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HUD switches from B (barometric distance) to F ( FCR) to slant range when mountain comes in line of sight. returning false values


darkman222

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As title says.

Does that make sense? As the manual says F means that the FCR is used for ranging. But if it switches to F if a mountain blocks the line of sight, the values must come out wrong. I would suppose it is to improve measurement, not like you can see here provide false information. Seems like the behavior is swapped.

At the end of the video you can see how it even counts down the distance to the mountain I am approaching. Only when I pull up and the line of sight to the target is restored it changes back to B, giving correct information again.

See a track: https://www.dropbox.com/sh/5ed6ht7b14jgj62/AAAvK_F-90bl9Jd6-MKnfYGLa?dl=0

And a video:

 


Edited by darkman222
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This is what the manual says:

The Slant Range is the direct, straight-line distance from the aircraft to the current target or SPI location. For range values greater than 1.0 NM, the range is displayed as a four-digit value to the nearest tenth of a nautical mile (i.e., 15.2 NM is displayed as “015.2”). For range values less than 1.0 NM, the range value is displayed as a three-digit value to the nearest hundred feet (i.e., 5500 feet is displayed as “055”). The letter on the left of the display indicates the method the range is determined.
• B. The slant range is determined based on the barometric altitude and steerpoint elevation.
• R. The slant range is determined based on the radar altimeter.
• F. The slant range is determined based on ranging data from the FCR.
• M. A Manual range is being used in an air-to-air mode or in air-to-ground CCIP mode.

So even if it shows B it means slant range. B and F both mean slant range, but just with different measuring methods. If you wanted to offset the target box from the waypoint might need to use slant range instead of the measurement to the waypoint I guess.

You can always work around a bug if you know its there of course 😉 But the use of measuring methods when a mountain blocks the sight seems to swapped.

 


Edited by darkman222
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This is an educated guess based on trying around, but it seems to switch from B to F when there is no LOS and the nearest LOS obstacle is within 10.0 NM. And back to B when outbound and reaching 10.0 NM. See my attached track.

The track has a simple LOS test using https://wiki.hoggitworld.com/view/DCS_func_isVisible. It's not really relevant for my track (it's clear there's no LOS in my track), but I left it in in case others are interested to test with it. It's pretty much the example script from the Hoggit wiki put in a repetitive trigger.

F-16 slant range and LOS.trk

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In my video it uses FCR ranging even for a distance about 4 nm. But thats not the problem.

Why does it switch to FCR ranging when the line of sight is blocked, and now measures the distance to the obstacle blocking the line of sight? And when the line of sight is free it uses barometric. Shouldnt it be the other way around?

 


Edited by darkman222
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When FCR is providing range like AGR (terrain locked) or with a FTT/GMTT in GM then an "F" appears. Simply having the cursors over a portion of the GM map doesn't mean the FCR is tracking and won't be providing slant range. I.e. it will be in "B" mode calculations throughout the conditions shown in the video.

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  • darkman222 changed the title to HUD switches from B (barometric distance) to F ( FCR) to slant range when mountain comes in line of sight. returning false values
  • ED Team

moved and tagged. 

I will need to get feedback from the team. 

thanks

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  • 2 months later...

I have posted this bug for 2.8, I thought it was labelled as "investigating" but I cant find that thread any more. I think the behavior was never explained or verified as correct. So I am bringing it up again.

With 2.9 I was hoping for those kind of inconsistencies in the F16 being adressed, but it does not seem so.

As you can see in the video, the slant range is calculated by steerpoint / barometric elevation in CCRP. As soon as the line of sight is blocked by a hill, it switches to FCR radar measurement which is wrong of course because it measures the distance to the hill now.

At the beginning and at the end of the video (sorry in the middle its obscured due to head tracking) you can see that the "distance to steerpoint" is the same when in baro, and when its in FCR there is a difference between the two.

 

image.png

Track attached as well.

baro radar.trk


Edited by darkman222
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