Next-generation Global Navigation Satellite Systems Research Articles

CCE-OMBOC: A Simple and Efficient Constant-Envelope Technology for Multicarrier Navigation Modulation by Clipping

Multicarrier navigation modulation is a trend within next-generation global navigation satellite systems (GNSS) aiming to enhance navigation performance, but it forces amplifiers to work in nonsaturation zones, resulting in low power efficiency. This paper presents constant-envelope multiplexing (CEM) based on clipping to overcome the low transmission efficiency of orthogonal multi-binary offset carriers (OMBOCs). The clip constant-envelope OMBOC (CCE-OMBOC) features a hard limit for the original OMBOC signal, and its cross-correlation function (CCF) has a fixed ratio with the CCF of the original OMBOC. Thus, the clipping process has no adverse effect on navigation performance. Additionally, the expression of transmission and multiplexing efficiency is presented according to OMBOC’s amplitude distribution. A low sampling rate is suggested for the CCE-OMBOC, which reduces the cost of signal generation. For OMBOC, the CCE-OMBOC provides multiplexing efficiency comparable to that of constant-envelope multiplexing via intermodulation construction (CEMIC). CCE-OMBOC has a straightforward generation process; in contrast, the complexity of CEMIC rises significantly with increasing subcarriers. Moreover, the CCE-OMBOC is a multicarrier CEM modulation tool that has good tracking performance and excellent compatibility. The greater the number of subcarriers, the more navigation services and the higher the navigation data rate. The CCE-OMBOC can be used in next-generation GNSS and integrated communication and navigation systems.

Investigation of the aberrant record from bus GPS receiver onboard FORMOSAT-7/COSMIC-2 satellite constellation in low Earth orbit

The FORMOSAT-7/COSMIC-2 (F7/C2) satellite constellation, a joint Taiwan-US collaboration mission, was launched on 25 June 2019. Following the FORMOSAT-3/COSMIC, the F7/C2 mission aims to provide global users the next-generation global navigation satellite system (GNSS) radio occultation data. The mission consists of six identical small satellites with an inclination angle of 24°, launched to the parking orbit of 720 km, and then transferred to the mission orbit of 550 km. Each satellite bus is equipped with two commercial off-the-shelf global positioning system receivers manufactured by Surrey Satellite Technology Ltd. for positioning, navigation, and timing functions. This receiver generates “GPS Receiver (bus GPSR) Warm Start Requested (WSR)” messages after commissioning, which could be considered as a kind of single event effect (SEE). More than 7000 WSR events have been recorded in 3 years after launch, providing another opportunity to investigate the SEE with the commercial off-the-shelf bus GPSR. In addition, the F7/C2 mission is the first satellite constellation that allows scientists to investigate the SEE using the same constellation at different altitudes of Low-Earth orbit. This study investigates and discusses the cumulative rate, distribution of WSRs, and their relationships between altitude and geomagnetic field. The statistical method is applied to define a warning area of the high probability occurring the SEE of this kind of component based on the geomagnetic field, whose corresponding strength is around 21,200/20,550 nT and close to the South Atlantic Anomaly area. Moreover, the relationships with space weather, including the disturbance of the Earth's magnetic field (Kp index) and the solar activity (F10.7 index), are also analyzed and reported.

Satellite Availability and Service Performance Evaluation for Next-Generation GNSS, RNSS and LEO Augmentation Constellation

Positioning accuracy is affected by the combined effect of user range errors and the geometric distribution of satellites. Dilution of precision (DOP) is defined as the geometric strength of visible satellites. DOP is calculated based on the satellite broadcast or precise ephemerides. However, because the modernization program of next-generation navigation satellite systems is still under construction, there is a lack of real ephemerides to assess the performance of next-generation constellations. Without requiring real ephemerides, we describe a method to estimate satellite visibility and DOP. The improvement of four next-generation Global Navigation Satellite Systems (four-GNSS-NG), compared to the navigation constellations that are currently in operation (four-GNSS), is statistically analyzed. The augmentation of the full constellation the Quasi-Zenith Satellite System (7-QZSS) and the Navigation with Indian Constellation (11-NavIC) for regional users and the low Earth orbit (LEO) constellation enhancing four-GNSS performance are also analyzed based on this method. The results indicate that the average number visible satellites of the four-GNSS-NG will reach 44.86, and the average geometry DOP (GDOP) will be 1.19, which is an improvement of 17.3% and 7.8%, respectively. With the augmentation of the 120-satellite mixed-orbit LEO constellation, the multi-GNSS visible satellites will increase by 5 to 8 at all latitudes, while the GDOP will be reduced by 6.2% on average. Adding 7-QZSS and 11-NavIC to the four-GNSS-NG, 37.51 to 71.58 satellites are available on global scales. The average position DOP (PDOP), horizontal DOP (HDOP), vertical DOP (VDOP), and time DOP (TDOP) are reduced to 0.82, 0.46, 0.67 and 0.44, respectively.

Design of a Compact, Multifrequency, Multiconstellation GNSS Precise Point Positioning Correction Format

High-accuracy, high-precision global navigation satellite systems (GNSSs) have been utilized by specialized user markets for over two decades. Traditional high-accuracy GNSS systems are not scalable as they require local infrastructure. Precise point positioning (PPP) is a GNSS position estimation technique that allows positioning and navigation with centimetre-level accuracy without local infrastructure. The caveat of PPP is the timely transmission of corrections to errors in the GNSS signals. Next-generation GNSS satellites such as Galileo, BeiDou, QZSS, and other local systems have plans to broadcast corrections for PPP services broadcast corrections for PPP services. However, since these services are expected to have limited channel capacity, the development of efficient formats for PPP corrections is needed. In this article, a messaging strategy for PPP corrections and testing its performance, in terms of data rate and obtained positioning accuracy, is proposed. Message formats in the literature require 522 bps or more to transmit corrections for 70 satellites. By contrast, the message format proposed in this article can transmit information corrections for more than 70 satellites using 52% of the standard satellite-based augmentation system channel's 212 bps capacity, while incurring less than 1 cm of positioning accuracy degradation.

FH-BOC: generalized low-ambiguity anti-interference spread spectrum modulation based on frequency-hopping binary offset carrier

ObjectiveThe paper aims to present a generalized modulation scheme that can improve the anti-interference performance of global navigation satellite systems (GNSS) and mitigate the ambiguity problem in BOC modulation.Summary background dataWith the exponential growth of location-based services, there is a need to improve the positioning accuracy and the capability to resist against external interference in challenging environments, such as urban canyons, forested terrains, and indoor areas, in which signal attenuation, interference, and multipath fading can seriously degrade the positioning accuracy of global navigation satellite systems (GNSS) and GNSS-like systems. The binary offset carrier (BOC) modulation has been adopted in GNSSs because of its good spectral isolation from heritage signals, high accuracy, multipath interference resistance, and flexibility in signal implementation compared with BPSK-R modulation. However, for high-order BOC modulation, the main drawback is the ambiguity in tracking due to the multiple side peaks of the autocorrelation function (ACF). The receiver may incorrectly lock onto one of these side peaks, causing intolerable measurement bias, and this undesirable behavior limits the application of this modulation scheme in navigation systems.MethodsWe present a generalized low-ambiguity anti-interference spread spectrum modulation based on frequency-hopping BOC (FH-BOC). First, we formulate the mathematical model of FH-BOC modulation and derive the analytical expressions for the normalized ACF and PSD, and we analyze the time and frequency properties of several representative FH-BOC signals. Next, we present recommended parameter selections, a generation and detection scheme for FH-BOC modulation. Finally, we analyze the characteristics of the ACF and PSD, the tracking performance, the spectral separation, and the anti-narrowband interference and multipath interference performance for several specific BOC and FH-BOC signals.ResultsThe results show that FH-BOC with the largest frequency-hopping band has lower ACF ambiguity, better anti-interception performance, and better anti-intrasystem interference, narrowband interference, and multipath interference performance than BOC modulation with the same main lobe bandwidth (MLB). The tracking and anti-interference performance of FH-BOC is similar to that of BOC modulation with the same ACF main peak width.ConclusionsFH-BOC is a generalized type of modulation that includes BOC modulation. The proposed FH-BOC signal improves the anti-interference performance and mitigates the ACF ambiguity problem of BOC modulation. The acquisition time and complexity of the receiving process for the proposed FH-BOC signal are the same for the BOC signal with the same MLB. The new modulation scheme which we proposed can serve as a new paradigm for the next-generation GNSS signal design, especially military signal design. It can also be used in the signal design for GNSS-like systems, such as systems for indoor positioning, GNSS enhancement, and pseudolite-based positioning.

Review of the state of the art and future prospects of the ground-based GNSS meteorology in Europe

Abstract. Global navigation satellite systems (GNSSs) have revolutionised positioning, navigation, and timing, becoming a common part of our everyday life. Aside from these well-known civilian and commercial applications, GNSS is now an established atmospheric observing system, which can accurately sense water vapour, the most abundant greenhouse gas, accounting for 60–70 % of atmospheric warming. In Europe, the application of GNSS in meteorology started roughly two decades ago, and today it is a well-established field in both research and operation. This review covers the state of the art in GNSS meteorology in Europe. The advances in GNSS processing for derivation of tropospheric products, application of GNSS tropospheric products in operational weather prediction and application of GNSS tropospheric products for climate monitoring are discussed. The GNSS processing techniques and tropospheric products are reviewed. A summary of the use of the products for validation and impact studies with operational numerical weather prediction (NWP) models as well as very short weather prediction (nowcasting) case studies is given. Climate research with GNSSs is an emerging field of research, but the studies so far have been limited to comparison with climate models and derivation of trends. More than 15 years of GNSS meteorology in Europe has already achieved outstanding cooperation between the atmospheric and geodetic communities. It is now feasible to develop next-generation GNSS tropospheric products and applications that can enhance the quality of weather forecasts and climate monitoring. This work is carried out within COST Action ES1206 advanced global navigation satellite systems tropospheric products for monitoring severe weather events and climate (GNSS4SWEC, http://gnss4swec.knmi.nl).

Two-Dimensional Compressed Correlator for Fast Acquisition of <inline-formula> <tex-math notation="TeX">$\hbox{BOC}(m, n)$</tex-math></inline-formula> Signals

The long spreading code and the binary offset carrier (BOC) modulation technique are used in the next-generation Global Navigation Satellite System (GNSS) to improve positioning performance and to reduce inter-GNSS interference; however, the signal acquisition process of a GNSS receiver can take more time and requires additional hardware resources than legacy Global Positioning System (GPS) receivers. This paper presents the BOC 2-D compressed correlator (TDCC) technique for the fast acquisition of the next-generation GNSS signals. In the proposed BOC-TDCC, signal power in neighboring code-phase hypotheses and Doppler frequency hypotheses can be coherently combined and tested by a single compressed correlator in the first stage, and the conventional correlator-based serial search technique is employed in the second stage to test each hypothesis combined in the first stage. The performance of the BOC-TDCC is demonstrated with numerous Monte Carlo simulations and tested with real data. The BOC-TDCC has much lower mean acquisition time (MAT) than other conventional search schemes, which demonstrates that the BOC-TDCC is an effective search scheme for next-generation GNSS signals.

Determination of Detection Parameters on TDCC Performance

Due to the longer PRN code employed in next-generation GNSS (Global Navigation Satellite System), receivers in the signal acquisition process should test a larger number of code phase hypotheses. Recently, to reduce acquisition time and computational complexity, TDCC (Two-dimensional Compressed Correlator) is introduced, and this technique is different from the conventional double dwell search technique in terms of parameters used to reduce acquisition time and computational complexity. Studies for optimizing the performance of TDCC have not been introduced yet, therefore, in this paper, detection thresholds of TDCC are investigated to optimize its acquisition performance. The optimal detection thresholds minimizing the MAT (Mean Acquisition Time) and MAC (Mean Acquisition Computation) are numerically evaluated and analyzed for widely used detection strategies in its serial and parallel search schemes. It is demonstrated that the minimized MAT and MAC are much smaller than the MAT and MAC using a detection threshold based on CFAR (Constant False Alarm Rate).

Reconfigurable Dual-Channel Multiband RF Receiver for GPS/Galileo/BD-2 Systems

A fully integrated dual-channel multiband RF receiver is designed and implemented for next-generation global navigation satellite systems (GNSSs) in a 0.18-μm CMOS process. Its two reconfigurable signal channels can simultaneously process any two types of 2-, 4-, or 20-MHz bandwidth signals mainly located around the RF bands of 1.2 and 1.57 GHz for GPS, Galileo, and BD-2 (aka Compass) systems, while achieving better performance (die area, noise figure, gain dynamic range) than other state-of-the-art GNSS receivers. A digital automatic gain control loop consisting of a variable gain amplifier and nonuniform 4-bit ADC is utilized to improve the receiver's robustness and performance in the presence of interferences. While drawing 25-mA current per channel from a 1.8-V supply, this RF receiver achieves a total noise figure of 2.5 dB/2.7 dB at 1.2/1.57 GHz, an image rejection of 28 dB, a maximum voltage gain of 110 dB, a gain dynamic range of 73 dB, and an input-referred 1-dB compression point of -58 dBm, with an active die area of 2.4 mm2 for single channel.

Development of a GPU-Based Two-Way Time Transfer Modem

We have developed a new two-way time transfer modem to improve the time transfer precision of remote clock comparison. As a timing signal, we apply a binary offset carrier, which is similar to those signals used for the next-generation Global Navigation Satellite Systems. We took advantage of versatile A/D and D/A converters, and most of the digital signal processing stages were realized by software, running on an off-the-shelf PC. This enabled us to realize the complete system with cheaper equipment, leading to an affordable low-cost modem. For the real-time digital signal processing stages implemented in software, we relied on a graphics processing unit (GPU) developed for computer game enthusiast. The developed modem can receive four channels at the same time with a single GPU card. We performed two-way satellite time transfer experiments using these modems between Japan and Taiwan. The obtained results are consistent within 200 ps with respect to the results of GPS carrier phase time transfer. As a consequence, we improved the time transfer precision by nearly one order of magnitude as compared to a conventional two-way modem without increasing the connection fees caused by commercial communication satellites.

An analytical model of code-tracking performance for next-generation GNSS modulations

Precorrelation bandlimiting, sampling, and quantization (BSQ) are three fundamental functions of receivers for global navigation satellite system (GNSS) signal processing. Analytical models have been developed to evaluate the implementation losses due to different combinations of these parameters for legacy modulations such as BPSK. However, the performance metric was losses of effective carrier-to-noise ratio in dB, instead of performance degradation of a delay-locked loop (DLL) used for code-tracking. This paper proposes an analytical model to predict the DLL performance with the loss of RMS tracking error in meters as the performance metrics. The signal model & assumptions and canonical form of DLL are first introduced. Then, analytical model is derived for DLL tracking accuracy and the performance metrics, tracking loss is defined. Finally, simulation results of two next-generation modulations for GPS and Galileo, namely CBOC and TMBOC, are presented.

Positioning with OFDM signals for the next- generation GNSS

Instead of spread spectrum signals used for positioning in current global navigation satellite systems (GNSS), this paper presents a novel positioning method using OFDM signals for the next-generation GNSS. In contrast to the existing positioning methods with OFDM signals based on timing synchronization or super resolution algorithms, the proposed scheme uses the property of the OFDM signal itself in the frequency domain with respect to transmission delay, resulting in the accurate time of arrival (TOA) estimation. Computer simulations verify that the positioning accuracy of the proposed algorithm with bearable complexity can reach the Cramer-Rao lower bound (CRLB) in the range of medium to high signal-to-noise ratio (SNR). The accuracy achievable is in the order of centimeters, higher than that of the state-of-the- art positioning methods using OFDM signals in the literature.

A methodology for the characterization of environmental effects on global navigation satellite system (GNSS) propagation

In this paper a methodology for the characterization of environment-related (shadowing and multipath) propagation effects on multi-satellite systems is presented. This work is focused on availability and performance evaluation of global navigation satellite systems (GNSSs). The definition of next-generation GNSSs specially designed for civil applications requires the precise understanding of propagation effects; these include not only ionospheric and tropospheric but also environmental effects, since a large group of their expected users will no longer be located in open areas with clear visibility of several satellites but in densely built-up areas subject to high blockage probabilities. Issues such as constellation definition, signal design, codeword structure, etc. require the aid of powerful analysis tools. The basic ideas for the specification of such multi-satellite propagation analysis tools are presented. This paper describes part of the work being carried out under European Space Agency (ESA) contract ESA/ESTEC 12223/96/NL/JSC. © 1998 John Wiley & Sons, Ltd.