User's Manual Part 1

Positioning Modes of Operation Chapter 5
OEMV Family Installation and Operation User Manual Rev 5B 81
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5.2.2 SBAS Commands and Logs
The command SBASCONTROL, enables the use of the SBAS corrections in the position filter. In
order to use this command, first ensure that your receiver is capable of receiving SBAS corrections.
Several SBAS specific logs also exist and are all prefixed by the word WAAS except for the
RAWWAASFRAME log.
The PSRDIFFSOURCE command sets the station ID value which identifies the base station from
which to accept pseudorange corrections. All DGPS types may revert to SBAS, if enabled using the
SBASCONTROL command.
Refer to the OEMV Family Firmware Reference Manual for more details on the SBAS commands and
logs mentioned above.
5.3 Pseudorange Differential
There are two types of differential positioning algorithms: pseudorange and carrier phase. In both of
these approaches, the “quality” of the positioning solution generally increases with the number of
satellites which can be simultaneously viewed by both the base and rover station receivers. As well,
the quality of the positioning solution increases if the distribution of satellites in the sky is favorable;
this distribution is quantified by a figure of merit, the Position Dilution of Precision (PDOP), which is
defined in such a way that the lower the PDOP, the better the solution. Pseudorange differential is the
focus of this section. Carrier-phase algorithms are discussed in Section 5.5, Carrier-Phase Differential
starting on Page 91.
5.3.1 Pseudorange Algorithms
Pseudorange algorithms correlate the pseudorandom code on the GPS signal received from a
particular satellite, with a version generated within the base station receiver itself. The time delay
between the two versions, multiplied by the speed of light, yields the pseudorange (so called because
it contains several errors) between the base station and that particular satellite. The availability of four
pseudoranges allows the base station receiver to compute its position (in three dimensions) and the
offset required to synchronize its clock with GPS system time. The discrepancy between the base
station receiver’s computed position and its known position is due to errors and biases on each
pseudorange. The base station receiver calculates these errors and biases for each pseudorange, and
then broadcasts these corrections to the rover station. The rover receiver applies the corrections to its
own measurements; its corrected pseudoranges are then processed in a least-squares algorithm to
obtain a position solution.
The “wide correlator” receiver design that predominates in the GPS industry yields accuracies of 3-5
m Spherical Error Probable (SEP
1
). NovAtel’s patented PAC technology reduces noise and multipath
interference errors, yielding accuracies of 1 m SEP.
1. SEP: The radius of a sphere, centred at the user’s true location, that contains 50 percent of the
individual three-dimensional position measurements made using a particular navigation
system.