This paper reports the design, proof-of-concept implementation and preliminary performance assessment of a low-cost, real-time, portable, low power, and small form factor GNSS rebroadcaster. This device can be used both as a GNSS signal generator and as a GNSS signal regenerator. This device can be used to test the addition of new features in GNSS signals, such as new signals for ranging, and to characterize the performance of new and existing spoofing countermeasures for GNSS receivers in real time. This device does not require the use of post-processed GNSS signals, enabling the testing with live signals, for instance in a vehicular test campaign replicating the correct dynamic and channel impairments.
Even though the remarkable growth of the last years, the global GNSS market will still continue to expand in the next decade both in terms of devices and service. The introduction of a new generation of mass market chips based on multi GNSS dual frequency measurements, already being commercialized and integrated in smartphones by major manufacturers, is contributing to a new level of positioning accuracy in the mass-market location-based services. Better positioning and navigation experience in urban environments are the key challenges of dual-frequency GNSS enabled receivers. Thanks to the shape of the E5 signal provided by Galileo satellites as well as the other wide-band signals available on the same band, as provided by GPS satellites, the multipath effect could be greatly reduced with significant improvement on the position accuracy. In this paper a low complexity uncombined dual frequency Extended Kalman Filter (EKF) filter is presented. The goal is to deliver continuous and smooth navigation results, even in challenging environment conditions, with a low complexity and stable solution for the GNSS mass market. The proposed EKF follows the standard and simple approach of the single frequency EKF, but exploiting the measurements from both L1 and L5 bands to take benefits of the straightness of L5 measurements against multipath. Therefore, a simple uncombined L1 and L5 measurements approach is used and it is preferred in area with not significant ionospheric error. For each epoch and for each satellite in view, only the measurements coming from one frequency are selected. To exploit the resilience of L5/E5a measurements against multipath and to overcome the visibility limitation due to lower number of satellites available on L5 band than on L1 band, a “visibility check” on the measurements available is done. To show the greater resistance of L5/E5a measurements to the effects of multipath and so to test the proposed EKF, the raw measurements of a mass market dual frequency chipset for smartphone of two kinematic data collections are used. To cover different signal reception conditions, an urban/sub-urban and a rural environment are chosen. Different PVT post processing configurations have been applied to the GNSS observations, to evaluate the impact of carrier phase pseudorange smoothing and of the different frequency selections on the position accuracy and solution availability. In addition, the performance of the proposed standard single point positioning EKF using uncombined dual frequency signals is also compared to the performance obtained by processing the same raw measurements with an external Precise Point Positioning (PPP) tool. The results are promising and show that a positioning improvement from few meters up to tens meters is possible using dual frequency mass market receivers with respect to single frequency receivers, both in mild and harsh environments.
Lunar exploration is going to have a central role in the future of space industry. A large number of missions, both scientifically and economically oriented, are planned for the next decade, many of which are consolidated and structured within international agencies, such as the ESA/SSTL Lunar Pathfinder and NASA Artemis program. This opens the door to the exploration of new technologies and techniques for missions to the moon, with the objective of improving autonomy and robustness, while reducing the costs. The use of weak-GNSS signals to navigate spacecraft in cislunar space has been investigated in several publications, bringing the advantage of being less expensive and based on already existing infrastructure, allowing to improve accuracy, robustness and autonomy. ESA is planning to demonstrate this technology as part of the Lunar Pathfinder mission. Despite the tremendous advantages brought by the use of GNSS signals in lunar orbit, this concept has some limitations, for example it cannot serve users on the far side of the Moon and the service for users experiencing Moon occultation would be compromised. Within the Moonlight initiative, ESA is currently working to define a Lunar Communication and Navigation Service (LCNS), which aims to provide advanced communication and navigation services, covering the limitations of Earth GNSS and improving further the navigation performances in cis-lunar space. This paper presents the outcome of a research aimed to perform a representative end-to-end assessment of what are realistic navigation performance achievable by an autonomous GNSS navigation system for two Moon mission scenarios. In addition, preliminary performances using a potential LCNS constellation are assessed, together with Earth GNSS or standalone. The test campaign shows the performances of two different key scenarios: Lunar Pathfinder and Low Lunar Frozen Orbit (LLFO). For the Lunar Pathfinder, it resulted that Earth-GNSS signals can be successfully utilized to reach satisfactory navigation performance: positioning errors as low as 50 meters 3D Root Mean Square (RMS) were obtained using Earth GNSS signal alone, with a slight improvement introduced by LCNS constellation. For the LLFO, the usability of Earth GNSS signal is heavily compromised by the scenario inherent characteristics. In these case, the introduction of the LCNS constellation significantly improves navigation performance, allowing to obtain results similar to the Lunar Pathfinder. This work has been performed as part of the ESA activity NAVISP-EL1-023 “Earth-Moon Navigation: System Study and development of a high-sensitive spaceborne receiver”.
The detection of evil waveforms (EWF) in GNSS signals is crucial to refrain from using anomalous signals in the PVT solution, which could degrade significantly its accuracy. The EWF detectors are, in general, based on the computation of the distortion of the code autocorrelation function. Thus, the multipath effect, an independent mechanism that also distorts the autocorrelation shape, can be incorrectly assumed by the EWF detector as the presence of EWF, leading to a major increase of the probability of false alarm. In the paper we analyze the robustness of the main EWF detectors and modulations to the presence of multipath.
Unmanned aerial vehicles have become a crucial technology in a wide variety of fields. The availability and integrity of exact positioning feedback information are of paramount importance for the safety of many UAV applications. Global navigation satellite system receivers provide reliable and accurate positioning solutions in outdoor environments. However, they suffer strong performance degradation in harsher scenarios such as forests, urban canyons, or indoor environments. This paper addresses the performance of GNSS receivers installed on a drone. They were tested in different scenarios such as under open sky or in adverse conditions of signal reception. The solutions offered by the receivers were compared to RTK reference trajectories. Moreover, the advantage of including a stereo tracking camera in the setup to obtain a more accurate reference trajectory in some scenarios was analyzed. The results give insights on the accuracy and the quality of the measurements of current GNSS technologies in aerial applications.
Evil waveforms (EWF) are anomalies in the GNSS transmitted signals that can degrade significantly the accuracy of the PVT solution. The cross-correlation function of the incoming signal disturbed by EWF distortion and the locally-generated code signal is obtained analytically for threat models TM-A, TM-B and TM-C. These results are useful to evaluate efficiently the performance of EWF detectors, namely the detectability and hazard regions.
Multipath phenomenon is directly linked to the geometry of the environment in the vicinity of the antenna. For dual GNSS antenna systems, this dependence on the local environment can makes the multipath error contributions on each antenna dependent. This feared event is called common modes. The present document gives a first assessment of this phenomenon considering two static antennas. By defining a common metric and a detection methodology suited for the phenomenon, common modes are analyzed with both simulated and real data set. Analyses focus on the impact of the distance between the two antennas on the amplitude of the common modes under specific scenarios of test. First results show that the phenomenon requires a short distance between antennas to impact the independence of the MP errors contributions.
This paper presents a robust tracking strategy for global navigation satellite system (GNSS) receivers in sounding rockets. The continuous tracking of sounding rockets using GNSS signals becomes challenging due to the high signal dynamics experienced, especially during lift-off. Therefore, robust tracking techniques are crucial to keep the lock of the satellite signals and achieve a continuous position, velocity, and time (PVT) solution of the sounding rocket. This research presents a robust adaptive tracking technique: the loop-bandwidth control algorithm (LBCA)-based frequency locked loop-assisted-phase locked loop (FAP). This technique is compared with state-of-the-art robust tracking techniques using fixed tracking configurations. Both methods are tested in a simulated rocket launch scenario. Results show that the LBCA-based FAP is a reliable option for sounding rocket scenarios due to its robustness against high dynamics.
STARE, a real-time SofTwAre REceiver for positioning with the long-term evolution (LTE) and fifth-generation (5G) new radio (NR) cellular downlink signals, is presented and demonstrated. The real-time operation is achieved by interfacing directly with the software-defined radio (SDR), therefore avoiding the requirement to store the captured signal on a drive and allowing to process signals continuously over arbitrarily long periods. STARE supports multi-channel SDRs and parallel execution of an arbitrary number of tracking channels, which independently acquire and track the desired signals. During the acquisition stage, the tracking channel applies a path selection criterion based on the signal-to-noise ratio (SNR) of the earliest path to prevent incorrect delay and phase estimation, which may occur when the channel order is overestimated. The design of the tracking stage follows a closed-loop architecture providing a continuous estimation of the delay, Doppler, phase, and SNR. The real-time operation of STARE is demonstrated by monitoring downlink signals of a commercially operated LTE base station for an uninterrupted period of one week. For this purpose, STARE is deployed on a static monitoring setup composed of a processing unit, an SDR, an omnidirectional antenna, and a high-precision Rubidium reference clock. The collected measurements are used to study the SNR and delay errors. The delay errors are estimated using the code-minus-carrier (CMC) technique and are observed to achieve a sub-meter standard deviation.
Spoofing attacks against global navigation satellite system (GNSS) receivers are a serious threat to secure navigation, also in autonomous driving. Cars typically include, beyond the GNSS receiver, also an inertial measurement unit (IMU), whose data can be used to detect GNSS spoofing attacks. We consider a specific spoofing attack, with the spoofed trajectory that gradually diverges from the true trajectory, and we propose a spoofing detection method based on machine learning. First, a feature vector is designed, collecting the difference of two estimates of the device velocity, obtained from the GNSS receiver and the IMU. Then, a neural network (NN) is trained over a set of true and spoofed trajectories to detect the attack. We compare the proposed solution with an approximated Neyman-Pearson test and a literature reference direct comparison method, confirming the low error probabilities of our novel solution.
Radio Frequency fingerprinting (RFF) methods are gaining popularity as physical-layer identification or authentication methods in various navigation and communication applications. Traditionally, RFF has been used in terrestrial communications to identify the genuine transmitters from spoofers and jammers. In recent literature, RFF has gained attention also in the context of satellite navigation and Low Earth Orbit (LEO) satellite communications, though this research area is still in an incipient phase. RFF studies in the context of satellite transmitters (or transceivers) are typically hindered by the challenges in acquiring high-quality raw measurement data. In this paper, we analyze via RFF methods, in both pre-correlation and post-correlation domain, the raw GNSS data collected at three locations: Tampere (Finland), Nottingham (UK), and Nuremberg (Germany). The datasets for the first two scenarios have been collected by the authors, while the third dataset is available in open access. We show that we are able to reach average classification probabilities of spoofer versus GNSS up to 99.99% (i.e., Nuremberg measurements) with pre-correlation data and up to 87.72 % (i.e., Nottingham measurements) with post-correlation data. We also discuss the challenges and limitations of RFF in the context of GNSS.
Current Satellite Based Augmentation Systems (SBAS) enable precision approach operations with Global Navigation Satellite Systems (GNSS) up to CAT-1. A definitive agreement on CAT-2 requirements has not been achieved yet. However, it is foreseeable that the current SBAS Time-To-Alert (TTA) performances of 6 seconds could be a limiting factor for the enabling of GNSS-based CAT-2 operations. Unfortunately, the improvement of such performance looks very challenging and expensive. Therefore, alternative solutions should also be investigated, where the SBAS TTA is, at least partially, compensated by the user algorithms. On-board sensors like Inertial Measurement Units (IMU) could be the ideal aiding technology for the compensation of the SBAS TTA. Therefore, an Extended Kalman Filter (EKF) was designed in this work for implementing an SBAS-GNSS/IMU sensor fusion framework. The application of SBAS-augmentation to an EKF-based algorithm, as well as the countermeasures proposed to solve the critical issues that this leads to, represented one of the most innovative aspects of the present work. Integrity and continuity fault trees were derived for the proposed system, with correspondent risk budget allocation and Protection Levels (PL) formulas. A simple TTA compensation scheme was then described where all the GNSS and IMU measurements in the last X seconds (X smaller or equal to TTA) are buffered and the EKF runs X seconds in the past. Each estimate is then propagated up to the current time instant through the strapdown integration of the inertial measurements only. The compensation window X represents a trade-off between the amount of TTA compensated and the accuracy degradation/PLs inflation due to the strapdown integration. Numerical evaluations of such trade-off were performed in this work by considering different IMU grades in simulated scenario based on a real landing trajectory. As expected, the results showed that the accuracy degradation depends on both the amount of TTA compensation and the IMU grade. In particular, for the high IMU grade no significant impact was observed when compensating 4 seconds out of SBAS TTA (overall TTA equal to 2 seconds).
We present in this paper a design for a monitorization station that will be part of a network of receivers in charge of providing a service for detection, characterization, and localization of interfering signals in safety critical GNSS environments. The purpose is to cover a wideband frequency range from 800 MHz to 1900 MHz with a power level mask taken from EGNOS and DFMC GBAS requirements. The proposed design combines two four-element arrays with two USRP units for collecting signals within the desired SNR margins. A comparative simulation study has been done to evaluate different techniques for interference detection, characterization, and estimation of its angle of arrival. The results show that a combination of power and kurtosis monitorization, both in temporal and frequency domains, is a proper solution to detect different types of interfering signals. In order to assure the best signal characterization, the use of the three selected techniques serves to overcome their respective limitations. For angle of arrival estimation, the fact of working in a suboptimal array configuration provides more robustness against outliers for certain geometries.
This paper introduces Tiira - an open-source hardware-based Global Navigation Satellite Systems (GNSS) receiver and multi-sensor navigation system. Its unique features include system modularity, an openness of software and hardware designs, signal processing running in dedicated soft-core proces-sors supported by an accelerator implemented in hardware logic. The work on the platform is ongoing; this paper describes the overall system, its architecture and structure, design choices and challenges, and the development status. We also provide some preliminary results.
Dedicated and aerial fifth generation (5G) networks, here called 5G overlay networks, are envisaged to enhance existing positioning services, when combined with global navigation satellite systems (GNSS) and other sensors. There is a need for accurate and timely positioning in safety-critical automotive and aerial applications, such as advanced warning systems or in urban air mobility (UAM). Today, these high-accuracy demands can partially be satisfied by GNSS, though not in dense urban conditions or under GNSS threats (e.g. interference, jamming or spoofing). Temporary and on-demand 5G network deployments using ground and flying base stations (BSs) are indeed a novel solution to exploit hybrid GNSS, 5G and sensor algorithms for the provision of accurate three-dimensional (3D) position and motion information, especially for challenging urban and suburban scenarios. Thus, this paper first analyzes the positioning technologies available, including signals, positioning methods, algorithms and architectures. Then, design considerations of 5G overlay networks are discussed, by including simulation results on the 5G signal bandwidth, antenna array and network deployment.
The use of constant envelope signals for global navigation satellite systems (GNSS) has a long tradition. These signals allow the satellite payload's high power amplifier (HPA) to be driven in saturation, hence with a very low output power backoff (OBO) and high efficiency. For non-constant envelope signals, the HPA's input power must be carefully chosen near the transition of linear region and saturation region, not to distort the shape of the code-division multiple access (CDMA) waveforms too much - distortions would otherwise lead to a correlation loss at the GNSS receiver. In this paper, we revisit some nonconstant envelope CDMA waveforms with raised-cosine (RC) or prolate spheroidal wave function (PSWF) pulse shapes, and try to give a fair comparison with constant envelope waveforms with conventional binary phase-shift keying (BPSK) or frequencyhopping binary offset carrier (FHBOC) spreading. We also address how the operation of the HPA requires a careful trade-off between losses caused by OBO, filtering losses by the payload's output multiplexer (OMUX) and the receiver front-end, as well as receiver correlation loss. Performance is compared in terms of achievable receiver signal-to-noise ratio (SNR) and time of arrival (TOA) estimation error variance. The results suggest that the constant envelope signals' advantage of low OBO is often expended by filtering losses, such that band-limited non-constant envelope spreading waveforms may be an interesting option for future GNSS signal design.
Galileo has started authenticating its navigation message through OSNMA. In order to support OSNMA implementation by receiver manufacturers and application developers, this paper presents OSNMAlib, an open Python library implementing OSNMA functions. OSNMAlib processes the Galileo I/NAV pages in decoded SBF, hexadecimal or other formats, and performs the required operations to authenticate Galileo navigation data: OSNMA status handling, cryptographic functions required for the Merkle tree, digital signatures, keychain management and tag authentication. It handles the up-to-date data authentication status and performs the public key and chain renewal and revocation processes. This paper describes OSNMAlib architecture and its main functions and presents the first test results.
The presence of unintentional or intentional Radio Frequency Interference (RFI) signals in the Global Navigation Satellite Systems (GNSS) frequency bands is by far one of the main vulnerabilities of every GNSS receiver. This known threat can cause severe positioning performance degradation and even a complete service unavailability. Complementary to time and frequency-domain mitigation techniques, it is well known that antenna-array based receivers can benefit from spatial domain processing. By exploiting spatial diversity, an array-based smart antenna can selectively attenuate the RFIs Direction of Arrival (DOA) and provide high gain towards the legitimate GNSS signals. In this work, we propose a receiverindependent GNSS smart antenna architecture that implements a real-time automatic and autonomous null-steering spatial filtering for GNSS bands. The platform uses Commercial Off The Shelf (COTS) components including a multichannel receiver frontend, a System on Chip (SoC) hybrid FPGA/CPU digital signal processor, and an up-converter to shift the spatially filtered GNSS signal back to its carrier frequency. In this way, the proposed smart antenna can be connected to any conventional singleantenna GNSS receiver. The paper includes both the theory of operation, the implementation details, and the prototype performance in a real-life open field scenario.
The demand for Position Velocity and Time (PVT) assurance is growing across the Global Navigation Satellite System (GNSS) industry. One of the approaches to assist in providing a reliable, authentic and resilient PVT service is to encode some unpredictable elements into the broadcast GNSS signals. In this paper we take a high level view of what is meant by PVT assurance, what can be achieved by such system-side, signal-level contributions to the solution, and what remains to be performed by the receiver. In particular, we discuss the importance of maintaining tight time bounds, develop a high level attack model and show that the usual correlation function approach may not be sufficient to detect a spoofing attack. We conclude with a number of high-level recommendations for signallevel PVT assurance schemes.
During the past few decades, the use of global navigation satellite systems (GNSSs) has become the primary and sometimes only way of providing a positioning, navigation, and timing (PNT) solution for many outdoor applications. Furthermore, GNSS is playing an important role on the development of smart cities and Internet of things (IoT) applications. For this reason, seamless navigation has become very crucial for numerous PNT-dependent applications. Unfortunately, GNSS is a technology that is vulnerable to several threats. All these ingredients boil down to the need for alternative PNT solutions to backup GNSS in case of miss performance or denial of service. The use of low earth orbit (LEO) constellations has been considered in the literature to provide global solution, but more importantly because it will bring some benefits with respect to medium earth orbit (MEO); which is the constellation used in GNSS. Based on these considerations, in this paper we focus on the design of a new PNT signal for LEO constellations. Furthermore, a comprehensive performance analysis is carried out with the aim of reducing the receiver complexity. For that, a chirp spread spectrum signal design is considered.
