ION GNSS 2009
Session E2: GLONASS Modernization, QZSS, & Other GNSS

Title: Development of a Multi-Antenna Real-Time GPS/Beidou Receiver for Troposphere/Ionosphere Monitoring
Author(s): J-C. Juang, C-T. Tsai, & Y-H. Chen, National Cheng Kung University, Taiwan
Date/Time: Wednesday, September 19, 2012, 4:46 p.m.
Room: Room 102/103/104

In addition to providing accurate PVT service, GNSS signals have also been utilized in science research for probing the earth and atmosphere. Some researchers are investigating the strong earthquakes precursor by variations of the ionospheric total electron content (TEC) observed with GPS signal. In addition, in the Taiwan/US joint science mission FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate), six satellites are equipped with GPS receiver for Radio Occultation (RO) experiment. This mission has successfully provided a significant amount of RO data for weather forecasting. Several such kind of GPS signal applications are developing for more accurate weather or disaster prediction, especially for weather anomaly.
With the emerging of other satellite navigation system, such as Galileo, GLONASS and Compass, more GPS-like signals are available and provide more observation on the above researches. At the beginning of 2012, 10 Beidou satellites have been deployed into service, 1 medium earth orbit (MEO), 5 inclined geosynchronous orbit (IGSO) and 4 geostationary earth orbit (GEO). Its service covers China and parts of Asia-Pacific regional. In Taiwan, the Beidou IGSO and GEO satellites are in the field of view at most time. The high availability of Beidou signals may lead to more GNSS-based observations. Indeed, the geometric relationship between Taiwan and Beidou satellites also provides nice opportunity to observe the severe ionospheric anomaly at low latitude region.

The paper describes the development of a GPS/Beidou receiver. The frequency and signal structure of GPS L1 and Beidou B1 are different. GPS broadcasts L1 signal in 1575.42 MHz with chip rate 1.023MHz. In contrast, Beidou broadcasts B1 signal in 1561.098 MHz with chip rate 2.046MHz. In addition, the signals from Beidou MEO and IGSO are also modulated with secondary code. The goal is to receive signals from all satellites for RO application. As a single antenna cannot track the satellite over a wide range of azimuths. To accounting for weak signal of low elevation or occultation environment, antenna array and digital beam forming approach are adopted in the GPS/Beidou receiver to enhance the signal power.

This array receiver collects the signals by 4 identical multi-band GNSS antennas. The signals captured are first amplified by LNA and then downconverted with common LO with the frequency well decided to arrange the IF frequency. The output IF signals have I/Q components and are digitized by a 12-bit analog to digital converter (ADC). The digital I/Q IF signals are then packaged by the FPGA and send to the processor for signal acquisition and tracking. Considering the requirement of real-time, the correlators of acquisition are implemented with SIMD instructions in assembly language to speed up the execution. The tracking loop is designed to deal with both GPS and Beidou signals, especially considering the secondary code of Beidou. The tracking status is controlled by a state machine.

As the requirement above mentioned, the antenna array beamforming is primarily to enhance the carrier-to-noise ratio of desired signals. The reception pattern is controlled in digital beamforming approaches by weighting and summing the signals of antenna array. Several algorithms are already developed to conduct these functions in different conditions. Some of them optimize certain conditions with known signal structure of the desired signal. Others algorithms do not need prior knowledge of signal structure and minimize output power to certain constraints such as Minimum Variance Distortionless Response (MVDR) and Constrained Least Mean-Squares. The constraints can be set to form a beam in the direction of satellite or suppress the interference. This array receiver adopts the Space-Time Adaptive Processing (STAP) with adaptive MVDR Beamforming algorithm. Its main goal is to adaptively enhance the signals from low elevation and without losing the signals from high elevation. The steering vector associated with the direction of satellite needs to be obtained by carrier phase differences between elements of antenna array. The Minimum Variance Distortion Response (MVDR) algorithm computes the weights iteratively. Then, combine the signal of antenna array and track the signal by either closed loop or open-loop. The physical allocation of antenna elements is also considering to implement the receiver which is capable to receive all GPS/Beidou signals from all sky.

The validation of this proposed array receiver is conducted by receiving real GPS/Beidou signals. The discussion will compare the performance of GPS only, Beidou only and GPS/Beidou combined reception to understand the benefit of new GNSS signal on troposphere/ionosphere monitoring and provide ground based RO and weather forecast observation data.

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