Modern GNSS Signals Research Articles

Data + pilot biases in modern GNSS signals

While traditional GNSS signals are always modulated with navigation data, various modern signals provide a distinct pilot channel without data modulation to support long coherent integration times. Intra-signal biases between the data and pilot components of such signals are evaluated for satellites of the GPS, Galileo, BeiDou-3, and QZSS constellations using measurements from a dedicated set of receivers. Peak values of about 2 ns are obtained for the GPS L5 signal, while slightly smaller values of up to 1 ns apply for the B1C and B2a signals of BeiDou-3 as well as the QZSS L1C signal. For Galileo E1 and E5a/b, data + pilot biases are confined to less than 0.1–0.3 ns, which is typically less than other pseudorange errors for these signals. Fully negligible values of < 0.05 ns are obtained for the L1C and L2C signals of GPS as well as the L2C signal of QZSS in accord with expectations for time-multiplexed or interlaced modulations. To support consistent processing of multi-GNSS data in heterogeneous networks, biases between combined data + pilot tracking and pilot-only tracking are derived in a dedicated zero-baseline receiver test bed. The analysis confirms the general understanding that biases between combined and pilot-only pseudoranges amount to a fixed fraction of the corresponding data + pilot biases. This fraction depends on the power sharing of the data and pilot component in the respective signals and amounts to 50% in most cases. The results of this study are expected to remove the prevailing problem of two distinct receiver groups in the generation of satellite clock and bias products by the International GNSS Service and to enable a rigorous and consistent use of these products by multi-GNSS users.

The Potency of the Modernized GNSS Signals for Real-Time Kinematic Positioning

GNSS positioning has become popular in the past decade as an efficient method of precise and real-time positioning. It is relatively low cost and ease-of-use. Up to now, several parameters were defined to characterize the performance of real-time positioning: availability, precision, accuracy. This article evaluates the performance of signal linear combinations for real-time positioning, both for static as well as the kinematic positioning. This article starts with the investigation of linear combinations (LC) rising from the carrier frequencies of the GNSS systems. Some Linear Combination shows potential benefits in carrier phase integer ambiguity resolution, particularly utilizing the Galileo and Beidou signal phase carrier. For each system, a set of combinations was studied, analyzed, and then selected during the development of GNSS positioning method utilizing the Least-squares Ambiguity Decorrelation Adjustment (LAMBDA). Special signal selection can affect the estimated position and its standard deviation. To further analyze, the results obtained from data processing are compared with respect to baselines and signals. The ambiguity fixing rate is correlated with the baseline length and the method as well as the signals that were used. The analysis of the measurement noise level was first conducted to set a baseline for the real-time GNSS positioning application. According to the results and to assess the data quality and positioning performance of GNSS in respect with GPS (Global Positioning System), an experimental test has established using MGEX data. This research investigates the satellite visibilities, multipath, Signal to Noise Ratio (SNR), and positioning performance. It is shown that in every epoch, at least 8 satellites are visible. The SNR’s are up to 60 dBHz, the code multipath residuals varies within ~1 m, while the phase residuals varies by about ~2 cm. hence the modernized GNSS signals have potencies to improves the RTK positioning.

Amelioration of Acquisition of a Degraded Signal in a GNSS Receiver Embeded in an Observation Satellite by Using Double Bloc Zero Padding Method (DBZP)

In this paper, we study the acquisition of signal in a GPS (Global Positioning System) receiver on board of an observation satellite that receives information on a carrier wave L1 frequency 1575.42 MHz .We simulated GPS signal acquisition. This allowed us to see the behavior of this type of receiver in AWGN channel. Block Zero-Padding (DBZP) acquisition method, known for his efficiency, is deeply analyzed to highlight the acquisition of the modernized GNSS signals can be seriously degraded by the presence of bit sign transitions at each spreading code period. It appears clear that there is a need to use an acquisition method which is bit sign transition insensitive. To tackle this problem an improved and innovative acquisition method, the Double-Block Zero-Padding. Besides this major improvement, other developments are proposed to limit losses on the acquisition performance, in. This paper we will focus on the study of the acquisition of weak signals. The performance study and the results point out the efficiency of this method for the acquisition signals, in comparison with a classical acquisition method which is also data insensitive.

Derivation of the Cramér-Rao Bound in the GNSS-Reflectometry Context for Static, Ground-Based Receivers in Scenarios with Coherent Reflection.

The use of the reflected Global Navigation Satellite Systems’ (GNSS) signals in Earth observation applications, referred to as GNSS reflectometry (GNSS-R), has been already studied for more than two decades. However, the estimation precision that can be achieved by GNSS-R sensors in some particular scenarios is still not fully understood yet. In an effort to partially fill this gap, in this paper, we compute the Cramér–Rao bound (CRB) for the specific case of static ground-based GNSS-R receivers and scenarios where the coherent component of the reflected signal is dominant. We compute the CRB for GNSS signals with different modulations, GPS L1 C/A and GPS L5 I/Q, which use binary phase-shift keying, and Galileo E1 B/C and E5, using the binary offset carrier. The CRB for these signals is evaluated as a function of the receiver bandwidth and different scenario parameters, such as the height of the receiver or the properties of the reflection surface. The CRB computation presented considers observation times of up to several tens of seconds, in which the satellite elevation angle observed changes significantly. Finally, the results obtained show the theoretical benefit of using modern GNSS signals with GNSS-R techniques using long observation times, such as the interference pattern technique.

Stability Criteria for GNSS Receiver Tracking Loops

This paper analyzes the stability criteria for closed loop tracking in a GNSS receiver using Bode analysis techniques. This defines the stable limits of predetection integration time (T) for a given loop filter design, including its order and noise bandwidth (Bn) in combination with all of the other components involved in the closed receiver tracking loop. The loop filter design used in this paper is based on a traditional analog design converted to digital using the bilinear transform. The shortcomings of this traditional design are briefly described, but the traditional design suffices for small T and ample phase margin. Clearly, a pilot channel in modern GNSS signals permits much longer T than allowed by the original GPS data channels, but the stability assurance must be considered in either case. The methodology applies to both data (Costas) and data-less (pilot) tracking loops. Additional insight is provided using the results of this analysis. Copyright © 2014 Institute of Navigation.

Generalised constant‐envelope DualQPSK and AltBOC modulations for modern GNSS signals

Two QPSK-like global navigation satellite system signals with unequal power are multiplexed as a constant-envelope signal analytically. It is further generalised as an extended AltBOC signal that has unbalanced power on the upper and the lower sidebands by the phase rotating technique. The modulations provide additional power control flexibility than the traditional DualQPSK and AltBOC signals and are demonstrated as optimum analytical implementations that maximise the power efficiency.

Acquisition of modern GNSS signals using a modified parallel code-phase search architecture

The acquisition of global navigation satellite system signals can be performed using a fast Fourier transform (FFT). The FFT-based acquisition performs a circular correlation, and is thus sensitive to potential transitions between consecutive periods of the code. Such transitions are not occurring often for the GPS L1 C/A signal because of the low data rate, but very likely for the new GNSS signals having a secondary code. The straightforward solution consists in using two periods of the incoming primary code and using zero-padding for the local code to perform the correlation. However, this solution increases the complexity, and is moreover not efficient since half of the points calculated are discarded. This has led us to research for a more efficient algorithm, which discards less points by calculating several sub-correlations. It is applied to the GPS L5, Galileo E5a, E5b and E1 signals. Considering the radix-2 FFT, the proposed algorithm is more efficient for the L5, E5a and E5b signals, and possibly for the E1 signal. The theoretical number of operations can be reduced by 21%, the processing time measured on a software implementation is reduced by 39%, and the memory resources are almost halved for an FPGA implementation.

Soil Moisture &amp; Snow Properties Determination with GNSS in Alpine Environments: Challenges, Status, and Perspectives

Moisture content in the soil and snow in the alpine environment is an important factor, not only for environmentally oriented research, but also for decision making in agriculture and hazard management. Current observation techniques quantifying soil moisture or characterizing a snow pack often require dedicated instrumentation that measures either at point scale or at very large (satellite pixel) scale. Given the heterogeneity of both snow cover and soil moisture in alpine terrain, observations of the spatial distribution of moisture and snow-cover are lacking at spatial scales relevant for alpine hydrometeorology. This paper provides an overview of the challenges and status of the determination of soil moisture and snow properties in alpine environments. Current measurement techniques and newly proposed ones, based on the reception of reflected Global Navigation Satellite Signals (i.e., GNSS Reflectometry or GNSS-R), or the use of laser scanning are reviewed, and the perspectives offered by these new techniques to fill the current gap in the instrumentation level are discussed. Some key enabling technologies including the availability of modernized GNSS signals and GNSS array beamforming techniques are also considered and discussed.

Joint Determination of Precorrelation Bandwidth, Sampling Frequency and Quantization in Wideband Compass Receivers

Precorrelation filtering, sampling, and quantization are three fundamental functions for GNSS receivers. Analytical model and numerical simulation methods have been developed to evaluate the implementation losses due to different combinations of these parameters, first in the context of white noise and later non-white interference. However, the performance metric was loss of effective carrier-to-noise density ratio (C/N0). This is indirect in predicting the performance degradation of a delay-lock loop (DLL). This paper proposes an analytical model to predict the DLL performance with loss of root-mean-squared (RMS) tracking accuracy in meters. In the current project, a wideband Compass receiver capable of processing modern GNSS signals is required to form the core of the GNSS Vulnerabilities Assessment and Validation Platform (VAVE). The method proposed was used to determine the optimal combination of precorrelation bandwidth, sampling frequency, and quantization to guide the design of VAVE receiver hardware. Here optimality means highest possible performance with minimized implementation cost. Copyright © 2012 Institute of Navigation

Multiplexing Performance Assessment of POCET Method for Compass B1/B3 Signals

Compared with traditional GPS signals, modern GNSS signals are much more complex and have various new modulations. This introduces a difficulty in combining multiple signal components into a constant-envelope signal that maximises the power efficiency of High Power Amplifiers (HPA) on satellites. This paper first describes the fundamental Phase-Optimised Constant-Envelope Transmission (POCET) technique that searches the optimum combining solution for multiple binary navigation signals. Then the Compass B1/B3 signals are modelled by POCET. For the B1 band, a binary complex sub-carrier is adopted to implement the centre frequency difference between regional and global Compass navigation systems. Regional B1 Open Service (OS) signals and global TMBOC signals are combined with optimum loss of 1·0 dB. For the B3 band, Interplex modulation is proved to be the optimum method to combine QPSK (10) and BOC (15, 2·5) signals. Signal quality in the presence of finite word-length effects of Digital-Analog (DA) converters is analysed. Simulations for signal model validation are conducted. The result indicates that relative amplitude error less than 0·01 and angle error less than 0·1 degree can be achieved with 10 bit DA converters. The POCET method is demonstrated as an efficient solution for Compass signals.

Effects of Signal Deformations on Modernized GNSS Signals

Satellite-based navigation requires precise knowledge of the structure of the transmitted signals. For GPS, accurate knowledge of the shape of the code correlation peaks is required to ensure no biases are introduced into the position solution. It is generally presumed that all GPS-like satellite signals are virtually identical. However, in 1993 a satellite malfunction introduced significant distortion onto one of the satellite C/A codes. That distortion caused range errors to vary with receiver filter characteristics and code tracking loop implementation. As a result, high-integrity systems such as the Wide Area Augmentation System (WAAS) must implement signal deformation monitors to detect and remove signals that become anomalously distorted. In the future, WAAS will rely on modernized signals from both GPS (L5) and Galileo (E1/L1/E2 and E5A/E5B). This should increase performance for users; however they must still protect against potential signal deformations. Although the International Civil Aviation Organization (ICAO) has agreed on a threat scenario for GPS L1 signals, no such agreement exists for modernized signals. In addition, each of these signals will have different chipping rates and their correlation peak structures will be quite different from that of the GPS C/A code. Their code tracking loop implantations are as yet not well- defined, but may differ somewhat from traditional architectures. An additional complication is the unknown receiver filter characteristics that the new receivers will employ. Each of these factors may render a given signal and/or receiver configuration more or less sensitive to signal deformations. This paper analyzes the range error sensitivity of several modernized signals subjected to distortions of the type considered in the ICAO threat model for signal deformations. To isolate the effects of the signal-in-space deformation errors, it assumes an ideal, wideband receiver filter and basic early-minus-late code tracking implementations for the new codes. It also compares the distortion-induced range errors for the new codes to those currently modeled for the C/A code. Finally, these results are used to motivate threat model refinements and receiver tracking loop constraints that minimize the affects of this error source for the modernized GNSS signals.