NCYankee wrote:
GPS altitude is not expected to match the aircraft altimeter except when on the ground. [snip, snip]I know all this. I just reported that during this flight (at 3000 ft), there was not a major discrepancy.
1000 foot difference is less common, but 500+ occurs frequently at low altitudes. We just went thru a real cold spell and there were big enough differences at some airports to require temperature correction, and these are for relatively low altitudes, particularly the intermediate segment which is often only 1500 above airport elevation. It has everything to do with temperature at airport elevation and making sure one is not comparing a pressure altitude with a GPS altitude, which can build in 100’s of extra feet of error.Throw in an inversion or smog layer and things get worse as it throws off the altimeter with the non linear lapse rate that it is not designed to handle.
NCYankee wrote:
GPS altitude is not expected to match the aircraft altimeter except when on the ground
Sure, but I’ve never seen any discrepancies as the OP sees. I don’t have an iPad with built-in GPS or barometer but use an external GPS (Dual 160). The devistaion is perhaps in the 100-200ft region max, normally less.
Airborne_Again wrote:
Yesterday, I took the new iPad Mini 4 flying. Again the GPS altitude was less than 100 feet off from the altimeter.
GPS altitude is not expected to match the aircraft altimeter except when on the ground. In flight, the error is almost all in the aircraft altimeter, as it is only sensing static pressure and converting it to an altitude based on a model of the standard atmosphere. The atmosphere rarely is willing to cooperate in matching the standard.model. The GPS calculates altitude geometrically by triangulation and has a much better accuracy which is independent of altitude and is more likely to be closer to true altitude than a barometric altimeter. They generally agree on the ground when the altimeter is set to the local altimeter setting. I never expect the two types of altitude to match in the air, particularly when surface temperature deviates from ISA.
Assisted GPS related posts moved to the Assisted GPS thread
Airborne_Again wrote:
I’ve been playing around with an new iPad Mini 4 (on the ground, in a car) for a few days now. I haven’t checked the altitude error carefully, but definitely less than 100 feet.
Yesterday, I took the new iPad Mini 4 flying. Again the GPS altitude was less than 100 feet off from the altimeter.
Well that eliminates that!
Exactly. The old one doesn’t, the new one does, and it would be very interesting to know what it does with the barometric information.
Built-in baro sensor is not the explanation of my occasional altitude hickups. Apparently my old mini iPad does not have a baro sensor!
I recently changed from an iPad 2 to a 4. The 4 has a baro sensor, the 2 does not. The 2 always gave me correct altitude, pressurized airliner or own plane. But, no baro information. I haven’t tried it on the 4…..
