RE: Accuracy of GPS with respect to surveys

[mephillips12@magnox.co.uk]

     The main problem with differential GPS that we have encountered over
     many radiological surveys is that it works fine in open field sites,
     giving accuracies of a few tens of cm, but becomes more unreliable
     with even quite sparse tree cover or near buildings.

     As an alternative in such areas we use laser ranging using a standard
     surveyor's Automatic Total Station. We have packaged up our kit into a
     smalll FWD vehicle, so that we can readily switch between the DGPS and
     ATS technologies, and "position stamp" high resolution gamma spectra /
     data log count rates in real time.

     I'd be happy to discuss further by direct email.

     Martin

     Martin Phillips
     Plant & Environmental Radiological Measurements
     BNFL Magnox Generation Division
     email: m.e.phillips@magnox.co.uk

     ---------------------------------------
     Date: Sat, 2 May 1998 09:14:30 -0400
     From: "Clement, Christopher" <clementc@aecl.ca> To:
     radsafe@romulus.ehs.uiuc.edu
     Subject: RE: Accuracy of GPS with respect to surveys
     Message-ID: <6D043CD3ABCFD011B47600805F3160BCE753B9@crs27.crl.aecl.ca>
     GPS precision can be greatly improved using several techniques, the
     most common of which is differential correction.  This technique
     relies on receiving GPS information at a stationary, known location.
     In simple terms, the difference between the known and measured
     readings at the stationary location can be applied to the data from
     your moving GPS either in real-time (often via radio link) or later.
     The US Coast Guard operates many differential stations which transmit
     the corrections over radio over most if not all of the US coast, as
     well as near the great lakes and possibly other locations.  Using
     equipment <$1000 we have been able to achieve <10 m precision using US
     Coast Guard signals (which are free to use), and with equipment
     <$10,000 we have been able to achieve <2 m precision, all in real
     time.  For another $10,000 or so you can set up your own differential
     station anywhere.