are you wondering why there is a ZDF in Pulse mode?

I think the easiest way to look at it is the name itself: Zero | Doppler | Filter. Simplifying a lot, if there is no Doppler ( = Zero), then a Filter kicks in.
PD stands for Pulse Doppler, and it uses the Doppler principles to work, so you need a difference in relative speed. If two objects move at the same speed, there is no speed difference, so no Doppler. No Doppler → apply the filter.
I suppose there may be some returns, but they might be hard to process, or perhaps appear aleatory to the radar processing them (clutter from sidelobes?), and so they are removed - you need someone more expert on the matter for a good technical explanation. I never had the time to look into the technical side of radar processing, although I wish I could at some point.

This is how ZDF looks in-game:

 

Back to the AWG-9, if you switch from PD to Pulse, the logic changes and, de facto, there are no filters. That's why you see everything (even too much if you used the bugged Gain knob). In fact, the raison d'être of Pulse is, among other things, to allow crews to operate in an environment where the target aspect can quickly change. MPRF would have been the better choice (but it comes with its own set of issues, range especially) but, from my understanding, MPRF processing with analogue hardware was not feasible in the Tomcat.

I hope this helps and not complicates things instead

[Ivandrov]

4 hours ago, WarthogOsl said:

Yeah, I don't get why you'd need the zero doppler filter for that reason then.

The manual states that it is not possible for targets with no doppler shift to become contacts in the Pulse Doppler modes. That and it being described as a hardware limitation implies that it is a drawback of the technique itself, possibly in combination with the radar components.

[Muchocracker]

On 6/15/2025 at 3:16 PM, WarthogOsl said:

Is PD any more prone to seeing the inside of the radome versus pulse radar?

The radome is designed to be transparent to radio waves in the frequency range that the radar operates in. It will not reflect any notable amount of energy back at the antenna that wouldn't be just folded under the transmitter spillover. 




I would like to provide a clarification on the topic of "Zero doppler" because i believe calling the MLC filter this is not only misleading but factually incorrect, and further contributes to confusion and misunderstanding on how pulse doppler radars work. 

For a moving platform, the ground will always have an amount of radial velocity with exception to 90's down, and 90 degree's to the side of the platform's velocity. And the doppler shift produced is a direct relationship of that. Straight ahead of the aircraft the doppler shift of the ground is going to be nearly inverse of its own velocity and it decreases closer to zero as the angle increases towards 90.


stimsons introduction to airborne radar

 

For a scanning radar this results in the spectra of the main lobe clutter to shift as the antenna is moving and as the velocity of the aircraft changes. It almost never stays in the exact same place (in doppler frequency)
 

 

What "Zero" doppler actually looks like is the altitude return, coming from the sidelobes straight below the aircraft. And the transmitter spillover. It's not impossible to detect targets in this region but it requires the targets to be very close to the radar in order to have its return strength overpower the clutter. 





The main reason why the MLC region is wholesale excluded from further processing is not because of the MLC itself (you're not detecting targets in there anyway), but ground moving targets. GMT's are a gigantic clutter and airborne target discrimination issue (albeit not as much of an issue for HPRF as MPRF) that is not considered or even known about by most of the DCS space. The actual MLC spectra is typically only going to encompass an envelope of a few hundred Hz, but the MLC filter has to be made a few Kilo-Hz to cover the region where GMT's are likely to appear and to reject them. All of this generally applies to search modes and not Single Target Track or illumination modes for reasons not relevant here. 


 

On 6/16/2025 at 7:31 PM, Ivandrov said:

The manual states that it is not possible for targets with no doppler shift to become contacts in the Pulse Doppler modes. That and it being described as a hardware limitation implies that it is a drawback of the technique itself, possibly in combination with the radar components.

It's not hardware, it's a fundamental property of EM physics. When Discriminating targets based on doppler alone it is impossible to detect targets if their doppler shift equals and is buried by main lobe clutter. There is nothing the radar can do that will change that. Range discrimination will be the only way to detect this target, and that's where MPRF tries to bridge the gap between LPRF and HPRF (but is not immune to notching and creates its own drawbacks).

[Ivandrov]

7 hours ago, Muchocracker said:

It's not hardware, it's a fundamental property of EM physics. When Discriminating targets based on doppler alone it is impossible to detect targets if their doppler shift equals and is buried by main lobe clutter. There is nothing the radar can do that will change that. Range discrimination will be the only way to detect this target, and that's where MPRF tries to bridge the gap between LPRF and HPRF (but is not immune to notching and creates its own drawbacks).

So, yeah. That's kind of what I said. The thing is that for the AWG-9, all of the Pulse Doppler modes are HPRF only. It makes me skeptical that moving ground vehicles is the main reason for such a large MLC filter if you're saying that is not as much of an issue for HPRF modes. As a reminder the filter encompasses a region of +-133 knots. Which is far more than you can reasonably expect ground vehicles in a combat zone to be moving.

Edited July 9 by Ivandrov

[Muchocracker]

3 hours ago, Ivandrov said:

So, yeah. That's kind of what I said. The thing is that for the AWG-9, all of the Pulse Doppler modes are HPRF only. It makes me skeptical that moving ground vehicles is the main reason for such a large MLC filter if you're saying that is not as much of an issue for HPRF modes. As a reminder the filter encompasses a region of +-133 knots. Which is far more than you can reasonably expect ground vehicles in a combat zone to be moving.

It has to be wide enough to encompass the spectra at any point in time or you get leakage past the clutter canceller notches and the rejection filter. When the antenna is pointing farther and farther away from the aircraft's velocity the main lobe clutter spectra significantly widens. GMT's and their center doppler frequency also get shifted farther away from the MLC. Close range sidelobes also have to be considered with this. It's 133 knots wide for added tolerance. 

Edited July 9 by Muchocracker