Satellite-based positioning has been experiencing a rapid evolution in recent years due to the inclusion of two new Global Navigation Satellite Systems (GNSSs), the Chinese BeiDou and the European Galileo. These two new systems are still under development to offer a worldwide anytime positioning service along with the existing giants, the US GPS and the Russian GLONASS. The two existing systems have also been undertaking modernisation programmes due to the reliance of more and more applications to GNSS services, more demanding requirements from users, and the need to mitigate interference and disturbances to the radio signals used by these systems.
Researchers at the Finnish Geodetic Institute are following signals from all the visible BeiDou satellites with their developed software-defined satellite navigation receiver FGI-GSRx. The developed software receiver platform is now capable of receiving signals from all BeiDou satellites (GEO/IGSO/MEO), and it can offer a BeiDou-only navigation fix when at least 4 satellites are in sight. In addition to BeiDou signal reception, the FGI-GSRx can receive signals from GPS and Galileo satellites in L1 and E1 bands, respectively.
BeiDou has a mixed space constellation that has, at the moment, 5 Geostationary Earth Orbit (GEO) satellites, 5 MEO (Medium Earth Orbit) satellites and 5 Inclined Geosynchronous Satellite Orbit (IGSO) satellites. The IGSO and MEO satellites broadcast D1 messages that contain basic navigation information, whereas the GEO satellites broadcast D2 messages that contain basic navigation and augmentation service information (BeiDou integrity, differential and ionospheric grid information). More importantly, the D1 and D2 have different signal structures. For example, the D2 has a data rate of 500 bits per second, whereas the D1 has a data rate of 50 bits per second with anextra tier of Neumann-Hoffman (NH) code modulation resulting in a data rate of 1000 bits per second. This eventually makes the receiver design more challenging as compared to that of the legacy GPS L1 C/A (Coarse/Acquisition) receiver. The use of NH code and the resultant increase in the data bit rate has pros and cons. On the positive side, the NH code can boost the ability of anti-narrowband interference and improve the cross-correlation property of satellite signals and the bit synchronization process; whereas on the negative side, the existence of NH code makes the acquisition and tracking of the modernized GNSS signals more challenging .
Software-defined GNSS receiver
A GNSS software-defined receiver consists of three major components, an RF front-end unit, a signal processing unit, and a navigation processing unit. The RF front-end module is responsible for signal amplification, noise filtering, down-conversion, automatic gain control, and analogue-to-digital conversion. The front-end module converts the received analog data to digital Intermediate Frequency (IF) data at a rate which is several times more than the code chipping rate.
The digital IF data are then processed by a signal processing unit whose main responsibilities include signal acquisition, code and carrier tracking and data demodulation. The demodulated data and the resulting pseudorange measurements are then utilised by a navigation processing unit in order to offer a Position, Velocity and Timing (PVT) solution, along with some other relevant information. The software-defined receiver differs from a conventional receiver in the sense that the functions of the processing and navigation units, including correlation/tracking and navigation tasks, are delivered by software, leading to a more flexible design with potential savings in cost and power. A software-defined GNSS receiver platform, FGI-GSRx, has been developed at the Finnish Geodetic Institute for the analysis and validation of novel algorithms for an optimised GNSS navigation performance. The basic version of FGI-GSRx is based on an open-source software receiver platform , and it has been adapted recently to be BeiDou-compatible with a dual-frequency front-end from Nottingham Scientific Limited (NSL). The NSL front-end, ‘Stereo v2’, is used to capture the BeiDou data.
Front-end, Stereo v2, from Nottingham Scientific Limited
Live Data Collection
For the purpose of demonstrating the capability of the FGI-GSRx with BeiDou signals, BeiDou data was collected on January 31, 2014 at around 9:30 AM UTC time at a static position with a roof antenna at the Finnish Geodetic Institute, Kirkkonummi, Finland.
There are 1 GEO satellite (PRN 05), 3 IGSO satellites (PRNs 7, 9, 10), and 3 MEO (PRNs 11, 13 and 14) satellites available at the moment of data collection. The FGI-GSRx receiver can acquire, track and compute a navigation solution with all the visible satellites.
A novel long coherent acquisition scheme in the presence of NH codes was implemented to FGI-GSRx to acquire the BeiDou satellites .
Acquisition metric for the available BeiDou IGSO and MEO satellites
After the signal acquisition, the main control is handed over to tracking loops to track the variations of carrier phase and code offset due to the line of sight movement between the satellites and the receiver. The main objective of signal tracking is to wipe off the code and the carrier. A conventional narrow correlator Delay Lock Loop (DLL)  is used to synchronise the code phase of the local replica with the incoming signal. The DLL synchronises the code phase of the local replica with the incoming signal. In addition, a Frequency Lock Loop (FLL) assisted Phase Lock Loop (PLL) is used to synchronise the carrier frequency and phase with that of the incoming signal. For more information about GNSS receiver design, please refer to .
Channel tracking status for BeiDou PRN 14
The Carrier-to-Noise density ratio (C/N0) in the receivers is often calculated based on the ratio of the narrow band and the wideband power . If this C/N0 estimation technique is used, the NH code must have to be wiped off before the narrow band power is calculated. Otherwise, the narrow band power calculation will be erroneous due to the presence of bit transition within the 20 ms bit boundary. In the implemented software-defined FGI-GSRx BeiDou receiver, the C/N0 is estimated based on the ratio of the signal’s narrowband power to its wideband power as mentioned in .
The error statistics were computed for a stand-alone code-phase based position solution after applying standard environmental error corrections with the broadcasted parameters from D1 navigation messages. The ionospheric grid parameters transmitted in the D2 messages by the BeiDou GEO satellites (i.e., PRNs 1-5) cannot be utilised in Finland due to its location at high latitude (above 600 N). The position error statistics were computed with respect to the known true position. The horizontal and vertical mean errors for the above dataset were 2.68 and 11.23 meters respectively with a mean PDOP (position dilution of precision) of 2.07.
Google-Earth view of BeiDou-only Navigation Fix with FGI-GSRx software receiver
The developed FGI-GSRx software receiver is a unique platform in Finland for analysing the current and new GNSS signals. Work is currently under progress on developing a multi-GNSS navigation solution with three different GNSS constellations, the GPS, Galileo and BeiDou, by utilising their software receiver platform. The multi-GNSS navigation solution will offer a better availability, accuracy and reliability due to the increase in the number of visible satellites.
This research has been conducted within the projects DETERJAM (Detection analysis, and risk management of satellite navigation jamming) funded by the Scientific Advisory Board for Defence of the Finnish Ministry of Defence and the Finnish Geodetic Institute, Finland and FINCOMPASS, funded by the Finnish Technology Agency TEKES with the Finnish Geodetic Institute, Nokia Corporation, Roger-GPS Ltd., and Vaisala Ltd.
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