Leveraging Spatial Diversity to Mitigate Partial Band Interference in Undersea Networks through Waveform Reconstruction

Many acoustic channels suffer from interference which is neither narrowband nor impulsive. This relatively long duration partial band interference can be particularly detrimental to system performance. We survey recent work in interference mitigation as background motivation to develop a spatial diversity receiver for use in underwater networks. The network consists of multiple distributed cabled hydrophones that receive data transmitted over a time-varying multipath channel in the presence of partial band interference produced by interfering active sonar signals as well as marine mammal vocalizations. In operational networks, many “dropped” messages are lost due to partial band interference which corrupts different portions of the received signal depending on the relative position of the interferers, information source and receivers due to the slow speed of propagation. Our algorithm has been tested on simulated data and is shown to work on an example from a recent undersea experiment

A Multiple COTS Receiver GNSS Spoof Detector -- Extensions

Spoofing refers to the intentional (and considered malicious) interference to a GNSS user\u27s inputs so as to distort the derived position information. A variety of approaches to detect spoofing have been proposed in the literature. Much of this prior work has focused on the conceptual level with limited analysis of the resulting detection performance, and/or has proposed fundamental redesign of the receiver itself. Little eort has been directed towards using existing, commercial-off-the-shelf (COTS) stand-alone receiver technology to perform spoof detection. At ION ITM 2013 these authors proposed a simple spoof detection concept based on the use of multiple COTS receivers and analyzed the performance of several ad hoc detection algorithms from a Neyman-Pearson perspective assuming Gaussian statistics. At ION GNSS+ 2013, by restricting attention to a horizontal platform and assuming an independent measurement error model, we were able to develop the optimum Neyman-Pearson hypothesis test. That paper also included an analysis of performance, yielding closed form expressions for the false alarm and detection probabilities and an optimization of the performance over the locations of the receivers\u27 antennae. This current works extends the earlier results by considering more realistic statistical models, considers the processing of several sequential outputs from the receivers and addresses 3-D receiver antennae patterns

A Temporal Algorithm for Satellite Subset Selection in Multi-Constellation GNSS

GNSS receivers convert the measured pseudoranges from the visible GNSS satellites into an estimate of the position and clock offset of the receiver. For various reasons receivers might want to process only a subset of the visible satellites; it would be desired, of course, to use the best subset. In general, selecting the best subset is a combinatorics problem; selecting m objects from a choice of n allows for (n m) potential subsets. And since the typical performance criterion (e.g. Geometric Dilution of Precision) is nonlinear and non-separable in the satellites’ locations in the sky, finding the best subset is a brute force procedure; hence, a number of authors have described sub-optimal algorithms for choosing satellites. This paper revisits this problem, especially in the context of multiple GNSS constellations. The paper begins with a review of the existing subset selection algorithms. We note that all of these algorithms are what might be called “snapshot” in nature, selecting a subset for a single, fixed skyview of satellites. Through an example with the GPS constellation, we examine the time-sequential, or temporal, characteristics of the best subset selection noting: That the best subset at a particular point (snapshot) in time is also the best subset for a significant time interval around that point (typically measured in minutes). That the changes in the best subset over time are primarily, but not always, due to the loss or gain of a satellite crossing the horizon (or, more precisely, the receiver’s mask angle). Based upon these observations this paper develops several time-sequential, or temporal, algorithms that attempt to track the optimum subset of satellites over time at low computational cost. The accuracy and complexity of the algorithms are assessed with GPS constellation data. On a larger scale, these algorithms are then tested on combined GPS, GLONASS, and Galileo constellations with the resulting performance compared to optimal solutions found via exhaustive search

Spoof Detection Using Multiple COTS Receivers in Safety Critical Applications

While the GPS is well known to be an accurate provider of position information across the globe, its low power level makes it susceptible to spoong. Given its status as the primary (perhaps only) provider of position in many safety critical applications, this susceptibility is of great concern. Several possible methods to detect a spoong event at a single GPS receiver have been proposed in the literature. We note, however, that almost all of this prior work has been on the conceptual level; there has been very little analysis of the resulting detection performance. Recognizing that redundant equipment may already exist for some users, we have proposed to detect spoong by comparing the position solutions from two or more COTS receivers mounted on the same platform (ION ITM, Jan. 2013). The concept is that the existence of a spoofer would make the statistical relationship of the observed positions dierent than it would be during normal, nonspoofed, operation. The primary advantage of such an approach is that its implementation does not require receiver hardware modication or even access to software GPS methods; a separate processor could easily monitor the positions generated by each of the receivers and decide spoof versus no spoof. Our earlier paper initiated a performance analysis of the approach; this paper continues and extends the investigation

Applying Spatial Diversity to Mitigate Partial Band Interference in Undersea Networks

Many acoustic channels suffer from interference which is neither narrowband nor impulsive. This relatively long duration partial band interference can be particularly detrimental to system performance. We survey recent work in interference mitigation and orthogonal frequency division multiplexing (OFDM) as background motivation to develop a spatial diversity receiver for use in underwater networks. The network consists of multiple distributed cabled hydrophones that receive data transmitted over a time-varying multipath channel in the presence of partial band interference produced by interfering active sonar signals as well as marine mammal vocalizations. In operational networks, many “dropped” messages are lost due to partial band interference which corrupts different portions of the received signal depending on the relative position of the interferers, information source and receivers due to the slow speed of propagation

Limits on GNSS Performance at High Latitudes

As climate change will likely lead to more of a human presence in the higher latitudes, it is important to consider how well our safety-critical positioning systems work near the poles. The orbits of the GPS (and other GNSS) preclude satellites with high elevation in these regions; hence, it is clear that at least vertical accuracy is impacted. This paper characterizes this positioning performance loss by developing lower bounds on GDOP as a function of receiver latitude. Examples with actual ephemeris data are included for comparison to the bounds

Rethinking Star Selection in Celestial Navigation

In celestial navigation the altitude (elevation) angles to multiple celestial bodies are measured; these measurements are then used to compute the position of the user on the surface of the Earth. Methods described in the literature include the classical “altitude-intercept” algorithm as well as direct and iterative least-squares solutions for over determined situations. While it seems rather obvious that the user should select bright stars scattered across the sky, there appears to be no established results on the level of performance that is achievable based upon the number of stars sighted nor how the “best” set of stars might be selected from those visible. This paper addresses both of these issues by examining the performance of celestial navigation noting its similarity to the performance of GNSS systems; specifically, modern results on GDOP for GNSS are adapted to this classical celestial navigation problem

Modernized eLoran: The Case for Completely Changing Chains, Rates, and Phase Codes

First deployed in the U.S. in 1957, Loran-C dominated radio-based navigation for many years. In 2000 the FAA began a significant recapitalization of Loran in the U.S.; the 2001 Volpe report on the vulnerability of the GPS reinforced the need for a revamped Loran. What emerged was an enhanced or evolved version, so called eLoran, aiming to achieve, for example, 10- 20 meter absolute positioning accuracy, RNP 0.3 mile required navigation performance, and stratum 1 time. After 10 years of development, in 2010, this U.S. e ffort was halted and the U.S. transmitters were silenced; since that time, eLoran is still being developed in Europe and deployed in Asia. Earlier this year U.S. Government interest in eLoran has again stirred (evidenced by a U.S. Army request for information and a U.S. Dept. of Transportation request for public comment); the rest of these initiated much conversation at the 2015 ION ITM. The prior U.S. (and continuing European) development of eLoran kept many of the 1950\u27s system design choices so as to be compatible with legacy Loran receivers. These include the pulse shape, groups, chains, rates, phase codes, emission delays, etc. Chosen to suit 1950\u27s technology, many of these restrictions are no longer necessary given the advances in transmitter and receiver technology (e.g. software defined radio) over the last half century. It is the opinion of these authors that as Loran, per se, no longer exists in the U.S., any re-emergence of a low frequency radio navigation system need not be held to these performance limiting constraints. In prior work these authors have promoted more significant changes to eLoran to improve system performance; specifically, single-rating all stations, reconquering the chain/rate structure within the continental U.S., and changing the phase codes. The current paper expands on these prior e fforts. Specifically, we propose putting all of the eLoran transmitters on the same repetition period and employing unique phase codes for each transmitter. To effectively choose new phase codes for eLoran, and assess their performance, we rely on the auto- and cross-correlation metrics. These metrics describe how well a receiver can both acquire and track a specific signal when contaminated by multi- path interference, the existence of other signals, and noise. While a perfect auto-correlation function, large at zero lag corresponding to the actual arrival of the signal and zero elsewhere, and a perfect cross- correlation function, zero for all lags, are preferred, it is impossible to find such codes. However, limiting the size of the window for which we require perfect auto- and cross-correlations, such codes can be found. To create such codes for eLoran we adapt results from the CDMA literature on complementary sequences and Large Area Synchronized (LAS) codes. This paper begins with a brief review of the relevant characteristics of Loran-C, including a discussion of the effects of sky wave and cross rate interference. This is followed by a survey of previously published ideas/concepts on how elements of the system could be changed so as to improve performance. Finally, details on the proposed rate/chain/phase code structure are presented. The reader should recognize that these ideas and results are not intended to define what the best eLoran system would be; rather, if eLoran soars again in the U.S., we hope to initiate a dialogue that looks beyond the decisions made in the 1950\u27s

APNT for GNSS Spoof Detection

Global Navigation Satellite Systems (GNSS) are well known to be accurate providers of position, navigation, and time (PNT) information across the globe. With capable receivers and well-populated satellite constellations, GNSS users typically believe that the position and time information provided by their GNSS receiver is perfectly accurate. More sophisticated users look beyond accuracy and are also concerned with the integrity of the GNSS information. Advances in electronics technology have enabled the creation of malicious RF interference of GNSS signals. Inexpensive jamming devices overpower or distort the GNSS receivers input so as to completely deny the GNSS user of PNT information. A second threat to GNSS integrity is spoofing, the creation of counterfeit GNSS signals. This type of attack is considered more dangerous than a jamming attack since an erroneous PNT solution is often worse than no solution at all. The detection of spoofing is the subject of this paper. A variety of approaches have been proposed in the literature to recognize spoofing; many of these are based on the RF signal alone, including multi-antenna and multi-receiver methods. Another class of spoof detection algorithm is to compare the GNSS result to data from another, non-GNSS (hence, non-spoofed) sensor. In this paper we imagine that the trusted signal is the output of an Alternative PNT (APNT) receiver. APNT refers to stand alone, non-GNSS systems that are intended to provide PNT information during periods in which GNSS is unavailable The wide recognition of the vulnerabilities of the GPS in the Volpe report spurred the search for APNT systems; examples include the development of eLoran in the U.S. and Europe, general work on signals of opportunity ranging, DME-DME positioning, and, quite recently, R-Mode in Europe (we note that none of these systems is currently operational). The intent is that an integrated receiver, either loosely or tightly coupled, would merge the two systems’ observables to yield the best PNT information possible; in practice, since the APNTs’ solutions are typically of lower accuracy than the GNSS solutions, the combined result is nearly equal to the GNSS-alone solution. The goal of this paper is to show that these APNT solutions should be used at ALL times; as a substitute for GNSS PNT when GNSS is unavailable and as an integrity check (e.g. spoof detector) when GNSS is available. At a cursory level spoof detection using APNT appears simple; just compare the two position outputs to see if they are close. This paper looks deeper, considering the questions: How can we use the time estimates to detect position spoofing? How close is close enough in this context? What is the probability of error in the decision? How do the geometries of both systems impact the test itself and its resulting performance? What happens if the receivers are providing different information

A Comparison of a Single Receiver and a Multi-Receiver Techniques to Mitigate Partial Band Interference

Many acoustic channels suffer from interference which is neither narrowband nor impulsive. This relatively long duration partial band interference can be particularly detrimental to system performance. We survey recent work in interference mitigation as background motivation to develop a spatial diversity receiver for use in underwater networks and compare this novel multi-receiver interference mitigation strategy with a recently developed single receiver interference mitigation algorithm using experimental data collected from the underwater acoustic network at the Atlantic Underwater Test and Evaluation Center. The network consists of multiple distributed cabled hydrophones that receive data transmitted over a time-varying multipath channel in the presence of partial band interference produced by interfering active sonar signals. In operational networks, many dropped messages are lost due to partial band interference which corrupts different portions of the received signal depending on the relative position of the interferers, information source and receivers due to the slow speed of propagation