(GNSS) Receiver

Implementation of a USRP based Real-Time Global Navigation Satellite System (GNSS) Receiver M. Aamir, M. Hassan Sajjad, Umar Iqbal Bhatti, Salma Zaineb Farooq, Moazzam Maqsood Department of Electrical Engineering Institute of Space Technology Islamabad, Pakistan [email protected], [email protected], [email protected], [email protected], [email protected] Abstract— This paper describes the implementation of a Global Navigation Satellite System (GNSS) receiver based on the Software Defined Radio approach. A software receiver has more flexibility as compared to the conventional hardware receiver since the intermediate signals are available for processing and analysis at each stage. A software receiver can also be configured as a multi-constellation GNSS receiver with slight modifications. The GNSS Software Defined Receiver consists of two parts: The Radio Frequency front end and the signal processing software in a computer. The following implementation uses the Universal Software Radio Peripheral (USRP) as the front end and a GNU Radio based open source software for signal processing. The RF front end has an active Global Positioning System (GPS) antenna with a gain of 30 dB, a custom made Bias-Tee with a loss of 1.17dB to power up the active antenna, a custom made Low Noise Amplifier with a gain of 11 dB to amplify the GNSS signals and USRP-N210. USRP-N210 is paired with an appropriate daughter board WBX for GPS signal reception. USRP acts as a downconverter and a sampler. The operating system used for signal processing is Ubuntu 12.10 (32 bit) on Corei5 System. GNSS satellites Acquisition, Tracking and Navigation Solution computation is performed in the PC by the use of GNU-Radio based open source software. The Position fix for GPS Satellites is obtained in real-time and the signal is analyzed at various stages of signal processing. The position fix obtained is analyzed by importing the KML file into Google Earth. The real time signal acquisition results are analyzed and presented. The software is tested to acquire eight satellites simultaneously in real-time to give very precise position fix. The software can be re-configured to obtain any application specific parameters when needed. Keywords— Global Navigation Satellite System, Universal Software Radio Peripheral, Software Defined Radio, Signal Acquisition, Tracking, Navigation Solution, Phase locked loop, Delay locked loop, Low Noise Amplifier.

I. INTRODUCTION Knowing the position and location has always remained a source keen interest for human beings. In the past human explored star constellations to determine his position. On the advent of industrial revolution compass and other hand held instruments were used to determine the direction and position. With further advancement of technology, humans utilized the ancient concept of stars constellations and developed their own navigation satellites constellation for timing and position information for the whole globe [1].

Up till now industry has made much advancement in communications and navigation fields, but all those solutions were provided on very large scale integrated chips. During the past decade Software Defined Radio (SDR) technology has emerged and gained widespread popularity due to its capabilities like reconfigurability and flexibility. SDR has made it possible to design a single system to perform multiple jobs at multiple frequencies. Development of more than one Global Navigation Satellite Systems (GNSS) and incorporation of more advanced signals for existing navigation systems, raised the need of a reconfigurable receiver to cover all current and future signals and GNSS Systems. SDR solved this problem and provided interoperability between different GNSS systems for more accurate and reliable Position Velocity and Time (PVT) estimates. SDR enables to simulate different scenarios and environment conditions for the development of GNSS receivers for a specific application. GNSS SDR’s are also being used for education and research purposes for the development of new applications. This paper describes the implementation of GNSS SDR for Global Positioning System (GPS) at L1 band. The SDR is implemented on Universal Software Radio peripheral (USRP). A USRP is a general purpose SDR, available in different variants for different application requirements. This implementation uses USRP-N210 with WBX daughter board. It provides 40MHz bandwidth and can be modified in firmware to give 50MHz bandwidth to cover all GNSS signals at L1 band. Custom made Low Noise Amplifier (LNA), custom made Bias-Tee and active GPS antenna are used for GPS signal reception [2], [3]. The paper is organized as follows: section 2 compares the hardware and software based receivers, section 3 describes the components of Radio Frequency (RF) front end and modules of software defined receiver along with signal processing results taken at various stages of signal processing, and finally conclusion is drawn in section 4.

II.

COMPARISON BETWEEN HARDWARE AND SOFTWARE BASED GNSS RECEIVERS

A. Hardware based Receiver Hardware based GNSS receivers are presented in the form of large scale integrated chips. These receivers are power efficient, small in size and require less processing power as compared to SDR, because the number of channels and other parameters are fixed in these receivers. B. Software defined receiver Software defined GNSS receivers have two parts: RF-front end and the signal processing software. RF-front end is the external hardware which is used to capture GNSS Signals. It down-converts the RF signal to IF or Baseband and then after sampling, digital signal is fed to the host system for signal processing. Acquisition, Tracking and Navigation Solution computation are done in the signal processing software. These receivers can be modified according to the application requirements. Generally, GNSS SDR’s are used for simulations of different scenarios and research purposes. C. Advantages of Software Defined Receiver Commercially available GNSS receiver chips or GNSS receivers, in which all the signal processing is done in the hardware chips, are limited in terms of their Doppler frequency search band, the sampling frequency, the Phase Locked Loop (PLL) Noise Bandwidth and the algorithm used to process the incoming GNSS signal. The above parameters cannot be changed in hardware based receivers; the final received output from hardware based receivers is the Navigation Solution only. SDR is a modular based system, in which signal can be analyzed at any processing stage and signal processing algorithms can also be changed. Different scenario behaviors can be simulated and analyzed. Different signal processing parameters can also be changed like Doppler frequency search band, Sampling rate, any filter configuration, PLL Noise Bandwidth, Delay Locked Loop (DLL) Noise Bandwidth, number of Satellites to be tracked and other parameters for a specific algorithm. III. IMPLEMENTATION OF SOFTWARE DEFINED GNSS RECEIVER The Software Defined GNSS receiver consists of hardware and software parts. Hardware part has an active GPS antenna, Bias tee, LNA and USRP N210. Software part consists of GNU radio based open-source libraries[4],[11] patched up in modular form. First module consists of Universal Hardware Driver (UHD) for data reception from USRP-N210, The second module is the Acquisition module for searching satellites in view, The third module is the Tracking module for extraction of Navigation message and the fourth module is the Navigation Solution computation module to compute the position coordinates of the receiver. Software Defined GNSS receiver flow diagram is shown in Fig. 1.

Fig. 2.

Flow Diagram of Software Defined GNSS Receiver

A. Active GPS antenna The Active GPS antenna used is a commercially available Right Hand Circular Polarized GPS car antenna with 30dB gain and 50Ω impedance. Its model number is LCGPS01. [5]. B. Bias-Tee Custom made Bias tee design is based on TCBT-14+ from Mini-Circuits® [10]. PCB was designed on the pcb designing software Diptrace™. The design has only one external component that is 0.01µf capacitor. Bias tee has the insertion loss of 1.17dB and is designed for 50Ω impedance system. Its functional schematic and picture is shown in Fig. 2.

Fig. 3.

Functional Schematic and picture of custom made Bias Tee

C. Low Noise Amplifier Custom made LNA used, is based on BFP640ESD, with a gain of 11dB [6],[7]. Bias-T mentioned in the schematic is for 50Ω impedance system. Its functional schematic and picture are shown in Fig. 3.

Fig. 1.

Functional Schematic and picture of LNA

D. USRP-N210 USRP N210 is a powerful flexible Software Radio Peripheral used to develop and implement SDRs; It has 100MS/s dual ADC, 50Mbps Gigabit Ethernet connection and 2.5ppm TCXO reference clock. This device is used along with WBX daughter board which provides 40MHz bandwidth capability with 50-2200 MHz frequency range. Its noise figure is 5dB [2],[3].

2) Tracking Tracking module gets rough estimates of Doppler frequency and Code Phase from Acquisition module and then refines these parameters for complete removal of Carrier and Coarse Acquisition code to get Navigation Data at baseband. Tracking module keeps lock of the changing code and carrier Doppler shift. The algorithm used is “Phase locked loop plus Delay locked loop tracking” [8],[9]. The Tracking module flow diagram is shown in Fig. 6.

E. Signal Processing Software The Host System used is Corei5 with Ubuntu 12.10. The software consists of open-source libraries based on GNURadio [4],[11]. The algorithms used for each module are as follows: 1) Acquisition Acquisition process gives the rough estimates of Doppler Shift and Code phase of visible satellites, the algorithm used for acquisition of GPS satellites is “Parallel Code Phase Search Acquisition Method” [8],[9]. The Acquisition process flow diagram is shown in Fig. 4.

Fig. 7.

Tracking module flow diagram

The PLL band is set to 50Hz and DLL band is set to 2Hz. The chip spacing for Early, Late and Prompt signals is set to 0.5 chips. Results at various point of tracking algorithm are given below. Fig. 4.

Acquisition Process flow diagram

Software works at a sampling rate of 4Msps; The Doppler frequency search band is set to ±10KHz and Doppler frequency search step size is 250Hz. Acquisition results at point ‘A’ in Fig.4 for PRN32 and PRN18 are shown in Fig. 5.

Fig. 7 shows the Coarse Acquisition (C/A) code frequency variations over time at point ‘B’ in Fig. 6. This variation is due to the Doppler shift. This is the error plus initial bias signal of PRN code generator in code tracking loop (DLL).

Fig. 8.

Fig. 6.

Acquisition results of PRN18 (Right) and PRN32 (Left)

Coarse Acquisition (C/A) code frequency over time

Fig. 8 shows the Doppler shift of carrier signal from point ‘C’ in Fig.6. This is the error signal of the carrier tracking loop (PLL). Numerical control oscillator adds initial bias to it and

Decision of presence of any satellite is made by comparing the highest peak with the second highest peak in Doppler frequency and Code phase search. The distinct peak of PRN18 can be seen in Fig.5 while there is no distinct peak present for PRN32.

Fig. 5.

Carrier frequency doppler shift

generates the exact replica of carrier signal. The PLL plus DLL loop discriminators computes the above given code and carrier Doppler shifts from the output of Early, Late and Prompt correlators. The prompt correlator of in-phase arm gives the Navigation data at base band; the overall procedure’s goal is to maximize the energy in prompt correlator of in-phase arm.

3) Navigation Solution The Navigation Solution module gets navigation data from PLL plus DLL loop and after estimating pseudoranges computes the receiver position in real time [8],[9]. Fig. 11 shows the computed pseudorange of PRN1.

The correlator results for Early, Late and Prompt signals of in-phase arm from point ‘D’ in Fig. 6 are shown in Fig. 9.

Fig. 12.

Pseudorange of PRN1

The decreasing Pseudorange shows that the satellite identified with PRN1 is coming head-on towards the receiver and its elevation angle is increasing. Fig. 9.

Correlation results of Early, Late and Prompt signals

The large amplitude of prompt signal can be seen in Fig. 9 as compared to early and late signals.

Fig. 13 shows the actual position of receiver at Institute of Space Technology with computed coordinate points in Google Earth.

Fig. 10 shows the Navigation message bits from point ‘E’ in Fig. 6. The Navigation message is taken out from Prompt signal of in-phase arm after correlation.

Fig. 13.

Fig. 11.

Navigation Message bits

Fig. 10.

Navigation Solution analysis from “u-center v8.11™”

Receiver Position in Google Earth

Fig. 12 shows the detailed information about the tracked satellites and receiver performance. This analysis was done in GNSS evaluation software by u-blox “u-center™ v8.11”. The Position Dilution of Precision (PDOP) was 6.1m, Vertical

Dilution of Precision (VDOP) was 4.1m and Horizontal Dilution of Precision (HDOP) was 4.6m. Satellite vehicles with PRN IDs PRN9, PRN17, PRN26 and PRN28 were being tracked during the analysis. Maximum C/N0 received was 53dB from PRN9. IV. CONCLUSION This paper presented implementation of software defined GNSS receiver on USRP-N210. GPS signals were received and analyzed. Results at various stages of signal processing can be taken as shown above. The receiver is providing accuracy of 4m-8m. The SDR implementation is working fine and can be utilized for educational and research purposes. The custom made Bias-Tee and LNA designs are also verified as they are producing satisfactory result with this receiver. The real-time intermediate signals were saved while running the software and then the data was plotted and analyzed in Matlab™. The researchers and students can see the effect of changing different software parameters, as described in section 2, on receiver performance in real-time. This will help a lot in understanding the GNSS systems and signal processing algorithms working for GNSS related studies. New algorithms can be added and tested also, in the modular structure of the software receiver. Future work includes the addition of Galileo, Beidou and Glonass systems and interoperability for these systems.

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