Good GPS reception need a good antenna. Many phones (e.g. my BB 8830) use so called assisted GPS which reply on basestations and triangulation. So they don't work well in the air if at all.

Some phones (e.g. Nokia N95) are true GPS devices but they use a tiny antenna so reception is poor causing a long satellite lock even not moving on the ground.

True GPS devices use a large antenna about one inch square making an ugly phone or watch. I can get a good reception in the air with a Garmin etrex and Quest. Unless I got a fix on the ground first, even these GPS devices will take a while to get satellite lock once the plane is at cruising speed. Some GPS models (e.g. Garmin eTrex "H" versions) have higher sensitivity and can get a fix even away from windows.

In Northern Hemisphere, south facing windows have view of more satellites than north facing windows and can get a satellite lock easier.

Most true GPS devices can record and export a track even without maps loaded.

Does this post need to be edited?

I've used my Garmin Forerunner 205 numerous times and it tells me the airspeed, altitude, etc. I've heard the pilot read our altitude and it was accurate with what my GPS was saying. It has a little bit of a hard time if I'm not near the window.

GPS measures ground speed. The receiver has to be calibrated to calculate the air speed.

The GPS receiver can calculate speed without being on the ground although it will take longer to get a fix while it is moving. Position of the receiver is calculated from position of the satellites and time. So it will work even at airplane altitudes.

http://tycho.usno.navy.mil/gpsinfo.html
http://www.trimble.com/gps/howgps.shtml
http://www8.garmin.com/aboutGPS/

You do know what ground speed is? Ground sped does not mean the object is on the ground.

Positioning data alone can only give you ground speed. Other data is needed to calculate air speed.

GPS receiver doesn't know where the ground is and they don't need to be "calibrated". It just know the receiver location calculated from satellite messages it received. Satellites are traveling in fixed orbits. Receiver then calculates its position from the satellite positions with respect to absolute time. Each satellite is essentially a 3rd order time standard. First order being the universal time standard in Paris, 2nd order being the US National time standard at NIST near Denver.

GPS receiver calculates the travel speed from the next position and elapsed time. Each location point is recorded as altitude, longitude, latitude and time. So no calibration on the "ground" is needed. Once it got a fix anywhere, on the ground or in a plane, it is "calibrated" if you want to call it that.

GPS maps are overlays and tied to receiver location via datum set by USGS and other national surveying agencies. The latest one in use in the US is from 1984 thus called WGS84.

Below is a recorded track FRA-SFO. The GPS device was turned on after the plane took off and left on for the entire flight until near SFO. Each leg recorded represents the distance between two points (Alt, Long, Lat, time) and speed is calculated from it.

FRA-SFO

assisted gps simply means it uses the cellular network to shorten the time for the gps to get a fix on its position. instead of 30-60 seconds or more, it's near instant. after that, the gps tracks as it would normally and the cellular network is no longer needed.

if a gps unit can pick up gps satellites, it's a 'true gps.' the antenna has nothing to do with it, although better antennas will obviously help. the gps devices with the sirfstar iii chipset are amazingly sensitive, and i've heard of them getting a fix (no assist either) inside buildings.

that's true.

These are good comments but I won't go so far as antenna has nothing to do with it. No matter how good the chipsets had become, they need an antenna to get signals from the satellites.

High sensitivity is not free either. They have shorter battery life. I use a Garmin eTrex Venture Cx for travel and it has 32 hours of battery life, good enough for a trip across the ocean. The best high sensitivity in the eTrex series (e.g. H, HC, HCx versions) have about 25 hours battery life. So I opt for longer battery life over higher sensitivity.

yes of course. i did not mean that no antenna is needed, just that a true gps does not indicate any particular type of antenna.

Geez. Do you not understand simple English? I never said the GPS needs to know where the ground is. You are the one who brought location of the receiver into play?

You are absoultely confused. There is no way the GPS can give you air speed without other data. Since, GPS,. fundamentally measures position, and the rate of change of position is the ground speed, not air speed, it measures ground speed. It needs airspeed data to calculate airspeed. If you don;t get that simple principle, you need to learn a bit about GPS, and about ground speed, as well as airspeed.

The only speed that can be calculated from position data, without any additional input, is the ground speed. That's why I asked you if you knew what ground speed is. You yourself imagined that it had anything to do with location of the receiver.

Yes, I suspected that is what you meant.

Nevertheless there are physical limitations to antennae design. A good antenna takes space and affects aesthetics and design of the end product. There are just no space for a real GPS antenna in cellphones and watches etc.

Satellites are very far away. They use limited power sources such as solar panels and small nuclear power plants. The signal reaching the earth is very weak. Better GPS chipset is not only better sensitivity but also employing DSP technology to separate signal from noise or multi-path issues.

Since cellphone BST tower always has a GPS to serve as a time base, A-GPS works fine. But these devices lacking a true GPS doesn't work in area of no service, such as in foreign countries. My BB8830 is one of these.

I have a Garmin GPS 10 BT pug. It supports cellphone OS such as Palm, Symbian so my Treo and Nokia N72 can load up a mapset on flash cards and work as GPS. But the functionality tends to be quirky thus limiting their usefulness as a real tool. Plus I will need to carry another charger in the travel kit.

If I may ask sir, what is that "other data"?

Let's start from what you know so that we are on the same page.
What do you think you can calculate from position data?
Do you know what ground speed and ow it differs from air speed? I find it unimaginable that someone wouldn't know that.

My sincere apologies sir...

There are no difference between air speed and ground speed as long as direction of travel are parallel.