blankblank blank

One Final Look At GPS Positional Accuracy With SA Watch

In previous posts (one, two), I’ve compared GPS positional accuracy for a Garmin 60Cx handheld GPS unit and a Holux M-1000 Bluetooth unit and seem to have found that:

– The Garmin’s average position seem to be better, but the distribution in measured positions is large

– The Holux’s positions have a much tighter distribution, but the average is further off from the true position than the Garmin

Just for the heck of it, I took a look at results from both receivers with one more program that’s been around a while: SA Watch (shareware, $20; 30-day demo evaluation version available). SA stands for Selective Availability, the pre-2000 government policy of degrading GPS signal accuracy to non-military GPS receivers. The idea was that you could run SA Watch for a prolonged period of time, hours if not days, and the effects of SA would get cancelled out leaving you with a reasonably accurate position. SA is now turned off, but SA can still be used to evaluate the stability of a GPS-measured position, and calculate an average. While VisualGPS gives you an arithmetic and least-squares mean, SA Watch lets you calculate a mean weighted by the Dilution Of Precision, a measure of the amount of error in the position that’s due to GPS satellite geometry.

Here’s a plot of 119 positions measured by the Holux M-1000 GPS unit; the position grouping is pretty tight:


And a plot of  412 positions measured over the same time period by the Garmin:


As with previous measurements, the distribution of Garmin positions is far larger than that of the Holux. Based on all this, I’d have to say that a single position from the Holux is likely to be more accurate than a single measurement from a Garmin, even if the Garmin’s average value is better. And for my applications, that’s what really matters; you may have a different requirement. If I get the time, I may do a similar analysis in the future from a different location, and see if the results are comparable.

Looking for something else? Enter some keywords below, then click "Search".    

2 Responses to “One Final Look At GPS Positional Accuracy With SA Watch”

  1. 1 Peter Guth

    The NW-SE trend of positions from your Garmin unit is very similar to what I’ve seen, again mostly with Garmin units. Clearly their spatial distribution is not random.

  2. 2 PMarc


    maybe that is an issue with the DOP accepted for position fixes by the garmin software. It could be that Holux’ software ir stricter in this sense.

    Again, it should be nice to try with a pro receiver and see what different settings yeld in terms of positional accuracy and precision.

Comments are currently closed; feel free to contact me with questions/issues.