Observations on using a Garmin GPS III on a Commercial Aircraft
By Paul Jaffe

I have had my GIII for a month or so, and I just had the occasion to take it on a Continental Airlines B737 flight from the Los Angeles area to Houston. On the return flight, at night, I had it turned on most of the way back to LA. The following are some observations:

1) When I was boarding the plane, one of the pilots was standing next to the cockpit door. I asked him if I could use the GPS while underway. Without hesitation, he agreed.

2) I had a window seat with no external antenna on the GIII. You must have a window seat. If you pull back the GPS from the window even a few inches, you lose satellite contact almost immediately.

3) I turned on the GIII when the flight attendant announced that it was now OK to use laptop computers, etc. Likewise, I turned it off at the same point in the descent.

4) It took about 5 minutes at first to acquire satellites. I had previously initialized by map. Later, after I would turn the GIII off for awhile, it would acquire five or six satellites very quickly when re-started...in no more than a minute.

5) You get acquisition faster when the GIII is located close to the bottom of the window frame rather than in the upper part.

6) After a while, it becomes awkward to keep holding the GPS. I found that if I lowered the food service tray and rested my hand on the tray and my arm on the armrest, I could have the GIII sit comfortably and well positioned on my wrist. This put the unit lower than the bottom of the window, but it was easy to have the antenna sticking up no more than an inch above the bottom edge. This proved to me a comfortable position and the GIII worked fine this way, for hours on end.

7) The pilot announced our initial cruise altitude and early into the flight descended somewhat due to turbulence, In both cases, the altitude readout on the GIII was 1000 to 1500 ft. in error, always on the high side.  (This is because the pilot sets his altimeter to 29.92" above 18,000 feet no mater what the actual barometer pressure is -causing the plane to be at a somewhat different altitude than the altitude he is holding and reporting).  The DOP was around 2.0 and the EPE around 150 ft. during these readings

8) It's very entertaining to use the GIII when flying, especially at night. You see circles of light off in the distance and you can scan and zoom around and readily determine the cities and towns you're looking at.

9) Altitude is deceiving in determining lateral distance by eye. For example, it is easy to underestimate the distance of the track of the aircraft from cities. When looking almost straight down at a landmark city, I was guessing it was 1-2 miles displaced from our track. In fact, it was more than 5 miles, using the scale on the GPSIII screen.

 Paul M. Jaffe