[sac-forum] Re: GPS Reading

  • From: "Robert Parks" <rparks@xxxxxxxxxxxxxxxxx>
  • To: <sac-forum@xxxxxxxxxxxxx>
  • Date: Thu, 28 Oct 2010 09:18:42 -0700

To really get an accurate position, the satellite constellation should
change positions a large amount.  The surveyors doing a static session
with their high $$ systems, will collect data for a few hours at a
certain spot, then return a week later. By then the entire constellation
of satellites would have shifted around the globe, and a totally new
geometry will be measured.

I have a set of survey grade GPS receivers and equipment, if you need
really accurate readings. They could be used to collect at least 2 hours
of data, that could be processed by the OPUS program on the web.
Accuracy to 5-8 centimeters or better. 

To checl your hand held, look on the Maricopa County Dept of
Transportation website (mcdot survey), to locate the nearest accessible
GDACS station. The location is published so you can place your receiver
on the monument and check its readings.  There is on on the west end of
the Adobe Dam, near 35th Ave & Deer Valley accessible from 43d avenue.
My WAAS capable receiver was about 8 ft HZ and 5 ft vertical, very good

Bob Parks

--- Begin Message ---
  • From: <stanlep@xxxxxxxxxxxxx>
  • To: <sac-forum@xxxxxxxxxxxxx>
  • Date: Wed, 27 Oct 2010 18:40:50 -0700
I hope I haven't asked this question before. If I take 10 GPS readings
(latitude and longitude) of a point location on Earth all within a half
hour time interval, will the average of the 10 readings be an *unbiased*
estimate of the true latitude and longitude? Or does one have to average
over all possible positions of the satellites to get an unbiased estimate
(which still may not be unbiased if all possible satellite positions are
not possible in that location)? I don't know how GPS positions are
calculated and so I do not know if when the GPS unit gives a 9 meter
accuracy for a reading, for example, whether this is unbiased deviation
from the true location.

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