Radionavigation Laboratory

Permanent URI for this communityhttps://hdl.handle.net/2152/15722

At the University of Texas at Austin Radionavigation Laboratory, we explore novel ways to exploit and protect radionavigation systems such as GPS. We develop technologies that advance software-defined GNSS receivers, enable opportunistic navigation, ensure navigation security and integrity, and explain ionospheric phenomena.

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Accuracy Limits for Globally-Referenced Digital Mapping Using Standard GNSS
(IEEE, 2018) Narula, Lakshay; Murrian, Matthew J.; Humphreys, Todd E.
Exchange of location and sensor data among connected and automated vehicles will demand accurate global referencing of the digital maps currently being developed to aid positioning for automated driving. This paper explores the limit of such maps’ globally-referenced position accuracy when the mapping agents are equipped with low-cost Global Navigation Satellite System (GNSS) receivers performing standard codephase-based navigation. The key accuracy-limiting factor is shown to be the asymptotic average of the error sources that impair standard GNSS positioning. Asymptotic statistics of each GNSS error source are analyzed through both simulation and empirical data to show that sub-50-cm accurate digital mapping is feasible in moderately urban environments in the horizontal plane after multiple mapping sessions with standard GNSS, but larger biases persist in the vertical direction.
Achieving the Cramer-Rao Lower Bound in GPS Time-of-Arrival Estimation: A Frequency Domain Weighted Least-Squares Estimator Approach
(2013) Wesson, Kyle; Humphreys, Todd E.
Adaptive Estimation of Signals of Opportunity
(2014-04) Kassas, Zaher M.; Ghadiok, Vaibhav
To exploit unknown ambient radio frequency signals of opportunity (SOPs) for positioning and navigation, one must estimate their states along with a set of parameters that characterize the stability of their oscillators. SOPs can be modeled as stochastic dynamical systems driven by process noise. The statistics of such process noise is typically unknown to the receiver wanting to exploit the SOPs for positioning and navigation. Incorrect statistical models jeopardize the estimation optimality and may cause filter divergence. This necessitates the development of adaptive filters, which provide a significant improvementover fixed filters through the filter learning process. This paper develops two such adaptive filters: an innovationbased maximum likelihood filter and an interacting multiple model filter and compares their performance and complexity. Numerical and experimental results are presented demonstrating the superiority of these filters over fixed, mismatched filters.
Advances in GNSS Equipment
(2010) Humphreys, Todd E.
Analysis of Ionospheric Scintillations using Wideband GPS L1 C/A Signal Data
(2004) Humphreys, Todd E.; Ledvina, Brent M.; Psiaki, Mark L.; Kintner, Paul M.
A non-real-time GPS receiver has been developed and tested for use in scintillation analysis. The receiver consists of a digital storage receiver and non-real-time software acquisition and tracking algorithms. The goal of this work is to shed light on the behavior of strongly scintillating signals: signals which cause conventional GPS receivers to lose carrier lock. The receiver collects wideband GPS L1 digital data sampled at 5.7 MHz using an RF front-end and stores it on disk for post-processing. It processes the data off-line to determine carrier signal amplitude and phase variations during scintillations. The main processing algorithms are traditional code delay and carrier frequency acquisition algorithms and special signal processing algorithms that effectively function as a delay-locked loop and phase-locked loop. The tracking algorithms use non-causal smoothing techniques in order to optimally reconstruct the phase and amplitude variations of a scintillating signal. These techniques are robust against the deep power fades and strong phase fluctuations characteristic of scintillating signals. To test the receiver, scintillation data were collected in Cauchoeira Paulista, Brazil, from December 4 to 6, 2003. The data set spans several hours and includes times when one or more satellite signals are scintillating. The smoothing algorithm has been used to determine the carrier amplitude and phase time histories of the scintillating signals along with the distortion of the pseudorandom noise (PRN) code’s autocorrelation function. These quantities provide a characterization of scintillation that can be used to study the physics of scintillations or to provide off-line test cases to evaluate a tracking algorithm’s ability to maintain signal lock during scintillations.
Assessing the Spoofing Threat: Development of a Portable GPS Civilian Spoofer
(2008) Humphreys, Todd E.; Ledvina, Brent M.; Psiaki, Mark L.; O'Hanlon, Brady W.; Kintner, Paul M.
A portable civilian GPS spoofer is implemented on a digital signal processor and used to characterize spoofing effects and develop defenses against civilian spoofing. This work is intended to equip GNSS users and receiver manufacturers with authentication methods that are effective against unsophisticated spoofing attacks. The work also serves to refine the civilian spoofing threat assessment by demonstrating the challenges involved in mounting a spoofing attack.
Assured Navigation and Timing
(2017-09-28) Humphreys, Todd
Automotive Collision Risk Estimation Under Cooperative Sensing
(IEEE, 2020) LaChapelle, Daniel; Humphreys, Todd; Narula, Lakshay; Iannucci, Peter; Moradi-Pari, Ehsan
This paper offers a technique for estimating collision risk for automated ground vehicles engaged in cooperative sensing. The technique allows quantification of (i) risk reduced due to cooperation, and (ii) the increased accuracy of risk assessment due to cooperation. If either is significant, cooperation can be viewed as a desirable practice for meeting the stringent risk budget of increasingly automated vehicles; if not, then cooperation—with its various drawbacks—need not be pursued. Collision risk is evaluated over an ego vehicle’s trajectory based on a dynamic probabilistic occupancy map and a loss function that maps collision-relevant state information to a cost metric. The risk evaluation framework is demonstrated using real data captured from two cooperating vehicles traversing an urban intersection.
Automotive-Radar-Based 50-cm Urban Positioning
(IEEE, 2020) Narula, Lakshay; Iannucci, Peter A.; Humphreys, Todd E.
Deployment of automated ground vehicles (AGVs) beyond the confines of sunny and dry climes will require sub-lane-level positioning techniques based on radio waves rather than near-visible-light radiation. Like human sight, lidar and cameras perform poorly in low-visibility conditions. This paper develops and demonstrates a novel technique for robust 50-cm-accurate urban ground positioning based on commercially-available low-cost automotive radars. The technique is computationally efficient yet obtains a globally-optimal translation and heading solution, avoiding local minima caused by repeating patterns in the urban radar environment. Performance is evaluated on an extensive and realistic urban data set. Comparison against ground truth shows that, when coupled with stable short-term odometry, the technique maintains 95-percentile errors below 50 cm in horizontal position and 1 degree in heading.
A Blueprint for Civil GPS Navigation Message Authentication
(2014-05) Kerns, Andrew J.; Wesson, Kyle D.; Humphreys, Todd E.
A proposal for civil GPS navigation message authentication (NMA) is presented with sufficient specificity to enable near-term implementation. Although previous work established the practicality and efficacy of NMA for civil GPS signal authentication, there remains a need for a detailed proposal that addresses several outstanding considerations regarding implementation. In particular, this paper (1) provides a definitive evaluation of the tradeoffs involved in the choice of cryptographic protocol, and (2) optimizes the placement of digital signature bits in the GPS CNAV message stream. By offering GPS engineers and policymakers a detailed blueprint for civil NMA, this work advances the possibility of NMA implementation on modernized civil GPS signals.
CASES: A Novel Low-Cost Ground-based DualFrequency GPS Software Receiver and Space Weather Monitor
(2011) Crowley, Geoff; Bust, Gary S.; Reynolds, Adam; Azeem, Irfan; Wilder, Rick; O'Hanlon, Brady W.; Psiaki, Mark L.; Powell, Steven; Humphreys, Todd E.; Bhatti, Jahshan A.
GPS receivers can be used for monitoring space weather events such as TEC variations and scintillation. This paper describes the new GPS sensor developed by ASTRA, Cornell and UT Austin. The receiver is called “Connected Autonomous Space Environment Sensor (CASE)”, and represents a revolutionary advance in dual frequency GPS space-weather monitoring. CASES is a paperback-novel-sized dual-frequency GPS software receiver with robust dual-frequency tracking performance, stand-alone capability, and complete software upgradability. The receiver tracks L1 and L2 civilian signals (specifically L1 C/A, L2 CL and L2 CM). The sensor measures and calculates TEC with a relative accuracy of a few 0.01 TECU at a cadence of up to 1 Hz (post-processing up to 100 Hz). It measures amplitude and phase at up to 100 Hz on both L1 and L2-C, for up to 14 satellites in view. It calculates the standard scintillation severity indicators S4, τ0, and σΦ, and a new index, the Scintillation Power Ration (SPR), all at a cadence that is user defined. It is able to track through scintillation with {S4, τ0, amplitude} combinations as severe as {0.8, 0.8 seconds, 43 dB-Hz (nominal)} (i.e., commensurate with vigorous post-sunset equatorial scintillation) with a mean time between cycle slips of 480 seconds and with a mean time between frequency-unlock greater than 1 hour. Other capabilities and options include: Various data interface solutions; In-receiver and network-wide calibration of biases, and detection and mitigation of multipath; Network-wide automated remote configuration of receivers, quality control, re-processing, archiving and redistribution of data in real-time; Software products for data-processing and visualization. CASES has been designed and developed by the ionosphere community rather than adapting a commercial receiver. The low price of the sensor means that many more instruments can be purchased on a fixed budget, which will lead to new kinds of opportunities for monitoring and scientific study, including networked applications. Other potential uses for CASES receivers include geodetic and seismic monitoring, measurement of precipitable water vapor in the troposphere at meso-scale resolution, and educational outreach
CASES: A Smart, Compact GPS Software Receiver for Space Weather Monitoring
(2011) O'Hanlon, Brady W.; Psiaki, Mark L.; Powell, Steven; Bhatti, Jahshan A.; Humphreys, Todd E.; Crowley, Geoff; Bust, Gary S
A real-time software-defined GPS receiver for the L1 C/A and L2C codes has been developed as a low-cost space weather instrument for monitoring ionospheric scintillation and total electron content. The so-called CASES receiver implements several novel processing techniques not previously published that make it well suited for space weather monitoring: (A) a differencing technique for eliminating local clock effects, (B) an advanced triggering mechanism for determining the onset of scintillation, (C) data buffering to permit observation of the prelude to scintillation, and (D) data-bit prediction and wipe-off for robust tracking. The receiver has been tested in a variety of benign and adverse signal conditions (e.g., severe ionospheric scintillation, both real and simulated); the results are presented here. The custom hardware platform on which the software runs is compact while remaining flexible and extensible. The CASES platform consists of a digital signal processor, an ARM microcontroller, and a custom-built narrow-band dualfrequency front end. Because the receiver is softwaredefined, it can be remotely reprogrammed via the internet or another communications link.
Centimeter Positioning with a Smartphone-Quality GNSS Antenna
(2014-09) Pesyna, Kenneth M. Jr; Heath, Robert W. Jr.; Humphreys, Todd E.
This paper demonstrates for the first time that centimeteraccurate positioning is possible based on data sampled from a smartphone-quality Global Navigation Satellite System (GNSS) antenna. Centimeter-accurate smartphone positioning will enable a host of new applications such as globally-registered fiduciary-marker-free augmented reality and location-based contextual advertising, both of which have been hampered by the several-meterlevel errors in traditional GNSS positioning. An empirical analysis of data collected from a smartphone-grade GNSS antenna reveals the antenna to be the primary impediment to fast and reliable resolution of the integer ambiguities which arise when solving for a centimeter-accurate carrierphase differential position. The antenna’s poor multipath suppression and irregular gain pattern result in large timecorrelated phase errors which significantly increase the time to integer ambiguity resolution as compared to even a low-quality stand-alone patch antenna. The time to integer resolution—and to a centimeter-accurate fix—is significantly reduced when more GNSS signals are tracked or when the smartphone experiences gentle wavelength-scale random motion.
Characterization of Receiver Response to Spoofing Attacks
(The Institute of Navigation, 2011-10) Shepard, Daniel; Humphreys, Todd
Test procedures are developed for characterizing the response of civil GPS receivers to spoofing attacks. Two response characteristics are analyzed in detail for four representative GPS receivers: (1) the spoofer power advantage over the authentic signals required for successful receiver capture, and (2) the aggressiveness with which a spoofer can manipulate the victim receiver’s time and position solution. Two of the tested receivers are commonly used in critical infrastructure applications, one in smart power grid regulation and one in telecommunications networks. The implications of the test results for these critical infrastructure applications are discussed.
Characterization of Receiver Response to Spoofing Attacks
(2011) Shepard, Daniel P.; Humphreys, Todd E.
Test procedures are developed for characterizing the response of civil GPS receivers to spoofing attacks. Two response characteristics are analyzed in detail for four representative GPS receivers: (1) the spoofer power advantage over the authentic signals required for successful receiver capture, and (2) the aggressiveness with which a spoofer can manipulate the victim receiver’s time and position solution. Two of the tested receivers are commonly used in critical infrastructure applications, one in smart power grid regulation and one in telecommunications networks. The implications of the test results for these critical infrastructure applications are discussed.
Characterization of Receiver Response to Spoofing Attacks
(2011-05) Shepard, Daniel
Test procedures are developed for characterizing the response of civil GPS receivers to spoofing attacks. Two response characteristics are analyzed in detail for four representative GPS receivers: (1) the aggressiveness with which a spoofer can manipulate the victim receiver’s time and position solution, and (2) the spoofer power advantage over the authentic signals required for successful receiver capture. Two of the tested receivers are commonly used in critical infrastructure applications, one in “smart” power grid regulation and one in telecommunications networks. The implications of the test results for these critical infrastructure applications are discussed.
Civilian GPS Spoofing Detection based on DualReceiver Correlation of Military Signals
(2011) Psiaki, Mark L.; O'Hanlon, Brady W.; Bhatti, Jahshan A.; Shepard, Daniel P.; Humphreys, Todd E.
Cross-correlations of unknown encrypted signals between two civilian GNSS receivers are used to detect spoofing of known open-source signals. This type of detection algorithm is the strongest known defense against sophisticated spoofing attacks if the defended receiver has only one antenna. The attack strategy of concern starts by overlaying false GNSS radio-navigation signals exactly on top of the true signals. The false signals increase in power, lift the receiver tracking loops off of the true signals, and then drag the tracking loops and the navigation solution to erroneous, but consistent results. This paper develops codeless and semi-codeless spoofing detection methods for use in inexpensive, narrow-band civilian GNSS receivers. Detailed algorithms and analyses are developed that use the encrypted military P(Y) code on the L1 GPS frequency in order to defend the open-source civilian C/A code. The new detection techniques are similar to methods used in civilian dualfrequency GPS receivers to track the P(Y) code on L2 by cross-correlating it with P(Y) on L1. Successful detection of actual spoofing attacks is demonstrated by off-line processing of digitally recorded RF data. The codeless technique can detect attacks using 1.2 sec of correlation, and the semi-codeless technique requires correlation intervals of 0.2 sec or less. This technique has been demonstrated in a narrow-band receiver with a 2.5 MHz bandwidth RF front-end that attenuates the P(Y) code by 5.5 dB.
Collaborative Opportunistic Navigation
(2012) Kassas, Zak; Pesyna, Kenneth M. Jr; Humphreys, Todd E.
Collaborative Opportunistic Navigation
(2013) Kassas, Zaher M.; Humphreys, Todd E.
A Collection of Observations and Advice on University Teaching
(2011) Humphreys, Todd

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