Precise Orbit Determination of Global Navigation Satellite System of ...

Chapter 9 Software and Simulation Results
situation. In this section, the satellite visibility will be considered in the dynamic orbit determination. The
satellite visibility, however, is a big problem for resolving initial ambiguity. The visibility of a IGSO satellite
located at λ =-10° with eight tracking stations has been shown in Figure 9-5.
From Figure 9-5 it can be seen that except the tracking stations Kourou (5) and Libreville (6), which are located
near the equator, the observations of carrier phase from other tracking stations and IGSO satellites are
discontinuous due to satellite visibility. After re-tracking the satellite at the tracking stations, the new initial
ambiguities must be introduced in the observations. For example, from Figure 9-5, for tracking station 7 and 8,
the first session of continuous observation is only about three hours. It is not enough time for this three-hour
observation session to resolve all initial ambiguities due to the high altitude of the IGSO satellite (refer to §7.2.1,
Chapter 7). Then observations are interrupted because the satellite is not visible from these two tracking stations.
After a six hour interruption, the satellite will be seen again and re-locked, the new initial ambiguities must be
introduced and solved, which will affect the accuracy of satellite orbit determination (see Figure 9-30, Figure 9-
31 and Figure 9-32). But, if the accuracy with centimeter level of orbit determination has been achieved before
observation interruption, the satellite visibility will not be a problem, because the satellite orbit can be precisely
predicted by satellite dynamic models. The new initial ambiguities can be computed according to precise satellite
orbit and the coordinates of ground tracking stations.
For convenience in simulation, the true initial ambiguities will be set according to Table 9-2. When the satellites
are re-tracked, the same true initial ambiguities will be set as before. This assumption will not affect the actual
accuracy of orbit determination. It is only convenient for graphical illustration. In order to save text volume, in
the following figures only the ambiguity solutions from the tracking stations 1 and 5 are given. The observation
from the tracking station 1 was not continuous, whereas tracking station 5 was continuous. The initial
ambiguities solved as float ambiguities are shown as follows respectively.
Ambiguity
Ambiguity
21
20.8
20.6
20.4
20.2
20
19.8
19.6
19.4
19.2
19
101
100.8
100.6
100.4
100.2
100
99.8
99.6
99.4
99.2
99
0 24 48 72 96 120 144 168 192 216 240
Tim e (hour)
Figure 9-30 Float Ambiguity Solution of a IGSO Satellite at λ =-10° at Tracking Station 1
0 24 48 72 96 120 144 168 192 216 240
Tim e (hour)
Figure 9-31 Float Ambiguity Solution of a IGSO Satellite at λ =-10° at Tracking Station 5
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