Jabbers - DCS World - F-14 Tomcat - Back Seat - RIO AWG-9 Tutorial

[Jabbers_]

From the video:

 

"The two examples, where it disappears, shows the weakness of the pulse Doppler".

 

No, it doesn't.

 

One example (the first) shows the weakness of the pulse Doppler. The other (second) shows the weakness of ED's radar simulation.

 

IRL, co-speed targets show up on a PD radar just fine, and I really wish ED would fix this.

 

To further back myself, from the HB F-14 Manual

 

Because of the way the radar operates the doppler filters it will have two blind ranges. The main lobe clutter (MLC) region which contains most of the ground returns, those returning with zero groundspeed is one of them and is 266 knots wide, centered around own aircraft groundspeed (133 knots slower and 133 knots faster). This is the reason that the radar can be notched as a target with the same relative groundspeed as the ground will also be filtered out. This is however only true for look-down conditions as when the radar antenna looks up into the sky this filter isn’t necessary and can be turned off. If the MLC switch on the DDD panel is in AUTO the radar will automatically turn off the MLC filter if looking more than 3 degrees above the horizon. It can also be turned off manually by the RIO but if the antenna looks down this can make the displays unusable in RWS and TWS as all of the ground returns will be sent to the computer for tracking. In whichever case, with the MLC filter off, the target cannot notch the AN/AWG-9 if it is above the radar.

The second filter, and second blind spot, of the radar is the zero doppler filter. This blind area is centered around a closure rate of negative own groundspeed, meaning a target moving away from own aircraft at the same speed as own aircraft. This blind area is a hardware limitation as it is a doppler radar mode it cannot detect targets without a doppler shift. 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. This means that when chasing a fleeing target it may very well be necessary to use the pulse modes instead.

[Naquaii]

From the video:

 

"The two examples, where it disappears, shows the weakness of the pulse Doppler".

 

No, it doesn't.

 

One example (the first) shows the weakness of the pulse Doppler. The other (second) shows the weakness of ED's radar simulation.

 

IRL, co-speed targets show up on a PD radar just fine, and I really wish ED would fix this.

 

Both of these filters are purely implemented in our code from our F-14 documentation.

 

The zero doppler filter is very real in the AWG-9 and per definition is a weakness of a pure pulse-doppler radar as a target showing no relative doppler has no doppler and for the radar to see that it needs to switch out of pulse doppler.

 

A more modern fighter radar, while using pulse doppler likely does trickery like automatically switching between radar modes to get around this weakness.

 

The AWG-9 can't do this as it's purely pulse doppler in the PD modes an thus can't see targets with zero doppler shift.

[Steve Davies]

I stand corrected on the AWG-9, but the limitation for 'modern' radars in DCS is incorrect - I have spoken directly with ED about this, and they've acknowledged it.

 

Thanks for the info.

[Naquaii]

I stand corrected on the AWG-9, but the limitation for 'modern' radars in DCS is incorrect - I have spoken directly with ED about this, and they've acknowledged it.

 

Thanks for the info.

 

Yeah, I agree, it's unlikely that modern radars like APG-63 and APG-73 didn't have ways around that, but probably also classified! :-)

[Steve Davies]

Yeah, I agree, it's unlikely that modern radars like APG-63 and APG-73 didn't have ways around that, but probably also classified! :-)

 

Indeed. I know the APG-73, APG-65, APG-70 and APG-63 have no issue with co-speed radar contacts.

 

Ref. the AWG-9, it does raise the question of whether the F-14 guys ever flew a radar trail departure or arrival... perhaps you can ask one of your SMEs?

[Naquaii]

Indeed. I know the APG-73, APG-65, APG-70 and APG-63 have no issue with co-speed radar contacts.

 

Ref. the AWG-9, it does raise the question of whether the F-14 guys ever flew a radar trail departure or arrival... perhaps you can ask one of your SMEs?

 

I guess they had to use pulse if so, maybe possible as the RIO could've dedicated themselves to that while the pilot did his stuff.

 

I'll see if I can forward it the next time we talk to them.

[Steve Davies]

I guess they had to use pulse if so, maybe possible as the RIO could've dedicated themselves to that while the pilot did his stuff.

 

I'll see if I can forward it the next time we talk to them.

 

Yeah, that might be it.

 

I suppose that operating at sea, timing and marshalling would allow deconfliction. But if they were shore based, they must have had to have some capability.

[Larkis]

A Noob question, which purpose has the DDD? As i understand DDD and TID got both input from the radar. The DDD show only the approachspeed but nothing more, while the TID show everything as a more modern radar.

 

So while should i use the DDD when the TID provide more information from the same source? Sorry i don't understand that part in the video.

[Jabbers_]

A Noob question, which purpose has the DDD? As i understand DDD and TID got both input from the radar. The DDD show only the approachspeed but nothing more, while the TID show everything as a more modern radar.

 

So while should i use the DDD when the TID provide more information from the same source? Sorry i don't understand that part in the video.

 

Only RWS and TWS show anything on the TID, PD Search and Pulse do not. Pulse is a very necessary thing to learn so using the DDD is also important, DDD is essentially raw data, TID is translated when available to translate.

[Victory205]

Good grief, the F14 radar worked just fine for co speed targets. Quit believing rumors and Facebook loudmouths who have never touched a radar.

 

A dedicated RIO and an AWG9 is tough to escape. I have no idea what DCS will emulate.

[Steve Davies]

Good grief, the F14 radar worked just fine for co speed targets. Quit believing rumors and Facebook loudmouths who have never touched a radar.

 

A dedicated RIO and an AWG9 is tough to escape. I have no idea what DCS will emulate.

 

Ah!

 

So, what is this co-speed filter that HB have implemented? I expect that they have good cause given that they have the manuals?

[gyrovague]

A Noob question, which purpose has the DDD? As i understand DDD and TID got both input from the radar. The DDD show only the approachspeed but nothing more, while the TID show everything as a more modern radar.

 

So while should i use the DDD when the TID provide more information from the same source? Sorry i don't understand that part in the video.

 

The DDD is closer to a raw radar display, with some additional symbology. The TID is purely processed targets only, and can only show targets in P-STT, PD-STT, RWS and TWS (not P-SEARCH or PD-SEARCH). Both have their uses, and DDD is quite useful in pulse search for rudimentary ground mapping, navigation fixes and finding carrier group at sea for instance. See here for some details:

[Naquaii]

Good grief, the F14 radar worked just fine for co speed targets. Quit believing rumors and Facebook loudmouths who have never touched a radar.

 

Of course it did! In the Pulse modes...! :-)

[Naquaii]

Both of these filters are purely implemented in our code from our F-14 documentation.

 

The zero doppler filter is very real in the AWG-9 and per definition is a weakness of a pure pulse-doppler radar as a target showing no relative doppler has no doppler and for the radar to see that it needs to switch out of pulse doppler.

 

A more modern fighter radar, while using pulse doppler likely does trickery like automatically switching between radar modes to get around this weakness.

 

The AWG-9 can't do this as it's purely pulse doppler in the PD modes an thus can't see targets with zero doppler shift.

 

In before someone corrects me, just to clarify, the main reason for the zero doppler filter is that the AWG-9 needs it to reduce sideband doppler returns from the ground beneath the aircraft as these would overwise be an issue for the receiver.

 

A more modern radar could compensate for the doppler shift of own transmission and also by having better sideband surpression. This is what makes more modern radar systems disregard the weakness/difficulty with detecting co-speed targets.

[OnlyforDCS]

Hang on Naquaii, I thought that this second "filter" is actually a hardware limitation of the physical properties of the way the radar works itself? It sees doppler returns. A target that has a relative 0 speed to the radar, doesn't have any doppler shifts, therefore no target is detected. Or am I missing something?

Edited March 12, 2019 by OnlyforDCS

[Steve Davies]

A target that has a relative 0 speed to the radar, doesn't have any doppler shifts, therefore no target is detected. Or am I missing something?

 

This stuff is absolutely not something I claim to speak with any authority about (beyond knowing, because I have seen it, that co-speed targets show up just fine on AI radar sets that date from 1970).

 

However, I think that you may be missing the fact that for beamed contacts the radar is filtering out returns that have a Doppler shift (in technical terms, the frequency of echoes for these radar contacts increases), not filtering out returns that have no shift (frequency of echoes remains the same).

Edited March 12, 2019 by Steve Davies

[Nikodemuz]

Hang on Naquaii, I thought that this second "filter" is actually a hardware limitation of the physical properties of the way the radar works itself? It sees doppler returns. A target that has a relative 0 speed to the radar, doesn't have any doppler shifts, therefore no target is detected. Or am I missing something?

 

 

A target that is co-speed will typically have a doppler shift indeed. :)

 

I can use a couple of examples to explain. (Assuming look down and doppler filtering enabled)

 

If you are flying at 400kts transmitting at 10000Mhz, your radar will actually send signals at a slightly higher frequency due to doppler shift. For this example let's say that your current speed equates to a doppler shift of 1Mhx (exagerated).

 

If you then have an approaching target going at 400kts you will recieve an echo with a "blue shifted" 10002Mhz freq. Ground clutter would be at 10001Mhz and filtered out by the doppler filter. (since we know what speed we are going, we know what doppler we introduce ourselves)

 

If the target instead would be beaming you, i.e. going at 90 degrees to your heading right in front of you with zero radial speed. The echo would be non-shifted at 10001Mhz. This is the same freq as ground clutter, hence the radar would filter the target echo as clutter,

 

If on the other hand, the target would be co -speed, i.e. tail chase. The echo would be red-shifted at 1Mhz. So echo would be comming back to you at 10000Mhz. This is not the same as ground clutter and would then not be filtered out as ground clutter (we still know what speed we have and what doppler we introduce to the signal)

 

However, since there are alot more things going on, like side lobe clutter and general signal processing, radars will typically have weak spots in other areas. Above is very generalized to describe doppler. Thge newer the radar, the more tricks it can pull off in terms of mitigating these weaknesses.

 

In this example i've actually left out that there is a doppler shift when the echo is recieved by the radar as well... but it doesn't matter in this case.

 

So it basically boils down to the fact that doppler is always introduced to a signal if a transmitter, target or reciever has a speed component in the signal propagation path. But since we are looking for the target and not ourselves we need the target to have a speed component in the signal propagation path, i.e. it needs to be going either towards you our away from you.

 

 

All of the above is very simplified but I think it serves the purpose of explaining doppler in radars.

 

Hope it clarifies things a bit. :)

 

 

N

Edited March 12, 2019 by Nikodemuz

[Steve Davies]

Thanks for the explanation, N.

[OnlyforDCS]

So it basically boils down to the fact that doppler is always introduced to a signal if a transmitter, target or reciever has a speed component in the signal propagation path. But since we are looking for the target and not ourselves we need the target to have a speed component in the signal propagation path, i.e. it needs to be going either towards you our away from you.

 

 

N

 

But isn't this exactly what happens in a co-speed (tail chase)? The target won't be moving either away from you or towards you, it's relative speed will be 0 to you so it won't have either a red or blue shift relative to your own radar?

[OnlyforDCS]

However, I think that you may be missing the fact that the radar is filtering out returns that have a Doppler shift (in technical terms, the frequency of echoes for these radar contacts increases), not filtering out returns that have no shift (frequency of echoes remains the same).

 

Hmmm, this is exactly the opposite of how I believed doppler radars work. Now Im really confused :noexpression:

Edited March 12, 2019 by OnlyforDCS

[Nikodemuz]

But isn't this exactly what happens in a co-speed (tail chase)? The target won't be moving either away from you or towards you, it's relative speed will be 0 to you so it won't have either a red or blue shift relative to your own radar?

 

Well both yes and no. The key thing here is if doppler is added to the signal when it bounces off the target. And a target that is receding from the signal will do just that.

 

The sum of your doppler and the target induced doppler will be zero if you are co-speed, but since you know what doppler you introduce it's pretty easy math to figure out what doppler the target added and the most important thing here is that the target will be doppler shifted as compared to the ground which means that you can filter that out without filtering out the target.

 

It is not the speed relative to you that is most important but the velocity component in the propagation path of the signal.

 

If your radar signal hits something that is going away from you the reflection will be "red-shifted" regardless of how fast/slow/co-speed you are going. The trick is being able to filter out ground clutter and a target moving away will have a different doppler than the ground clutter.

 

Only way to get lost in the ground clutter is to have a "zero" speed in the propagation path, i.e. radial velocit vector.

 

 

A doppler radar is not blind to "zero doppler" targets, it just filters them out to avoid ground clutter. So as long as you know your own speed. That part of the equation is known and can be accounted for... The only signals you want to filter out are the ones that have the same doppler that you gave it when it left your radar since those signals are likely ground clutter. And a target in a co-speed tail chase will have a different doppler than the ground, hence it is not filtered out.

 

Again this is a gross simplification but still true for basic doppler filtering.

Edited March 12, 2019 by Nikodemuz

[OnlyforDCS]

Ok i think I get it now and Naquaii's explanation now makes sense. Thanks for the great clarification Nikodemuz!

Edited March 12, 2019 by OnlyforDCS

[Nikodemuz]

Ok i think I get it now and Naquaii's explanation now makes sense. Thanks for the great clarification Nikodemuz!

 

:thumbup:

[Steve Davies]

We're making progress!

[Steve Davies]

Hmmm, this is exactly the opposite of how I believed doppler radars work. Now Im really confused :noexpression:

 

I have edited my post to qualify that I was talking specifically about contacts on the beam :)

 

Somewhere I have a Doppler diagram for the APG-73 that shows this stuff visually. Will try to dig it out and post it.