Geneq SXBlue II Technical Reference Manual page 146

Japanese MT-SAT Satellite-based Augmentation System (MSAS) and the Indian Space
Research Organization's GPS Aided Geo-Augmented Navigation system (GAGAN)
system (still in test mode at the time of print) use this data format. The SXBlue II GPS
receiver is compatible with each of these differential services.
Note – When using a differential correction service, the resultant position may be
referenced to a local datum rather than the current WGS-84 ellipsoid. Please refer
to your signal provider for more information.
Post Processing
Post processing is a method to compute accurate positions in post mission by logging
raw measurement data at the base station and the rover simultaneously. The differential
processing can then be performed later in the office using sophisticated processing
software tools. There are a small variety of methodologies available to the operator,
such as static, rapid static, kinematic, etc.
Describing in detail the various post processing techniques is beyond the scope of this
document, however, generally, post processing is more complicated from a procedural
perspective and requires more skill to successfully operate than real-time systems.
Factors Affecting DGPS Accuracy
Many factors affect the positioning accuracy that a user may expect from a DGPS
system. The most significant of these influences include:
• Proximity of the remote user to the reference station (atmospheric and orbit errors)
• Age of the received differential corrections
• Atmospheric conditions at the beacon and remote user locations
• Satellite constellation geometry, often expressed as a Dilution of Precision (DOP)
• Magnitude of GPS signal multipath present at the remote station
• Quality of the GPS receiver being used at both the reference and remote stations
Proximity of the Reference Station
The distance between a remote user and the reference station can sometimes be
considerable, such as when using 300 kHz DGPS radiobeacons. Consequently, some
of the errors associated with GPS at the base station differ somewhat from those at the
remote user's location. This spatial decorrelation of errors can result in a relative
position offset from the absolute coordinates of the remote receiver. This offset may be
as much as one meter for every 100 km (62 miles) between the base station and remote
receiver.
The causes of decorrelation are:
• GPS satellite orbit errors (significant)
• Ionospheric errors (potential to be most significant depending on level of activity)
• Tropospheric errors (less significant)
SXBlue II GPS Series Technical Reference Manual 131