Radio Altitude determination

[dfournie]

I see that the offset 31E4 supplies a derived (aircraft altitude minus ground elevation) radio altitude for the aircraft. Does this number take into account changes in pressure and temp from 29.92 and 59F? Or, another way, where does the value "aircraft altitude" come from in the 31E4 derivation? Is it just an "aircraft above ground" value, or does this "aircraft above ground" value vary with pressure and temp? In a real aircraft this is obviously a radio transmission/reception by the antenna on the bottom of the aircraft. Not affected by pressure. I'm just trying to get some insight on how you are calculating it.

[Pete Dowson]

I see that the offset 31E4 supplies a derived (aircraft altitude minus ground elevation) radio altitude for the aircraft. Does this number take into account changes in pressure and temp from 29.92 and 59F?

No, as it says is subtracts the ground altitude from the aircraft altitude.

Or, another way, where does the value "aircraft altitude" come from in the 31E4 derivation? Is it just an "aircraft above ground" value, or does this "aircraft above ground" value vary with pressure and temp?

It takes the aircraft altitude (the one that states aircraft altitude, in the place where you'll also see latitude, longitude, pitch,bank, heading -- it's one of the 6 coordinates) and subtracts the ground altitude.

Sorry if this sounds obscure. I really didn't think it could be a lot clearer? :wink:

Pete

[dfournie]

The explanation is clear. However, the GPS datum used for Flight Sim 2004 (WGS84) uses a mathmateically derived altitude reference. GPS altitude measures the users' distance from the center of the satellites in orbit. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent. What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned. In my setup this is very relevant. It's just an accuracy thing. Sorry for the GPS lesson, but it looks like I need to change the code on my end. Thanks for the info.

[Pete Dowson]

Sorry for the GPS lesson, but it looks like I need to change the code on my end. Thanks for the info.

None of what you say is unknown to me, but nor is it at all relevant as far as I can see. I have no idea why you need to change any code at all -- how did you think FS got the ground altitude?

In FS there's only the ground elevation given by FS. It isn't using a GPS in real time to get this, it gets it from the scenery files which contain whatever data the person who made it put into it, possibly after modification by things like AFCAD altered runway elevations if you just happen to be over an airfield.

How the elevations were originally measured or computed is not really relevant to the calculation involved, which as I say is a simple subtraction.

In FS if the ground elevation says -15 then it is -15 -- the mesh it uses for the ground tells it so, and the rendering will place it so. What it actually is in that spot in the real world is really nothing to do with it. This is a simulated world.

There is some likely small discrepancy with the aircraft altitude though. I'm not exactly sure where it is measured to -- some datum in the specific model you are flying I think. It won't be your eye level or the lowest point of the extended gear.

Regards,

Pete

[dfournie]

You are 100% correct. This may shed some light on what I'm doing: I have FS 2004 interfaced into an actual Rockwell Collins Pro Line 21 system. It is a cockpit upgrade for an analog system we are tearing out. It will be making its way into a US Navy King Air at some point. Our sim lab uses a GPS satellite constellation simulator that models the GPS system and sends output as an RF signal to drive a real world GPS. We have the positional output from FS2004 "driving" the simulation around. I was trying to narrow down the cause of the error I am seeing in altitudes in various places. I now believe it is due to WGS84 GPS altitude as it is provided by my GPS simulator. The worldwide altitude data I'm using just doesn't match the FS2004 world exactly. As to be expected. I'm just doing some stuff that is slightly past the homebuilt sim stage.

[Pete Dowson]

I now believe it is due to WGS84 GPS altitude as it is provided by my GPS simulator. The worldwide altitude data I'm using just doesn't match the FS2004 world exactly. As to be expected. I'm just doing some stuff that is slightly past the homebuilt sim stage.

Depending wherabouts in the world you are needing this, there are some rather more accurate terrain meshes available for FS. Whether that helps or not I don't know.

Regards,

Pete