how accurate is a GPS for speed

Accuracy of GPS SYSTEM
GPS units very all over the lot as far as precise accuracy. Cheap consumer units very with type and speed of the processors and number of satellites received, the manufacturer, etc. etc. etc. As price goes up (to many thousand dollars), generally so does the accuracy. The price of cheap consumer units is generally tied to more bells and whistles though. I’m not talking about kayak applications here but just about the accuracy of the system.



Hand-held GPS units for mapping (‘drawing maps’) not only receive the GPS signal, but also receive a correction signal via radio from a Corp of Engineer base station This allows for accuracies of three feet and under, and is used for mapping trails, wetlands, well locations, or any other application where limited accuracy is needed. The information is collected in a data logger which is then downloaded to a GPS software program. The data can then be exported into a GIS usable format for map drawing.



Survey GPS units are designed for acquiring very accurate coordinates and measurement. Accuracy of hundredths of a foot can be obtained in just a few seconds of observation. This is achieved by setting a mobile base unit on a control monument and measuring various points with rover GPS units. By using RTK (real time kinematics) method you get instantaneous coordinates on measurements. This is accomplished by having both the base receiver and the rover receiver collecting GPS signals from the same satellites at the same time and then the base station broadcasting a correction signal to the rovers via radio signal. This is somewhat similar to what the old DGPS units did.

depends on how many satelites you have t



my 2c
With (comercial) correction acuracy 3 meters.

Speed = Distance/time. If you have error of 3 meters in distance, relative error in speed goes as



Delta(speed)/Speed=Delta(distance)/distance - errors in time dropped for obvious reasons.

BTW 1m/s = 3.6 km/h ( ~2knots )

Possible cruising speed ~4knots - 2m/s, you travel ~2 m each second, you error is ~3m.



Bottom line - instant readout can have huge error.



Garmin website claims 0.1 m/s RMS error in STEADY STATE - that is going at the same speed in a straight line.


accuracy
The GPS is extremely accurate with speeds over 10 miles per hour. If you compare your GPS speed with the speed on the speedometer of your vehicle you will find that it is an exact match. However when you are moving at very slow speeds such as when you are walking in the woods you will see a great fluctuation in speeds from about 2.5 mph to 5 mph as you wind your way through the woods.

Todd

could a gps tell you how fast
you were moving if you were falling? say skydiving?

not well
the altitude is significantly less accurate, + or - one hundred feet on average as I have read, not an expert though.

Evans is correct …
Using SPS (Standard Positioning Service), which is what is available to the public, vertical accuracies are about 1.5 times less accurate than horizontal (e.g. a position calculated with a horizontal accuracy of 10 meters would have a vertical accuracy of about 15 meters).



For those who are interested Peter Dana has a wonderful website which explains much about GPS development and how it works.



http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html



Enjoy!

~wetzool