[OSM-talk] Vista eTrex 10,20,30 series

[Mike N]

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.