GPS World, May 2009

INNOVATION System Design Test 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 49 48 47 46 45 44 43 42 41 40 C N0 dB Hz GPS Time hours Å FIGURE 1 Carrier power to noise density ratio C N 0 The magenta and green dots represent the PRN135 C N 0 values for L1 and L5 respectively The red and blue dots represent the PRN 138 C N 0 values for L1 and L5 respectively channels of the entire 32 channel configured receiver were assigned to the two WAAS GEOs for L1 and L5 dual frequency tracking and the other 28 channels were used for general GPS L1 and L2 dual frequency tracking With this capability the observation quality of the WAAS L5 signals could be directly compared with the WAAS L1 signals and the simultaneous GPS dual frequency measurements could be used for other purposes such as comparing the differences of the estimated DCBs for GPS and for WAAS Because PRN135 is seen at the low elevation angle of 76 an elevation cutoff angle of 0 was used to collect data from all satellites The collected data set for the continuous four days of August 24 27 2008 has been used to assess the quality of the L1 C1 and L5 C5 code measurements pseudoranges Test Results and Analyses In this section we investigate the overall quality of the WAAS L5 signal by comparing the C N 0 values provided directly by the receiver The computed MP1 and MP5 observables are also compared We then discuss the characteristics of the WAAS GEO satellite DCBs and the possible benefit of using WAAS GEO ranging measurements in the positioning domain Carrier Power to Noise Density Ratio According to the official signal specifications the transmitted signal power of the GPS L5 signal should be 06 dBW higher than that of the L1 C1 signal see also Table 2 To see the differences in the transmitted power of the actual WAAS L5 signal versus the L1 signal the observed C N 0 values of the L1 and L5 signals from both GEOs PRN135 and PRN138 are illustrated in FIGURE 1 In the figure we can first see that the overall C N 0 values for the L1 and L5 signals from both GEOs vary in time in the range of about 1 dB Hz Those variations might be explained by atmospheric effects or actual transmitted power fluctuations By comparing the C N 0 values between PRN138 and PRN135 we can also see that the C N 0 values have clear elevation angle dependence Also the C N 0 values from the higher elevation angle GEO PRN138 have smaller variations in time than those of PRN135 The results illustrated in Figure 1 show that the observed WAAS L5 C N 0 values are comparable with the L1 C N 0 values for both GEOs The compiled statistics of the observed C N 0 values shown in TABLE 3 also show that the C N 0 values of the L1 and L5 signals are comparable with the used equipment Code Multipath and Noise Level Analysis In this sub section the multipath and noise level MP of C1 and C5 codes for both WAAS GEOs are analyzed The MP observables referred to frequencies L1 and L5 were computed by combining code and carrier phase observations in the usual way See Evaluation of the New WAAS L5 Signal in Further Reading for details To see a detailed view of each step of the computations the step by step results are illustrated in FIGURES 2 and 3 These figures show single day results of the computed MP1 and MP5 values for PRN138 on August 25 2008 In the top panels the C1 L1 observable and C5 L5 observable which contain twice the ionospheric delays ambiguity satellite and receiver differential code bias and combined carrier phase and pseudorange MP values are illustrated The compared results show that the noise level of the C1 L1 observable is higher than that of C5 L5 observable By comparing the second and third panels in Figure 3 we can see that the twice ionospheric delay terms which are properly scaled to each observable C1 and C5 could be the main source of the low frequency time variations in the C5 L5 observable After removing the ionospheric term the remaining terms are only the constant ambiguity and slowly varying hardware delays Therefore the MP5 observable is more or less like a constant plus noise even though there exists a certain amount of bias that is caused by the carrier phase ambiguities 10 0 0 6 12 18 24 0 6 12 18 24 40 30 20 0 6 12 18 24 10 0 0 6 12 18 24 Å FIGURE 2 MP1 and related quantities for PRN138 on August 25 2008 The y axis range of all subplots is fixed at 20 meters GPS World May 2009 www gpsworld com 44