Aviation In Canada

Thursday, March 10, 2005

GPS Nav Error

I was searching for an airfield off the beaten track in Nova Scotia where a PA28 I had worked the week before for flight following did an emergency landing due to engine failure. Since I was totally unfamiliar with the roads and where this field may be, I plucked the lat/long out of the Canada Flight Supplement and entered them into my handheld GPS receiver.

I made my way to the community, feeling confident about the direction the road signs were giving me, until eventually, the roads turned to dirt roads, then across a single lane, deteriorating bridge and up a hill to a four-way intersection. No more guidance from signs, so I decided to guess. Besides, if I had to backtrack and had difficulty, I had the GPS. The first road I took I knew was leading me northwest, based on the position of the sun and the time of day. I drove this road for a couple of minutes before figuring this wasn't be the one I wanted. I decided it was time to look to the GPS for direction. Something was amiss.

The GPS was showing me heading southeast at 60 km/h, climbing out of 15,000 feet. I was heading NW at about 30, and ground level here should be around 100-150. The first thing I did was what anyone who encounters a position error with a GPS unit does: Check the geometry and number of satellites. I was surprised to find that of the 11 satellites above the horizon, the unit had a lock on 7 of them. Scattered nicely around the sky with one above, one NW, one NE, two S, and two SE at various elevations. Battery life was also not an issue, since it was powered off the car's cigarette lighter. It would be hard to get any better. I found the field anyway, and decided to let the unit go to see if it would right itself. After about 20 minutes, I finally had enough. I turned the unit off when it showed me about 33 km southeast of where I should be, still heading SE at 50 km/h, and climbing out of 21,000 feet. Immediately after powering it down, I turned it on again. 15 seconds later, it locked on what appeared to be a good location, including a good elevation, and zero groundspeed.

I've talked with a number of people, including my GPS guru George Dewar. There was no theoretical explanation of GPS for this error, other than an electronic error in the unit. I had owned this unit for two years prior to this, and for two years since, and had never before or afterward seen anything other than precise and consistent positions. I still have the unit, and it is still very reliable.

It is because of this story, and one similar to it from a local flight instructor (in his case, it was the unit installed in the plane), that I will refuse to use a GPS unit without a backup paper map, and will refer to this map periodically while enroute.