Inside GNSS Media & Research

NOV-DEC 2017

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Page 36 of 67 N O V E M B E R / D E C E M B E R 2 0 1 7 Inside GNSS 37 times to a full L1 carrier cycle of nearly 20 centimeters, well outside of the level which could be plausibly attributed to higher order terms ignored by (4). On close inspection, the data shown in Figure 4 does not appear to contain any stepwise transitions of a magni- tude commensurate with a full or half cycle slip on any of the carriers, mean- ing that this decorrelation is unlikely to be a signal tracking error. Unlike static group delay errors, it is not possible to measure and estimate this error contri- bution a priori. It is effectively an addi- tional noise source present only during scintillation. Since it will influence the magnitude of the residual error in the case of multi/dual frequency process- ing it is interesting to analyze this phenomena and attempt to quantify its expected magnitude by considering the level of correlation between carriers during a cross section of scintillation events affecting modernized civil sig- nals believed to be free of cycle slips. To quantify the correlation level between the scintillation effects on GNSS frequencies, the phase cor- relation coefficient can be calculated for the observed scintillation events according to the following relationship: where the terms δφ 1 and δφ 2 repre- sent epoch to epoch changes in the detrended phases. Figures 5 and 6 show the results for the events observed at 69.5° latitude (Tromsø, Norway) and 21° latitude (Hanoi, Vietnam). In Fig- ure 6 the level of correlation versus the intensity of the phase variation is plot- ted for L1CA vs. L2CM, and in contrast to the high latitude example shown in Figure 5, where increasing phase insta- bility leads to an increasing level of phase correlation between the two car- riers, for the Hanoi data the outcome is entirely different. Indeed, the phase correlation between the two carriers appears to be nearly non-existent on average, as the distribution of correla- tion measures is bifurcated with half the distribution tending towards high- er positive correlation levels, while the other half of the sampled distribution tends towards anti-correlated results. FIGURE 3 Correlated phase variation observed on each of the three civil signals transmitted by the Block II-F satellite PRN30/NORAD 39533 during phase scintillation observed from Hanoi. 64 66 68 70 Time [sec] Correlation of L1, L2, L5 carriers absent fading Decorrelated Phase [cycles] 0.2 0.1 0 –0.1 –0.2 FIGURE 4 During amplitude fading events, the variation of the indi- vidual carriers becomes uncorrelated, (Hanoi, 8th April, 2015.). 88 89 90 91 Time [sec] Decorrelation of L1, L2, L5 carriers during fading Decorrelated Phase [cycles] 1 0.5 0 –0.5 –1 FIGURE 5 Phase correlation between GLONASS L1 and L2 frequencies during high latitude phase scintillation event, based on 0.5 second averaging intervals, GLONASS SVID 59, Tromsø, (69.5º N), 14th of November 2012. 0 0.4 0.8 0.2 0.6 1 1.2 Carrier Phase STD [rad] Tromsǿ, 14.11.2012: GLONASS SVID 59 Phase Scintillaion Correlation: L1 and L2 L1/L2 Correlation Coefficient 1 0.5 0 –0.5 –1 FIGURE 6 Phase correlation between GPS L1CA and L2CM during low latitude phase and amplitude scintillation event, based on 0.5 second averaging intervals, GPS PRN 6, Hanoi, (21o N), 2nd of April 2015. 0 0.4 0.8 0.2 0.6 1 1.2 Carrier Phase STD [rad] Hanoi, 2.04.2015: GPS PRN 6 Phase Scintillaion Correlation: L1CA and L2CM L1/L2 Correlation Coefficient 1 0.5 0 –0.5 –1

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