posted 02-08-2009 10:49 PM ET (US)        

"The GPS is accurate for the determination of speed over ground (about 10 times more accurate than a car odometer) when moving at relatively constant speed in straight lines and is competent at determining speed on curved paths, although some overshoot does occur during transitions. Absolute error increases slightly at higher speeds but in percentage terms is less. In addition, when the system is tested under conditions of sudden changes in speed some inadequacies become evident. The system smooths the peaks and troughs of rapid accelerations and decelerations, which is attributed to inherent smoothing within the mathematical algorithm and the one sample per second output of the system."

http://www.rvc.ac.uk/sml/people/documents/WitteJBio2004GPS.pdf

Another study, perhaps not as rigorous, concluded:

"Higher speed sampling rate eliminates aliasing errors, increases the accuracy of the average speed and produces data that better represents acceleration. Speed measurement accuracy approaching 0.01 knot becomes possible."

http://mtbest.net/speed_sailing_helmet.html

Witte and Wilson revisited the topic in 2005, this time with precision-fix GPS receivers with WAAS enhancement. They then concluded:

"In conclusion, GPS units employing WAAS/EGNOS technology provide much more accurate position data and display enhanced accuracy of speed determination compared to non-WAAS GPS units."

http://www.rvc.ac.uk/sml/people/documents/WitteJB2005WAASGPS-final.pdf

The speed error in the GPS WAAS test mentioned above was found to be less than 0.2-meters/second for roughly two-thirds of the measurements. An error of 0.2-meters/second is

0.2-meters/second x 2.2369-MPH/meter/second = 0.45-MPH

I think you can say that the speed shown by a GPS receiver with WAAS enhancement when a boat is traveling in a straight line is accurate within about 0.5-MPH most of the time.

There are some anecdotal reports that manufacturers of GPS receivers are claiming that their devices are accurate to 0.1-MPH, but I could not find a direct citation of that claim being made in the OEM literature.

I also found some speculation that consumer-grade GPS receivers may compute speed only based on a two-dimensional fix, so if there is any vertical component to the motion, the speed may be in error. Vertical position calculation seems in general to be less accurate than horizontal. I often watch the altitude reading on my GPS receiver jump around when in fact the receiver is quite stationary. In marine applications, one would hope there was not much vertical motion involved, so speed errors from vertical velocity should not be a problem.

posted 02-10-2009 12:07 AM ET (US)            

So there you have it: 1/2 MPH accuracy.

posted 02-10-2009 09:22 AM ET (US)            

In calculating the speed, it seem clear that the influence of any error in position will be greater at lower speeds, at least in terms of a percentage error. If the speed calculation is based on track points that are only a short distance apart, any error in those positions will have more influence on the calculated speed than if the two track points were, let's say, ten times farther apart. Speed is likely to be more accurate as a percentage error at 30-MPH than at 3-MPH.

Errors in position would have the most impact if they resulted in the incorrect position being exactly on the course line track, where they would either add to subtract from the distance travelled. The errors should tend to be random, and therefore they will likely not be on the track line course, but displaced to one side or the other. Over time the errors probably tend to be distributed in a random distribution about the true position. This means that if a boat holds a straight course line for a reasonable period, the position errors will probably tend to be random and their influence on the speed calculations will tend to cancel out, as long as some sort of averaging of the speed calculation is being performed. This detail is probably unique to each brand and design of GPS receiver. I suspect that the speed averaging, if any, is probably only done over a short period, perhaps over a second or two.

Some newer GPS receivers are able to produce calculations of position and speed at much faster rates than older receivers. Some new GPS receivers are operating at 5-Hz, that is, they produce five position calculations each second. Older receivers are not as fast, perhaps more like 1-Hz. If the speed averaging algorithm uses a fixed period, say five seconds, then a receiver with higher data rates will be averaging more measurements. This may lead to higher accuracy in the speed calculations.

posted 02-10-2009 09:29 AM ET (US)            

http://albums.phanfare.com/4892322/2159339_2459167#imageID=31003240

posted 02-14-2009 10:14 AM ET (US)            

ACCURACY: Speed-- 0.3 knots RMS

Converting to statute miles, this is 0.345-MPH. The notation "RMS" generally is interpreted as "root mean square." RMS is a statistical measure of the magnitude of a varying quantity. You might consider it to be an averaging technique. This is similar to the notation of statistical averaging of Witte and Wilson when they specify the measured speed error being within a particular range for 66-percent of the time.

This figure for speed accuracy (0.345) from a marine GPS receiver manufacturer is in close agreement (-0.105) with the measured accuracy reported in the tests of Witte and Wilson (0.45). This seems to reaffirm the notion that the accuracy of the speed indication on a consumer-grade GPS receiver is in the order of about plus or minus 0.5-MPH most of the time

posted 02-14-2009 02:48 PM ET (US)            

Why do all read high? Because the automakers don't want a
lawsuit from speeding tickets?

posted 02-15-2009 04:49 AM ET (US)            

Do we know from all this referenced material if the modern technology consumer marine GPS units provide the same speed sampling rates as the Navi GT-11 and Velocitek?

Only asking because I can't really figure it out. Thanks.

posted 02-15-2009 09:09 AM ET (US)            

http://www.u-blox.com/products/Data_Sheets/LEA-4x_Data_Sheet(GPS. G4-MS4-06143).pdf

of any sort of advanced feature like a Doppler frequency shift measurement. The sales literature of the manufacturer says:

"The LEA-4A is a low-cost GPS module featuring the new u-blox 16-channel ANTARIS 4 receiver technology...."

I don't see any basis to infer that this GPS receiver is going to be more accurate than the ones tested or rated above, that is, about 0.345 to 0.45-MPH speed accuracy most of the time.

posted 02-15-2009 10:26 AM ET (US)            

--measurement of track point distance and time

--measurement of GPS carrier Doppler frequency shift

The track point method seems to have been the method generally in use with most marine GPS receivers. This method is an indirect measurement of speed. It deduces speed by calculating the distance between position fixes, then dividing that distance by the difference in time at those positions. This method seems to resolve to an accuracy of about 0.45-MPH when the positions are deduced with precision enhancement such as from WAAS or differential GPS (DGPS) ground stations. The time information is highly accurate, but errors in the position will introduce errors in the speed. As we all know, the position data is only accurate to about three meters, so any speed deduced from these positions will have error. As far as I can tell, this is the method used in most marine GPS receivers to compute the boat speed.

The Doppler frequency shift speed calculation is a direct measurement of speed. The L1 carrier frequency of the GPS satellite will appear to be shifted in frequency by the relative motion between the satellite and the receiver. Measurement of this shift is a direct measurement of speed. From what I can tell, speed measurement using this technique is a relatively recent development and may not be directly implemented in the GPS receiver chip itself. It seems that some newer GPS receiver chips can provide data output which gives a measurement of Doppler shift, and from this data, by using additional algorithms and software, a very accurate speed measurement can be obtained.

There is a quite readable discussion of speed measurement using Doppler in

http://www.gps-results.com/GPS_Speed.pdf

which concludes:

"The error margins of handheld-GPS measured speeds can be drastically improved by the use of Doppler-speed-based NMEA-data as compared to the traditionally used position-based evaluations. About one order of magnitude in accuracy can be gained...."

Curiously, much of this interest in more accurate speed measurement seems to be driven by another marine use: wind surfing.

There is software available which can be used to produce enhanced speed data from a GPS receiver which outputs the necessary Doppler information in its NMEA data stream. One such software package is RealSpeed:

http://intellimass.com/RealSpeed/Index.htm

which is available at modest cost (20-euro or €20). I am not certain if this software works on real time data or only on recorded data.

posted 02-15-2009 10:56 AM ET (US)            

http://www.locosystech.com/product.php?zln=en&id=30

This receiver mentions specifically that it provides the Doppler data output necessary for enhanced speed calculation:

"Major features include...support Doppler speed."

Bundled with the GT-31 is software for enhanced speed measurement:

Value-added Software bundled with GT-31

--RealSpeed (from www.intellimass.com): excellent software to get your real speed for your GPS tracks

--GPSResults (from www.gps-speed.com) : powerful software to analyze your windsurfing data and report results to the GPS speeding community

posted 02-15-2009 11:57 AM ET (US)            

"The GT-31 has been specifically developed for speed accuracy. It is light years ahead of every other GPS in this area as well," says Andrew Daff, one of the participants from that forum. He also says, "It is possible that there is another GPS manufacturer that is inspired by the capabilities of the GT-31 to try to build something to match it but I have not heard of it."

posted 02-15-2009 01:48 PM ET (US)            

Use of Doppler techniques in boat speed measurement has been applied to SONAR devices. Furuno manufactures a Doppler speed log which is able to provide very precise measurement of boat speed. This device is often used aboard very large vessels to help judge their speed when making up to a dock or other mooring.

http://www.wmjmarine.com/ds-30.html