Re: GPS Altitide Accuracy?
non standard temp-pressure??
EDIT:
I was curious, ran accross this at Anywhere map:
GPS Altimetry
Many people are surprised to find that their GPS altitude often disagrees with the altimeter’s altitude by a significant amount. This effect is more pronounced at higher altitudes. It is important to understand the limitations of GPS (and barometric) altimetry before safely using either in an aircraft.
GPS Altitude accuracy
GPS devices read what we will call the GEOMETRIC altitude of the aircraft, relative to a sea-level baseline that is defined in the WGS84 coordinate system. This altitude is unaffected by atmospheric conditions. Aircraft Altimeters derive altitude by measuring the air pressure.
There is a compensating setting in an altimeter (the Kollsman window) that allows the pilot to adjust the altimeter to the proper altitude at an airport, compensating for barometric pressure and temperature effects AT THE SURFACE.
At low altitudes GPS altitudes will generally agree with the aircraft (barometric) altimeter, so long as the altimeter is set to the local altimeter setting. It is not uncommon to see an error of plus or minus 100 feet. Many imported GPS receivers (the inexpensive ones commonly sold over the internet or by computer stores) have about a plus or minus 200 foot altitude accuracy. This is due to a miscalculation performed in a very common inexpensive GPS chipset that is used in virtually all of these receivers.
Most of these GPS altimetry errors are random and depend on satellite geometry and a host of other factors beyond the control of the pilot. These errors apply at any altitude as well.
It is because of these errors that GPS must not be used as an altitude reference when operating close to the ground. You should NEVER use the virtual ILS function or VNAV functions of any GPS within several hundred feet of the ground under any circumstance.
Barometric Altimetry Errors
At higher altitudes, the GPS altitude and Indicated (barometric) altitude often diverge. This is caused by the limitations of barometric altimetry, not by any error in the GPS calculations. All pilots know that temperature affects air density, and that on a hot day, the air is less dense – reducing aircraft performance.
This also affects the accuracy of the altimeter. Setting the Altimeter setting makes the indicated altitude agree with field elevation when the aircraft is on the ground at that airport. At ISA Standard temperature, the altimeter will be accurate as the aircraft climbs away from the airport elevation as well. However, when the air is very cold (more dense) or very hot (less dense) the altimeter will accumulate errors as we climb away from the field elevation where the altimeter setting was taken.
On a day that is 30 degrees Celsius colder the altimeter will show the aircraft about 10% higher than it’s AGL altitude above the station altitude where the altimeter setting was recorded. 30 higher results in the indicated altitude being 10% lower than the actual TRUE (GPS) altitude would indicate. Thus, on hot summer days, the GPS will show a higher cruise altitude than the altimeter, typically by about 500 feet at 10,000 feet. This has an impact on using the VNAV target altitudes and should be compensated for by the pilot. Air traffic control is based on Indicated altitude, not on GPS altimetry.
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