GPS Altitude

[nigelspeedy]

Hi Dynon,

I understand that GPS altitude displayed on the map is calculated as height above the WGS 84 geoid. But height above mean sea level is referenced from the EGM 96 Geoid and as a result are often quite different, in my location in sunny Mojave CA there is over a 100' difference for example. As a result there are quite rightly always warnings to users about these differences. To make the GPS derived altitude more useful how hard would it be to measure the height above the WGS 84 ellipsoid and then do a coordinate conversion to present height above the EGM 96 Geoid? You could call it MSL subscript G to let folks know it is referenced to mean sea level but is GPS derived. If this is possible could you include an option for AGL subscript G which could be height above ground level GPS derived, calculated as GPS altitude minus the terrain altitude for that lat/long, perhaps with a little smoothing/filtering.
Keep up the good work.
Cheers
Nigel

 

[Dynon]

Our terrain data is actually WGS 84 referenced, so actually this isn't an issue for SkyView's GPS-computed AGL measurement.

 

[nigelspeedy]

Thanks for the reply.
So I understand the displayed GPS Alt is WGS-84 referenced as is the Terrain Data so a pseudo rad alt is conceptually possible. Still the question of making GPS Altitude referenced to the MSL reference of EGM 96 instead of WGS-84 to make it more useful. Not sure which you display on the D-1 but it could be a possible improvement to that product as well.
Is the time shown on the top of the display UTC or GPS, noting that they differ by approx. 16 seconds due to the leap seconds that are added to UTC but not to the GPS. The reason I ask is that I have some tests in mind where I wish to be able to sync the Skyview log files to those from a DGPS.
Cheers
Nigel

 

[dynonsupport]

Assuming you are using Dynon's GPS puck. If you are using some other GPS, all bets are off:

Time in SkyView is UTC, not GPS time. There may be small offsets however, due to our Serial parser and other things going on in SkyView. We make no statements how close it is to UTC. If you need it to be within 1 second, it probably won't work.

I think you are mixing up WGS 84 and EGM 96 a bit. They aren't exclusive. As Wikipedia says:

Presently WGS 84 uses the EGM96 (Earth Gravitational Model 1996) geoid, revised in 2004. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100 km horizontal resolution). The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about [ch8722]105 m to about +85 m. EGM96 differs from the original WGS 84 geoid, referred to as EGM84.

And:

The latest major revision of WGS 84 is also referred to as "Earth Gravitational Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004. This model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).

So technically, WGS 84 can be either. So the real question is which geoid is inside the GPS. In Dynon's case, the GPS outputs altitude based on the EGM96 fidelity Geoid grid, and our terrain database matches.

Of course a psuedo-radar altimeter is possible in software. However, it will have errors of 100+ feet at times for various reasons, so using it for precision terrain clearance or for touchdown assistance is a fool's errand. We do already allow users to select a point on the earth with the map pointer and it will tell you how high above or below it you are. I've watched competing EFIS units read "0 AGL" until you were 300 feet in the air with all of the errors stacked up.

On top of all of this, there is a newer model than EGM96. EGM84 and EGM96 are partially limited by data storage size and compute power, which are less of an issue now than 18+ years ago. There's already newer, more accurate models with more points. These will differ from EGM96, but are theoretically "better". So it's not like EGM96 is perfection, it's just a common reference.

So, what are you really trying to use "MSLg" to do? As pilots we don't really use MSL unless we are on the ground, since the instant you depart the field the errors begin to stack up. However, we all fly the same baro altitude so we don't crash into one another, even if our MSL is varying as we travel. So I'm interested in why this data is interesting to such high accuracy.

If you really want to see the Geoid correction that is in use in SkyView, go to your serial port for your GPS, choose "navigation data" at the bottom, and go to "GPGGA". A bit below that will be listed the geoid correction for your current location.

 

[ckurz7000]

There are several airports where VFR approach segments are referenced as AGL altitudes (e.g., LJMB, LJPZ). Minimum flight altitudes are specified as AGL, too.

Using AGL based on GPS altitude and the usual elevation data as a replacement for a radar altimeter is inviting disaster. However, there are many ways to be stupid in an aircraft and consequently kill yourself. The pilot bears the responsibility.

Therefore I would also like to see an AGL info item selectable for the map page.

-- Chris.

 

[dynonsupport]

Chris,
We understand the usefulness of AGL. The question was how accurate MSL is useful.

How are pilots without an AGL display expected to fly the approaches that require AGL altitudes? This will help us determine what people really mean by "AGL".

 

[ckurz7000]

Accurate MSL is only useful when you're in the air and don't have a baro setting. It's useful as a backup altimeter, although it doesn't ensure that you're at the same altitude as someone else who is flying on an offiial baro setting. Limited use, I'd say.

The "poor man's radar altimeter" is more useful, in my opinion. How pilots without this are supposed to fly, you ask? Well, it's by gosh and by golly, really. With minimum altitudes you have to add a generous safety factor so as not to violate them. During approaches which specify AGL altitudes, it is the pilot's responsibility to familiarize himself with every aspect of the flight before embarking on it. That's the official answer you get. However, if I had an AGL readout I would feel much better. It takes a lot of guesswork out.

-- Chris.

 

[RVDan]

Could this replace or at least simulate radalt?

 

[dynonsupport]

Only if you need a radar altimeter that is only accurate within 300 feet. It will not work to get those last few feet before touchdown, which is the generic use of a radar altimeter. So it all depends why you want a radar altimeter.

 

[nigelspeedy]

When originally posting my intent was twofold: First to make sure the GPS altitude was as close to baro alt as possible (hopefully increasing its usefulness) by taking into account differences between the frames of reference, and it seems that this has been done. This feature has been available for a while and is common on lots of GPS devices and aps but I struggle to find a use for it. Perhaps if both my AHRS fail or my both my static ports get blocked and the alternate static source does not work I could use GPS altitude as a backup. When do other people use GPS altitude in flight?

Seconds, I realize that a GPS derived AGL would not be as accurate as a Rad Alt, but it would be close enough for some simple tasks like: ensuring you are complying with minimum height above ground restrictions for flight over built up areas or noise sensitive areas or national parks or wildlife preserves, or helping you answer the question 'how far could I glide from here?'. I agree using a GPS derived AGL for low level precision terrain clearance or for touchdown assistance would be a fool's errand. I guess you could say the same about the use of synthetic vision as it is all GPS derived as well. A pity as I was really hoping for a terrain following autopilot mode some time in the future.
Cheers
Nigel

 

[ckurz7000]

Yes, synthetic vision, in IMHO, is one a prime example of video game aera applications in a place where it encourages potentially very dangerous behavior. It probably is only a "must have" for EFIS because people who don't know a lot about flying find think it is cool.

-- Chris.