General Question MFD Ground speed wrong?

[jevans]

I don't know if I'm just being stupid, but I'm trying to get ground speed using the formula:

GS = TAS * (PR /(PR+A))

Where PR is the radius of the reference body, and a is altitude. While this gets a value that seems correct, it disagrees slightly with the GS readout on the default surface MFD.

On experimenting with the orbiter surface MFD, it seems that the GS readout takes account of vertical velocity too. For instance, I went to Brighton Beach, applied full hover thrust for a bit, no translations, and eventually came back down to land( crash, anyway) into the same pad that I had taken off from. However, during this, the GS readout indicated the same as the VS readout - despite the fact that I wasn't going anywhere across the Moon's surface. This is at odds with the way I've always thought GS should work, ie. velocity across the surface.

Is this a bug in the default MFD, or am I being thick in some way?:facepalm:

Anyway, I presume to calculate the GS value, I take the length of the vector defined by the x and z velocity relative to the surface?

 

[Urwumpe]

Ground speed in Orbiter = Velocity vector relative to a fixed point on the surface of Earth, directly below the spacecraft.

The official ground speed is just the projection of the true velocity on the surface of Earth.

 

[Hielor]

You are correct, the "GS" mode in the default surfacemfd is just total velocity relative to the ground instead of speed over the ground which is what "groundspeed" usually refers to in aviation. I touched on groundspeed but didn't elaborate in my thread at HTTP://www.orbiter-forum.com/showthread.php?p=81415.

I suspect that the ground-relative speed is more useful for SurfaceMFD than traditional groundspeed because it's essentially a pfd, and over-the-ground speed is more of a navigational concern...

 

[Urwumpe]

I suspect that the ground-relative speed is more useful for SurfaceMFD than traditional groundspeed because it's essentially a pfd, and over-the-ground speed is more of a navigational concern...

also, the angular ground speed in Orbiters Surface MFD is much more useful, together with the angular distance from Map MFD...

 

[jevans]

I see. I've spent too long on FSX, then...

Urwumpe,

So which one should I use for the SSU PFD? 'Orbiter' GS or 'True/aviation' GS? (This is for the M/VR mode, I sorted the others out fine).

 

[Hielor]

I see. I've spent too long on FSX, then.

What do modern aviation GPS units give for the groundspeed when in a vertical dive? I could test this when I get back to Seattle but that won't be for another week.

 

[Urwumpe]

I see. I've spent too long on FSX, then...

Urwumpe,

So which one should I use for the SSU PFD? 'Orbiter' GS or 'True/aviation' GS? (This is for the M/VR mode, I sorted the others out fine).

Space Shuttle GS of course. :lol:

There is no indication of Ground Speed, that I know of, the GS in the PFD is called the GSI, the Glide Slope Indicator.

The VREL or VR is what you would call surface relative velocity, it is calculated relative to the rotating surface of Earth.

---------- Post added at 06:59 PM ---------- Previous post was at 06:57 PM ----------

What do modern aviation GPS units give for the groundspeed when in a vertical dive? I could test this when I get back to Seattle but that won't be for another week.

The indicator will flash "1AN\0U70FHERE1111"

 

[jevans]

Well, the Garmin in FS would give 0 if you were traveling vertically. I don't know if this is true for real ones too, but I would imagine it is. It's modeled quite accurately.

 

[Urwumpe]

Note, that VR is not at all Ground Speed - It is velocity (vector, not just magnitude) and relative to the surface. It should indicate 150 KEAS, if you are dropping vertically at 150 knots equivalent airspeed (less than Mach 0.9). If you are dropping vertically on the equator at 5000 ft/s, it should read 5.2 kft/s in M/VI mode and 5.0 kft/s at M/VR mode.

 

[statickid]

I was landing on the moon the other day (I could be wrong) but I think I used the Orbital Speed to come to stop with a speed indication of zero that did not take the fact that I was falling into account, which would be more like the GS that you are originally describing.

I was only trying to come to a stop vertically above the landing pad but the vector to ground GS in orbiter can be confusing in this situation.

 

[jevans]

Note, that VR is not at all Ground Speed - It is velocity (vector, not just magnitude) and relative to the surface. It should indicate 150 KEAS, if you are dropping vertically at 150 knots equivalent airspeed (less than Mach 0.9). If you are dropping vertically on the equator at 5000 ft/s, it should read 5.2 kft/s in M/VI mode and 5.0 kft/s at M/VR mode.

So it's like in Orbiter, VR is speed relative to the point on the surface below the ship? And VI has the extra .2 because of the rotation of the Earth.