RE: Low cost method to increase the accuracy of GPS data

["Clement, Christopher" <clementc@aecl.ca>]

I caution against using the method of improving GPS data described
below.  We tried something like this with two GPS units, one stationary
and the other moving, both continuously logging location data and time.
After collecting the data, we used the difference between the mean and
instantaneous measured stationary location to correct the measured
moving location data.  The net effect was no obvious improvement.  My
understanding is that true differential corrections correct the data
from each available satellite, not the overall location.

If someone has had some documented success with the method described
below I'd love to hear about it.  If it works, it would be a great way
to improve location precision at very little cost.

------------------------------------------------------------
Christopher H. Clement
Scientific Specialist / Radiation Protection Program Manager
Low-Level Radioactive Waste Management Office
Atomic Energy of Canada Limited
clementc@aecl.ca


> ----------
> From:
> LASHLEYT@detroitedison.com[SMTP:LASHLEYT@detroitedison.com]
> Sent: 	May 4, 1998 10:46 AM
> To: 	Multiple recipients of list
> Subject: 	Low cost method to increase the accuracy of GPS data
> 
>  
> There has been a lot of information presented on RadSafe concerning
> improving 
> the accuracy of GPS data.  Almost all of the suggestions have been to
> purchase 
> additional equipment that range from $1,000 to $10,000.  If you are
> working 
> with a small budget, like myself, and can't afford the additional
> equipment, 
> there is a manual method to improve the accuracy of GPS data.  This
> method
> was 
> used to perform geological surveys (gravity and magnetic) before the
> days of 
> GPS technology.  The only additional equipment that you need to
> purchase is
> a 
> 7.5 minute U. S. Geological Survey topographical map of the area you
> want to 
> survey.  The USGS maps show the locations of USGS benchmarks.  These 
> benchmarks are brass plates attached to cement markers that have the
> latitude, 
> longitude and altitude of that point on the ground.  When you start
> your 
> survey use the USGS map and locate the nearest benchmark to the area
> you
> want 
> to survey.  At the benchmark take a GPS reading (I suggest an average
> of
> three 
> readings) and calculate a correction factor for the latitude and
> longitude
> and 
> note the time.  At this point you can start collecting you survey data
> and 
> recording the time of each data point.  Periodically and at the end of
> your 
> survey return to the benchmark and take another correction reading.
> At the 
> end of the day, determine the equations for the lines that represent
> latitude 
> and longitude correction factors vs. Time and apply these equations to
> you
> GPS 
> survey data.  This technique is not as accurate as real-time
> differential
> GPS, 
> but gives you improved data at a fraction of the cost. 
>  
>  
> ================================ 
> Thomas M. Lashley 
> Radiological Environmental Engineer 
> Fermi 2  Nuclear Power Plant 
> LashleyT@DetroitEdison.com 
> (734)-586-1697
>