Laptop GPS World

In every GPS forum on the internet, the question is often asked, "How accurate is the model XYZ-999 GPS?" There's no simple answer. Or, actually, there are multiple answers. .

What aspect of the GPS do you want to measure?

Absolute positional accuracy? I think this is the one most people sort of have in mind when they ask the question. When the GPS says I'm standing at a particular location on the face of the planet, how close is it - really - to where I am actually standing?

It turns out that it's not terribly accurate. Most manufacturers will claim between 10 meters and 5 meters. I have occasionally seen as good as 2.5 meters with WAAS enabled.

So, what does that mean, really? Well, if the GPS reports that it currently has an accuracy of 7 meters, if you draw a circle with a diameter of 7 meters centered on the coordinates being reported, odds are that you are somewhere in that 7 meter circle. That's plenty good enough to help you find Ikea in a strange city, find the nearest donut shop or to get you home or back to the campsite. But it's not likely to get a survey engineer very excited.

Speed? Most manufacturers will report something like 0.1 meters per second accuracy for the speed reading. That's right up there with the cop's radar gun. It's good enough to show you that your car's speedo is reading about 5% faster than you are really going. (Car manufacturers do this intentionally so you can't sue them when you get a speeding ticket.)

Relative Positional Accuracy? If you put a stick in the ground and take a reading, move somewhere else and take another reading, this is the distance between those two points. This would show up, for example, as the distance traveled on your trip meter and also as the direction you are traveling as you move from the first point to the second.

This is as accurate as speed. Has to be, if you think about it. Speed is calculated by taking two readings, marking the location and time at each reading, computing the distance between the two locations and the time it took to travel that distance, then dividing the distance by the time to get the speed. So to have very high accuracy for speed, you must have equally high accuracy for the distance travelled. And for the time.

Which brings us to the final measurement of GPS accuracy:

Time? The usual spec you will see will be "1 microsecond synchronized to GPS time".

As with the relative positional accuracy, this factor has to be very accurate to get the kind of speed accuracy claimed (and delivered). This is largely a function of the GPS satellites themselves, plus a little computation in the GPS receiver. Each GPS satellite has a cesium clock in it that is hugely accurate. Just one step short of the nuclear clocks like at Greenwich or Denver. It doesn't get much better than this. Definitely better than your average Timex (or Rolex, for that matter).

Y'er welcome.

...ken...

Good information. Thanks.