Inside GNSS Media & Research

JUL-AUG 2018

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52 Inside GNSS J U L Y / A U G U S T 2 0 1 8 www.insidegnss.com D k is the navigation data sequence, f k L1 is the nominal value of the FDMA L1 carrier frequencies, f d is the Doppler frequency seen in the ground, and n is the received noise. e nominal values of the FDMA L1 carrier frequencies are defined by Equation (2): where: k = represents the frequency channel, f 0 L1 = 1602 MHz for the GLONASS L1 band, and ∆f L1 = 562.5 kHz frequency separation between GLONASS car- riers in the L1 band. Since a total of 24 satellites populate the constellation and only 14 frequency channels are available, GLONASS satellites will share some frequency channels but only when in antipodal positions, i.e., satellites in opposite position on Earth. General parameters of this signal are defined in Table 2. Signal Source & Signal Conditioner e Signal Source block hides the complexity of accessing each specific signal source, providing a single interface to a variety of different implementations. Each implementation will target the parsing of data from specific front ends or custom formats in a raw file. It will then transform it to the standard used by the receiver. Using the NT1065 front end (N. C. Shivaramaiah et alia) data was collected across the GLONASS L1 frequency band and minor modifications to the signal source blocks were added to parse the data previously stored in a file. e Signal Conditioner block oversees adapting the sample bit depth to a data type tractable at the host computer running the soware receiver, and optionally intermediate frequency to baseband conversion, resampling, and filtering. Regardless of the selected signal source configuration, this interface delivers a sample data stream to the receiver processing channels, acting as a facade between the signal source and the synchronization channels. Acquisition e role of an Acquisition block is the detection of signals from a given GNSS satellite. As per Equation (1), in the case of a posi- tive detection, it should provide coarse estimations of the code phase (τ) and the Doppler shi (f d ) to initialize the delay and phase tracking loops. Since GLONASS FDMA uses a gold code to detect time delay, acquisition techniques developed for GPS L1 C/A were modified to accommodate GLONASS processing. GLONASS signal acquisition can be seen as that of GPS L1 C/A, but instead of looping over different Pseudo Random Noise (PRN) code values, the code will loop over a single code at dif- ferent frequency channels k. Aer frequency channel removal, the typical Parallel Code Phase Search (PCPS) algorithm (D. Akopian) ( Figure 3 ) can be used for acquisition. Most impor- tantly, this approach reuses previously developed blocks in the platform, which allows for this flexible level of abstraction within the internal soware architecture. Figure 4 shows the acquisition results for a GLONASS sat- ellite in real data collection. e significant peak in the figure indicates a positive signal detection on a frequency channel. Tracking e Tracking block is also receiving the data stream x IN , but does nothing until it receives a "positive acquisition" message from the control pane, along with the coarse estimations τ acq and f dacq . en, its role is to refine such estimations and track their changes along time. ree parameters are relevant for signal tracking: the code phase (τ), Doppler frequency (f d ), and carrier phase (ψ). As with the signal acquisition, GLONASS L1 C/A signal tracking reuses blocks developed for the legacy GPS L1 C/A signal tracking. e main difference to consider is the removal of the frequency channel offsets from IF due to its FDMA prop- erties. Aer carrier removal happens, tracking for GLONASS could be treated as a typical GPS L1 C/A tracking module ( Fig- ure 5 ). Re-usability of existing blocks reduces code complexity and highlights the benefits of flexibility within the platform. Tracking results are shown in Figure 6 . e values for the C/ N 0 , carrier frequency (relative to nominal frequency channel), and code frequency are shown in the bottom, while a discrete time scatter plot showing phase lock with bits of navigation is shown up top. Due to the effect of the meander sequence pres- ent in the GLONASS Navigation Message (GNAV) message (see Additional Resources), bits of navigation need further process- ing before decoding can be applied to the signal. WORKING PAPERS FIGURE 3 Generic PCPS acquisition implementation in GNSS-SDR Incoming signal Output Buffering Local oscillator Circular Shift Gold Code IFFT 1 | 2 FFT FFT 90 deg conj Q r FIGURE 4 Acquisition Results for GLONASS Satellite Number 22 in the GNSS-SDR platform Code Delay (chips) 0 –10000 –5000 0 5000 100 200 300 400 Dopler Shift (Hz) Acquisition metric (µ) ×10 13 18 16 12 10 8 6 4 2 0

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