GPS Accuracy

[jgillott]

I've been using a Garmin GPS for years and have been very happy. Saw this on my screen when I got home tonight and had to laugh.

Makes you wonder how it came up with that speed. Nothing I own could be that fast unless you dropped them out of an airplane.

 

[MichaelOverfield]

Yes.. I have a bicycle with a speedometer that displays maximum speed over 75 MPH (if I really did that it would be for sure scary fast on a bike!).. in its case, it has a wireless signal, and interference caused it to read high.. so that is the cause there.

Similar here. Your GPS uses various satellites to compute a 3 dimensional position in space, based upon range from satellites, literally thousands of miles away (if directly overhead they'd be 26,500 miles actually). GPS signals are pretty weak on a good day, and as the military is finding out, pretty easy to jam.

So, your GPS uses best position and speed is calculated by time and distance covered between known positions.. all you need is a period of interference, and/or low accuracy fix (only 2 satellites gets you what is called 2D), followed by a more accurate fix, much further from the last, and it computes a speed from distance between the last 2 fixes and known time difference... and from there you get 183 MPH!

Heavy clouds, large trees, large buildings in cities, and of course RF interference all affect GPS signals and can lower fix quality/accuracy.

I have a Garmin too, it doesn't show the number of satellites of quality of fix so, hard to figure this out, but I have a DeLorme unit that does show this and even maps which satellites it is tracking and their positions, it shows current position and a circle indicating expected accuracy, that circle can be large or very small depending on how many satellites it tracks and where they are positioned. .

OK, I know this because I've navigated ships at sea, using GPS and other methods to determine ship position, thus it was all part of my training and qualification.

 

[jgillott]

Thank you for an excellent explanation. I would imagine that driving around in downtown Pittsburgh with all the lousy rainy weather lately would account for the interference.

 

[MichaelOverfield]

Yes, both the rain and city could be a factor... out here in WA state, large trees/forest do it often too, trees here are massive.

 

[crispyboy]

I have never had my GPS tell me that I was going 183 mph but it has "lost it's mind" from time to time. I was driving north of Indianapolis last summer and the dumb thing wanted me to get off the 4 lane highway and drive around a country road through a corn field......... I give this Magellan about a 65% accuracy rate. I will never give up paper maps. My GPS stays on the dashboard because of it's entertainment value and it does have a wireless backup camera for hooking up trailers.