[aldogoreng]

what is maximum altitude that can be recorded by blackberry GPS?
I use bbTracker and GPSLogger on 8800 and record my flight. Maximum altitude that can be recorded only 19862 ft even the plane fly over 32000 ft.

[Dubdub]

Keep in mind that altitude using GPS isn't accurate at all as it isn't corrected for barometric pressure whatsoever. Don't know what the max is however.

[arieltf]

Originally Posted by Dubdub

Keep in mind that altitude using GPS isn't accurate at all as it isn't corrected for barometric pressure whatsoever. Don't know what the max is however.

I would think that GPS would be more accurate than the regular altimeter that uses pressure to calculate altitude. The pressure at any altitude/location varies depending on the weather. If a high pressure weather system is present, the barometric pressure is higher than if a low pressure weather system is present in the same altitude/location. That would make a regular altimeter vary considerably.

GPS uses triangulation between satellites. The more satellites detected by the GPS unit, supposedly, the more accurate it is. Barometric pressure does not come into play.

--Ariel

[Dubdub]

You can't fly using GPS altitudes as it isn't corrected for pressure. Plus I have always been told by GPS engineers that the altitude isn't very good. Location, movement and speed are very good, altitude isn't.

Check this link for more info: Altitude Accuracy

[hrbuckley]

The reason you can't fly using GPS derived altitudes has nothing to do with correcting for pressure, as alluded to the GPS altitude is not measured using barometric pressure. However due to geometry the altitude solutions for civilian grade GPS receivers are generally not as good as horizontal positions.

There is some confusion over this because TSO C129a requires the pilot to "manually insert the barometric pressure" unless "the automatic altitude input utilizes barometric corrected altitude data".

For the OP, the maximum reportable altitude depends on the chip used.

[Dubdub]

I would think flight using GPS altitudes has everything to do with pressure as all aircraft operate at altitudes based upon and referenced by a pressure altimeter.

I bet you could use it to get a line of position however, using the old pressure navigation techniques. Will have to to try that sometimes to see if it works.

[hrbuckley]

A GPS fix locates the receiver in 4 dimensions (x,y,z,time), the device then converts those into geographic coordinates (of the selected variety and datum) and altitude above mean sea level. They do this independent of any other data source. The way this is done is by measuring the time of arrival of the signal from each satellite and comparing it to the time of transmission encoded in the signal. Using the speed of light, this give the receiver what is called a pseudo range from the satellite. The receiver then computes the point in space that best matches all the ranges from the know positions of the satellites. Nothing is perfect, the atmosphere delays the signals, the clocks on the satellites drift minutely with respect to each other, so you end up with a volume of space in which the receiver is likely to be located. Because it is much more likely that the majority of the satellites involved in the fix are closer to the horizon than the zenith, most of the pseudo ranges are going to be at shallow angles to the horizon, so the volume looks more like a foot ball with its long axis vertical, than a sphere.

All this leads to the fact that civilian grade GPS units can not outperform a sensitive barometric altimeter that is correctly adjusted. So airplanes do indeed use barometric pressure to determine altitude, not the GPS derived altitude. The only reason to enter the altimeter setting into a GPS unit is if that unit also has a barometric altimeter built in that it can then use to give vertical guidance to the pilot. This is very handy to have because complex routes and approaches have vertical profiles, so the GPS can easily compute what altitude the plane should be at given it knows the horizontal position. If it also has a 'legal' altitude it can tell the pilot he is too high, or too low.

[G M Fude]

Great posts, Buckley! I especially like the visual description of the football shaped space. I wonder if wristwatches like a Suunto X10 that have both a pressure transducer and a GPS receiver combine data from the two to give a more accurate altitude? I suspect not.

[hrbuckley]

We had a Garmin Etrex at work that had an altimeter. It did something like that, but I think it made some assumptions that depended on it being on the ground. It seemed to get confused when I took it flying.

[sparks]

Buckley is close, but not close enough for aviation.

At altitudes below 18,000 feet we are mostly interested in our height above the ground, referenced to sea level. The traditional instrument used to measure height is the pressure altimeter, which the pilot must frequently adjust to the local barometric pressure reading supplied during the flight by Air Traffic Control. The altitude shown on a GPS in this environment would indicate the same as a properly set pressure altimeter, plus or minus the errors outlined by Buckley. In fact, GPS altitudes can and are used for various phases of flight in this airspace.

Above 18,000 feet however, the aviation world is less interested in height above the ground and more interested at cruising at uniform levels for traffic separation. To achieve this, all altimeters are adjusted to a standard pressure setting (29.92" Hg). Altitudes are then referred to as "flight levels". Although the actual height of the "flight level" will vary due to local pressure differences, aircraft will always be cruising at the same relative "flight level" without the need for a local altimeter setting. In this airspace, a GPS altitude reading would not give a proper "flight level" reading unless corrected to mimic the barometric variances experienced by the pressure altimeter as the flight progresses.

So, pressure does enter into the equation, depending on the airspace and the instrument used. Safe flying.