GPS vs ADIRU Altitudes

[Traffic]

Along with the torch and the digital camera, I recently added a handheld GPS to my toy box.

I have used the handheld regularly on hiking treks and found it provides accurate elevation/altitude readings.

Once in a while I have switched it on in he cruise and found it very accurate in the lateral mode but just noticed it invariably gives an altitude reading substantially higher than the Air Data & Inertial Reference Units that drive the airshow readout.

In the past two weeks I have bounced this question off a number of colleagues and got many differing answers..all from guys with 15,000+ hours.

Yesterday the ADIRU indicated 37,000' while the GPS showed about 39,200'.

Likewise the on board GPS read about 39,200'.

FL vs true altitude differences are accounted for by pressure and temp differences.

The most plausible reason I have heard to date is that the GPS has the 'true' surface of the earth in its memory...it being a flat reference point This varies from location to location, and it is from this non-spherical surface that it takes its height reading.

Can someone put me out of my misery because this is an airine with 85 heavies that doesn't seem to know the answer?

[tej823]

GPS altitude readouts are figure above a standard referance datum. The WGS-84 Ellipsoid surface was created as a point of referance as a standard measuring point that more or less represented the shape of the earth (not really a sphere, but a bit squashed).

Hope this is a help.

[411A]

One must remember that the ADC is providing a pressure altitude, with the altimeters set to 1013.2/29.92.

So, the true altitude will vary a bit.

In the good 'ole days, the actual altitude above MSL could be ascertained (on long over water routes) with the HRRA, as this was very useful for pressure pattern navigation.

[Intruder]

Also, GPS is inherently less accurate in altitude than lateral measurement. It is possible that the update rate on your handheld is a bit slow to keep up with the airplane movement, and altitude errors are the result.

Try putting the GPS in 2-D nav mode with the current altitude (corrected to sea level pressure, if possible) preset, and see what it does...

[Old Smokey]

Assuming that the QNH was at or close to standard, you could expect an indicated altitude of 37000 feet to translate to a true altitude of 39200 feet in an atmospheric temperature of ISA+12.8°C.

That sounds pretty right for South-East Asia (I note from your profile that your location is Asia).

[HotDog]

Hi Traffic, I didn't really wanted to get involved in this discussion but here is my two cent's worth. The optimum config for best accuracy with a GPS RX is having four sattelite signals at 45 degrees above the horizon and one in each direction, ideally N, E, S, W, for an accurate horizontal fix.
For a vertical fix, the optimum set up is one sattelite overhead & three on the horizon, ideally 120 degrees apart in azimuth. This is impossible to achieve inside the cabin of an aircraft. Stick to the Airshow, it's much more accurate. Cheers, HD.

Old Smokey, QNH and teperature have nought to do with GPS.

[Romeo Tango Alpha]

Yep, spot on, GPS has NOTHING to do with QNH and temp, but...

Re-read Old Smokey. He said that a TRUE height of 39,200 (as shown by GPS), will translate to an INDICATED 37,000 at ISA+12.8, in near equatorial regions. INDICATED altitude IS effected by temperature and pressure...

[Blacksheep]

As always RTFQ. Traffic asked the reason for the difference between GPS and the ADIRU altitude. The answer is that the two heights are unrelated - GPS being a measure of physical altitude while the altitude output from the ADIRU is a standardicsed estimate of altitude based upon atmospheric pressure.

GPS - either Traffic's own hand held or the aircraft's onboard GPS - indicate the height of the receiver above the WGS- 84 surface of the earth. What use this may be during cruise flight is debatable, for not all aircraft are fitted with GPS and a cruise altitude of more than 31,000 feet provides adequate safety height for even the tallest lumpy bits of the planet.

The ADIRU provides an electical output to drive an electrical altimeter or generate a pressure altitude reading on the PFD. During cruise flight this is standardised using QNH [29.92 inches of mercury/1013.2 millibars] as the atmospheric standard for ambient pressure at ground level. The actual pressure at ground level is most likely quite different, but the objective is to provide all aircraft with a common standard to work to.

Using a standard setting means that all aircraft indicating the same pressure altitude on their altimeters will be at the same level regardless of their physical altitude above the earth's surface - which is after all the objective of the whole exercise.