Inside GNSS Media & Research

JUL-AUG 2018

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54 Inside GNSS J U L Y / A U G U S T 2 0 1 8 be the same in both cases. e result is a measurement of the range rate defined as: Doppler Shift Measurement e Doppler effect is the change in frequency for an observer (in this case, the GNSS receiver i) moving relative to its source (in this case, a given GNSS satellite k). Equation (6) gives the relationship between observed frequency f i and emitted fre- quency f k : Since the speeds of the receiver v i (t) and the satellite v k are small compared to the speed of light c, the difference between the observed frequency f i and emitted frequency f k can be approximated by en, the Doppler Effect measurement can be written as where r r (t) and v r (t) are the position and velocity of the receiver at the instant t. e term v (s) (t (s) ) − v r (t r ) T is the radial veloc- ity from the receiver relative to the satellite, and and are the receiver and satellite clock dri, respectively. e Doppler Effect measurement is given in Hz. PVT e PVT solution in GNSS-SDR currently uses a module cre- ated based on the RTKLib library. As such, the GLONASS integration to this module was only in charge of providing the necessary conversion tools to translate from the GNSS-SDR modules to the RTKLib input. All conversion parameters were developed following the description of the RTKLib API library (T. Takasu and A. Yasuda). To test the code developed and presented in this work, a GNSS data logger using the NT1065 front end was used. e data logger of this device allowed for multi-frequency, multi- band collection at the same time. For this study, the specific configuration loaded in the device's firmware targeted a data collection for GPS L1 C/A, GLONASS L1 C/A, GLONASS L2 C/A, and GPS L2C, simultaneously. Figure 8 shows the first position solution for GLONASS L1 CA signals ever achieved by GNSS- SDR. e figure shows the position of the receiver in the Earth Centered Earth Fixed (ECEF) coordinate frame with the position covariance for each of the components. e figure also plots the clock variation error, the number of observations used during the position computation, and the esti- mated position across the X and Y components relative to the true antenna position. Finally, the 90% Circular Error Probable (CEP) statistic, as per the description by G. M. Siouris, is shown. Case Study: Performance under RFI A Radio Frequency Interference (RFI) tone was introduced in the data set collected for the GPS L1 C/A signal, simulating a scenario where a common Continuous Wave (CW) PPD device will null the operating band of the signal. Typical RFI devices, such as those described in Table 1, were studied and a pulse mimicking their behavior was generated to null the band. e simulated interference in the band was inserted 60 seconds into the collected data (allowing initial position solution computa- tion) and lasted for about 20 seconds. Figure 9 shows the position solution generated by GNSS- SDR under the presence of a nulling RFI tone. Aer 60 sec- onds of data processing, the position solution stops, resum- ing around 50 seconds later. is indicates the inability of the receiver to compute its position. Also of interest is the fact that after the RFI resumes, the receiver will need to spend time to decode the ephem- eris data before being able to compute a position solution unless some intelligent signal processing technique is applied to reuse the previous decoded ephemeris. If under the assumption of this experiment, a PPD device as in Table 1 is used, then the GLONASS FDMA signals could be used to keep the position solution active, even during the jam- ming period. Results of this scenario are shown in Figure 10 . e receiver, when using a combined solution of GPS L1 C/A and GLONASS L1 C/A, is able to provide position estimates even though the RFI tone is nulling the GPS L1 band. Due to the reduction in the number of observations when the GPS L1 band is nulled, a small performance hit is seen but no loss of the position solution happens, which is the cumbersome mission of the proposal. Conclusion This article presents the f irst-ever position solution of GLONASS L1 C/A in the GNSS-SDR platform. is addition will allow the receiver to take advantage of the FDMA signals available in the radio navigation spectrum and will open a new set of tools for the scientific community to use in the diverse field of GNSS processing. Addition of the GLONASS L1 C/A in a combined position solution is a simple but effective tech- WORKING PAPERS

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