Inside GNSS Media & Research

JUL-AUG 2018

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Page 52 of 59 J U L Y / A U G U S T 2 0 1 8 Inside GNSS 53 Telemetry Decoding T h e T e l e m - etr y Decoding block decodes t he nav igation message for the signal. Once the signal is prop- e r l y t r a c k e d , t h e Tr a c k i n g block will start to populate the required f ields i n t h e g n s s _ sy ncro objec t, w h i c h s e r v e s as the pipeline b e t w e e n t h e implementation of the blocks in the channel. The symbols popu- lated in gnss_syncro will be used to decode the GNAV message aer the meander sequence has been removed, as shown in Figure 7 . GNAV decoding is a very straight- for wa rd process t hat requ i res careful bookkeeping between the bit position for each of the fields in the string, which is carefully described in its Interface Control Document (ICD) (see Additional Resources). Following the design pattern of GNSS-SDR, the decoded data was divided into four objects, named Glonass_Gnav_Ephemeris, Glonass_Gnav_Navigation_Mes- sage, Glonass_Gnav_Utc_Model, and Glonass_Gnav_Almanac, holding the relationships described in Figure 2. Observables The Observables block generates by default three types of measurements for processing in the navigation solution com- putation. ese measurements include pseudo-ranges, accu- mulated carrier phase, and Doppler frequencies. All of this is generated through a single object called Hybrid_Observables, which computes these measurements in a generic way across all channels. As such, work in this area was minimalistic and only consisted of managing the information flow between the Telemetry Decoder and Observables block. Nevertheless, a proper definition of the measurements as per the GNSS-SDR platform is offered for clarity. Pseudorange Measurements e basic observation equation for the pseudorange as given by K. Borre et alia, assuming that is geometric pseudorange from satellite k to the receiver i, c is speed of light, δt i is receiver clock offset, δt k is satellite clock offset, is tropospheric delay, and is ionospheric delay, is Accumulated Carrier Phase Measurements Following the same definition of pseudorange, the carrier phase can be defined as a measurement of ranges as defined in Equation (4) e last term of the equation represents the initial unknown ambiguity in the cycle number relating to the distance between receiver and satellite. However, when applying a differentia- tion of continuous carrier phase measurements, ambiguities in the number of cycles are eliminated due to the fact that for continuous carrier phase measurements, the ambiguity will FIGURE 5 Generic tracking implementation architecture for GNSS receivers FIGURE 6 Tracking Results for GLONASS Satellite Number 22 in the GNSS-SDR platform Discrete-Time Scatter Plot I prompt –1000 0 1000 Q prompt ×10 6 500 0 –500 Bits of the navigation message 1000 0 –1000 Carrier to Noise Ratio Time (s) 50 100 150 200 250 300 CN0 (dB-Hz) 60 40 20 0 Carrier Freq Time (s) 0 100 200 300 400 Freq (Hz) –2.813 –2.8135 –2.814 –2.8145 –2.145 ×10 5 Code Freq Time (s) 100 200 300 50 50 100 150 200 250 300 Correlation results Time (s) 1500 1000 500 0 50 100 150 200 250 300 150 250 Freq (Hz) 5.109996 5.109995 5.109994 5.109993

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