GNSS Software Receiver Research Articles

Airborne GNSS reflectometry for coastal monitoring of sea state

Sea level rise and sea state variability, resulting from climate change and global warming, are critical research areas. However, current techniques for observing and monitoring these phenomena have limitations in terms of spatial and temporal resolution, particularly in dynamic coastal zones. GNSS Reflectometry (GNSS-R) is an emerging bistatic radar-based technique that utilizes the GNSS direct (transmitter-receiver) and reflected (transmitter-reflection point-receiver) signals to extract properties of the reflecting surface. This study explores the potential of airborne GNSS-R as a means to monitor sea state in coastal areas by using the Doppler spread and reflectivity as observables. The paper aims to derive a sea state factor from the reflected signal power and the Doppler shift distribution to analyze its correlation with wind speed and significant wave height data obtained from the ERA5 model. The experiment involved four flights conducted along the coast between Calais and Boulogne-sur-Mer, France, in July 2019. A GNSS software receiver processes the direct and reflected signals, tracking and re-tracking the reflected signals with the aid of a specular reflection model. The resulting in-phase and quadrature components are analyzed in the spectral domain every minute to estimate the power, the surface reflectivity, and the relative Doppler shift. The findings reveal that the sea state factor and Doppler spreading are sensitive to sea state conditions, correlated with the ERA5 parameters, and influenced by the elevation angle of GNSS satellites. At low elevations (E<10°), the sea state factor demonstrates an inverse relationship (anti-correlation) with the wind speed and significant wave height, while the Doppler distribution shows a correlation with these parameters. Both correlations decrease with increasing elevation angle. This research underscores the potential of airborne GNSS-R for monitoring sea state variability in coastal areas enhancing our understanding of the relationships between GNSS-R measurements and sea state parameters.

A Wide Range Distributed Multi-Node Receive Chain Calibration Method Based on GNSS Software Receiver

The phase calibration of distributed multi-node receiving system is required considering the phase synthesis working mode of distributed radar system for echo signals. Whereas, in respect to the traditional calibration tower and unmanned aerial vehicle scheme, there are many disadvantages. The wide range distributed multi-node receive chain calibration method based on GNSS software receiver was proposed in this paper, taking advantage of the software receiver to calculate and obtain the actual phase difference among the receiving nodes as well as the coordinate information of satellites in transmitting signal. Therefore, the phase difference resulted from different transmit path can be deleted, and the phase inconsistencies among the receiving nodes can be obtained. Besides, the fingerprint maps database regarding inconsistencies among nodes and two-dimensional angle of relative receiving node for satellites was constructed in this paper, the correspondence between the omnidirectional two-dimensional angle and the inconsistencies among different nodes was established by the actual results and two-dimensional fitting, and the phase inconsistency results among nodes can be obtained only by solving the satellite coordinate information. There are many advantages for the method proposed in this paper, such as slight influence of multipath effect, capability of all-weather all-day calibration, low costs.

Interference Mitigation for GNSS Receivers Using FFT Excision Filtering Implemented on an FPGA

GNSS receivers process signals with very low received power levels (<−160 dBW) and, therefore GNSS signals are susceptible to interference. Interference mitigation algorithms have become common in GNSS receiver designs in both professional and mass-market applications to combat both unintentional and intentional (jamming) interference. Interference excision filters using fast Fourier transforms (FFTs) have been proposed in the past as a powerful method of interference mitigation. However, the hardware implementations of this algorithm mostly limited their use to military GNSS receivers where greater power and resources were available. Novel implementation of existing FPGA technology should make interference mitigation feasible with limited hardware resources. This paper details the practicalities of implementing excision filters on currently available FPGAs trading off the achievable performance against the required hardware resources. The hardware implementation of the FFT excision mitigation algorithm is validated with the GNSS software receiver. The results indicate that the desired performance of the developed algorithm has achieved the expectations and can provide significant improvement on mitigation techniques in current GNSS receiver hardware. Two hardware implementation designs (fixed-point and float-point data type format) are developed and compared to achieve the optimal design that can provide the best performance (C/No) with the possible minimum hardware resources.

Low-Cost GNSS Software Receiver Performance Assessment

The Software-Defined Receiver (SDR) is a rapidly evolving technology which is a useful tool for researchers and allows users an extreme level customization. The main aim of this work is the assessment of the performance of the combination consisting of the Global Navigation Satellite Systems Software-Defined Receiver (GNSS-SDR), developed by CTTC (Centre Tecnològic de Telecomunicacions de la Catalunya), and a low-cost front-end. GNSS signals were acquired by a Nuand bladeRF x-40 front-end fed by the TOPCON PG-A1 antenna. Particular attention was paid to the study of the clock-steering mechanism due to the low-cost characteristics of the bladeRF x-40 clock. Two different tests were carried out: In the first test, the clock-steering algorithm was activated, while in the second, it was deactivated. The tests were conducted in a highly degraded scenario where the receiver was surrounded by tall buildings. Single-Point and Code Differential positioning were computed. The achieved results show that the steering function guarantees the availability of more solutions, but the DRMS is quite the same in the two tests.

Automated power spectrum analysis of low-latitude ionospheric scintillations recorded using software GNSS receiver

The impact of equatorial plasma bubbles (EPBs) on signals from navigation satellites has remained unpredictable. EPBs could be tracked to some extent with prior knowledge of zonal drift and other important parameters associated with scintillations for a given local time. Aiming for this, we analyze high-rate carrier-to-noise (C/N0) GPS L1 signals recorded using a software GNSS receiver during February–April 2015. In the absence of an automated algorithm in this domain, a new algorithm is developed for estimation of power spectra, Fresnel frequency, noise floor, spectral slope, and ionospheric zonal irregularity drift using per minute scintillation patches. Importantly, since multipath and scintillation can co-occur at lower elevation angles, a new criterion for multipath identification and separation from scintillation and fading is defined as part of the automated algorithm. The C/N0 data above the threshold value of 0.17 of the scintillation index (S4) are only considered by this algorithm. Details of the approach and method are presented with example sets. Results for the estimated zonal drifts, spectral index, and decorrelation length of the post-sunset irregularities are illustrated. Variations in these parameters are presented along with local time, latitude, and longitude of the ionospheric pierce points. The drastic variations that a GPS user and a forecaster would encounter in equatorial latitudes are explained in terms of the challenge they pose. This study is probably the first in terms of automation of the process that shall be highly useful for single-station scintillation observations.

A SIMD intrinsic correlator library for GNSS software receivers

An open-source implementation of a code division multiple access software correlator library that leverages single instruction multiple data (SIMD) is presented. We initially discuss the key aspects involved in the correlation operation for software radio applications. Afterward, we present the state-of-the-art application programming interface that provides SIMD capable methods for each of the components in a correlation operation, including the first of its kind parallelized code and carrier generation using lookup tables and SIMD instructions. The library was developed using SIMD intrinsic instructions, which are a C-type nomenclature offering access to the assembly instructions originally designed for the SIMD extensions in the processor. This design paradigm presents an advantage in terms of readability and simplified code development to accommodate future modifications. Recorded data were used with a standalone global navigation satellite system software receiver where the methods hereby presented were tested and profiled to validate theoretical assumptions.

Deformation monitoring using GNSS-R technology

GNSS reflectometry (GNSS-R) has been widely studied in recent years for various applications, such as soil moisture monitoring, biomass analysis, and sea state monitoring. This paper presents the concept of a novel application of using GNSS-R technology for deformation monitoring. Instead of installing GNSS on the deformation body to sense the movement, GNSS-R deformation monitoring system estimates the deformation from receiving GNSS signal reflected by the deformation body remotely. A prototype of GNSS-R deformation monitoring system has been developed based on GNSS software receiver technology. A 3D geometrical model of GNSS signal reflection has been used to reveal the relationship between the change of carrier phase difference and deformation. After compensating the propagation path delay changes caused by satellite movement, the changes in the remaining carrier phase difference are linked to the deformation. Field tests have been carried using the GNSS-R system developed and the results show sub-centimeter level deformation can be observed with the new technology. Unlike other GNSS deformation monitoring methods, GNSS-R receivers are not installed on the slope which makes this new technology more attractive.

Reflectometry With an Open-Source Software GNSS Receiver: Use Case With Carrier Phase Altimetry

An open-source GNSS software receiver allows to have full access to the signal processing and to make add-ons to the source code in order to obtain the desired GNSS reflectometry processing. The direct signal is processed in the standard way, its tracking loops replica are tapped to have a robust processing of the reflected signal in a master-slave configuration, with the very same carrier replica used to correlate the reflected signal. In addition, the data bit sign is wiped off, which allows to extend the coherent integration time (CIT) well beyond the usual 20-ms limit on the reflected way. This allows having a straightforward and accurate measurement of the Amplitude Ratio and Differential Carrier Phase between the direct and reflected signals. The possible applications are precise carrier phase altimetry and any application requiring signal amplitude ratio, or reflected signal Delay Map, with single or dual polarization, this include code altimetry, humidity, biomass, soil roughness, ocean surface wind and wave height, and snow and ice characteristics retrieval. This software is intended to be used as a research tool. It has been tested for carrier phase altimetry on real data sets collected in rather calm water conditions: at the 60-m Cordouan Lighthouse, and during a 600- m high ATR42 flight over a lake. In both cases, continuous carrier phase measurement with a centimeter level precision was obtained when extending the CIT up to 500 ms. Increasing the CIT beyond 20 ms is the key to improve carrier phase altimetry robustness.

Correlation acceleration in GNSS software receivers using a CUDA-enabled GPU

The correlation process in a GNSS receiver tracking module can be computationally prohibitive if it is executed on a central processing unit (CPU) using single-instruction single-data algorithms. An efficient replacement for a CPU is a graphics processing unit (GPU). A GPU is composed of massive parallel processors with high floating point performance and memory bandwidth. It can be used to accelerate the burdensome correlation process in GNSS software receivers. We propose a novel GPU-based correlator architecture for GNSS software receivers, which is independent of the GPU device, the number of the processing channels, the signal type, and the correlation time. The proposed architecture is implemented and optimized using CUDA, a parallel computing platform and programming model for GPUs. We focus on the following aspects: the design and the time complexity analysis of the proposed GPU-based correlator algorithm, the tests that verify the correctness and the optimization of the implementation, and the performance evaluation of the optimized GPU-based correlator. Moreover, we introduce some new CUDA features that can be applied in a GPU-based correlator.

Applied Technology in "GNSS if Signal Acquisition System" and Software Receiver Design

The paper completed the software and hardware design of GNSS intermediate frequency signal acquisition system under Linux operating system, realized the GNSS software receiver under Linux. Basing on QT C++ graphical user interface application development framework and Qwt plug-in, the software and data graphical display interface was also accomplished. By caching the data of RF module and controlling read and write signals of CY7C68013A through FPGA, the high-speed data transmission function was completed. The test results show that the data transmission rate of this data acquisition system can reach more than 200 Mbps, software receiver satisfies the requirement of positioning accuracy, has good flexibility and higher adaptability, and the algorithm of which was easy to be expanded and upgraded.

ソフトウェアGNSS受信機の開発( GPS/GNSS研究会)

ソフトウェアGNSS受信機の開発( GPS/GNSS研究会)

Self-Contained Antenna Array Calibration using GNSS Signals

Antenna array processing techniques require calibration algorithms that often rely on the availability of signal sources at known locations, a good knowledge of the array manifold, or a reference antenna. An alternative is provided by GNSS signals that provide the location of their sources as part of their navigation data. In this paper, a projection methodology using GNSS signals is proposed for the calibration of antenna arrays. The Gram Schmidt process is used along with the properties of the signal steering vectors to determine linear relationships between the recovered signals and the calibration parameters. The obtained system of equations is then solved in the Minimum Mean Square Error (MMSE) sense leading to the estimated calibration parameters. The proposed algorithm accounts for signal gain/phase mismatches and mutual coupling between array elements. Finally, the effectiveness of the proposed technique and its suitability for beamforming and Direction-of-Arrival (DoA) applications is supported by several experiments performed using live GPS signals and a GNSS software receiver. Copyright © 2012 Institute of Navigation.

Demonstration of a high sensitivity GNSS software receiver for indoor positioning

Advances in signal processing techniques contributed to the significant improvements of GNSS receiver performance in dense multipath environments and created the opportunities for a new category of high-sensitivity GNSS (HS-GNSS) receivers that can provide GNSS location services in indoor environments. The difficulties in improving the availability, reliability, and accuracy of these indoor capable GNSS receivers exceed those of the receivers designed for the most hostile urban canyon environments. The authors of this paper identified the vector tracking schemes, signal propagation statistics, and parallel processing techniques that are critical to a robust HS-GNSS receiver for indoor environments and successfully incorporated them into a fully functional high-sensitivity software receiver. A flexible vector-based receiver architecture is introduced to combine these key indoor signal processing technologies into GSNRx-hs™ – the high sensitivity software navigation receiver developed at the University of Calgary. The resulting receiver can perform multi-mode vector tracking in indoor environment at various levels of location and timing uncertainties. In addition to the obvious improvements in time-to-first-fix (TTFF) and signal sensitivity, the field test results in indoor environments surrounded by wood, glass, and concrete showed that the new techniques effectively improved the performance of indoor GNSS positioning. With fine GNSS timing, the proposed receiver can consistently deliver indoor navigation solution with the horizontal accuracy of 2–15m depending on the satellite geometry and the indoor environments. If only the coarse GNSS timing is available, the horizontal accuracy of the indoor navigation solution from the proposed receiver is around 30m depending on the coarse timing accuracy, the satellite geometry, and the indoor environments. From the preliminary field test results, it has been observed that the signal processing sensitivity is the dominant factor on the availability of the indoor navigation solution, while the GNSS timing accuracy is the dominant factor on the accuracy of the indoor navigation solution.

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

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++ class-based 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/N 0) 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/N 0 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.

GNSS software receiver for the MAGIA (Missione Altimetrica Geofisica GeochImica lunAre) mission

In recent years a number of missions have been conceived to acquire and track signals from GPS satellites at altitudes higher than the GPS constellation itself. The main purpose of SDR is to reduce the number of hardware components, projected for specific unmodifiable uses, and utilize general purpose units: on board CPU, DSP or FPGA. In this paper he study and design of a GNSS Software Receiver for space application, applied to a Lunar Mission, is presented with particular attention on Space Segment device and on Ground Facilities.

China Galileo test range research and status

This article introduces the research and development for the China Galileo Test Range (CGTR) system, which includes indoor test environment (ITE) based on the microwave anechoic chamber, outdoor test environment (OTE) based on the pseudolite network and multimedia demonstration and training facility (MDTF) with multimode GNSS software receiver, application simulating facility, and multimedia platform. This article presents the key technical topics of CGTR and the system status by now. The topics are valuable for GNSS receiver testing, in indoor and outdoor test environment designing, multimode GNSS software receiver designing and GNSS application simulations.

Interoperable GPS, GLONASS and Galileo software receiver

The new and modernized GNSS navigation systems will provide various navigation services and signals for civil user. The system operators work on basic interoperability agreements which simplify multi-system navigation receiver architecture and reduce its cost. The most current GNSS signals of GPS, GLONASS and Galileo can be processed by one or several classical E-L correlators with a look up table PRN generator of a length of 10,230 chips. Most signals are processed optimally, but for some signals, like Galileo E1b and E1c or GPS L1C, this signal processor does not utilize all signal components and properties. The proposed E-L correlator was implemented to the FPGA of the experimental software GNSS receiver and was tested on the Galileo E1b, E1c, and E5a signals. The target multi-system GNSS receiver architecture based on an ExpressCard peripheral card for the standard PC computer or notebook is described herein. Up to one hundred universal correlators and signal snapshot capture unit for DSP acquisition are programmed to the receiver FPGA.

Interoperable GPS, GLONASS and GALILEO software receiver

The new and modernized GNSS navigation systems will provide various navigation services and signals for civil user. The system operators work on basic interoperability agreements which simplify multi-system navigation receiver architecture and reduce its cost. The most current GNSS signals of GPS, GLO- NASS and GALILEO can be processed by one or several classical E-L correlators with a look up table PRN generator of length 10230 chips. Most signals are processed optimally, but for some signals, like Galileo E1b and E1c or GPS L1C, this signal processor does not utilize all signal components and properties. The proposed E-L correlator was implemented to the FPGA of the experimental software GNSS receiver and was tested on the GALILEO E1b, E1c and E5a signals. The target multi-system GNSS receiver architecture based on an ExpressCard peripheral card for the standard PC computer or notebook is described in the paper. Up to hundred universal correlators and also signal snapshot capture unit for DSP acquisition are programmed to the receiver FPGA.

Implement and Analysis of a COMPASS/GPS Software Receiver

A multi-channel, multi-system and multi-frequency GNSS software receiver was developed with C++ code. The software receiver takes digital intermediate frequency (IF) data as its input. It performs acquisition, tracking and navigation solution on a standard PC with C++ programs instead of hardware. It is capable of acquisition and tracking of COMPASS B1C code and GPS L1C/A code. An IF software signal generator was developed to verify functions and performances of the software receiver. This generator can simulate the COMPASS B1(C code) and GPS L1 (C/A code) constellation signals. The outputs of the signal generator were compared with the outputs of the receiver. The results show that there is less error. The correctness of the software receiver was inspected.

Distributed Arithmetic for Efficient Base‐Band Processing in Real‐Time GNSS Software Receivers

The growing market of GNSS capable mobile devices is driving the interest of GNSS software solutions, as they can share many system resources (processor, memory), reducing both the size and the cost of their integration. Indeed, with the increasing performance of modern processors, it becomes now feasible to implement in software a multichannel GNSS receiver operating in real time. However, a major issue with this approach is the large computing resources required for the base‐band processing, in particular for the correlation operations. Therefore, new algorithms need to be developed in order to reduce the overall complexity of the receiver architecture. Towards that aim, this paper first introduces the challenges of the software implementation of a GPS receiver, with a main focus given to the base‐band processing and correlation operations. It then describes the already existing solutions and, from this, introduces a new algorithm based on distributed arithmetic.