70 OEM4 Family Installation and Operation User Manual Rev 12Chapter 6 Positioning Modes of OperationA SBAS-capable receiver will permit anyone within the area of coverage to take advantage of itsbenefits.6.2.2 SBAS Commands and LogsThe command SBASCONTROL, enables the use of the SBAS corrections in the position filter. In orderto 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 theRAWWAASFRAME log.The PSRDIFFSOURCE command sets the station ID value which identifies the base station fromwhich to accept pseudorange corrections. All DGPS types may revert to SBAS, if enabled using theSBASCONTROL command.Consult Volume 2 of this manual for more details on individual SBAS commands and logs.6.3 Pseudorange DifferentialThere are two types of differential positioning algorithms: pseudorange and carrier phase. In both ofthese approaches, the “quality” of the positioning solution generally increases with the number ofsatellites 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 isdefined in such a way that the lower the PDOP, the better the solution. Pseudorange differential is thefocus of this section. Carrier-phase algorithms are discussed in Carrier-Phase Differential on Page76.6.3.1 Pseudorange AlgorithmsPseudorange algorithms correlate the pseudorandom code on the GPS signal received from aparticular satellite, with a version generated within the base station receiver itself. The time delaybetween the two versions, multiplied by the speed of light, yields the pseudorange (so called becauseit contains several errors) between the base station and that particular satellite. The availability of fourpseudoranges allows the base station receiver to compute its position (in three dimensions) and theoffset required to synchronize its clock with GPS system time. The discrepancy between the basestation receiver’s computed position and its known position is due to errors and biases on eachpseudorange. The base station receiver sums these errors and biases for each pseudorange, and thenbroadcasts these corrections to the rover station. The rover receiver applies the corrections to its ownmeasurements; its corrected pseudoranges are then processed in a least-squares algorithm to obtain aposition solution.The “wide correlator” receiver design that predominates in the GPS industry yields accuracies of 3-5m (SEP). NovAtel’s patented Narrow Correlator tracking technology reduces noise and multipath
