Doppler Nav Question

[Avio]

Does the DCS Doppler system takes into account measurements when going up slopes of mountains, as well as terrain objects such as tall buildings? Are these modelled?

 

[Dograw75]

Im not sure what you mean, but elevation is not a factor because its is just measure x/y not z.  so if you travel 10km west dead on the correct heading and gain 2 km altitude it just shows 10km distance traveled.  Completely linear, not slant range.  As far as Ive experienced anyway.

[Avio]

But technically the doppler reflected signal would be affected by every metre of movement detected, and by that shouldn't it be reading the "slant range" when going up a really steep mountain slope? That would give an inaccurate readout of the actual horizontal range that one is more interested in. Question is, is this being modelled in game?

 

[Blackeye]

That's assuming the doppler navigation has no way of figuring out that it is going uphill. Throw the altimeter into the mix and now assessing the vertical component of the movement is possible.  Even without that you could probably draw some conclusions about the ground's orientation from the distances of each beam.

 

Now I don't know what the capabilities of the real system in the Mi-8 are and it is quite possible that DCS is cheating here, but I think it's not impossible to take hills and slopes into account to some extent.

 

 

 

Edited April 10, 2021 by Blackeye

[QuiGon]

1 hour ago, Blackeye said:

That's assuming the doppler navigation has no way of figuring out that it is going uphill. Throw the altimeter into the mix and now assessing the vertical component of the movement is possible.  Even without that you could probably draw some conclusions about the ground's orientation from the distances of each beam.

 

Now I don't know what the capabilities of the real system in the Mi-8 are and it is quite possible that DCS is cheating here, but I think it's not impossible to take hills and slopes into account to some extent.

 

I don't think the Mi-8 doppler nav system does take input from other sensors like the baro altimeter into account. It's a system on its own AFAIK.

Edited April 10, 2021 by QuiGon

[Dograw75]

The point you are travelling to is not designated by altitude, it only determines direction, so there is no slant range.  How could it if you have not designated an altitude,  from your position it determines your lateral movement in relation to the ground, altitude is not factored.  Its not modeled in game.

 

Edit.  I am referring specifically to the nav portion when I say vertical speed or altitude is not factored into it.  it measures lateral speed and heading to determine how far you have travelled and heading in relation to the ground.  vertical speed is not a factor.

Edited April 10, 2021 by Dograw75
Additional clarification

[Frederf]

Since there is a forward and rear sensor going forward over an upward slope will increase the radial velocity of the fore return but also decrease the radial velocity of the aft return. If the system averages the two then the horizontal velocity calculation is unchanged.

[randomTOTEN]

On 4/8/2021 at 10:50 PM, Avio said:

Are these modelled?

After thinking about it, I question if there's anything here that actually needs to be modeled. You assert that terrain contours and obstructions/buildings will introduce errors into the DISS, but I don't think I agree, and can't quite understand why you think that would be the case. Why exactly do you think these terrain features cause errors in the calculations?

[Blackeye]

Imagine the helicopter flying over a level field for a mile (with markers) following the terrain. Now if that field was tilted down (or up) a few degrees the only thing that would change from the helicopters perspective is the relation between attitude, collective and speed because the gravity vector is now tilted and slightly adding (or subtracting) to the forces pushing it forward.

However as far as a radar looking at the ground is concerned nothing has changed and the distance between the markers is still a mile (which it is). However since the field is now tilted the distance measured parallel to the sea level is actually less. Since the latter is what is usually given as distance between two points the radar would indicate a longer distance than you would measure on a map.

 

That difference is very small for lower angles (less than 1% for a 5 degree slope) but for steeper flanks it could be noticeable.

[randomTOTEN]

I think this is what lead to some confusion for OP too,

 

3 hours ago, Blackeye said:

However as far as a radar looking at the ground is concerned nothing has changed and the distance between the markers is still a mile (which it is). However since the field is now tilted the distance measured parallel to the sea level is actually less. Since the latter is what is usually given as distance between two points the radar would indicate a longer distance than you would measure on a map.

It's a fundamental misunderstanding of the equipment.

The unit doesn't measure distance. Thus distance measurement errors on uneven terrain cannot cause errors.

 

EDIT2: Cool, I just realized there's a set of preflight tests using the aft panel I've never tried before! I gotta give those a go!

Edited April 12, 2021 by randomTOTEN

[Blackeye]

We're not talking about uneven we are talking about slanted. The radar can only measure movement relative to the ground - there is no way the radar itself can detect how an otherwise even surface is oriented in space (unless it has access to other sensors).

 

If the helicopter was capable of going straight up with its nose pointing upwards along a vertical surface the radar would measure that in the same way as if it was flying horizontally over a similar surface. In the former case the "map distance" would be zero though as you're going straight up.

Obviously no helicopter is capable of that but following a 15 degrees slope should not be a problem and the distance measured that way would be about 3.5% longer than the distance on the map.

Edited April 12, 2021 by Blackeye

[randomTOTEN]

The terrain is slanted, which I am referring to when I call it "uneven"

4 minutes ago, Blackeye said:

and the distance measured

I'm telling you buddy, it doesn't measure distance.

 

Quote

If the helicopter was capable of going straight up with its nose pointing upwards along a vertical surface the radar would measure that in the same way as if it was flying horizontally over a similar surface.

It would not, because the system is gyro stabilized, and will cut out measurement when the stabilization limits are exceeded.

Edited April 12, 2021 by randomTOTEN

[Blackeye]

It analyses the Doppler shift of radar beams which is indicative of the relative speed to the ground, which then is used to determine the distance traveled. Without knowing the orientation of the ground or your flight path altitude I don't see a way to derive the location without that error.

Unless you're talking about storing the ground profile and then trying to match features to stored data like the Viggen does, but that\s not how the radar on the MI-8 works afaik.

Edited April 12, 2021 by Blackeye

[randomTOTEN]

10 minutes ago, Blackeye said:

It analyses the Doppler shift of radar beams which is indicative of the relative speed to the ground, which then is used to determine the distance traveled.

Correct. It does not measure distance. An upsloping hill, flat ground, a sheer cliff face. They all approach the unit at the same speed. They all produce the same doppler frequency shift. The unit will correctly measure the groundspeed using all of them. The angle of terrain doesn't matter.

The distance calculation (of the route, not the ground below) is entirely internal. There is no distance measured to ground, thus distance measurement error from hills cannot cause errors.

10 minutes ago, Blackeye said:

Without knowing the orientation of the ground or your flight path altitude I don't see a way to derive the location without that error.

The system knows flight path attitude because it is gyro stabilized. It does not care what the orientation of the ground is. Ground orientation does not factor because it does not influence the doppler shift of the signal. The doppler principle eliminates the need for knowing the shape of the ground below.

 

10 minutes ago, Blackeye said:

Unless you're talking about storing the ground profile and then trying to match features to stored a data like the Viggen does, but that\s not how the radar on the MI-8 works afaik.

TERNAV is a separate system from the Viggen's Doppler Nav, so the principles are irrelevant here, as you correctly state this Hip (and soon Hind) don't have it.

Edited April 12, 2021 by randomTOTEN

[Blackeye]

18 minutes ago, randomTOTEN said:

The doppler principle eliminates the need for knowing the shape of the ground below.

Correct but it doesn't solve the orientation problem.

 

Quote

Ground orientation does not factor because it does not influence the doppler shift of the signal.

 

 Which is exactly my point of needing to know the orientation, because it does not change the signal but will influence the distance on the map.

 

 

Quote

The system knows flight path attitude because it is gyro stabilized. It does not care what the orientation of the ground is.

 

Which explains where the additional required information about orientation comes from and solves the problem nicely, but it is not part of the Doppler radar itself.

Edited April 12, 2021 by Blackeye

[randomTOTEN]

27 minutes ago, Blackeye said:

the distance on the map.

what map?

[Blackeye]

The one you project your flight path on, i.e. the distance traveled in the horizontal plane.

[randomTOTEN]

1 hour ago, Blackeye said:

Correct but it doesn't solve the orientation problem.

Yeah, it does.

1 hour ago, Blackeye said:

because it does not change the signal but will influence the distance on the map.

the only thing influencing the distance on the map is the signal. the signals are stabilized to the horizon with the AGB-3K attitude gyro, and oriented with respect to direction using the GMK-1A. The ground distance doesn't matter. the ground shape doesn't matter. The gyro systems orient the horizontal plane, the doppler measurements move the navigation solution. It doesn't make sense when you claim that a sloping terrain doesn't affect the signal, but changes the map. It can't simultaneously do some thing and yet not do that thing at the same time.

1 hour ago, Blackeye said:

but it is not part of the Doppler radar itself.

it's not part of the radar, but it is part of the DISS. It's mentioned in the first paragraph of the system in the manual. I just tried it. Shutting off the left pilot's attitude indicator results in an immediate failure of the DISS. Maybe the actual radar unit is still operative. Who cares. It can't provide valid information.

 

Edited April 12, 2021 by randomTOTEN

[randomTOTEN]

As another topic of understanding,

 

2 hours ago, Blackeye said:

If the helicopter was capable of going straight up with its nose pointing upwards along a vertical surface the radar would measure that in the same way as if it was flying horizontally over a similar surface.

In this situation, the helicopters groundspeed (relative to the horizon) is 0kmh. If the gyro stabilization was perfect, the doppler signals would still be oriented by the gyros in relation to the horizon. Instead of detecting motion along the vertical surface, they would detect no doppler shift from the surface which has no speed relative to the unit, and the resulting groundspeed would be 0kmh. Which would be correct. Even though the helicopter is in a vertical climb, and pointed straight up.

Edited April 12, 2021 by randomTOTEN
see even I make the mistake of using "distance" for measurement lol

[Blackeye]

And now imagine a Doppler radar without a gryo keeping it pointed downwards - it would happily track the vertical speed because it now would be pointed sideways.

 

Which again is my point: You need some additional device besides the radar itself. Whether it is coupling to the ADI, a gyro stabilized platform or some altitude information.

 

The misunderstanding seems to be that you are talking about the DISS system as fully implemented in the Mi-8, while my comments refer to a Doppler radar without those features, stating that the DISS system needs something beyond that if you want it to be correct over sloped surfaces. Which apparently it has in form of at least gyro stabilization.

Edited April 12, 2021 by Blackeye

[randomTOTEN]

Yeah I'd actually forgotten that the system used Gyros until this thread. Now I need to run those preflight tests I just discovered.

[Blackeye]

Didn't know that either but in hindsight it makes a lot of sense to have them.