reported Time on Target (TOT) function slant range bug

[Fresh]

The TOT function in the Hornet calculates wrongly if the sequenced waypoints are not at 0 feet (sea level). If your waypoints are at altitude, e.g. FL400, it includes the slant range to the next waypoint, showing increasing required ground speed as you approach the waypoint. Edit: if the waypoint is at ground level the erroneous slant range to sea level is calculated/included... 

Basically the mission computer calculates the time to the next waypoint in 2 dimensions correctly, but measures the distance in 3 dimensions incorrectly, this causes a false required speed as the slant range (3D) becomes different to the 2D time to the next waypoint in your sequence.

Workaround: place all your sequence waypoints at ground/sea. (Or at least your IP and TGT WYPT at ground/sea level). I haven't tested the difference between AGL and Sea Level.

Please see attached tracks:

TOT 40K A: This is with the waypoints programmed at FL400

TOT 40K B: Only the TGT waypoint (WYPT 5) and the preceeding WYPT is at sea level

TOT 40K C : All waypoints are a sea level.

Thanks in advance for fixing this.

Fresh.

TOT 40K A.trk TOT 40K B.trk TOT 40K C.trk TOT Test 100 feet.miz TOT Test 40000 feet b.miz

Edited May 4, 2024 by Fresh

[Fresh]

Further testing confirms this is a bug, because in Nevada with terrain at ~5000 feet the same error appears as you approach your waypoint. Since you cannot enter a 0 feet waypoint (below ground) the MC calculates wrongly.

TOT 40K A Nevada.trk TOT 40K C Nevada.trk TOT Test 40000 feet Nevada A.miz TOT Test 40000 feet Nevada C.miz

[Phantom711]

Thanks for bringing this up.

I did another test at KOLA with several waypoints at a fow hundred feet MSL. Even though not 0 feet, but somewhere between 0 and 1000ft, there was also no noticable problem. So my impression is, that the problem gets aggrevated the highter the elevation of the waypoints. 

It´s almost as if the MC, in it´s attempt to calculate the required groundspeed, puts the aircraft at 0ft MSL but then still takes the waypoint elevation into account in its computation. 

@BIGNEWY Kindly asking devs to take a look at this.

[Phantom711]

Am 3.5.2024 um 23:16 schrieb Fresh:

Workaround: place all your sequence waypoints at ground/sea. (Or at least your IP and TGT WYPT at ground/sea level).

Problem is, that there is nothing you can do about the elevation in some areas like NEVADA, where it already becomes noticable.

[Fresh]

Yes, agreed, it is only a half-workaround :-)

[Phantom711]

@BIGNEWY

Sorry, I would like to *bump* this, since it has not gotten any acknowledgement so far.

[Phantom711]

-Bumpedeebump-

[Fresh]

Hi there,

here's another attempt at explaining the TOT bug.. please see attached drawing.

Basically the required speed to meet TOT, should be the speed needed to cover the horizontal distance in the time remaining. However, the Hornet TOT function is using the slant range to the sea level position of the waypoint. ("Slant Range B" in my drawing). This means, unless the Hornet is skimming the sea, there is always a mistake in the calculation as you approach your TGT.

If you overfly your target at 6000 feet AMSL (ca 1 NM) with 360 knots ground speed. 10 seconds before your TGT with 10 seconds before TOT your required speed is 360 and distance is 1 NM. But in the bugged case now, the Hornet calculates you need to fly 509 knots. 5 seconds later, at a distance of 0.5 NM to TGT, the slant range is 1.11 NM. You are perfectly on speed and distance to reach TOT, but the Hornet says you need to fly at 804 knots. 1 second before TOT the Hornet will say you need to fly at 3617 knots, even though 360 would be correct.

Thanks for looking into this,

Fresh

 

 

[Lord Vader]

Thank you all for your patience and collaboration.

I've reported this internally for analysis.