GPS L2C Research Articles

Binary Offset Carrier (BOC) and Binary Phase Shift Keying (BPSK) Modulation in Indoor Drones GNSS Recievers using Multipath Error Envelope MEE Technique

The applications of unmanned aerial vehicles (UAVs) have greatly eased the lives of human being. Due to the mass commerial production of UAVs (or Drones) and the support of the ongoing scientific research, it can now be used in disaster monitoring, surviellance, fire conrolling and many outdoor usages. However, the indoor applications have stricter requirements for the autonomous positioning capability of UAVs, requiring its positioning accuracy to be within the narrower areas and aisles. The Global Navigation Satellite System (GNSS) is currently the only guidance and autorecovery method that can be applied directly and consistently to UAV positioning. The current GPS L1C and/or Galileo E1 open services signals are being widely used for this purpose. Moreover, the Binary Offset Carrier (BOC) techniques have been recently adopted in Galileo and GPS BIII as one of the efficient mitigation methods to decrease the interference/multipath delays and to increase both the Position Accuracy and the immunity of GNSS interferences. This research aims to present a software method of assessing the improvements of the Accuracy, and yet the Availability, by producing the MEE for BOC technique. The used methodology is analyzing the theoretical equations behind the interfernece multipath error envelopes in both BPSK and the new BOC signals. The Results show a higher performance of BOC GNSS Recievers over BPSK ones, and less delay.

Global Navigation Satellite System Channel Coding Structures for Rapid Signal Acquisition in Harsh Environmental Conditions

<h3>Abstract</h3> In this article, we present the design of a new navigation message system that includes an error-correcting scheme. This design exploits the “carousel” nature of the broadcast navigation message and facilitates (i) a reduction in the time to first fix (TTFF) and (ii) enhanced error-correcting performance under both favorable and challenging channel conditions. We show here that this combination design requires error-correcting schemes characterized by maximum distance separable (MDS) and full diversity properties. Error-correcting Root low density parity check (Root-LDPC) codes operate efficiently to block various channels and thus can permit efficient and rapid recovery of information over potentially non-ergodic channels. Finally, to ensure appropriate data demodulation in harsh environmental conditions, we propose the use of Root-LDPC codes endowed with a nested property which will permit them to adjust the channel coding rate depending on the number of information blocks received. The proposed error-correcting combination design was then simulated and compared with the well-known GPS L1C subframe 2 using several different transmission scenarios. The results of these simulations revealed some enhancement of the error-correcting performance and reductions in TTFF in several specific situations.

Randomly Flipped Chip based signal power authentication for GNSS civilian signals

The navigation signal authentication schemes protect GNSS services from spoofing attacks by attaching unforgeable information to signals. In this study, Randomly Flipped Chip based Signal Power Authentication (RFC-SPA), is proposed to jointly authenticate the navigation message and spreading code of GNSS civilian signals. Designed to support two modes of authentication (fast channel and slow channel), the proposed scheme employs a fixed amount of flipped chips, which are randomly distributed in some areas of the spreading code to form a relative fluctuation in the correlation result of the signal. The fast channel is a spreading code authentication (SCA) of these flipped chips, which is the same as the Chimera. In the slow channel, since the correlation results between the local periodical spreading code and the signal in areas with flipped chips will be smaller than those results in areas without flipped chips, receivers in the tracking correlation can extract this fluctuation. Since the fluctuation modulates a digital signature, receivers can verify the authenticity of both the navigation message and spreading code through the authentication of this digital signature. Compared with mainstream schemes, RFC-SPA requires no extra tracking channel and has no impact on the navigation message. Besides, the proposed scheme maintains a similar robustness and security performance while significantly reducing the storage requirement compared with SCA. The scheme is demonstrated in a proposed implementation for Beidou B1C civilian signal, and the performance metrics are analysed. In the proposed implementation, the detection possibility is higher than 99.7% at 35dBHz of C/N0, and only 33.6% of the storage is required compared with the suggested configuration of the Chimera scheme for the GPS L1C civilian signal.

Experimental Analysis of GPS L2C Signal Quality under Various Observational Conditions

To appropriately handle the GPS L2C signal for the future development of positioning algorithms, this contribution has endeavoured to figure out its features by computing some quality measures of all the modernised satellites (i.e., twenty-three as of January 2022) and comparing them with those of the C/A and P2(Y) code. A series of quality analyses have been carried out to GPS measurements at twenty-four continuously operating reference stations (CORS) equipped with eight receiver models. In addition, experimental data acquired by a combination of high-end and cost-effective receivers have been intensively assessed to characterise the relative signal quality of the L2C to the legacy codes. The observational conditions considered in this study include different multipath environments and receiver dynamics. The results of the analyses, in general, indicate that the quality of the L2C-derived measurements is equivalent to the legacy civilian code (i.e., C/A) but superior to the encrypted military signal (P2(Y)). Hence, it is expected that the new civilian GPS signal enhances the positioning performance of dual-frequency measurements with both high-end and cost-effective receivers.

Area-Efficient Universal Code Generator for GPS L1C and BDS B1C Signals

Recently, multi-frequency multi-constellation receivers have been actively studied, which are single receivers that process multiple global navigation satellite system (GNSS) signals for high accuracy and reliability. However, in order for a single receiver to process multiple GNSS signals, it requires as many code generators as the number of supported GNSS signals, and this is one of the problems that must be solved in implementing an efficient multi-frequency multi-constellation receiver. This paper proposes an area-efficient universal code generator that can support both GPS L1C signals and BDS B1C signals. The proposed architecture alleviates the area problem by sharing common hardware in a time-multiplex mode without degrading the overall system performance. According to the result of the synthesis using the CMOS 65 nm process, the proposed universal code generator has an area reduced by 98%, 93%, and 60% compared to the previous memory-based universal code generator (MB UCG), the Legendre-generation universal code generator (LG UCG), and the Weil-generation universal code generator (WG UCG), respectively. Furthermore, the proposed generator is applicable to all Legendre sequence-based codes.

Low Complexity Robust Data Demodulation for GNSS.

In this article, we provide closed-form approximations of log-likelihood ratio (LLR) values for direct sequence spread spectrum (DS-SS) systems over three particular scenarios, which are commonly found in the Global Navigation Satellite System (GNSS) environment. Those scenarios are the open sky with smooth variation of the signal-to-noise ratio (SNR), the additive Gaussian interference, and pulsed jamming. In most of the current communications systems, block-wise estimators are considered. However, for some applications such as GNSSs, symbol-wise estimators are available due to the low data rate. Usually, the noise variance is considered either perfectly known or available through symbol-wise estimators, leading to possible mismatched demodulation, which could induce errors in the decoding process. In this contribution, we first derive two closed-form expressions for LLRs in additive white Gaussian and Laplacian noise channels, under noise uncertainty, based on conjugate priors. Then, assuming those cases where the statistical knowledge about the estimation error is characterized by a noise variance following an inverse log-normal distribution, we derive the corresponding closed-form LLR approximations. The relevance of the proposed expressions is investigated in the context of the GPS L1C signal where the clock and ephemeris data (CED) are encoded with low-density parity-check (LDPC) codes. Then, the CED is iteratively decoded based on the belief propagation (BP) algorithm. Simulation results show significant frame error rate (FER) improvement compared to classical approaches not accounting for such uncertainty.

Enhanced GNSS Authentication Based on the Joint CHIMERA/OSNMA Scheme

The authentication of the navigation signals can be considered as the contribution of the system to the robustness against spoofing attacks and it is becoming an important requirement for a growing number of user communities. GPS and Galileo systems are proposing evolutions of their civil signals to embed features of authentication. For Galileo, the Open Service Navigation Message Authentication (OSNMA) is integrated in the Galileo E1 OS signal. For the GPS, the Chips-Message Robust Authentication (Chimera) solution, designed for the GPS L1C signal, is foreseen to be tested soon. On the other hand, suitable signal processing techniques can be implemented inside the receiver to monitor the quality of the received signals and protect against spoofing attacks. Such techniques shall work as a complement to the authentication strategies, to further increase the signals’ robustness. Within this context, the paper presents the Joint Chimera/OSNMA scheme , designed to be adopted by a multi-constellation receiver that already exploits both OSNMA and Chimera enhancements. The idea is to further strengthen the robustness with respect to the individual use of the two solutions, to tackle sophisticated spoofing attacks, which are able to avoid detection from navigation message authentication (NMA) techniques. The manuscript proves the high performance of the joint scheme, presenting the results of a wide bench of tests, under different scenarios of spoofing, and user conditions.

Navigation Message Authentication Based on One-Way Hash Chain to Mitigate Spoofing Attacks for GPS L1

Global Navigation Satellite Systems (GNSS) is vulnerable to significant threats such as spoofing attacks. Data and signal authentication can be used as an effective approach to mitigate such security threats. This paper proposes a Navigation Message Authentication (NMA) method based on Timed Efficient Stream Loss-tolerant Authentication (TESLA), which is using one-way encryption chains. The contributions of this paper are two techniques for implementing TESLA using reserved bits for GPS L1 data. The TESLA algorithm is applied to the extracted navigation message of the GPS L1C/A. Due to the flexibility of the GPS L1C message structure, the protocol is also implemented on Civil Navigation signals (CNAV-2) of GPS L1C. In comparison with Elliptic Curve Digital Signature Algorithm (ECDSA), the Authentication Rate (AR) increases to two minutes for each authentication and performance in term of the Authentication Error Rate (AER) is optimized, by using the TESLA method.

Coherency evaluation of GNSS MBOC pilot and data signal in joint tracking

The MBOC signal uses a modulation in which the pilot and the data signals are separated. However, these modulation methods have evolved a variety of joint receiving algorithms. The paper introduces...

Receiving and Assessing L1C Signal for In-Orbit GPS III and QZSS Transmissions Using a Software-Defined Receiver

To avoid signal interference in L1 frequency and provide various services, GPS has updated a modern signal, called L1C, which has been tested with three QZSS satellites launched in 2017. In December 2018, the first GPS III satellite was launched, which implies improved joint positioning using GPS and QZSS L1C signal. The L1C signal offers a series of advanced designs in signal modulation, message structure and coding. We present complete methodologies for joint L1C signal receiving and processing. For the transmitted signals, we present a methodology and results from collecting and assessing Binary Offset Carrier (BOC) modulation and time-multiplexed BOC (TMBOC) modulation used in the L1C signal. Using the same omnidirectional antenna and test equipment, we collected the L1C signal in Xi’an and Sanya, China, respectively. The experiments in Xi’an verify the joint positioning method to complement the GPS III and QZSS satellite constellations. Our methodology evaluates the ranging difference and positioning error of BOC and TMBOC modulation under the same environment and satellite constellation configuration in Sanya. It is also verified that the joint positioning error is less than the QZSS-only positioning due to the optimization of the satellite constellation.

Copernicus Sentinel–3B – GPS L2C tracking tests during commissioning phase

Abstract The Copernicus Sentinel–3B, the twin satellite of Copernicus Sentinel–3A, was launched on 25 April 2018. During its commissioning phase, the radar altimeter satellite has been flying in tandem with Sentinel–3A, only 30 s apart. This was mainly done for calibration and validation of the instruments on-board Sentinel–3B, maximizing the correlation of measurements taken by both Sentinel–3 satellites. The commissioning phase of Sentinel–3B and thus the tandem flight with Sentinel–3A has also been used to do several tests with the RUAG GPS receivers on-board Sentinel–3B. Future missions like the Copernicus Sentinel–C and –D satellites and the Copernicus Sentinel–6 (Jason-CS) satellite will carry an enhanced RUAG GNSS receiver (PODRIX). The GPS receivers on Sentinel–3B already have the capability to track the GPS L2C signal, which will become important for the future when the number of GPS satellites transmitting the L2C signal increase from currently 19 satellites up to the full GPS constellation. To test the receiver’s L2C tracking performance, the redundant receiver on Sentinel–3B has been running in parallel to the main receiver with L2C tracking enabled, either exclusively or in a mixed P(Y)&L2C configuration. The performance of this new signal is analysed in detail. The Sentinel–3B POD processing chain running at the Copernicus POD Service is used to determine orbits based on the P(Y) signals from the main receiver and based on C/A plus the new L2C signal from the redundant receiver, all of them publicly available via the Copernicus Open Access Hub. The Sentinel–3B POD performance is compared to that of Sentinel–3A and the results of the Sentinel–3B redundant receiver are directly compared to the results of the main receiver. Results are compared in terms of observation metrics, estimated orbit parameters and differences to internal and external orbit products. External orbit validation is done with Satellite Laser Ranging (SLR) measurements to the satellites. Although only 19 GPS satellites are currently broadcasting the L2C signal, the resulting orbits are of equivalent quality as the operational orbit products based on P(Y) data from the full GPS constellation. SLR mean and standard deviation of 0.37 ± 1.32 cm and 0.39 ± 1.43 cm for the main and redundant receiver solutions, respectively, confirm the orbit accuracy independent of the signals used for POD. The percentage of more than five observations per epoch is still very low with approx. 15% considering the current L2C capable GPS constellation. Increase of this percentage is analysed based on a step-wise increase of the L2C capable GPS satellites up to a GPS constellation of 31 satellites.

Land surface characterization using BeiDou signal-to-noise ratio observations

China's BeiDou Navigation Satellite System (BDS) is providing new opportunities for GNSS reflectometry-related applications. We give the first and comprehensive description of the feasibility and potential of using BDS signal-to-noise ratio (SNR) data to characterize land surface in terms of the volumetric soil moisture (VSM), vegetation water content (VWC) and snow depth. BDS SNR-derived interferogram metrics (phase ?, amplitude A, and effective reflector height h) are investigated, and their correlations to the corresponding land surface parameters are established. Data collected from a geodetic-quality BDS/GPS compatible receiver for approximately 300-day period were used to validate the VSM retrieval. Results show that both BDS B1 and B2 frequencies can perform well to reflect the fluctuations of the VSM. Specifically, the B2-derived phase ? exhibits a slightly higher correlation with in situ VSM than that of B1 (R?=?0.83 vs. R?=?0.80), and the B2-derived amplitude A also exhibits a higher correlation with MODIS NDVI than that of B1 (R?=?0.49 vs. R?=?0.53); whilst for snow, the B1 and B2 results indicate qualitative agreement with concurrent in situ snow depth measurements. Furthermore, similar estimation performance can be obtained by comparing the results of BDS B1 and B2 against GPS L2C and L5. Therefore, BDS could be a new and powerful data source with comparable potential as GPS for effectively characterizing high-temporal resolution land surface.

Performance analysis of dual-frequency receiver using combinations of GPS L1, L5, and L2 civil signals

Processing of GNSS signals from more than one frequency band enhances the accuracy and integrity of a position solution in both standalone and differential positioning. The modern GPS program and newly launched GNSS systems such as GALILEO, BeiDou allow civilians to access signals from multiple frequencies in the L-band spectrum. While there are some advantages in triple-frequency processing in carrier phase applications, in general most of the standalone kinematic receivers get benefit from dual-frequency signals for ionosphere error correction. In implementing a dual-frequency receiver, it is necessary to select a combination of frequencies leading to an optimum performance of the existing civilian signals. In the current research work, we have analyzed the performance of dual-frequency receiver in terms of combined signal observation noise, sensitivity and robustness using analytical models by taking the combination of GPS L1, L2C and L5 signals as an example. Further, we have investigated the benefits of common Doppler estimate-based two-frequency signal tracking to reduce the noise in linear combination of observations. Through analytical and experimental results, it is confirmed that the L1/L5 signal combination in GPS system has low observation noise, which is suitable to use in high accuracy and precise positioning applications using standalone dual-frequency receiver. Further, it is shown that common Doppler estimate-based dual-frequency signal tracking has improved receiver tracking loop performance in terms of observation noise and multipath in linear combination of observations and enhanced receiver sensitivity and robustness. In GPS system, L1/L5 signals processed using common Doppler estimate-aided two-frequency signal tracking architecture, it is possible to effectively mitigate ionosphere delay and other receiver observation errors, to achieve less than 1 m position accuracy using unambiguous code phase observations. Proposed analysis is applicable of finding an optimal two-frequency signal combination in multi-frequency GNSS system and suitable signal processing architecture to obtain high accuracy and precise ionosphere-free position solution using code phase observations in standalone dual-frequency receiver.

Performance analysis of BDS-3 B1C and GPS L1C data/pilot component pseudo random noise codes

Abstract The Pseudo Random Noise (PRN) codes is an important part of the GNSS system based on Codes Division Multiple Access (CDMA), and it is the key to channel separation and satellite distinction. The GNSS system excellent performance characteristics of multiple access and spread spectrum are realized by the PRN codes. According to the officially released Interface Control Documents (ICDs), the Beidou global navigation satellite system (BDS-3) B1C and GPS L1C have chosen the Weil codes as PRN codes whose generation is based on the number theoretic method. The Weil codes has the advantages of flexible codes length, good performance and large candidate set, therefore, this paper mainly studies 63 pairs of data/pilot channel PRN codes for BDS-3 B1C and GPS L1C. On the basis of introducing BDS-3 B1C and GPS L1C PRN codes, mainly for the Weil codes generation principle, codes evaluation criteria and intra-system and inter-system odd/even auto-correlation and cross-correlation characteristics are simulated in detail. The experimental results show that the upper limit value, lower limit value, the mean and standard deviation of the odd auto-correlation and cross-correlation of the PRN codes of the BDS-3 B1C signal were better than those of the GPS L1C PRN codes. The performance of odd and even cross-correlation are almost similar between BDS-3 B1C and GPS L1C signal inter-systems, which are slightly worse than the intra-system, and the upper limit value of the inter-system is lower than 2.94 dB in the GPS L1C intra-system.

GNSS Code Multipath Mitigation by Cascading Measurement Monitoring Techniques.

Various measurement monitoring techniques are investigated to mitigate the effect of global navigation satellite systems (GNSS) code multipath through error correction, stochastic weighting of measurements and detection and exclusion (or de-weighting) of affected measurements. Following a comprehensive review of each approach, the paper focuses on detection/exclusion and detection/de-weighting techniques where several single and dual-frequency monitoring metrics are employed in a combination with time-averaging and the M of N detection strategy. A new Geometry-Free (GF) detection metric is proposed given its capability to be combined with a preceding Code-Minus-Carrier (CMC)-based error correction to reduce the number of excluded or de-weighted measurements and thus preserve the measurement geometry. Three geometry-based algorithms, namely measurement subset testing, consecutive exclusion and iterative change of measurement weights are investigated to address multipath scenarios with multiple simultaneously affected measurements. Experimental results are provided using GPS L1, L2C and L5 data collected in multipath environments for static and kinematic scenarios. For GPS L1, the proposed combined method shows more than 38% improvement over a conventional Carrier-to-Noise-density ratio (C/N0)-based Least-Squares (LS) solution in all but deep urban canyons. Lower performance was observed for L2C and L5 frequencies with a limited number of satellites in view.

An Unambiguous Delay-And-Multiply Acquisition Scheme for GPS L1C Signals.

The GPS provides positioning information almost anytime and anywhere on Earth, regardless of the weather conditions, and has become an essential technology for positioning and navigation. As a modernization program, the fourth civil GPS signal, denoted as L1C, will be transmitted from Block III satellites. One distinction of the L1C signal from the former signals is the use of binary offset carrier (BOC) modulation, which is necessary for compatibility and the reduction of interference between the legacy L1 signal and L1C signal, despite their use of the same carrier frequency. One drawback of using BOC modulation is the ambiguity problem, which comes from the multiple peaks in the correlation function and causes difficulties finding the code phase in the acquisition process. In this paper we suggest two delay-and-multiply (DM) methods for the L1C signal to solve the ambiguity problem. For the DM acquisition schemes we suggest the optimal delay time for the delay signal, and prove that the correlation function of the received DM signal and the generated DM signal has a triangular shape, as seen in the legacy GPS L1 signal. The noise characteristics of the decision variable are obtained and the performance of the DM acquisition scheme is given in terms of the probability of detection, and compared with that of the conventional method. We provide the procedure to find the Doppler frequency after obtaining the code phase through the proposed DM method.

Low Computational Signal Acquisition for GNSS Receivers Using a Resampling Strategy and Variable Circular Correlation Time

For the objective of essentially decreasing computational complexity and time consumption of signal acquisition, this paper explores a resampling strategy and variable circular correlation time strategy specific to broadband multi-frequency GNSS receivers. In broadband GNSS receivers, the resampling strategy is established to work on conventional acquisition algorithms by resampling the main lobe of received broadband signals with a much lower frequency. Variable circular correlation time is designed to adapt to different signal strength conditions and thereby increase the operation flexibility of GNSS signal acquisition. The acquisition threshold is defined as the ratio of the highest and second highest correlation results in the search space of carrier frequency and code phase. Moreover, computational complexity of signal acquisition is formulated by amounts of multiplication and summation operations in the acquisition process. Comparative experiments and performance analysis are conducted on four sets of real GPS L2C signals with different sampling frequencies. The results indicate that the resampling strategy can effectively decrease computation and time cost by nearly 90–94% with just slight loss of acquisition sensitivity. With circular correlation time varying from 10 ms to 20 ms, the time cost of signal acquisition has increased by about 2.7–5.6% per millisecond, with most satellites acquired successfully.

A Forward GPS Multipath Simulator Based on the Vegetation Radiative Transfer Equation Model.

Global Navigation Satellite Systems (GNSS) have been widely used in navigation, positioning and timing. Nowadays, the multipath errors may be re-utilized for the remote sensing of geophysical parameters (soil moisture, vegetation and snow depth), i.e., GPS-Multipath Reflectometry (GPS-MR). However, bistatic scattering properties and the relation between GPS observables and geophysical parameters are not clear, e.g., vegetation. In this paper, a new element on bistatic scattering properties of vegetation is incorporated into the traditional GPS-MR model. This new element is the first-order radiative transfer equation model. The new forward GPS multipath simulator is able to explicitly link the vegetation parameters with GPS multipath observables (signal-to-noise-ratio (SNR), code pseudorange and carrier phase observables). The trunk layer and its corresponding scattering mechanisms are ignored since GPS-MR is not suitable for high forest monitoring due to the coherence of direct and reflected signals. Based on this new model, the developed simulator can present how the GPS signals (L1 and L2 carrier frequencies, C/A, P(Y) and L2C modulations) are transmitted (scattered and absorbed) through vegetation medium and received by GPS receivers. Simulation results show that the wheat will decrease the amplitudes of GPS multipath observables (SNR, phase and code), if we increase the vegetation moisture contents or the scatters sizes (stem or leaf). Although the Specular-Ground component dominates the total specular scattering, vegetation covered ground soil moisture has almost no effects on the final multipath signatures. Our simulated results are consistent with previous results for environmental parameter detections by GPS-MR.

Enhanced GNSS Signal Tracking in Fading Environments using Frequency Diversity

A new methodology to enhance the GNSS parameter estimation accuracy in multipath fading environments using the frequency diversity reception is proposed herein. In such environments, fading occurrences observed in different frequency bands are independent of each other. Characteristics of frequency diversity reception using GPS L1 and L2C signals in dense fading conditions are first discussed and compared with those of spatial diversity reception. Comparative analyses of characterization metrics between frequency diversity and spatial diversity signals support the argument that the former is as effective as the latter. A frequency diversity based combined tracking approach is then proposed, and the performance is evaluated using the data collected in dense foliage and residential environments. Results show that frequency diversity based combined tracking results in improved performance compared to that of single frequency tracking. 3D position error reduction achieved by the proposed method is 52% in foliage conditions and 50% in residential environments. Copyright © 2017 Institute of Navigation

Spaceborne GNSS-R from the SMAP Mission: First Assessment of Polarimetric Scatterometry over Land and Cryosphere

This work describes the first global scale assessment of a Global Navigation Satellite Systems Reflectometry (GNSS-R) experiment performed on-board the Soil Moisture Active Passive (SMAP) mission for soil moisture and biomass determination. Scattered GPS L2 signals (1227.6 MHz) were collected by the SMAP’s dual-polarization (Horizontal H and Vertical V) radar receiver and then processed on-ground using a known replica of the GPS L2C code. The scattering properties over land are evaluated using the Signal-to-Noise Ratio (SNR), the Polarimetric Ratio (PR), and the width of the waveforms’ trailing and leading edges. These parameters show sensitivity to the effects of the Earth’s topography and Above Ground Biomass (ABG) even over Amazonian and Boreal forests. These effects are shown to be an important factor in precise soil moisture and biomass determination. Additionally, it is found that PR shows sensitivity to soil moisture content over different land cover types. In particular, the following values of the PR are found over: (a) tropical forests ~−1.2 dB; (b) boreal forests ~0.8 dB; (c) Greenland ~2.8 dB; and (d) the Sahara Desert ~3.2 dB.