|
personally, i think acquisition performance and sensitivity are the most impt performance metrics for a GPS. if you have to wait 5min every time you get turn on a GPS before it gets the initial fix, then it wastes too much of your time. also, the more satellites the GPS can keep a lock on while driving thru trees, urban canyons, and other obstructions, the more likely you aren't going to completely lose the lock on your position temporarily every time your sky view is obstructed. if you do lose your lock on the satellites, another impt thing is how fast you can get it back after you pass the obstructions. this is called a "hot start," which the SiRF units can usually do in <5-10sec, even if you cut the power off for 1/2 hr (like when you go into a store and come back).
the manufacturer's ratings in accuracy are doodoo as far as i'm concerned, because i've seen such wildly different specs on different units using the same hardware, and so many conditions beyond your control such as the geometry of the satellites relative to each other and you come into play to determine the "accuracy" at any given moment when you're driving. also, don't forget that the GPS is only spitting out a fix every 1sec, and that the fix was calculated using after the data were acquired, so when moving, your accuracy is further decreased by your velocity (and when using a garmin etrex in NMEA mode or a NAVMAN GPS sleeve, you get a fix only once every 2sec).
WAAS support definitely improves accuracy, but if i remember correctly, there are currently only 2 satellites and it's hard to get a clear sky view to them at all times.
as far as stumbling is concerned, the position of the AP is just an estimate based on the signal strength trajectory, so it adds so much error that talking about GPS error relative to stumbling is moot.
just my 2cents.
|