From: sprent@sol.surv.utas.edu.au (Tony Sprent)
Subject: Re: GPS, LORAN and inertial surveying
Organization: University of Tasmania
Date: Mon, 21 Dec 1992 04:31:49 GMT

In article <1992Dec14.201702.8041@tc.fluke.COM>, swifty@tc.fluke.COM (Steve Swift) writes:
> 
> 
> My father, who has been in the surveying business since 1951, keeps
> asking about the use of GPS, LORAN-C and inertial navigation in
> surveying.  Apparently the county he lives in (Thurston, Washington
> state) has used GPS and inertial navigation contrained to a precise
> control grid (using least-squares fits) to place a sub-grid of 270
> points.  These 270 points cover about 1000 square miles.  Using
> traditional methods (electronic transit) he has measured many of the
> points to be only about 0.01 feet off, or better, in a mile.  He
> wonders how they can be so close when GPS, LORAN-C and inertial are not
> specified that closely.  Anyone know?  How good are these navigation
> methods?
> -- 


In a nutshell, GPS relies on knowing the positions of a minimum of four
satellites and being able to measure the distance to each one. The positions of the
satellites are given in terms of where they will be in geocentric coordinates 
as a function of time. This information is transmitted by each satellite.

The distances are measured in more or less the same way that
Electromagnetic Distance Measuring Equipment (EDM) works except that it is a one way
measurement. Thus time as well as distance are required to be determined.
Hence the need for a minimum of four rather than three satellites. Three for position
(XYZ) and one for time, though in practice more are used. 

The main sources of error are in the uncertainties in the ephemeris, the resolution of 
the distance measurement due to the wavelength of the modulation of the carrier frequency
transmitted by each satellite, the effects due to the fact that the signals pass through
the atmosphere and other unknowns introduced by the US military who are
responsible for the GPS system.

With a single receiver of the type used for navigation, the accuracy of position will
vary between about 20 to 200 metres. If two receivers are used, one at a fixed known
position and the other at a point whos position is required, then the relative position of the
unknown point with respect to the fixed position can be determined to about 2 to 5 metres.
With special receivers commonly referred to as "geodetic receivers", the accuracy of 
position fix can be further refined by analysing the difference in phase between the two 
receivers of the carrier signals of the satellites. By doing this it is possible to obtain
position with a precision of the order of parts per million. Thus for example it is
possible to determine position to a few millimetres in tens or hundreds of kilometres.

These techniques are being used throughout the world now for surveying and I think you
will find that most courses in surveying will be leaning pretty strongly in the direction
of GPS technology. In Australia most of the Universities that run courses in surveying
have special extension courses in GPS for practising surveyors. I would imagine that a 
similar situation would exist in the States.

For information on GPS technology in general, I would suggest that you refer to the
 journal "GPS World" which is published in the States. 

For some time now I have been campaigning for a news group covering GPS. Other than 
sci.engr.civil which sometimes has a posting, the only other that I am aware of is 
comp.infosystems.gis which has a few more postings. There is a need for such a news group.

 ________________________________________________________
| Tony Sprent                                             |
| Department of Surveying and Spatial Information Science | 
| University of Tasmania                                  |     
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