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Oxford Technical Solutions
Figure 1. Differential GPS overview
For RTK (Real-Time Kinematic) carrier-phase measurements the principle is the same,
but the GPS in the car also has to figure out the difference in the number of carrier-
phase cycles between the GPS-Base and the car. The GPS-Base measures the carrier-
phase of the signals from each satellite and transmits it to the car.
Differential GPS works in real-time because the corrections from each satellite vary
slowly and predictably. The GPS in the car uses a model to predict the error from each
satellite. It can update its model when the radio link transmits new data. It is not
necessary for the GPS in the car to wait until the radio has transmitted the correction
before it outputs its latest value.
Correction types
The GPS-Base can transmit differential corrections in different formats. The
recommended format for OxTS products is RTCA (GPS-only) or RTCA2 (GPS and
GLONASS). The RTCM V3 format can also be used. Table 2 lists the different formats
supported by the GPS-Base and gives their suitability to other OxTS products.
