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gps altitude - best use of equipment

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gps altitude - best use of equipment

Old 10th Jan 2012, 20:33
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gps altitude - best use of equipment

ever checked the tolerance of your gps altimeter at the stand?
ever been worried about too complicated procedures for temperature correction?

would it make sense to use lower indication comparing pressure altitude and gps altitude when it comes to obstacle clearance, eg during approach or during engine failure procedures?
or is it better to distract yourself from flying by filling in excel spreadsheets with data and waiting for the erroneous outputs?

when gps altitude is made available to pilots on an airliner would expect tolerance of indication should be published...
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Old 10th Jan 2012, 20:48
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GPS altimetry errors are entirely based on satellite geometry, so will change over time as the satellites move. There is no residual error due to the GPS receiver itself. "Calibration" on the stand is meaningless.
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Old 11th Jan 2012, 10:11
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I fly GNS530 and check geometric alt often. At fls like 300 or 400 it will differ even more then 2000ft from pressure alt. I find it useless to use geometric alt.
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Old 11th Jan 2012, 10:49
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GPS height is, as a rule of thumb, up to three times less accurate than GPS horizontal position. The GPS horizontal position is typically less then 10 meters off, vertically it's typically accurate to about 30 meters or 100 ft.
Of course there are times that exceptional bad geometries of the GPS satellites cause outliers. The factor 3 is only a rule of thumb, not a guarantee.

To observe this at the stand is not a good idea. At the ground, especially in between buildings / tailplanes, the GPS signals are reflected and reach your GPS antenna via multiple paths. Hence the accuracy at the stand is not comparable to in-flight accuracy. During taxiing it is in general good.

To compare GPS height to barometric (QNH) altitude is a bit tricky. You already mentioned the temperature correction, which is an important factor. However, for obstacle clearance it could be beneficial to use GPS since you don't need to correct for temperature in that case.

Another important factor is the fact that GPS uses the WGS84 ellipsoid as height datum internally, while we are used to measuring height with respect to Mean Sea Level. The difference between the two varies depending on your location on the earth but be as much as +/- 100m. / 300ft. Europe is for example about 100 to 250 feet above the reference ellipsoid, the US is 20 to 100 feet below. This difference is accurately modeled by EGM96. Since that model is quite complex and requires heavy calculations a much simpler and slightly less accurate model is usually used inside avionics systems to convert for Height Above Ellipse (HAE) to MSL and vice versa.

The GPS height display in the cockpit may either show HAE or MSL. Only if MSL is displayed it could be suitable for obstacle clearance.

But please use the altimeter and correct that for temperature. Only use GPS height as a backup.

Best regards,

ATCast

See also:
Altimeter altitude vs GPS altitude
GPS altitude
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Old 11th Jan 2012, 15:48
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I have got a GPS datalogger which I use on my bicycle. After a ride, you upload the data onto your computer and you can use it to verify that you have folowed a prescribed route for certain cycling events (Audax).

The track it draws is pretty good.

The software will also give you elevation climbed, which cyclists are quite keen to know.

The results given are useless. One ride from my house, 600 feet amsl goes along a disused railway (i.e no steep grades) and then along a canal (flat) and then back.

It somehow reckons I climb 3900' in 18 miles - in reality 9 miles as it is downhill/flat all the way out.

If you export the data to mapping sftware that works out elevation gain by looking at countours, then I have climbed 794'. Not as impressive but much more realistic.

Looking at GPS altitude in the aircraft it doesn't vary hugely in cruise, but I wouldn't want to use it unless it was a dire emergency.
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Old 11th Jan 2012, 17:13
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ATcast,

There is also a program called VDatum Get VDatum

This is a program that you can enter the coordinates, and it gives the difference between datums, in the US, with the datum being NAVD 88 converting to the WGS84 ellipsoid....

Here is how its used with the FAA...


The different boxes use different methods to calc the GPS difference, Smiths uses a lookup table, while Honeywell calculates the difference.
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Old 11th Jan 2012, 18:19
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Thanks for that link FlightPathOBN, that will be useful one day sooner or later.

Regarding the various methods used by different boxes, I remember that a common method is described in a NATO STANAG (forgot the number). It involves both lookup and calculation (probably simple interpolation), so in the end Smith and Honeywell might be using the same method.
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Old 11th Jan 2012, 19:40
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Yes, they are somewhat similar, what Smiths does is they have created a grid of the geoid, then looks up the grid square location...
Honeywell uses the geoid directly...

One you upload that software, here are a couple of interesting points to look up..

one is DeadHorse, AK...another is Palm Springs,CA...

Enjoy!
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Old 11th Jan 2012, 20:29
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Serious question from an ex avionics eng. (been away from the biz since retirement!)

Is it even remotely legal to use GPS altitude in flight?
As I recall you need a minimum of 4 satellites in view to get anything like a reasonably accurate altitude figure and what you do get is essentially height above ground.
I can not envisage any in flight situation when GPS alt would be acceptable to ATC authorities...

pp
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Old 11th Jan 2012, 21:17
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My GPS manual says its GPS elevation data is accurate to + or - 50m
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Old 11th Jan 2012, 21:45
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GPS altitude is indeed essentially geometric height above the ground / sea / ellipsoid, and therefor it is not suitable for altitude keeping, for that you need a pressure altimeter. So from an ATC perspective GPS altitude is not be acceptable.

But GPS height is acceptable for measuring your vertical position relative to the ground. For that purpose it is used in aircraft systems, for example in EGPWS and in GPS / GBAS based landing systems.
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Old 11th Jan 2012, 23:35
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ATCast quote...

But GPS height is acceptable for measuring your vertical position relative to the ground. For that purpose it is used in aircraft systems, for example in EGPWS and in GPS / GBAS based landing systems.
Not disputing what you say in the least, as I said Ive been off the tools now for 5 years and things may have changed... but... when I last encountered EGPWS retrofitting our fleet of F20's with Proline 4 glass cockpits and lots of other technological goodies, so far as I recall the alt input was always Rad Alt.

Still cant see that altitude information from a GPS would be accurate enough, particularly for a critical piece of kit like EGPWS.

I will stand gratefully corrected if someone can disprove my suspicions. I may be retired but I try to stay current!

pp
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Old 12th Jan 2012, 03:49
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Not in the EU, perhaps. But in the US with WAAS, vertical precision is about 1.5m (7m or better 95% of the time). Lots of GPS LPV approaches in the US with Cat I minima (200' to 400').
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Old 12th Jan 2012, 08:58
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Phalconphixer,

In this document it is briefly explained how GPS height can be used in EGPWS.

Note that an aviation GPS receiver does not only calculate horizontal position and geometric height. It also calculates a number of quality indicators based on the satellite geometry. These quality indicators give the accuracy & integrity of the calculated position & height.

In the past the accuracy of the GPS signal was purposely degraded by the US govt for military strategic reasons, the so called Selective Availability (SA). This affected both horizontal and vertical position accuracy of GPS receivers. Since May 2000 SA is turned off and the accuracy is greatly improved.

Older generation GPS receivers (probably >80% of the population) have a hard coded assumption that SA is still ON, so the quality indicators they put out are worse that the actual quality of their position. New generation GPS receivers are SA-aware and give better quality indications, although they are still conservative.

ATCast
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Old 12th Jan 2012, 16:37
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Since that model is quite complex and requires heavy calculations a much simpler and slightly less accurate model is usually used inside avionics systems to convert for Height Above Ellipse (HAE) to MSL and vice versa.
Do you have more details please? I've found it quite difficult to discover what algorithms (if any) are used to convert between HAE and MSL in common GPS kit, and what the corresponding resiudual errors are.
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Old 12th Jan 2012, 17:16
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GPS height is, as a rule of thumb, up to three times less accurate than GPS horizontal position. The GPS horizontal position is typically less then 10 meters off, vertically it's typically accurate to about 30 meters or 100 ft.
Why is this so, any idea? If you see the mathematical equations and error sources for GPS they seem x,y,z agnostic. Wondering why z errors are high.
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Old 12th Jan 2012, 20:13
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Vertical error is higher as the satellite geometry dictated by the orbits and having the Earth getting in the way isn't ideal for determining vertical position.
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Old 13th Jan 2012, 13:48
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ATCast...

Thanks for the link and the other relevant information... Never too old to learn! Very informative.

pp
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Old 14th Jan 2012, 02:28
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There are other considerations for vnav....

From Boeing

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Old 14th Jan 2012, 17:08
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the best use of GPS Altitude so far: B.U.S.S

I forgot what the achronym means!

but it is a new gadget available in the airbus family.

When switching off all three ADRs, the PDFs speed tape and altitude tape are substituted by an angle of attack tape (similar to a fast slow indication) and a GPS altitude indication.

With this BUSS you are not afraid of unreliable speed anymore. You will not break the airplane into pieces, you can keep it within the envelope.

But I still have a question: In the unreliable speed procedure for airbus we have to level off. However we cannot trust the altimeter, since static probes could be the source of the unreliable speed indication. So GPS altimeter comes in very handy, then, but ¿Is it reliable to fly V/S zero?
and what if you don't have a GPS at all? but that is for another thread

ATCAst thanks for the likns and info!
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