AG radar modes don't respect the vertical coverage and elevation setting of the radar

[Northstar98]

Hi everyone,

 

Currently the F-16's air-to-ground radar has an overexaggerated radial footprint; the radar is able to display everything from basically ownship out to the extremes of the instrumented range and is able to do this regardless of antenna elevation angle (even with the antenna 60° above the horizon) or altitude.

Generally speaking, if the aircraft's altitude increases and the antenna elevation angle below the horizon increases, the smaller the footprint that the radar covers.

They say a picture speaks a thousand words so here's a diagram, that should more clearly illustrate what I mean:

 

In the F-16CM, the AG radar makes a 1 bar scan, with a bar corresponding to ~5° in elevation (determined through simple trig in the air modes). At an altitude of 20,000 ft, with a 15° look-down angle should give me a patch between ~10 and ~15 nmi.

Spoiler

If anyone is after the calculation, here it is:

d1 = A × tan(90 - (θ-ε/2))

d2 = A × tan(90 - (θ+ε/2))

 

d2 - d1 = radial patch footprint /m

 

Where:

d1 = inner distance /m

d2 = outer distance /m

A = altitude /m

θ = elevation angle below horizon /°

ε = vertical elevation coverage /°

 

As A and θ increase the value of d2 - d1 decreases and as A and θ decrease, the value of d2 - d1 increases.

 

Of course, the radar doesn't just have a mainlobe, it also has sidelobes, so the patch shouldn't abruptly terminate at those values but the brightness will gradually fall off to 0 (exactly how large it should be in total and how much it falls off by will depend on the radiation pattern of the antenna).

This can kinda be seen in this video (only here the elevation angle below the horizon is very small and the altitude is ~14,500 ft), you can see that the gain reduces gradually around ~27 nmi and there are no returns closer than 25 nmi (corresponding to an elevation coverage of about 3.5° - well within the range of a 1 bar scan).

 

However, in DCS, the footprint is currently unaffected by both altitude and elevation, leading to it being drastically overexagerrated (and even including areas the radar shouldn't be able to illuminate with its mainlobe). If I'm flying at 20,000 ft I can see everything from 0 - 80 nmi with everything at constant brightness (obviously brightness of specific objects will depend on their RCS), regardless of the elevation angle. In order to do this the radar would have to be able to scan an elevation angle of ~88°, way beyond what the radar should be able to achieve with a 1 bar scan.

Incidentally at 20,000 ft it shouldn't even be possible to see returns (apart from sidelobe returns, which should be more dim) inside of ~6 nmi, as the radar's elevation limits are ±60°.

 

Another related issue (not sure if this should be in a different thread) is that the radar doesn't seem to respect the elevation setting that well - if you dial all the gain all the way up and tilt the antenna elevation above the horizon to it's maximum, the image appears like it would normally if I turned the gain down. Go all the way to 60° down and the whole image goes more or less dark, despite the fact that the radar should be illuminating a section with its mainlobe. Essentially the antenna levation setting seems to behave as if it were a 2nd gain setting for the entire image.

Spoiler

Reduced gain, default elevation - notice how there's returns spanning the entire instrumented range:

 

Elevation set at +60°, max gain - not much difference:

 

Elevation set at -60°, max gain - image almost completely dark (despite the fact that the radar should still be illuminating a narrow patch of ground with its mainlobe):

 

APG-68_AG_coverage.trk

Edited August 12, 2024 by Northstar98
grammar, spelling

[SeaBass80]

This unrealistic ground mapping is present in both the f-16 an the f-18. For the F-18 see here:

Sadly there was no response to this since July. Let's see whether we can get a comment this time.

 

[Northstar98]

9 hours ago, SeaBass80 said:

This unrealistic ground mapping is present in both the f-16 an the f-18. For the F-18 see here:

Sadly there was no response to this since July. Let's see whether we can get a comment this time.

 

Indeed, thanks for linking me that thread, I wasn't aware a bug thread had been made for that and yes, these are the result of the same issue.

With the Hornet, you've also got the PEN/FAN setting, which, if I'm not mistaken increases the elevation scan (somebody who knows what they're talking about with the Hornet correct me).

[BIGNEWY]

This has already been reported 

However if you have public data to support it please PM me

[Northstar98]

2 minutes ago, BIGNEWY said:

This has already been reported 

Thank you, apologies I wasn't aware as I only searched the F-16 bug forum.

2 minutes ago, BIGNEWY said:

However if you have public data to support it please PM me

It's based on the gimbal limits of the radar as well as the elevation coverage of a 1-bar scan in DCS (so ±60° in the vertical, with an approximately 5° wide beamwidth in elevation, the former comes from DCS' manual on the F-16 and the latter comes from simple geometry of a 1-bar scan). It especially shouldn't be possible to see returns with the same brightness that are outside the gimbal limits of the radar.

The footprint not extending from ownship is supported in the video (and the coverage shown in said video does make sense based on the above 2 numbers, so nothing wrong on that front).

I'll try and hunt down some more videos of the AG radar in GM, would you prefer I posted them here or PM'd them to you?

[BIGNEWY]

Just now, Northstar98 said:

I'll try and hunt down some more videos of the AG radar in GM, would you prefer I posted them here or PM'd them to you?

Best to PM we dont want the 1.16 rule to be broken 

[Northstar98]

Just now, BIGNEWY said:

Best to PM we dont want the 1.16 rule to be broken 

Okay, will do.

[abelian]

I'm pretty sure that the OP is not correct. APG-68 should have a beam spoiler which creates a fan beam in the vertical direction when in air-to-ground modes. The height of the beam is not infinite, but it is much larger than 5°.

[Northstar98]

On 11/23/2022 at 11:27 AM, abelian said:

I'm pretty sure that the OP is not correct. APG-68 should have a beam spoiler which creates a fan beam in the vertical direction when in air-to-ground modes. The height of the beam is not infinite, but it is much larger than 5°.

If that's the case (is there a citation?), then the values for the specific examples are incorrect - thank you for the correction. Without knowing what the elevation coverage actually is, we won't be able to tell what range of distances should be displayed at x antenna elevation angle and altitude (it doesn't have to be infinite, but it would have to be significantly larger to have the entire display filled at every altitude and at just about every antenna elevation angle).

However, the whole point is that it still doesn't respect the elevation constraints (both the elevation coverage of the beam and the elevation angle it's set to).

Edited November 28, 2022 by Northstar98
made a sentence sound less stupid

[SeaBass80]

Is there any public info on the vertical beam divergence in GM mode? Can the APG68 switch off parts of the emitter surface to get it to emit from a narrow strip in order to achieve a high vertical beam divergence? [Edit: beam spoiling would probably be accomplished by phase scrambling.] Such beam shaping would likely severely reduce the available beam intensity (power per solid angle) due to using onmy a fraction of the planar antennas and due to the increased divergence. Thus the distance range would be limited, right?

The OP still has a point, even if there was this kind of extended divergence. At the moment, the ground radar does not respect the airframe orientation. You still get the same radar map even when you point the airframe vertically upwards. This is physically impossible and something that should be addressed for realism.

Edited November 23, 2022 by SeaBass80

[Northstar98]

5 minutes ago, SeaBass80 said:

Is there any public info on the vertical beam divergence in GM mode?

So far not having much luck finding one (one of the reasons why I went with values derived from air modes in DCS) - only thing I can say is that when looking at real videos, you can see regions where there are no returns (especially towards ownship).

5 minutes ago, SeaBass80 said:

The OP still has a point, even if there was this kind of extended divergence. At the moment, the ground radar does not respect the airframe orientation. You still get the same radar map even when you point the airframe vertically upwards.

Yeah, this is the main thing I want to bring up - the point pretty much remains as is regardless of what the numbers should actually be.

5 minutes ago, SeaBass80 said:

This is physically impossible and something that should be addressed for realism.

Agreed.

[abelian]

Unfortunately I don't have a public source. But I'm quite certain that the beam is going to be much taller than 5°. Probably at least 40°, maybe taller. For older radars this was apparently done by mechanically extended a spoiler in front of the antenna, which could create 60° or more beam spread.

Quote

Such beam shaping would likely severely reduce the available beam intensity (power per solid angle) due to using only a fraction of the planar antennas and due to the increased divergence. Thus the distance range would be limited, right?

Sure, the peak power at the main lobe would drop. But that's how ground mapping radar works: you want to spread the energy out over a large area. Note that the scan is much slower in GM than in air-to-air modes. I presume the beam is made narrower in EXP/DBS. But at the end of the day, the radar is still outputting the same average power.

Quote

This is physically impossible and something that should be addressed for realism

A simplified model for this would be to come up with an educated guess for the farfield radiation pattern of a fan beam, center the beam on the SPI as much as possible within the limits of the antenna elevation gimbal, and multiply the map (intensity? SNR?) by the projection of this curve onto backscatter return.

[SeaBass80]

Not sure what the status is - maybe ED can update us on the GM radar efforts. Currently (2.8.3.37556) the F-16 AG radar in GM mode can see everything between directly below the aircraft up to the horizon even with the aircraft nose pointing vertically up into the sky. This is not a matter of evidence needed, as it is simply impossible with the radar antenna. Would be great to see improved realism with the GM radar modes one day.

[Northstar98]

On 11/28/2022 at 12:13 AM, abelian said:

Unfortunately I don't have a public source. But I'm quite certain that the beam is going to be much taller than 5°. Probably at least 40°, maybe taller. For older radars this was apparently done by mechanically extended a spoiler in front of the antenna, which could create 60° or more beam spread.

Okay, the only thing I'll say though is looking at the above video, the illuminated area does make sense with the elevation coverage being small (in the video it corresponds to 3.5°), though it's difficult because the video was taken over mostly water, so we're only seeing a small island and some fairly dim sea clutter.