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General Discussion / Altitude Accuracy
« on: September 03, 2015, 12:23:29 pm »
Hi Lee,
I think your Pilotaware Initiative is brilliant, especially as visibility of non-transponder equipped aircraft will at last be possible if your device gains wide acceptance.
My question concerns altitude accuracy of "targets" on the GPS display. If the displayed target is from an ADS- B transponder, the encoding altimeter is referenced to QNE (1013mPa), and the data relates to Flight Level (FL). If the target is transmitting via P3I, then the altitude will be derived from the GPS unit. This depends on the user dialling in an offset, such as altitude of departure airfield in Easy VFR, to "calibrate" the GPS altitude against the WGS84? geoid. As such, both targets altitude display will be independent of air pressure changes that will affect altimeter settings.
Does your software convert FL to altitude without taking into account the barometric pressure e.g.FL 25x100ft is taken simply as 2500'altitude? Or is it the GPS software that converts this? The P3I targets should presumably be on the same reference as each other, the geoid. Do GPS and Glonass and whatever Europe comes up with, use the same geoid?
If the altitudes from the various targets sources cannot be directly related, caution will need to be used when interpreting the display. At least we will know that there is something out there,in which direction to look, and an approximation of height differences!
Regards,
Chris
I think your Pilotaware Initiative is brilliant, especially as visibility of non-transponder equipped aircraft will at last be possible if your device gains wide acceptance.
My question concerns altitude accuracy of "targets" on the GPS display. If the displayed target is from an ADS- B transponder, the encoding altimeter is referenced to QNE (1013mPa), and the data relates to Flight Level (FL). If the target is transmitting via P3I, then the altitude will be derived from the GPS unit. This depends on the user dialling in an offset, such as altitude of departure airfield in Easy VFR, to "calibrate" the GPS altitude against the WGS84? geoid. As such, both targets altitude display will be independent of air pressure changes that will affect altimeter settings.
Does your software convert FL to altitude without taking into account the barometric pressure e.g.FL 25x100ft is taken simply as 2500'altitude? Or is it the GPS software that converts this? The P3I targets should presumably be on the same reference as each other, the geoid. Do GPS and Glonass and whatever Europe comes up with, use the same geoid?
If the altitudes from the various targets sources cannot be directly related, caution will need to be used when interpreting the display. At least we will know that there is something out there,in which direction to look, and an approximation of height differences!
Regards,
Chris