Mainlobe Clutter on DDI

According to the manual, the DDI in pulse doppler modes is displaying closure range against the ground:

The display shows closure rate vs the ground (as if own aircraft was stationary) as opposed to relative closure rate.

What I don't understand is why the mainlobe clutter trace is showing as an arch:

I would understand this form if the display would show absolute closure range, in which case the closure of the ground relative to your aircraft us lower at higher azimuth. But since the display shows target closure range against the ground (with 0 meaning the target having the same closure as the ground and thus it being in the ground clutter), shouldn't the trace be a straight horizontal line?

I might have to clarify that passage in the manual but while it do indeed show a closure rate that compensates for own aircraft speed it doesn't compensate for the angle of the antenna.

So as the antenna sweeps towards the sides the observed speed of the ground will decrease due to looking at it at an angle. That's why it archs downwards.

I still don't quite understand. Does this mean that the closure rate scale is only accurate at an azimuth of zero?

Let's say in the image above there would be a target at azimuth of -60 with a closure rate of zero (center tick mark on the left scale). At this azimuth the MLC trace in the image displays at a closure rate of -600 knots. Is the target in the clutter or not?

I still don't quite understand. Does this mean that the closure rate scale is only accurate at an azimuth of zero?

 

Let's say in the image above there would be a target at azimuth of -60 with a closure rate of zero (center tick mark on the left scale). At this azimuth the MLC trace in the image displays at a closure rate of -600 knots. Is the target in the clutter or not?

Yes and no, the scale corresponds to observed closure rate, in effect being the part of the targets rate vector that points towards the radar.

That target would not be in the clutter and/or filter as the ground is not at 0 at that aspect.

Sorry for being slow to comprehend, but I would really like to understand how this display works.

In the example above, do I understand it correctly that both the red and the blue target would have the same 600 kts closure rate in relationship to the ground beneath them?

Therefore the rate scale at the left side is only valid for 0 azimuth targets. For all other azimuth you would have to guess the closure rate against the ground by judging the distance from the MLC trace.

And since the MLC trace is not displayed with the MLC filter enabled and since the MLC trace changes form depending on your own airspeed, for targets with an azimuth >0 you effectively have no reliable way to determine their actual closure rate in relation to the ground.

Consequently you also cannot accurately see on the display when a target is approaching the notch at azimuth >0.

Correct?

They would show the same observed closure rate vs the ground, yes.

The rate scale is correct relative the observed closure rate, but yes, you have to judge the relative speed to the ground from where the MLC trace would be.

Just keep in mind that you're only seeing the part of the targets speed vector that points towards the radar, i.e. if you were looking at a target that's moving 45 degrees left of your line of sight you'd only see half his real speed.

As far as determining when the target approaches the notch you'd have to guestimate when it's near to entering the MLC filter that follows the MLC. The only way really to see it visually is to turn off the MLC filter and try to see it visually. To do this accurately takes training.

Thank you very much, I think I got it now.

It seems to me to be a rather un-intuitive way to display things though. If we have the MLC filter enabled/auto and just the blue target appears on screen, the only real useful information that I can take away from it is azimuth. Closure rate really is just "yes, but just a wild guess how much". Why not just display absolute closure rate and be able to compare targets at different azimuth with a scale that works everywhere?

It's a function of how radar works more than a man machine interface consideration.

Using the DDD correctly in PD modes is quite challenging and takes practice.

I have a question for you Naquaii, should switching MLC filter OFF affect the processing of returned signals by WCS - and denning to generate TRACKS in TWS? Should it also affect the RWS signal processing?

My experience from testing is telling me, that switching it OFF gives me extra few knots for Doppler gate, where I can still detect notching target, without of affecting TWS, RWS functionality (Notching targets were lower than me, behind them was sea (water). I was able to generate tracks and fire AIM54 in TWS mode against almost notching targets.

oh now im confused. I thought the closure rate was on the right side? whats the rate scale on the right side then? where it says 0 at the bottom?

Once you start building a track file you can hook the contact and see absolute closure on the TID as a numeric.

I believe PDS and RWS are just that, search modes only to vector you towards contacts for further processing.

oh now im confused. I thought the closure rate was on the right side? whats the rate scale on the right side then? where it says 0 at the bottom?

The scales on the sides of the DDD has no set scaling, they're just visual references for the RIO. He still has to know what scale he has currently set. On the very early tomcats the scales were printed on the DDD itself but this was quite quickly removed. The 0 on the bottom is just a leftover.

And as the scales change depending on settings it doesn't really matter what side you use for reference.

I have a question for you Naquaii, should switching MLC filter OFF affect the processing of returned signals by WCS - and denning to generate TRACKS in TWS? Should it also affect the RWS signal processing?

My experience from testing is telling me, that switching it OFF gives me extra few knots for Doppler gate, where I can still detect notching target, without of affecting TWS, RWS functionality (Notching targets were lower than me, behind them was sea (water). I was able to generate tracks and fire AIM54 in TWS mode against almost notching targets.

The danger of manually disabling the MLC is that a lot of false targets in the MLC can get processed as tracks by the WCS, both placing unneccesary load on the computer and also using up the limited number of tracks it can hold. Not sure if this is currently modelled, if not, it will be.

The danger of manually disabling the MLC is that a lot of false targets in the MLC can get processed as tracks by the WCS, both placing unneccesary load on the computer and also using up the limited number of tracks it can hold. Not sure if this is currently modelled, if not, it will be.

Thanks very much for clearing things up.

Can you please writte down here, after WCS overloading is implemented?

So far, as I understand to MLC filter, it can(will) help you only in PD Search or RWS mode? When it comes to radar lookdown situation.

The scales on the sides of the DDD has no set scaling, they're just visual references for the RIO. He still has to know what scale he has currently set. On the very early tomcats the scales were printed on the DDD itself but this was quite quickly removed. The 0 on the bottom is just a leftover.

 

 

 

And as the scales change depending on settings it doesn't really matter what side you use for reference.

 

Ahh ok, now I get it! So left and right side is the same basically. Just a visual representation so the pilot can estimate the closure rate?

Thx for clarifying that!

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The danger of manually disabling the MLC is that a lot of false targets in the MLC can get processed as tracks by the WCS, both placing unneccesary load on the computer and also using up the limited number of tracks it can hold. Not sure if this is currently modelled, if not, it will be.

Thanks very much for clearing things up.

Can you please writte down here, after WCS overloading is implemented?

So far, as I understand to MLC filter, it can(will) help you only in PD Search or RWS mode? When it comes to radar lookdown situation.

The MLC filter is for lookdown situations yes, it's mostly visual in PD Search but like I said, in RWS and TWS it actually helps the computer by removing the clutter. Both TWS and RWS do compute target locations and are vulnarable to clutter from the MLC.

Ahh ok, now I get it! So left and right side is the same basically. Just a visual representation so the pilot can estimate the closure rate?

 

Thx for clarifying that!

Correct, it just to have a written scale on one side in early Tomcats but they removed that to allow the Aspect switch to change the scale shown instead.

Mainlobe Clutter on DDI

Ok. So to make this super clear. You should never ever use the right side of the scale now? You only use the left side of the scale where you also can change the closure rate scale?

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Ok. So to make this super clear. You should never ever use the right side of the scale now? You only use the left side of the scale where you also can change the closure rate scale?

 

 

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The scales to the sides of the DDD in the search modes can really only be used to help the RIO gauge the position on the screen of the returns. Use whichever you want for that, it's the DDD scale which is set by the Aspect switch that's important.

In the STT modes the right scale is used but it's still not set as you can change it using the Vc switch.

Remember that both these scales are physical indicator lines on the surface of the DDD itself, they can't change because of different settings.