[2.9.20.15384] Question about radar theory/implementation

[Stickler]

The attached track shows my aircraft at exactly 15 nm plan range from a discrete ground target and 3815 ft above it. The background terrain is completely flat and uniform (southernmost edge of Caucasus, over Türkiye).

From trigonometry we obtain that the target is 2.4° below the horizon.

2.4° happens to be the radar depression below the horizon obtained when asking Jester to scan 7500 ft below the aircraft at 30 nm, which I command him to do.

The target should therefore be in the center of the MAP 1-bar radar beam.

Looking at the track and the below screenshot which shows the radar picture immediately before exiting the aircraft, we can see that from visual interpretation of the radar, the reflected radar energy seems to be highest at around 11 nm slant range from the aircraft, and not at 15 nm, as I expected.

How can this be explained?

null

mbc.trk

Edited Sunday at 01:15 PM by Stickler

[Ivandrov]

Bring up the gain and switch to a circular polarization to find the full coverage of the beam on the ground. The LIN polarization makes many potential ground clutter returns less reflective and this dynamically changes as you roll the aircraft for instance.

It's also a case where the relative angle of the surface being painted by radar will increase as you try to look at the longer range ground returns and so less radar energy makes it back to the aircraft, either because it is attenuated by distance or diffuses/reflects in another direction.

That's what I can think of off the top of my head. MAP-PPI directly counters this by dynamically increasing the gain as the range of returns increases.

Edited Sunday at 04:25 PM by Ivandrov

[Stickler]

Thanks for the tip, however, MAP-PPI exhibits the same behaviour:

For completeness' sake, I included MAP-PPI with circular polarization, but the issue is the same. Plus, MAP only has linear polarization IRL, so the picture should not change (I reported this as a bug separately):

Before somebody asks: RDR 1-bar also apparently focuses its radar energy as seen above.

Edited Sunday at 05:38 PM by Stickler

[Ivandrov]

That's the only bug that I see so far is that STC doesn't seem to be functioning in PPI.

You say "focusing radar energy" but this is just an issue of the reflected radar energy weakening with distance. The manual gives the formula as 1/r^4. 

The brightness of radar returns is expected to diminish as the distance to them increases. As you turn the gain up, radar reflections from further distances start showing up. Gain is not a function that physically adjusted the antenna itself or how much power it is outputting, it is a modification to the returned signals that are received through one of the amplifiers so that you can have a useable picture to look at, because if you just turn the gain all the way up, the entire radar screen completely whites out and is unusable.

So, if you gradually swipe through the gain control and look at what happens to the radar picture, the returns should bias towards the closer distances of the aircraft as you turn it down, and as you increase it, the closer distances white out and the longer distance returns begin to show up and intensify.

 

You should be able to see that here. The only that has changed is that the receiver gain was turned up in the 2nd picture.

Edited Sunday at 06:57 PM by Ivandrov