A USRP2-based reconfigurable multi-constellation multi-frequency GNSS software receiver front end

Senlin Peng 0 1 2 Yu Morton 0 1 2 0 Y. Morton Department of Electrical and Computer Engineering, Miami University , Oxford, OH 45056, USA 1 S. Peng (&) Department of Electrical and Computer Engineering , Virginia Tech, Blacksburg, VA 24061, USA 2 Dr. Yu Morton is a professor in the Department of Electrical and Computer Engineering at Miami University. She holds a PhD in electrical engineering from the Pennsylvania State University and was a post-doctoral research fellow at the University of Michigan Space Physics Research Laboratory. Her cur- rent research interests are in high-accuracy and high-sensi- tivity GNSS receiver algo- rithms , ionosphere effects on GNSS performance, software- defined UWB radar for navigation, and navigation sensor integration We present a multi-constellation multi-band GNSS software receiver front end based on USRP2, a general purpose radio platform. When integrated with appropriate daughter boards, the USRP2 can be used to collect raw intermediate frequency (IF) data covering the entire GNSS family of signals. In this study, C?? classbased software receiver processing functions were developed to process the IF data for GPS L1, L2C, and L5 and GLONASS L1 and L2 signals collected by the USRP2 front end. The front end performance is evaluated against the outputs of a high end custom front end driven by the same local oscillator and two commercial receivers, all using the same real signal sources. The results show that for GPS signals, the USRP2 front end typically generates carrier-to-noise ratio (C/N0) at 1-3 and 1-2 dB below that of the high end front end and a NovAtel receiver, respectively. For GLONASS signals, the USRP2 C/N0 outputs are comparable to those of a Septentrio receiver. The carrier phase noise from the USRP2 outputs is similar to those of the benchmarking devices. These results demonstrate that the USRP2 is a suitable front end for applications, such as ionosphere scintillation studies. - A typical global navigation satellite systems (GNSS) receiver is composed of three major functional components: radio frequency (RF) front end, receiver signal processing, and navigation signal processing. The RF front end filters and amplifies the input RF signal and downconverts it to an intermediate frequency (IF) before an analog-to-digital converter (ADC) samples the signal for further processing. The RF front end is traditionally implemented in analog circuits. The receiver signal processing unit demodulates the signal to extract range and carrier phase measurements and navigation data messages that will be combined in the navigation signal processing stage to generate position, velocity, and timing solutions. A software-based GNSS receiver performs the demodulation function through software implementations on general purpose processors or FPGAs, while the traditional hardware-based receiver processing implements its functions on application specifics integrated circuits (ASIC). Compared to the hardware-based receivers, a software-based receiver offers more flexibility and allows more complicated algorithm implementations. As a result, software-defined GNSS receivers have gained much attention from both research and development communities in recent years (Akos 1997; Tsui 2004; Morton 2007). The focus is the realization and performance evaluation of a flexible GNSS receiver RF front end using a general purpose universal software radio peripheral (USRP) device for ionosphere scintillation data collection. USRP is a low-IF architecture radio designed to allow general purpose computers or digital signal processors (DSP) to function as high bandwidth communication devices. In recent years, the low-IF architecture has gained much attention due to the demand for integratable and flexible wideband low-cost receiver platforms that enable developers to build a wide range of communication systems with minimum cost and effort. With a maximum sampling frequency of 50 MHz and operating frequencies ranging from DC to 5.9 GHz, a properly configured USRP2 is capable of capturing all L band GNSS signals. Additionally, the device is equipped with a flexible data and control interface through a gigabit Ethernet port, making it ideal for field data collection and remote monitoring applications. The USRP2-based GNSS RF front end is a very attractive option as we enter a new era of satellite-based navigation with the recent GPS modernization that includes L2C, L5, and the planned L1C signals (Braschak et al. 2010), the increasing number of Russians GLONASS satellites and reformed signals (Revnivykh 2010), the emergence of Europeans Galileo (Hein et al. 2005) and Chinas Compass constellations (Cao et al. 2008), and a multitude of regional and spaced-based augmentation systems. The multi-constellation systems offer diverse signal structures over a wide span of frequencies and improve the spatial coverage at nearly every geographical location on the surface and in the near space of the Earth. In addition to enhanced continuity, availability, and integrity of navigation and timing solutions, the system will enable unprecedented scientific research of the dynamic atmosphere on a global scale. The USRP2-based software receiver presented is for the latter purpose. Specifically, we aim to establish an array of GNSS receivers at locations where GNSS signals traversing the ionosphere frequently experience scintillation. Existing deployment of ionosphere scintillation monitoring systems is limited to single-frequency GPS receivers or at most dual-frequency GPS receivers operating at the L1 and L2 bands (Groves et al. 2000; van Dierendonck et al. 1993, 2004; Skone et al. 2008; O Hanlon et al. 2011). The USRP2-based software receiver offers many advantages over these systems. First, GPS satellites have limited coverage at the high-latitude regions where scintillations frequently occur. GNSS satellites such as those in the GLONASS offer more high-latitude coverage and can be used to fill the GPS void (Wang et al. 2011). Second, the sheer number of combined satellites in all available constellations will increase the spatial resolution of the ionosphere tomography derived from a fixed size ground-based GNSS receiver array. Third, no field study has been conducted on the effect of ionosphere scintillation on the new GPS L5, GLONASS, Galileo, and Compass satellite signals. There is an urgent need to gain an understanding of the spatial correlation among the satellites and the frequency correlation among different signals under ionosphere scintillations (Seo et al. 2009, 2011; El-Arini et al. 2009). The USRP2 offers a flexible and reconfigurable platform for these studies. In our first phase of investigation, we have successfully developed and implemented software that controls the data collection system (Peng et al. 2010). A user can specify the USRP2 front end center frequency, receiver sampling frequency, and output data format through a software user interface. A (...truncated)