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Radar Altimeter


felixx75

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Why is there a difference between the radar altitude shown in the Radar Altimeter Gauge (120m) and shown in the HUD (108m)? Should it not be the same?

 

 

 

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Looks like you uploaded thumbnails and not photos, so it's hard to see what you are.

 

 

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Looks like you uploaded thumbnails and not photos, so it's hard to see what you are.

They show up fine for me :huh:

 

In regards to OPs question I don't know why there is such a difference :dunno:

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Does is make a difference if you are over land vs water with them matching up? I watched this older YouTube to see if they matched up over land and they seem to:

 

 

Edit: Here's one over water

 

 

I think you may have found a bug. I'll fire up the Ka-50 later and see what I see.

 

It seems to be always the same difference, over land and over water.

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Which altitude does the F2-view status bar show?

 

 

Is the difference always the same or does it change depending on the acutal altitude of the helo?

 

F2 always shows Baro altitude I believe. I checked it with Tacview and the HUD indication seems to be the correct one.

 

I always thought the gauge is analog and has some delay or lag than the digital one on the HUD.

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It's possible that the HUD is showing the Doppler system height while the round gauge shows the radar altimeter height. I'm surprised they are so different though. What are their values compared to true height (true altitude - terrain elevation)? Does it depend on helicopter attitude?

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Another noticeable difference is flying over objects such as buildings. The HUD Radar Altimeter jumps around depending on the objects below the aircraft, whereas the Radar Altimeter Gauge does not. This was a really nice find by the OP.

This really sounds like a bug.

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DCS Panavia Tornado (IDS) really needs to be a thing!

 

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Off by 10% definitely sounds like a bug. But how should it work in real life? If you fly over rough terrain, would the needle bounce rapidly exactly as the HUD number fluctuates? And it looks like low altitude warning is based off the analog gauge and not the HUD or ABRIS. Wouldn't this trip the low altitude warning all the time if it fluctuates rapidly?

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Beslan takeoff

 

Ground elevation 525m

F2 ALT 526m

RALT =<0m

HUD 001p

 

F2 ALT 625m

RALT 112m

HUD 100p

 

Suspiciously the yellow warning triangle is exactly equal to the needle when at actual 100m height going off when descending to F2 ALT 624m. I think the needle and yellow caret rotation angle in radians is miscalibrated compared to the texture art on the gauge background.

 

I had a hunch since rotational scale is non-linear. Scale is calibrated to 0m and 300m very well. If I fly at 826m (300m height) then 300m mark on A-036 instrument lines up perfectly. It is the middle heights which are not correlating so well.

 

\Mods\aircraft\Ka-50\Cockpit\Scripts\Devices_specs\RadarAltimeter.lua has line raltPointer = {valmin = 0, valmax = max_alt, T1 = 0.75, T2 = 0.316, wmax = 0, bias = {{valmin = 0, valmax = max_alt, bias = 0.1}}}

The A-036 "kicks" on and off about ~310m which can be seen by the needle jump. This corresponds to the distance_lim = 310.0 in the LUA. I don't know what the distance_lim_precise = 410.0 refers to. This controls the rotation of the white needle. "T1" and "T2" are calibration coefficients I think which may be enough to calibrate the input value to the needle output rotation correctly if different values are used.

 

Table of gauge background values to rotational angles:

m °

0 0

10 31

20 62

30 93

40 124

50 156

100 209

150 260

200 283

300 315

 

Compare this to the actual height value to rotational angles:

m °

0 0*

10 31*

20 65*

30 98*

40 132

50 172

100 224

150 269

200 290

300 315

*1m difference in F2 ALT height compared to ground elevation while landed is a significant fraction of the indication. Usually gauge is calibrated to show 0m when helicopter is landed even if object center is 1m above ground elevation.

 

Attached is graph visualizing the difference. In summary I think A-036 needle rotation is not rotated to the correct angular position for various heights.

1403263540_A-036GaugevsNeedle.thumb.gif.84f44364bd0cfa0ee14f64b2f37e4216.gif

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Another noticeable difference is flying over objects such as buildings. The HUD Radar Altimeter jumps around depending on the objects below the aircraft, whereas the Radar Altimeter Gauge does not. This was a really nice find by the OP.

 

 

Why would that be a bug? Isn't that your actual altitude when over a an object? If you were in a hover, wouldn't you want to know how far you can descend before you collide with an obstruction? That is working as intended.

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Why would that be a bug? Isn't that your actual altitude when over a an object? If you were in a hover, wouldn't you want to know how far you can descend before you collide with an obstruction? That is working as intended.

Sure, but the analogue gauge should jump as well. That's the bug he's talking about. ;)

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