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Author Topic:   GPS accuracy
kshoaps posted 08-12-2003 05:29 PM ET (US)   Profile for kshoaps   Send Email to kshoaps  
While looking at the site's FAQ file I saw that the speed function of the GPS can be in error as much as 2.5 mph.

Makes you wonder about some of the speeds we see on this site.

Aside from actually measuring a course (probably with a gps) and timing your passage are there any more reliable speed indicators?

jimh posted 08-12-2003 08:32 PM ET (US)     Profile for jimh  Send Email to jimh     
Many test reports say they used RADAR timing--just like the highway patrol.
Peter posted 08-12-2003 09:11 PM ET (US)     Profile for Peter  Send Email to Peter     
While underway I find that the speed readings on my GPS are remarkably consistent with the speed readings on the surface speed sensors such as the Yamaha speedometer and the fishfinder. The reading differences between the GPS and the surface speed sensors are never more than about 2 mph different and that is usually because I'm either heading into or going with a 2 mph current.

Perry posted 08-12-2003 09:53 PM ET (US)     Profile for Perry  Send Email to Perry     
I tested my handheld e-trex while driving my truck on the highway where there are 2 different radar (or laser?) speed readings. The results were within 1 mph at 65 mph.
JohnJ80 posted 08-12-2003 10:36 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I would assume thta the speed error tracks with the position error (DOP?). If the accuracy number is very good, then i would bet the speed is too. If the accuracy is out at 100 feet or something larger than the normal <20' you would have a larger speed error.

If you know were you are one second and the you know again a second later and those positions have an error of about 10' and you are going 30mph, then the error is small. If your position error is 100', then it is a problem.

However, if you measure your time over a long run with the GPS, get the distance, and calculate the speed, then it will be very accurate since the position error is a small (very small) portion of your actual distance run.

Hope that made sense.


triblet posted 08-12-2003 10:36 PM ET (US)     Profile for triblet  Send Email to triblet     
I'll bet that FAQ entry on Magellan's website datest to the
days when Selective Availability was still on. I'd see two
or three knots when the boat was on the trailer in the
parking lot

I've seen claims that with DGPS, the speed accuracy is about
.1 knot. Should be about the same with WAAS, and close with
vanilla (no DGPS, WAAS, or SA) GPS.


doobee posted 08-12-2003 11:12 PM ET (US)     Profile for doobee  Send Email to doobee     
That error is certainly related to selective availability, which you are not likely to encounter.
lhg posted 08-13-2003 05:46 PM ET (US)     Profile for lhg    
Even without SA, I've read that you need the differential correction signal (DGPS) for accurate SPEED indications.
Richard Quinlivan posted 08-13-2003 09:38 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Look at this link to a typical Garmin GPS unit. Speed accuracy is stated at 0.05 meters/sec steady state. That is pretty good.

where2 posted 08-20-2003 12:33 PM ET (US)     Profile for where2  Send Email to where2     
I tell you what, I trust the GPS speed even off a non-WAAS garmin more than I trust the speed on my mechanical speedometer in my car... IF I have a clear view of the sky.

On a long flat stretch of FL highway, I can set the cruise control in the car, note the GPS speed, start the stopwatch at a mile marker, and cruise for a couple of miles to get a good average for distance to remove my timing error, and to get a good average of GPS speeds. Run the stopwatch time versus the distance traveled, and it usually nails right at my average GPS speed.

If you're playing around trees and in canyons, multipath will mix up your GPS speed. On the open ocean with nothing around to obstruct the signal, it blows away the other instruments.

JBCornwell posted 08-20-2003 01:33 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
GPS units use time and distance to calculate speed, not doppler effect.

Positional error with or without DGPS or WAAS is irrelevant, because the error remains constant from one positional calculation to the next. Therefore the time and distance calculations are unaffected by positional error.

If my Garmin disagrees with any other measurement, including RADAR and hand-held stopwatch over a measured distance, I will believe my Garmin.

Red sky at night. . .

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