Understanding MLC vs Zero Doppler Filter

[Snappy]

Hi,

 

recently started to get aquainted with the RIO seat and diving into the radar basics (still just scratching the surface) and have

watched /read some of FlyandWires excellent material and the Heatblur manual part on this  but I still have problems understanding

the two filters.

Both filters seem to aim at filtering out returns that are within certain ground speed range, but what exactly is the difference between them?

I know the following contains massive oversimplifications and potential misconceptions on my side,   but I thought the big advantage

of Pulse Doppler vs classic Pulse was, that (besides different working principle) the PD radars were not suspectible to ground clutter returns since this gets filtered out inherently by removing everything that is own-speed +/- a certain variance.

 

But the two filters have me confused  and raised several questions:

The MLC makes sense to me, as it filters out the ground returns, the ground is "moving" relatively at my aircrafts own groundspeed  , so filtering

that speed range out in a look down situation makes sense.

But the Zero doppler doesn't make sense to me yet. Heatblurs F-14 manual states it is applied for targets with negative closure, i.e. moving away from ownship , but then when its talking about the filters speed range it says quote:

 

"The resulting blind area is 200 knots wide, meaning that a chased target moving at a speed of within 100 knots(+/-) of own groundspeed will be invisible to the radar"

 

Quesstion 1 :

 

That +/- has me confused.If a target is moving at -100 kt of own groundspeed it is actually not longer moving away its coming closer, or am I getting somethign wrong ? 

 

Question 2:

Also that speed range of +/- 100 kts of the  is within the speed range of the MLC filter with +/-133 kts. So why the need for an additional zero doppler filter if the value is already included within the MLC filter band?

 

Question 3:

And is the Zero Doppler Filter a Filter at all? Because in the manual it sounds more like a hardware/physical limitation of the radar that it just can't detect targets without a doppler shift.

 

 

Sorry if those questions are somewhat dumb. I'm not too technical a person and having some difficulties in getting my head wrapped around

this radar stuff.

 

If somebody could explain or answer my questions in a semi-simple way that would be great.

Kind regards,

 

Snappy

 

 

 

 

 

 

 

 

 

 

 

 

 

[Noctrach]

So an important distiction to make:
 

- The MLC filters out targets +/- 133 knots of F-14 groundspeed. The target's true closure is only relevant if its flying away from the F-14.

- The ZDF filters out targets +/- 100 knots of true closure. The F-14's groundspeed is not considered in this.

 

Let's look at the following scenarios:

1. Target flying perpendicular to the F-14's radar: This target has a closure similar to the F-14's groundspeed. If this target were to be flying at low altitude, its closure would be identical to the closure of the ground relative to the F-14. Let's say the F-14 is flying 400 knots. The target is filtered out by MLC, even though its closure falls well outside of the ZDF. Disabling MLC will reveal this target, as its closure is 400 knots and falls outside of the ZDF. Although it might be hard to find in the surrounding ground clutter.
2. Target flying away from the F-14: Consider the F-14 flying at 400 knots, but its target is running away at 450 knots. The resulting closure of -50 knots will bring it inside BOTH the MLC and the ZDF. Disabling the MLC will reveal ground clutter at 400 knots closure, but will not reveal the target as it is still inside the ZDF. This target is moving slightly away from the F-14, but the same would happen if it were flying 350 knots with the F-14 slowly overtaking. The true closure is just too low for the radar to process. Three things can be done to reveal it:
   1. Keep MLC enabled and change speed to get more than +/- 133 knots of closure
   2. Disable MLC and change speed to get more than +/- 100 knots of closure
   3. Use Pulse mode

As you can see, the MLC will reveal aircraft in scenarios where closure is much higher than +/-100 knots, but still hidden within other clutter sources.

The ZDF will only hide targets at very low positive or negative closure rates.

 

The rule of thumb here would be: If a target is flying towards you but you can't see it on radar, turn off MLC. If a target is flying away from you and you can't see it on radar, change speed or use pulse.

Edited February 26, 2021 by Noctrach

[Spiceman]

This might help a little as well.

 

 

[Saber2243]

Question 1: Correct, the ZDF filters out 100kts closure to 100kts separation

 

Question 2: As Stated by Noctrach, the ZDF is centered around zero knots closure, whereas the MLC is centered around own aircraft airspeed

 

for example if the F-14 was at 400kts ground speed
Targets with 100kts separation to 100kts closure would be filtered out by the ZDF

Targets with 101kts to 276kts closure would be visable

Targets with 277 to 533kts closure would be filtered out by the MLC
Targets above 533kts closure would be visible 

 

The most common situation to see both filters in action is a target flying a circular orbit a significant distance away from your radar, the target will be visible when facing toward you, disappear when moving perpendicular to you due to the MLC filter, and then disappear due to the ZDF when moving away from you.

 

Question 3: I believe this is an error in the manual, the ZDF is a filter designed to block out some of the side lobe clutter, which is a set of returns from almost directly below the aircraft, which has a closure of approximately zero when in level flight

 

[Snappy]

Hi Guys,

 

thanks for all your detailed answers! It has become semi-clear now , but still I 'm confused - sorry! The MLC is more or less clear now to me, but the ZDF still is

murky..

 

@Noctrach

Why do you say in regards to ZDF , that the F-14 own groundspeed is not considered with this? 

 

Heatblurs own manual states (bold underline by me)  re the zero doppler Filter

 

"The blind area is centered around a closure rate of negative own groundspeed..."     and later again

"The resulting blind area is 200 knots wide, meaning that a chased target moving at a speed of within 100 knots(+/-) of own groundspeed will be invisible to the radar"

 

Is this an error in the manual or what?

 

Btw I find the 2 sentences from the manual  in themselves somewhat contradictory, because the first sentence  makes it sound like if ownship is doing 400kts groundspeed the blind area is affecting targets pulling away  with -400 kts negative closure (+/- 100), while the second sentence makes it sound more reasonably that target of 300-500 groundspeed , not seperation /closure  will be filtered out.

 

@Noctrach @Saber2243
Your scenarios are both good , if the ZDF is centered on closure rate .

 

I think my confusion really hinges on whether the ZDF speed band  is based on own groundspeed or closure rate.The heatblur manual makes it sound like its own - groundspeed based.

 

@Spiceman,

Thanks for your link! Funnily I had just watched your video yesterday evening and it was part of my questions came up. (Not that anything was wrong or off  with your video, on the contrary,  it illustrated well the practical workarounds ,  its  just that  I kept thinking a bit more about it and trying to understand , unsuccessfully )

 

Kind regards,

Snappy

 

[Noctrach]

1 hour ago, Snappy said:

Why do you say in regards to ZDF , that the F-14 own groundspeed is not considered with this? 

 

Heatblurs own manual states (bold underline by me)  re the zero doppler Filter

 

"The blind area is centered around a closure rate of negative own groundspeed..."     and later again

"The resulting blind area is 200 knots wide, meaning that a chased target moving at a speed of within 100 knots(+/-) of own groundspeed will be invisible to the radar"

I think I phrased it poorly.

 

The intent was to clarify that the ZDF takes effect purely based on the relative closure between the two aircraft.

Whether flying at 100 or 500 knots groundspeed, the effect is the same if closure is within +/- 100 knots.

The keyword in this is closure, in first sentence and chased in the second.

Naturally groundspeed is a crucial factor to calculating closure. However, geometry has an equally significant effect.

 

The MLC on the other hand, can still take effect at closures well above  +/-133 knots. Specifically, it filters at groundspeed +/- 133 knots.

 

@Saber2243 's table illustrates this very well.

Edited February 26, 2021 by Noctrach

[Spiceman]

Think of it this way. The airplane always has a big, fat, constant target right below it, the ground. The ground is at zero Doppler or very close to it most of the time. Not when you’re climbing or descending, or rising/falling terrain, but that’s minimal. If you don’t filter that out, it will drown out everything else and you’ll have a constant target sitting right on top of you all the time. You’re getting your 10,000 watts of power reflected back from the ground just a few thousand feet away. You’ve got to filter that out. 

[Snappy]

@Spiceman

yes  I got that ground clutter part, of course you dont want to get this clutter messing up your radar picture  , but I thought it was the MLC that filters the ground clutter returns out, not the ZDF , despite the ground being close to or at zero doppler.

@Noctrach

No problem, ok it seems the wording in the manual isn't completely optimal either then. I'll just go with what you , Saber and Spiceman said.

ZDF filters a certain closure speed band and MLC filter a certain  groundspeed speed band with the accompanying implications for detection.

 

Thanks again everyone , for your help in explaining.

Kind regards,

Snappy

 

 

 

 

[Saber2243]

@Snappy

 

Just to clarify that final point, the zdf filters ground returns from directly below the aircraft, which have a zero closure rate, while the mlc filters ground returns from terrain out in front of the aircraft, which have a closure equal to the aircrafts ground speed 

 

Think about flying directly at a mountain, the closure of the mountain is your aircraft's ground speed