[MANUAL] Radar elevation in terrain avoidance mode

[Diac]

I messed around with the terrain avoidance (TA)/obstacle detection radar mode tonight, and noticed something:

 

It seems like what happens when the radar is put in TA is that the radar beam is narrowed in the vertical plane (tested by adjusting the elevation both in and out of TA mode), which I assume is what is supposed to happen. However, I also assume that the radar is supposed to be locked to boresight when TA mode is active, so that what shows up on the scope is only terrain/ojects at your altitude. This does not happen.

 

 

The way I tested this is as follows:

 

1. Bind the radar elevation axis to a rotary control with a middle detent (the viggen manual states that the elevation potmeter has a middle snap position).

 

2. Set radar in A1 mode, and turn weapon selector to AKAN (or any other mode where radar elevation is indicated on the scope).

 

3. Adjust radar elevation to the middle snap position on the elevation axis and observe that the middle does not correspond to boresight, but rather slightly above.

 

4. Fly towards a mountain or other terrain at your altitude.

 

5. Set radar in TA mode, and observe that nothing shows up on the scope.

 

6. Carefully fine-adjust radar elevation downwards (axis is inverted), so that terrain shows up on the scope.

 

7. Disable TA mode and go back to normal A1, and observe that radar elevation is now set to boresight.

 

 

So, TA mode seems to be working as intended, as it does narrow the beam in the vertical plane, however, the radar is not locked to boresight upon engagement of TA mode, so that manual fine-tuning of the radar elevation is needed in order to properly display terrain on the scope. Is this a bug? Or is the problem/bug that the middle position of the elevation axis is above boresight?

 

In addition, it also seems to depend on which weapon mode is selected.

 

Does anyone else share a similar experience?

Edited May 15, 2021 by RagnarDa

[renhanxue]

On the real aircraft, switching to TA mode sets the "neutral" antenna elevation (that is, the elevation that you get if the elevation knob is centered, in its middle snap position) to 0° relative to the aircraft X-axis, but since you have some positive alpha in level flight, your attitude is somewhat nose-up, so that ends up showing obstacles slightly above your actual flight altitude. In normal A1/A2 modes the neutral antenna elevation is actually slightly negative relative to the aircraft X-axis, between -0.5° and -3° depending on range setting and whether the altitude is above 600 meters or not (more negative at short range settings and at altitudes above 600 meters, there's a table in the SFI if you're really interested). For reference the elevation knob has a range of +/- 10°.

 

The antenna elevation knob adjusts the elevation relative to this neutral position, so switching modes or range settings changes the actual antenna elevation a bit even though the knob position stays the same.

 

That's all I know, I think.

 

 

edit: oh, one more thing: in A2A mode the neutral elevation is +1.5°.

Edited October 12, 2020 by renhanxue

[Diac]

On the real aircraft, switching to TA mode sets the "neutral" antenna elevation (that is, the elevation that you get if the elevation knob is centered, in its middle snap position) to 0° relative to the aircraft X-axis, but since you have some positive alpha in level flight, your attitude is somewhat nose-up, so that ends up showing obstacles slightly above your actual flight altitude. In normal A1/A2 modes the neutral antenna elevation is actually slightly negative relative to the aircraft X-axis, between -0.5° and -3° depending on range setting and whether the altitude is above 600 meters or not (more negative at short range settings and at altitudes above 600 meters, there's a table in the SFI if you're really interested). For reference the elevation knob has a range of +/- 10°.

 

The antenna elevation knob adjusts the elevation relative to this neutral position, so switching modes or range settings changes the actual antenna elevation a bit even though the knob position stays the same.

 

That's all I know, I think.

 

 

edit: oh, one more thing: in A2A mode the neutral elevation is +1.5°.

 

Very interesting, thank you for your insight!

 

Since the scope doesn't show relevant terrain when the elevation knob is set to the middle snap position, but has to be adjusted slightly downwards, I would guess that there is a bug causing the middle snap position to not be 0 deg relative to the aircraft axis, and that this is the reason why. Would you agree?

 

 

Edit: I just re-read your answer, and it seems to me that your opinion is that the lack of relevant terrain on the scope is due to the fact that you have some positive alpha in level flight, correct?

Edited October 14, 2020 by Diac

[renhanxue]

Very interesting, thank you for your insight!

 

Since the scope doesn't show relevant terrain when the elevation knob is set to the middle snap position, but has to be adjusted slightly downwards, I would guess that there is a bug causing the middle snap position to not be 0 deg relative to the aircraft axis, and that this is the reason why. Would you agree?

 

 

Edit: I just re-read your answer, and it seems to me that your opinion is that the lack of relevant terrain on the scope is due to the fact that you have some positive alpha in level flight, correct?

Yes, exactly. In level flight the aircraft isn't actually level with respect to the horizon, it's always somewhat nose-up, and so the radar beam points slightly up as well. I don't know why they did it like this on the real aircraft because it seems it removes some margin of error that you would otherwise have, but as far as I understand the SFI that's how it works.

[Machalot]

On 10/15/2020 at 6:31 AM, renhanxue said:

Yes, exactly. In level flight the aircraft isn't actually level with respect to the horizon, it's always somewhat nose-up, and so the radar beam points slightly up as well. I don't know why they did it like this on the real aircraft because it seems it removes some margin of error that you would otherwise have, but as far as I understand the SFI that's how it works.

 

Sorry for the necro, but I was just researching this again and remembered reading this thread a while back.  The manual on p. 117 says it's 0 deg relative to the horizon:

[renhanxue]

31 minutes ago, Machalot said:

 

Sorry for the necro, but I was just researching this again and remembered reading this thread a while back.  The manual on p. 117 says it's 0 deg relative to the horizon:

I think that's based on a misreading of the original Swedish flight manual (SFI) for the real aircraft, or it's based on documentation I don't have access to. Here's the relevant passage from the SFI (SFI fpl AJ 37 del 2 kap 1 sida 21):

 

 

All the normal search modes use a horizon-referenced coordinate system and the antenna elevation angles are explicitly stated to be relative to the horizon. However, note the very last sentence, after the table - in both air-to-air mode and in terrain avoidance mode, the angles stated are explicitly said to be relative to the aircraft x-axis, not relative to the horizon.

 

Note though that this is the original AJ 37 SFI from 1975 so things may have changed later on. I don't have access to a later version of this part of the flight manual though.

Edited April 12, 2021 by renhanxue