In the toy gadget category, there's the new Garmin eTrex 10,20, and 30
GPS Navigator / logger. I recently had a chance to get hands on one of
these to see if I could measure the effect of additional GLONASS
satellites on measurement. While a full analysis would be very
complex, I wanted to apply it to an area I knew to be problematic - a 9%
slope on a curved, heavily wooded trail. Several previous traces with
an eTrex Vista showed that a GPS logger can easily become confused,
resulting in a 20-50 Meter error at times.
http://greenvilleopenmap.info/SlidingRockCompare.png
With fewer leaves on the trees, I carried both units at the same
time, facing up and separated by about 1/2 Meter from each other and my
cell phone. The blue trace is the eTrex Vista, and the green trace is
the eTrex 30. In this single case, the eTrex 30 got 'confused' and had
a 20 Meter difference between the outbound and inbound trip. The traces
move in opposite directions and are separated in time by only about 20
minutes.
This is all very unscientific, but my conclusion is that even with
GLONASS, the basic constraints of position measurement via GPS still
apply. There might be times where additional GLONASS satellites
improve position, but they do not necessarily add accuracy in case of
interference from foliage or buildings.
The additional satellites do consistently reduce the time to acquire
a fix after power up.
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