Global Positioning System Reference
In-Depth Information
data were available are shown in the dashed areas in Figure 10. The temporary break in
reception of the NRTK data can be attributed to the use of GSM signals as the means of
communication between the DVRS centre and the aircraft at the time of the test. However,
the GSM signals were only used for testing purposes. In practice, the problem of breaks in
receiving the network corrections can be significantly alleviated by using more robust
means of delivering NRTK service to the aircraft.
25.27
25.26
25.25
25.24
25.23
25.22
25.21
25.2
25.19
25.18
55.34
55.36
55.38
55.4
Longitude (deg.)
Fig. 9. Trajectory of a fixed-wing aircraft test
When comparing the results of positioning obtained by the DVRS NRTK with the post-
mission double-difference positioning for the periods where NRTK data were received and
phase ambiguities were fixed, the average 2-D and height positioning discrepancies between
the two methods were at a few cm level, as they were 1.6 cm and 2.8 cm respectively. The
differences can mainly be attributed to the model assumptions and procedure in the two
techniques. During the period when phase ambiguities were only solved in a float solution,
the differences were 26.3 cm and 52.5 cm. However, when the DVRS NRTK data were lost,
positioning accuracy deteriorated to the metre level. In this case, supporting methods, such
as good prediction algorithms and integration with other sensors, e.g. a geodetic-grade
inertial system, are needed to cover the short periods when breaks in reception of the
measurement corrections take place. Several methods for prediction of NRTK observation
corrections as a time series were investigated in El-Mowafy, 2008. Different time-series
prediction methods were investigated for different types of errors. The double exponential
smoothing prediction approach performed best in most of the cases when studying the
satellite clock error corrections. Winters' method and the Autoregressive Integrated Moving
Average (ARIMA) model were the best methods for predicting the orbital and wet
tropospheric errors, respectively.
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