Ground collision warning using the radar and having radar altimeter off doesn't work

[TOViper]

OB 2.7.10.19473, SP from mission editor

The ground collision warning is not triggered when using the radar in mode A1 and the radar altimeter set to off and master mode NAV active.
According to RC2 it should work; the manual for this situation says:
"Ground collision warning is received when the radar is used (mode A1 or A2) when the calculated altitude and descent rate is such that the aircraft will impact the ground within 7 seconds. If the radar altimeter is available, the warning is always based on the radar altitude. This warning is blocked during aiming against ground targets, as well as when the landing gear is extended and the ground calibrated altitude is less than 50 metres."

 

AJS-37_Viggen_Ground_Collision_Warning_via_Radar_not_working.trk

[Machalot]

I can only get the ground collision warning light to shine if I have both the radar altimeter on and the radar in A1 or A2 mode.  Can you confirm this?  Once I hear back from you I'll submit a bug report.

[QuiGon]

2 hours ago, Machalot said:

I can only get the ground collision warning light to shine if I have both the radar altimeter on and the radar in A1 or A2 mode.  Can you confirm this?  Once I hear back from you I'll submit a bug report.

I can confirm exactly this. The light does not come on if either the radar altimeter is off or the radar is in mode A0. This is definitely a bug.

[MYSE1234]

Radar altimeter does indeed seem to have to be on to get the ground collision warning (Metspövarning), while in reality it should not be required.
Either A1 or A2 has to be used which is the correct behaviour, A0 does not enable this warning. God knows why, as it doesn't use the radar in anyway for the calculations. All it's doing is: 

HM+7*H < 0

H_M - the altitude, either from the altimeter or radar
7 - the seven seconds until collision
H - the descent rate in m/s. This is negative

Being required to have the radar altimeter on is a bug. Other than that it's working as expected from what I can see @Machalot

[Machalot]

@MYSE1234Does it measure altitude directly below the jet or out in front? It seems to see oncoming hills. 

[renhanxue]

15 hours ago, Machalot said:

@MYSE1234Does it measure altitude directly below the jet or out in front? It seems to see oncoming hills. 

To clarify here, ground collision warnings on the AJS 37 are a complex enough topic to have an entire chapter dedicated to them in the SFI. There are three main cases and then a bunch of special cases mostly related to weapon release altitude restrictions.

The first main type is bottenvarning (literally "bottom warning"). This requires the HÖJD CI/SI switch to be in position LD while the radar altimeter is enabled, and the trigger condition is radar altitude < 150m while also being less than half the air pressure altitude (e.g. it'll trigger if the radar altitude becomes less than 50m while the pressure altitude is 100m).

The second one is metspövarning, literally "fishing rod warning", and is the one referenced in the OP. As MYSE1234 says it doesn't actually use the radar nor does it require the radar altimeter to be active, but it is nevertheless only enabled if the radar is in mode A1 or A2. The manual explicitly says that the warning does not account for upcoming obstacles, only the current altitude. Presumably the thinking here was that if you're doing the Viggen thing at lowest altitude by the book, this warning would only be a potentially deadly distraction, while it's needed if you have to go head down to use the radar.

The third one is really a group of warnings that are enabled while the autopilot's altitude hold mode is engaged. The main case here is triggered if the altitude is less than 80% of what it was when the mode was engaged, but there's a whole bunch of cases here that spans a couple of pages of the SFI and I'm not going to go into all the details. It's notable though that if the radar altimeter is active it continuously updates the reference altitude. There's also special case that doesn't look like the other ground collision warnings (fixed ground collision light on the CI, no blinking lines on the HUD) in case of CK37 failure.

 

None of these cases are capable of warning of upcoming obstacles. If it does that in game then that's a bug. Do note though that in the transonic region (M 0.97 - M 1.05) the computer's idea of the altitude can be rather unreliable even if the radar altimeter is enabled, since in the transonic region the computer replaces the pressure altitude with a calculated altitude base on integration of the vertical acceleration.

Edited May 17, 2022 by renhanxue

[MYSE1234]

16 hours ago, Machalot said:

@MYSE1234Does it measure altitude directly below the jet or out in front? It seems to see oncoming hills. 

I also noticed that it seems to see in front of the aircraft a bit. The radar altimeter I'd imagine only measures directly below, but it doesn't explicitly say how it functions anywhere as far as I'm aware.