How realistic is this idea? Historically, aircraft measured their altitude with a kind of atmospheric barometer, compensated for QNH. I can’t help feeling that with today’s technology, it would be cheaper and more accurate to get true altitude from GPS and communicate that to ATC and/or to other pilots.
In the same line of thinking, one could report GPS derived course rather than heading, freeing the poor ATC operator of the task of converting heading to course, based upon whatever indication available of the wind.
And don’t come and tell that GPS can fail – of course it can, but in that unlikely case one can still revert the procedure, using the backup barometric altimeter and magnetic compass that IFR planes need to carry anyway.
Last in the same line of reasoning, when are we going to name runways after their true heading iso magnetic? That would save a fair amount of money and trouble by avoiding the renaming of runways due to magnetic drift. I understand it is already done in some areas with large magvar, such as Northern Canada.
All other considerations aside, GPS altitude from non-SBAS/non-GBAS receivers is much less precise than from a barometric altimeter with a known QNH. I observed the altitude readings on several GPS receivers, and fluctuations of ±100 ft at a stationary position were commonplace on all of them. There was also a fairly steady downward altitude drift by up to 200 ft for the first several minutes after satellite capture. With SBAS (e.g. EGNOS here in Europe), these errors should become reasonable, but I’d guess fewer than 20% of the GPS receivers installed in the European GA aircraft are SBAS-enabled.
Not very realistic. A barometric altimeter is cheaper than a GPS and does its job without the need of any power source. The purpose of an altimeter is to separate aircraft from each other vertically and to provide adequate separation from the ground or obstacles. To upgrade all of the aircraft worldwide would be an enormous waste of money to solve a problem that does not exist.
A barometric altimeter is cheaper than a GPS
No way! I bought several GPS sensors for Arduino projects for 5-10 Euros each. A GPS based altimeter can be made for less than 20 Euros for the parts (which means retail price 200 Euros…). Show me a mechanical altimeter with the required precision for that money. And regarding the power requirement: Such a device can run from a small backup battery a lot longer than the fuel will last.
one could report GPS derived course rather than heading
That’s what Mode S extended squitter is for. On a proper FMS, it also shows the controller the selected heading and altitude. Understandably, ATCOs love it…
On a proper FMS, it also shows the controller the selected heading and altitude…
And how it does… On “my” aeroplane we currently have a slight discrepancy between the no. 1 and no. 2 directional gyro, they are off by 3-5 degrees. Our tech staff has had no time yet to compensate the fluxgates and the limit is 5 degrees, so we still can fly. The transponder however only transmits the reading of the no. 1 gyro, so whenever the aircraft is flown from the right hand side, the heading asked for will be off some degrees. I have been told by controllers twice already: “Callsign 421, confirm steering 185 degrees, our readout shows 189”. Similar with the altitude, especially when flying manually. A very nitpicking Swiss controller recently reminded us that we were 80ft high (the allowed tolerance is 200ft!).
It will never happen. I get involved in a lot of useless discussions, but this one is beyond my patience. Y’all continue to tilt at them windmills.
But I wonder if things would be done this way if aviation was invented today 
@NCYankee: I always understood this forum wanted to be polite and positive. Please post along those lines, or not at all.
I think part of the issue is that there are a lot of very talented engineers and electronics/computer types on this board who are very enamoured of new technology. I see you all aware of what is possible at the cutting edge and chafing against the frustrating reins of regulation. We are generally, thanks to the makeup of the board at the IFR/hi tech end of aviation.
We are where we are because of proven historic technologies, and the certification standards. GPS altitude is very nice, but utterly useless given the way the current chart data have been set up in many cases. Altitude above the WGS84 ellipsoid may be helpful with stable solutions at high FLs, but barometric altitude is a proven solution at low level. It also works nicely at high level, so why mess with it for a solution that would require a lot of effort to certify it?
I just don’t see why one would wish to rely on the GPS constellation, with satellite coverage and geometry optimised for lateral positional accuracy, for your vertical positioning data.
