closed loop were at the sub-millimeter level for the whole period of the dataset.
ionospheric delay for the same two baselines. The higher ionospheric decorrelation over the longest baseline VTISLINJ can be seen when comparing the two graphs.
Figure 9 shows the double difference L1 and L2 residuals after fixing the network ambiguities, for baselines VTISCLAR and VTIS-LINJ, for the first and second highest elevation satellites, PRN 18 and PRN 21 respectively. The standard deviations of the residuals for both frequencies are summarized in table 2. It can be seen that for the 106 km long baseline VTIS-LINJ, the standard deviations of the L1 and L2 residuals are higher compared to the 72km baseline VTIS-CLAR.
(a)
Table 2: Standard deviations (m) of the double difference L1and L2 residuals Baseline
σ L1
σ L2
VTIS-LINJ VTIS-CLAR
0.017 0.012
0.027 0.022
(b)
(a)
Figure 10: Filtered Zenith Relative Ionospheric Delay for: (a) VTIS-CLAR (72 km) and (b) VTIS-LINJ (106 km).
(b)
To check the efficiency of the estimated ionospheric estimates, the double differenced post fit ionospheric residuals were computed from equation 17, referred as geometry free linear combination ( L4 ) in the literature.
L4 = ∇ΔΦ L1 − ∇ΔΦ L 2 − ∇ΔN L1 + ∇ΔN L 2 − ∇ΔI L1 + ∇ΔI L 2 (17) While figure 11a clearly indicates that the pattern of the time series is similar to the one of the residuals of figure 9a, figure b follows a closely similar pattern as the L1 and L2 residuals of figure 9b. This suggests that for both baselines, the double difference L1 and L2 ionospheric residuals dominate the other error sources present in the post fit observation double difference residuals.
Figure 9: Double Difference L1 and L2 residuals for satellite PRN pair 18-21 for: (a) VTIS-CLAR (72km) and (b) VTIS-LINJ (106km)
Following from equation 12, the relative zenith ionospheric estimates for baselines VTIS-CLAR and VTIS-LINJ are computed for the initial period. After the network ambiguity resolution, the ionospheric estimates are computed on an epoch by epoch basis. Figure 10 presents the time series of the filtered relative zenith
22nd International Meeting of the Satellite Division of The Institute of Navigation, Savannah, GA, September 22-25, 2009
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