Accuracy of GPS speed measurement
Posted: Sun Sep 29, 2019 2:40 pm
Can anyone tell me about the accuracy of GPS speed measurement - I ask because I am wondering if it is worth including GPS speed measurement in the unit I built to measure the drag of towed hulls at the recent AYRS event at the Bassingstoke Canal Centre - see posts here under 'Events'.
The American government provides information about the positional accuracy of GPS at the website gps.com, but there is nothing there about speed accuracy. I understand that gps determines speed by a doppler shift method, its not just a matter of timing between two positions.
A couple of writings I have found on the internet sugest gps speed error limits around 0.2 knots for a straight line course (higher on bends and wiggles but that is not relevant to our current application). Presumably that means +/-0.2 knots. The timing at Weymouth Speed Week is by use of GT31 GPS data loggers. The manufacture's (Locosys Technology Inc.) specification for this unit mentions a figure of 0.1m/s, again, about 0.2 knots.
Somewhere on the internet I have seen mention of a Garmin specification of 0.1knots for GPS speed accuracy but I havent been able to find this in a document from Garmin.
For the tow testing we are considering, for the time being at least, speeds will probably not exceed 5 knots, so +/0.2 knots is +/- 4% or more. We should be able to improve on that by refining the current timing method, so if the gps errors are really that great there may not be much point having the gps module in our equipment. Would be nice to know.
The American government provides information about the positional accuracy of GPS at the website gps.com, but there is nothing there about speed accuracy. I understand that gps determines speed by a doppler shift method, its not just a matter of timing between two positions.
A couple of writings I have found on the internet sugest gps speed error limits around 0.2 knots for a straight line course (higher on bends and wiggles but that is not relevant to our current application). Presumably that means +/-0.2 knots. The timing at Weymouth Speed Week is by use of GT31 GPS data loggers. The manufacture's (Locosys Technology Inc.) specification for this unit mentions a figure of 0.1m/s, again, about 0.2 knots.
Somewhere on the internet I have seen mention of a Garmin specification of 0.1knots for GPS speed accuracy but I havent been able to find this in a document from Garmin.
For the tow testing we are considering, for the time being at least, speeds will probably not exceed 5 knots, so +/0.2 knots is +/- 4% or more. We should be able to improve on that by refining the current timing method, so if the gps errors are really that great there may not be much point having the gps module in our equipment. Would be nice to know.