Is doppler nav impaired by low-altitude flying?

[Nealius]

I noticed something odd with the doppler nav and "moving map." While flying NOE, about 50~100m off the ground, the "moving map" was frequently off by tens of kilometers. Even as I updated my position over known landmarks, and even as it showed me perfectly on flightpath for short, two-minute legs, it would go off in a big way. 

 

However, while flying the same route back at around 1,800m altitude, the "moving map" was incredibly accurate and hardly needed any position update. 

 

Does the doppler nav and "moving map" simply function better at higher altitudes?

[Fri13]

It shouldn't. The radar detects your speed and drift etc. Combined with other sensors you get very accurate position for non-GPS system.

[Svend_Dellepude]

Also noticed that the map was off, but thought that it was how the system was. Doesn't look like an ultra precise system. But it's easy to update via landmarks.

[Nealius]

That's strange. My flight plan was:

 

Mezzah Departure

WPT1 at Ad Dumayr (the river between the city)

WPT2 at the SW threshold of An Nasiriyah runway

Target at the north-central part of CT23 (revetments along a highway)

Direct to Mezzah

 

I flew everything to the target below 100m AGL. The moving map showed me on flightpath, but WPT1 was a few kilometers to my left, so I corrected my course and I updated the moving map as I passed over WPT1. Continuing on to WPT2, it was again quite a few kilometers to my left. I corrected my course, and updated the moving map as I overflew WPT2. Flying on-course--according to the map--to the target, the target was again to my left, this time an even greater distance off than at WPT2 or at WPT1. 

 

The position error was consistently right of the waypoint, putting the waypoint to my left, regardless of cardinal direction. 

The degree of position error was inversely proportionate to the length of my legs. The long leg between Mezzah and WPT1 produced a small error. The medium leg between WPT1 and WPT2 produced a medium error. The short leg between WPT1 and Target produced a large error. 

 

I flew direct back to Mezzah at 1,800m. The longest leg of the entire route. The moving map showed almost zero error on that return leg. 

Edited June 21, 2021 by Nealius

[lmp]

According to RedKite, there may be an issue with the manual drift corrections being incorrectly applied between the two map scales:

 

 

Maybe this is what you're running into?

 

You could retry your experiment without applying any manual drift corrections (and/or any scale switching?) and see if the drift at different altitudes is still so different.

[Nealius]

No scale switching, at least not in flight. 

[TomChaai]

Maybe try turning off the switch on the left side panel of the cockpit, left of the 2 vertical gyros, there is an "airspeed to DISS" switch, this switch seem to mess up DISS real bad, the DISS even loses the proper heading and instead always assume the aircraft is moving north.

[Nealius]

9 hours ago, TomChaai said:

Maybe try turning off the switch on the left side panel of the cockpit, left of the 2 vertical gyros, there is an "airspeed to DISS" switch, this switch seem to mess up DISS real bad, the DISS even loses the proper heading and instead always assume the aircraft is moving north.

 

I never turn that switch on. 

[Fri13]

19 hours ago, lmp said:

According to RedKite, there may be an issue with the manual drift corrections being incorrectly applied between the two map scales.

 

More likely that the paper map we have is very inaccurate. 

 

 

So if you compare it by the look:

 

 

I have tested the map accuracy by flying to map small detailed element and then start a long route from it randomly. And then return back to that same location and compare the map marker position where started with, and it is very accurate that way. 

 

But you get to trouble if you try to navigate by using the map. It is way off visually that you can't fly by using the map topology and your position marker because the map is inaccurate and doesn't match the DCS World locations. 

This means that you can't look a map and say "There is the crossroad, I fly there" because you can be a 2 km off or there is no such crossroad.

 

This is why I have enjoyed from the KA-50 ABRIS system because it has realistic map look and capability, you know exactly what you are looking for. And I totally dislike all these old US navigation maps in Harrier, Hornet, Warthog etc that are digitalized versions from their 35mm film strips from paper maps the pilots had in the 70's or something. Not meant for combat operations but for generic navigation in long distances. 

And those maps are pushed everywhere where you are meant to do combat in 20 x 20 km area, but you are given map that has details for a 500 x 500 km routes in/out combat area. 

 

This is problematic for DCS because we do not have curved 3D terrain but all is flat. It causes not just major problems with the weather engine now where we should have clouds behind the horizon because curved earth, and this way avoid the jagged lines and aliasing in clouds because there should be usually so much moisture that blends all fine details (blurs), but we have trouble in the basic navigation process with magnetic and true headings, the map coordinates and like. 

 

So in one hand we have a proper topology maps that are worked with big efforts to real spherical earth, and then those maps are used for a DCS that draws the terrain as flat earth, so corrected paper map doesn't work!

 

And then you are given so lousy USA based map that is no good for anything else than cruising at 30 000 ft from one side of map to another. Missing maybe 5/6 of the details that should be there. 

 

I would like to see in Mi-24P a kneeboard implemented in the cardboard box. Like let us to add the wanted map boards from the editor. Click the handle to open the box, click the board to browse through the mission imported JPEG files and then click to select one, and click to close the box. Now we would have the wanted board inside the box, and we are required to move the position strip to location where we think we are on it. This way we would be operating basically with two boards only. One for generic in/out combat zone board, and then when we enter the combat zone we would swap to detailed one for area where we are flying.

 Leave the automatic board swapping to easy-mode or something.  

[Nealius]

1 hour ago, Fri13 said:

But you get to trouble if you try to navigate by using the map. It is way off visually that you can't fly by using the map topology and your position marker because the map is inaccurate and doesn't match the DCS World locations. 

 

I've suspected this, but wasn't sure if were a localised issue to some terrains. When flying in Syria I notice entire airfields missing from the moving map on the modern jets. 

[MAXsenna]

Good suggestion!@Fri13

Sent from my MAR-LX1A using Tapatalk

[VampireNZ]

It would just be nice to have sim topology that matched the maps - was trying to use a lake for a visual reference for positioning off the map and didn't see it when I expected to, so spent 10 minutes surveying the local area to try and find where I was....only to find the lake on the map didn't exist in the sim. 

[Fri13]

11 minutes ago, VampireNZ said:

It would just be nice to have sim topology that matched the maps - was trying to use a lake for a visual reference for positioning off the map and didn't see it when I expected to, so spent 10 minutes surveying the local area to try and find where I was....only to find the lake on the map didn't exist in the sim. 

 

Exactly the problem. 

 

I would like to have this excellent work from flappiefh as map boards

If I recall correctly, he has drawn the lakes and rivers by hand, checked the bridges and roads and all.

It might not be realistic printed map, but it is far more usable than US map as now. 

You would at least have accurate representation of towns,roads, hills, lakes, rivers and like. 

 

Purpose of the map is to be able position yourself to the surroundings with visual observation, and current map doesn't offer that:

 

Accurate map for DCS 2.5 Caucasus theatre - GeoTIFF (digitalcombatsimulator.com)

 

 

 

 

 

[VitMax]

Tens of kilometers seems a bit excessive for paper map imprecisions. Also, who knows, maybe topographic maps in DCS really are that far from reality.

As for the original question, DISS should operate just fine at low altitudes. I just did Syrian patrol instant mission and I flew the whole way at 5-25 meters over ground, and moving map was quite precise.

In Mi-8 manual it says that system's internal deviation should be less than 1.5 km per every hour of flight. The only limitations for DISS are speed < 400 KPH (which is always true in our case), bank < 30 degrees, and pitch < 7 degrees. If those limits are exceeded, system goes into memory mode, in which it calculates position using the last accurate data. When you back in the allowed envelope, it will continue to work fine.

[Nealius]

6 hours ago, VampireNZ said:

It would just be nice to have sim topology that matched the maps - was trying to use a lake for a visual reference for positioning off the map and didn't see it when I expected to, so spent 10 minutes surveying the local area to try and find where I was....only to find the lake on the map didn't exist in the sim. 

 

I only experience this on desert maps, where dried lake beds are misleadingly colored blue in the mission editor/planner "alt" map view.

[VampireNZ]

1 hour ago, Nealius said:

 

I only experience this on desert maps, where dried lake beds are misleadingly colored blue in the mission editor/planner "alt" map view.

 

Was on Cyprus...and not a small lake 

[LazyBoot]

8 hours ago, VitMax said:

and pitch < 7 degrees. If those limits are exceeded, system goes into memory mode, in which it calculates position using the last accurate data. When you back in the allowed envelope, it will continue to work fine.

doing a ferry flight with 0 wind, I hit the pitch limit at around 290 ground speed, as evidenced by the speed and drift on the drift indicator no longer updating.