10.2  DGPS CONCEPT

DGPS is based on the principle that receivers in the same vicinity will simultaneously

experience common errors on a particular satellite ranging signal.  In general, the user

(mobile) receivers use measurements from the reference receiver to remove the

common errors.  In order to accomplish this, the user (mobile) receivers must

simultaneously use a subset or the same set of satellites as the reference station.   The

DGPS positioning equations are formulated so that the common errors cancel.  The

common errors include signal path delays through the atmosphere, and satellite clock

and ephemeris errors.  For PPS users, the common satellite errors are residual system

errors that are normally present in the PVT solution.  For SPS users, the common

satellite errors also include the intentionally added errors from SA.  Errors that are

unique to each receiver, such as receiver measurement noise and multipath, cannot be

removed without additional recursive processing (by the reference receiver, user

receiver, or both) to provide an averaged, smoothed, or filtered solution.

Various DGPS techniques are employed depending on the accuracy desired, where the

data processing is to be performed, and whether real time results are required.  If real 

time results are required then a data link is also required.  For applications without a

real time requirement, the data can be collected and processed later.  The accuracy

requirements usually dictate which measurements are used and what algorithms are

employed.  Under normal conditions, DGPS accuracy is independent of whether SPS

or PPS is being used, although real time PPS DGPS can have a lower data rate than

SPS DGPS because the rate of change of the nominal system errors is slower than the

rate of change of SA.  However, the user and the Reference Station must be using the

same service (either PPS or SPS).

The clock and frequency biases for a particular satellite will appear the same to all

users since these parameters are unaffected by signal propagation or distance from the

satellite.  The pseudorange and deltarange (Doppler) measurements will be different

for different users, because they will be at different locations and have different relative

velocities with respect to the satellite, but the satellite clock and frequency bias will be

common error components of those measurements.  The signal propagation delay is

truly a common error for receivers in the same location, but as the distance between

receivers increases, this error gradually decorrelates and becomes independent.  The

satellite ephemeris has errors in all three dimensions.  Therefore, part of the error will

appear as a common range error and part will remain a residual ephemeris error. The

residual portion is normally small and its impact remains small for similar observation

angles to the satellite.   

The accepted standard for SPS DGPS was developed by the Radio Technical

Commission for Maritime Services (RTCM) Special Committee 104 (SC 104).  The

RTCM developed standards for the use of differential corrections, and defined the data

format to be used between the reference station and the user.  The stan dards are

primarily intended for real time  operational use and cover a wide range of DGPS

measurement types.  Most SPS DGPS  receivers are compatible with the RTCM SC 104

differential message formats. DGPS  standards have also been developed by the Radio

Technical Commission for Aeronautics  (RTCA) for special Category I precision

approach using ranging code differential. The standards are contained in RTCA

document DO 217.  This document is

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