A Real-Time Software Receiver for the GLONASS L1 Signal

Senlin Peng and Brent M. Ledvina

Abstract:	A GLONASS L1 receiver has been developed using real-time software receiver techniques in conjunction with COTS RF front-end hardware. The purpose of this research is to develop a functional GLONASS receiver that can take advantage of the replenishment of the GLONASS constellation of satellites. The new receiver will be used for engineering and scientific research. The receiver operates on the GLONASS L1 band, of which its lowest frequency is about 22.6 MHz above the GPS L1 band. The GLONASS constellation employs an FDMA (frequency division multiple access) modulation scheme for transmission of the ranging signals. The signals are assigned channel numbers from -7 to +13 (0 inclusive), which correspond to carrier frequency slots. The signal on each channel has a null-to-null bandwidth of 1.022 MHz, and the nominal offset between channels is 562.5 kHz. This leads to the range of center frequencies occupied by these 21 channels being 1598.0625-1609.3125 MHz. Two satellites can transmit on the same carrier frequency if they are in counter positions in an orbital plane. The maximum number of GLONASS satellites is 24, occupying three orbital planes; however this number is expected to increase to 32 in the coming years. The possible received frequency range of the transmitted GLONASS signals is approximately 1597.6-1609.8 MHz, or about 12.2 MHz in bandwidth. However, the current 15 GLONASS satellites on orbit only transmit signals on channels -2 to +7, covering the frequency range of 1600.4-1606.4 MHz, or about 6 MHz in bandwidth. This range of frequencies requires an RF front end with a sampling frequency of at least 12 MHz in order to satisfy the Nyquist criterion. The software receiver developed here makes use of a reconfigurable RF front end called the Universal Software Radio Peripheral (USRP) with a maximum real sampling frequency of 16 MHz. The USRP uses interchangeable daughter boards to down-convert and digitize RF signals in the range of DC to 2.9 GHz, where each daughterboard covers an overlapping subset of this range. This RF front end was chosen for its flexibility and ease of use. The output of the RF front end is 8-bit complex I/Q samples output via a USB cable. The software receiver processing of the RF front-end outputs is accomplished by using bit-wise parallelism, as described in References [1] and [2]. In order to process the incoming RF data in this manner, the 8-bit complex I/Q samples are quantized to 2 bits. This is performed in the software receiver prior to signal correlation. In-phase and quadrature accumulations are computed using bit-wise parallel techniques, and these accumulations are used to drive code tracking delay-lock loops (DLLs) and carrier tracking phase-lock loops (PLLs). The computation of accumulations and the implementation of DLLs and PLLs for the GLONASS PR ranging signals proceed exactly as described in Ref. [1]. The GLONASS PR ranging signal, which is the same for each satellite, is similar to the GPS C/A code signal. The GLONASS PR ranging signal is a m-sequence with 511 chips chipping at a rate of 511 k chips/sec, making the nominal PR ranging signal period 1 ms. Generation of the GLONASS PR ranging signal can be accomplished using shift registers in a fashion similar to how the GPS C/A codes are generated. In terms of signal tracking, The GLONASS accumulation calculations and DLLs use PR ranging code mixing that is standard. Base-band mixing with a PLL or FLL is identical to that of GPS. Additional contributions of the paper will provide a path towards building a GPS+ GLONASS software receiver, by illustrating differences between the two navigation systems. These contributions include descriptions of the transformations between the GLONASS and GPS geodetic coordinate systems and time reference system. GLONASS uses the PZ-90 geodetic coordinate system and a time reference that is synchronized to UTC offset by 3 hours plus a fractional offset. In addition, GLONASS´s relatively low number of maximum satellites on orbit means a 2-D navigation routine must be integrated into the receiver. A 12-channel real-time software receiver has been developed in C and runs on a personal computer. Correlation, acquisition, tracking, algorithms for decoding GLONASS message, and computation of pseudoranges and satellite positions, and the 2-D navigation solver will be discussed.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	2268 - 2279
Cite this article:	Peng, Senlin, Ledvina, Brent M., "A Real-Time Software Receiver for the GLONASS L1 Signal," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 2268-2279.
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