Inside GNSS Media & Research

NOV-DEC 2017

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38 Inside GNSS N O V E M B E R / D E C E M B E R 2 0 1 7 www.insidegnss.com Ionosphere-free Residual When scaled by their wavelengths, the carrier phase measurements on differ- ent GNSS frequencies appear to match closely when the scintillation effect is weak or moderate, but diverge from one another when the scintillation effect is strong regardless of whether the dominant scintillation effect is on the phase or amplitude of the signal. It is believed that these divergences occur when diffraction alters the phases by a factor that is not proportional to the wavelengths of their carriers leading to a residual in the ionosphere-free phase combination. An example of this phenomena is the so-called "canonical fade", which may be the cause of the decorrelation events presented here. Figure 7 shows the average absolute L1/ L2 ionosphere-free combination resid- ual from Tromsø (69.5° N) observa- tions, whereas results generated based on the data from Hanoi (21° N) are illustrated in Figures 8 , 9 and 10 . Noting that the range of carrier phase standard deviation considered in Figures 8, 9 and 10 is smaller than that considered in the high latitude plot, it is clear that the level of ionosphere- free residual present in the Hanoi data increases much more rapidly with ris- ing phase standard deviation than was the case with the high latitude observa- tions. While it is not unexpected that the L1/L5 combination residual is also substantial, as indicated in Figure 9, the more interesting observation is that the L2C and L5 signals also have con- siderable levels of decorrelation despite their relatively small 51 megahertz of spectral separation, compared to the nearly 350 megahertz of spectral sepa- ration between L1 and L2. In Figure 10, it is seen that for one of the tracked satellites during this event, the level of ionosphere-free residual in the L2CM/ L5Q combination seems to exceed one meter even while the underlying data shows no signs of cycle slips. Conclusion To summarize, it has been demon- strated that ionospheric scintillation phenomena tend to cause an additional measurement residual in the nominal ionosphere-free combinations that greatly exceeds the expected value of the neglected higher order terms and may be a substantial or dominant nui- sance term in some applications. While the residual is present with both phase and amplitude scintillation, and is more pronounced with strong scintilla- tion to a point, the relationship appears stochastic and not deterministic. It is tempting to assume that con- cerns about ionospheric effects during GNSS SOLUTIONS FIGURE 7 The averaged absolute ionosphere-free combination re- sidual of the L1 and L2 phase measurements on multiple GLONASS satellites during phase scintillation, Tromsø (69.5º N), 14th of November 2012. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Carrier Phase STD (L1) [rad] GLONASS L1/L2 IFree residual [m] 0.06 0.05 0.04 0.03 0.02 0.01 0 FIGURE 8 The averaged absolute ionosphere-free combination residual of the L1CA and L2CM phase measurements on multiple GPS satellites during phase and amplitude scintillation, Hanoi (21° N) 26th of March 2015. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Carrier Phase STD (L1) [rad] GLONASS L1/L2 IFree residual [m] 0.06 0.05 0.04 0.03 0.02 0.01 0 FIGURE 9 The averaged absolute ionosphere-free combination residual of the L1CA and L5Q phase measurements on multiple GPS satellites during phase and amplitude scintillation, Hanoi (21° N) 26th of March 2015. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Carrier Phase STD (L1) [rad] GPS, L1CA/L5Q IFree residual [m] 0.06 0.05 0.04 0.03 0.02 0.01 0 FIGURE 10 The averaged absolute ionosphere-free combination residual of the L2CM and L5Q phase measurements on multiple GPS satellites during phase and amplitude scintillation, Hanoi (21° N) 26th of March 2015. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Carrier Phase STD (L2) [rad] GPS, L2CM/L5Q IFree residual [m] 1.2 1 0.8 0.6 0.4 0.2 0

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