This newly revised and greatly expanded edition of the popular Artech House book Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems offers you a current and comprehensive understanding of satellite navigation, inertial navigation, terrestrial radio navigation, dead reckoning, and environmental feature matching . It provides both an introduction to navigation systems and an in-depth treatment of INS/GNSS and multisensor integration. The second edition offers a wealth of added and updated material, including a brand new chapter on the principles of radio positioning and a chapter devoted to important applications in the field. Other updates include expanded treatments of map matching, image-based navigation, attitude determination, acoustic positioning, pedestrian navigation, advanced GNSS techniques, and several terrestrial and short-range radio positioning technologies. The book shows you how satellite, inertial, and other navigation technologies work, and focuses on processing chains and error sources. In addition, you get a clear introduction to coordinate frames, multi-frame kinematics, Earth models, gravity, Kalman filtering, and nonlinear filtering. Providing solutions to common integration problems, the book describes and compares different integration architectures, and explains how to model different error sources. You get a broad and penetrating overview of current technology and are brought up to speed with the latest developments in the field, including context-dependent and cooperative positioning. DVD Included: Features eleven appendices, interactive worked examples, basic GNSS and INS MATLAB simulation software, and problems and exercises to help you master the material.

Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems, Second Edition

Detection and prevention of global navigation satellite system (GNSS) “spoofing” attacks, or the broadcast of false global navigation satellite system services, has recently attracted much research interest. This survey aims to fill three gaps in the literature: first, to assess in detail the exact nature of threat scenarios posed by spoofing against the most commonly cited targets; second, to investigate the many practical impediments, often underplayed, to carrying out GNSS spoofing attacks in the field; and third, to survey and assess the effectiveness of a wide range of proposed defences against GNSS spoofing. Our conclusion lists promising areas of future research.

A Survey and Analysis of the GNSS Spoofing Threat and Countermeasures

The aviation industry's increasing reliance on GPS to facilitate navigation and air traffic monitoring opens new attack vectors with the purpose of hijacking UAVs or interfering with air safety. We propose Crowd-GPS-Sec to detect and localize GPS spoofing attacks on moving airborne targets such as UAVs or commercial airliners. Unlike previous attempts to secure GPS, Crowd-GPS-Sec neither requires any updates of the GPS infrastructure nor of the airborne GPS receivers, which are both unlikely to happen in the near future. In contrast, Crowd-GPS-Sec leverages crowdsourcing to monitor the air traffic from GPS-derived position advertisements that aircraft periodically broadcast for air traffic control purposes. Spoofing attacks are detected and localized by an independent infrastructure on the ground which continuously analyzes the contents and the times of arrival of these advertisements. We evaluate our system with real-world data from a crowdsourced air traffic monitoring sensor network and by simulations. We show that Crowd-GPS-Sec is able to globally detect GPS spoofing attacks in less than two seconds and to localize the attacker up to an accuracy of 150 meters after 15 minutes of monitoring time.

Crowd-GPS-Sec: Leveraging Crowdsourcing to Detect and Localize GPS Spoofing Attacks

A method is developed to detect GNSS spoofing by processing beat carrier-phase measurements from a pair of antennas in a CDGPS-type calculation. This system detects spoofing attacks that are resistant to standard RAIM technique, and it can sense an attack in a fraction of a second without external aiding. The signal-in-space properties used to detect spoofing are the relationships of the signal arrival directions to the vector that points from one antenna to the other. In the un-spoofed case, there are a multiplicity of relationships between the inter-antenna vector and the arrival directions of the multiple signals, which results in a quantifiable multiplicity of carrier-phase single-differences between the antennas. In the spoofed case, there is a single direction of arrival, assuming a single spoofer transmission antenna, and the carrier phase single-differences are identical for all channels, up to an integer cycle ambiguity. A real-time implementation of this detection method has been developed, and it has been tested against live-signal spoofing attacks aboard a superyacht that was cruising around Italy en route from Monaco to Venice. The prototype system demonstrated an ability to detect spoofing attacks in a fraction of a second, though lags in the system’s signal processing lengthened the detection delay to as much as 6 seconds. The system experienced challenges during the initial phase of a spoofing attack if the spoofer power was not much greater than that of the true signal. The true and spoofed signals interfere in a beating pattern in this case, making the composite signal harder to track and harder to classify as being either spoofed or non-spoofed. After the spoofer drags the victim receiver off to an erroneous position or timing fix, the beating subsides, and the new spoofing detection system performs well.

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GNSS Spoofing Detection Using Two-Antenna Differential Carrier Phase

Spoofing is a serious threat to the widespread use of Global Navigation Satellite Systems (GNSSs) such as GPS and can be expected to play an important role in the security of many future IoT systems that rely on time, location, or navigation information. In this paper, we focus on the technique of multi-receiver GPS spoofing detection, so far only proposed theoretically. This technique promises to detect malicious spoofing signals by making use of the reported positions of several GPS receivers deployed in a fixed constellation. We scrutinize the assumptions of prior work, in particular the error models, and investigate how these models and their results can be improved due to the correlation of errors at co-located receiver positions. We show that by leveraging spatial noise correlations, the false acceptance rate of the countermeasure can be improved while preserving the sensitivity to attacks. As a result, receivers can be placed significantly closer together than previously expected, which broadens the applicability of the countermeasure. Based on theoretical and practical investigations, we build the first realization of a multi-receiver countermeasure and experimentally evaluate its performance both in authentic and in spoofing scenarios.

Multi-receiver GPS spoofing detection: error models and realization

In 2001, the Volpe National Transportation Systems Center completed an evaluation of the Global Positioning System (GPS) vulnerabilities and the potential impacts to transportation systems in the United States. One of the recommendations of this study was for the operation of backup system(s) to GPS; Loran C, which has been operated by the U.S. Coast Guard for the past 40 years, was identified as one possible backup system. The Federal Aviation Administration (FAA) has been leading a team consisting of members from industry, government, and academia to evaluate the future of Loran-C in the United States. In a recently completed Navigation Transition Study, the FAA concluded that Loran-C, as an independent radionavigation system, is theoretically the best backup for the GPS; however, in order for Loran-C to be considered a viable back-up system to GPS, it must be able to meet the requirements of non-precision approach (NPA) for the aviation community and the harbor entrance and approach (HEA) requirements for the maritime community. The accuracy requirements for Loran to be used as a backup system are 307 m for NPA and 20 m for HEA. In addition, there are integrity, availability, and continuity requirements. The current Loran system of 24 stations provides a stated absolute accuracy in navigation position of only 0.25 NM; however, enhanced Loran or eLoran has the capability of meeting the stringent requirements for NPA and HEA. In order to meet the accuracy requirements user receivers must use additional secondary factors (ASFs) in calculating the user position. ASFs are propagation time adjustments that are subtracted from the receiver's times of arrival (TOAs) to account for propagation over non-seawater paths. These ASFs vary both spatially and temporally and both types of variations need to be accounted for to meet the accuracy targets. The current approaches to meeting the needs of the aviation and maritime communities are slightly different. For maritime navigation, the spatial variations will be accounted for through the use of a grid of ASF values that is known by the receiver a priori. As one component of the eLoran system, a reference station located nearby the harbor will be used to estimate the temporal changes in the ASFs relative to the published spatial grid; these differences will be broadcast using the Loran data channel (9th pulse) to the user receiver. This general method to HEA navigation was discussed by the authors in 2003 (ION AM 2003). More recently (ION GNSS 2006) we developed a technique to process survey data into a harbor grid. For the aviation community the approach is to measure and publish a set of ASF values for each airport. These airport ASFs will be adjusted to be in the middle of the seasonal variation in order to minimize the maximum error. This approach has been discussed by the authors most recently in papers presented in 2005 (ILA 34) and 2006 (ION NTM 2006). In this paper we show results from both flight tests at various airports around the U.S. and maritime tests in the Thames River in CT. These results demonstrate the ability of eLoran to meet the accuracy requirements for both NPA and HEA using the ASF methods we have proposed.

An Evaluation of eLoran as a Backup to GPS

GPS spoofing is a hot topic of late; technical discussions vary widely based upon the assumed capabilities and a priori knowledge of the spoofer. For a single GPS receiver, various methods to detect a spoofing event have been proposed in the literature. These range from simple ideas (e.g. monitoring the power levels of the GPS signals) to more complex concepts (e.g. looking for vestigial peaks in the correlator outputs) to the comparison to non- GPS signals (e.g. an IMU). Much of this prior work has been on the conceptual level with limited experimentation; little appears to have been done to analyze the resulting detection performance. The detector of interest here monitors the GPS signals using not one, but two or more receivers with their antennas at known relative positions. The assumption is that during a spoofing event these multiple receivers will receive the same spoofer RF signal in that the satellites’ characteristics (i.e. relative times of arrival) are identical at all of the antennas. With no spoofer present, each antenna would receive a unique RF signal, consistent with its position in space. The concept of the detector, then, is that the presence of spoofing is discernible from the near equivalence of the receivers’ receptions. While one could compare these multiple receptions at the RF level, we compare the position solutions across receivers, declaring a spoofing event if the resulting position solutions are too close to each other as compared to the (known) relative locations of the antennas. The primary advantage of such an approach is that the hypothesis test does not require receiver hardware modification or even access to software GPS methods; a separate processor could easily monitor the positions output from the receivers. In this paper we analyze such a detector from a Neyman-Pearson perspective assuming Gaussian statistics on the position solution data. We consider four cases: (1) two receivers with fixed (known) locations, (2) two receivers with fixed separation and known orientation (but unknown absolute position), (3) two receivers with fixed separation and unknown orientation, and (4) a three receiver example.

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Analysis of a Simple, Multi-Receiver GPS Spoof Detector

The Federal Aviation Administration (FAA) is currently leading a team consisting of members from industry, government, and academia to provide guidance to the policy makers in their evaluation of the future of Loran-C in the United States. In a recently completed Navigation Transition Study, the FAA concluded that Loran-C, as an independent radio navigation (RNAV) system, is theoretically the best backup for the global positioning system (GPS). However, for Loran-C to be considered a viable backup system to GPS, it must be able to meet the requirements for non-precision approaches (NPAs) for the aviation community, and the Harbor Entrance and Approach (HEA) requirements for the maritime community. Through FAA sponsoring, the US Coast Guard Academy (USCGA) is responsible for conducting some of the tests and evaluations to help determine whether Loran can provide the accuracy, availability, integrity, and continuity to meet these requirements. A major part of assessing the suitability of Loran is in understanding the nature of Loran ground wave propagation over paths of varying conductivities and terrain. Propagation time adjustments, called "additional secondary factors" (ASFs), are used to adjust receiver times of arrival (TOAs) to account for propagation over non-seawater path(s). These ASFs vary both spatially and temporally, and unless understood and/or modeled, we lose accuracy and may not be able to guarantee a hazardously misleading information (HMI) probability of less than 1times 10-7. During the summer of 2003, the Coast Guard Academy, with flight support from the FAA Technical Center, conducted a series of tests to measure ASF variations in the vicinity of several selected airfields in Colorado, Arkansas, Florida, and California. In addition, approaches were flown at several airports in each of these areas. ASF and TOA data collected during these trials has been analyzed through post-processing to determine the Loran position accuracy during approaches. The accuracy of raw (uncorrected Loran) as well as ASF-corrected Loran positions are shown for a variety of ASF profiles. Using the BALOR ASF modeling software produced by the University of Wales at Bangor, we have also done some simulations of best-case performance analysis for receivers with little or no noise. In addition, we present some ideas for using a Kalman-filtered integrated Loran-INS receiver to smooth out Loran position errors. Finally, how all of these efforts lead toward meeting the accuracy requirements is shown

Can LORAN meet GPS backup requirements

Position, Navigation, and Timing (PNT) is part of the critical infrastructure necessary for the safety and efficiency of vessel movements, especially in congested areas such as the North Sea. Global Navigation Satellite Systems (GNSS), especially the U.S. Global Positioning System (GPS), have become the primary PNT sources for maritime operations. The GNSS position is used both for vessel navigation and as the position and timing source for other systems such as Automatic Identification System (AIS).
Unfortunately, GNSS is vulnerable to jamming and interference, intentional or not, which can lead to the loss of positioning information or, even worse, to incorrect positioning information. The user requirement is for dependable PNT information at all times. One potential source of resilient PNT services is Ranging Mode (R-Mode), an alternative PNT concept related to Signals of OPportunity (SoOP) PNT, which uses signals independent of GNSS.
In 2013 the German Federal Waterways and Shipping Administration contracted for a feasibility study of R-Mode using medium frequency (MF) Differential GNSS (DGNSS) and very high frequency (VHF) AIS signals as well as those signals in combination and in combination with eLoran. At ION GNSS+ 2014 some of the authors presented the results from that feasibility study and showed the projected performance using the signals individually and in combination. In most of the shipping lanes on the North Sea it appeared that 10m or better performance could be achieved.
Following up on that work, prototypes of a transmitter and receiver for MF-DGNSS R-Mode have been developed. The transmitter was installed in IJmuiden (Netherlands) and the receiver deployed along the Dutch coast to the south of the transmitter for initial on-air testing of the R-Mode concept; both were synchronized to UTC via GPS. While positioning is not possible with only one R-Mode transmitter, the combination of a synchronized transmitter and receiver pair allows for useful testing of the R-Mode concept. Specifically, the receiver could estimate a true range (rather than a pseudorange); hence, the stability of the range with environmental variations (e.g. weather, day/night skywave effects, etc.) can be studied.
To further study skywave effects, the R-Mode modulator was relocated to a more powerful transmitter at Heligoland (Germany) and the receiver moved to a location near the Kiel Canal (Germany). A second receiver was installed at a similar distance from the transmitter to enable simultaneous comparison of two different propagation paths. Later a third receiver was added in order to provide three paths of different lengths.
The original R-Mode feasibility study also examined the number of stations that it would be possible to receive in the North Sea area to ensure there were sufficient stations for positioning. However, it did not look at the converse question; how would a large number of R-Mode signals impact legacy users. As part of the prototype development, very limited experiments were performed to assess the impact of the new signals on legacy DGNSS receivers. Specifically, during the on-air testing mentioned above, a commercial DGNSS receiver was able to accurately demodulate and decode the on-air transmissions from the single prototype R-Mode transmitter; the impact of multiple signals was not examined. A new study, funded by the General Lighthouse Authorities of the UK and Ireland, has just been initiated to analyze how legacy equipment would respond to multiple R-Mode signals at different frequencies, both in-band and out-of-band.
This paper presents details of the prototype transmitter and receiver and includes statistical analyses of the range estimates recorded to date and the impact of skywave interference at night using data from the German test sites. Additionally, this paper includes preliminary results of the R-Mode signal interference study.

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Gregory W. Johnson

Papers

An Evaluation of eLoran as a Backup to GPS

Analysis of a Simple, Multi-Receiver GPS Spoof Detector

Can LORAN meet GPS backup requirements

Initial Results of MF-DGNSS R-Mode as an Alternative Position Navigation and Timing Service

Navigating using an ASF grid for harbor entrance and approach