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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Item 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.Item GPS Carrier Tracking Loop Performance in the presence of Ionospheric Scintillations(2005) Humphreys, Todd E.; Psiaki, Mark L.; Kintner, Paul M.The performance of several GPS carrier tracking loops is evaluated using wideband GPS data recorded during strong ionospheric scintillations. The aim of this study is to determine the loop structures and parameters that enable good phase tracking during the power fades and phase dynamics induced by scintillations. Constant-bandwidth and variable-bandwidth loops are studied using theoretical models, simulation, and tests with actual GPS signals. Constant-bandwidth loops with loop bandwidths near 15 Hz are shown to lose phase lock during scintillations. Use of the decision-directed discriminator reduces the carrier lock threshold by ∼1 dB relative to the arctangent and conventional Costas discriminators. A proposed variablebandwidth loop based on a Kalman filter reduces the carrier lock threshold by more than 7 dB compared to a 15-Hz constant-bandwidth loop. The Kalman filter-based strategy employs a soft-decision discriminator, explicitly models the effects of receiver clock noise, and optimally adapts the loop bandwidth to the carrier-to-noise ratio. In extensive simulation and in tests using actual wideband GPS data, the Kalman filter PLL demonstrates improved cycle slip immunity relative to constant bandwidth PLLs.Item Searching for Galileo(2006) Psiaki, Mark L.; Humphreys, Todd E.; Mohiuddin, Shan; Powell, Steven P.; Cerruti, Alessandro P.; Kintner, Paul M.Statistical analysis techniques have been used to find and decode the L1 BOC(1,1) signal of the first prototype Galileo spacecraft, GIOVE-A. The resulting pseudorandom number (PRN) codes can be used by receiver developers to test their devices on the GIOVE-A signals. The analysis has used codeless techniques to acquire the signal and to remove its carrier and binary offset carrier (BOC) components, and it has determined the timing and chip values of the PRN codes using optimal statistical signal processing methods. The resulting codes' per-chip error probabilities are less than 10-10. The period of the pilot PRN code is 200 ms, which is twice the length published in Galileo documentation.Item GNSS Receiver Implementation on a DSP: Status, Challenges, and Prospects(2006) Humphreys, Todd E.; Psiaki, Mark L.; Kintner, Paul M.; Ledvina, Brent M.A real-time GPS L1 C/A-code software receiver has been implemented on a Digital Signal Processor (DSP). The receiver exploits FFT-based techniques to perform autonomous acquisition down to a threshold of C/N0 = 33 dB-Hz. Efficient correlation algorithms and robust tracking loops enable the receiver to track an equivalent of 43 L1 C/A-code channels in real time with a tracking threshold of 25 dB-Hz. This accomplishment represents a milestone in an ongoing effort to develop a low-cost, flexible, and capable GNSS receiver for use as a scientific instrument and for GNSS receiver technology development. This paper reports on the current design and capability of the DSPbased receiver, provides an overview of the challenges that are particular to embedded GNSS software receiver design, and discusses the prospects of DSP-based GNSS software receivers in relation to the multiple frequencies and higher bandwidths offered by modernized GNSS.Item A Technique for Determining the Carrier Phase Differences between Independent GPS Receivers during Scintillation(2007) Mohiuddin, Shan; Humphreys, Todd E.; Psiaki, Mark L.A method for recovering the carrier phase differences between pairs of independent GPS receivers has been developed and demonstrated in truth-model simulations. This effort is in support of a project that intends to image the disturbed ionosphere with diffraction tomography techniques using GPS measurements from large arrays of receivers. Carrier phase differential GPS techniques, common in surveying and relative navigation, are employed to determine the phase relationships between the receivers in the imaging array. Strategies for estimating the absolute carrier phase disturbances at each receiver are discussed. Simulation results demonstrate that the system can rapidly detect the onset of scintillation, identify one non-scintillating reference signal, and recover the carrier phase differences accurate to 0.1 cycles.Item Considerations for Future IGS Receivers(2008) Humphreys, Todd E.; Young, Larry; Pany, ThomasFuture IGS receivers are considered against the backdrop of GNSS signal modernization and the IGS’s goal of further improving the accuracy of its products. The purpose of this paper is to provide IGS members with a guide to making decisions about GNSS receivers. Modernized GNSS signals are analyzed with a view toward IGS applications. A schedule for minimum IGS receiver requirements is proposed. Features of idealized conceptual receivers are discussed. The prospects for standard commercial receivers and for software-defined GNSS receivers are examined. Recommendations are given for how the IGS should proceed in order to maximally benefit from the transformation in GNSS that will occur over the next decade.Item 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.Item Evaluating GPS Receiver Robustness to Ionospheric Scintillation(2008) Hinks, Joanna C.; Humphreys, Todd E.; O'Hanlon, Brady; Psiaki, Mark L.; Kintner, Paul M.A method for testing GPS receivers for ionospheric scintillation robustness has been implemented using a GPS signal simulator and a statistical model that captures the characteristics of scintillation relevant to receiver performance. This technique will help GNSS equipment manufacturers and users prepare for the approaching solar maximum by enabling repeatable receiver performance tests under realistic scintillation conditions. Ionospheric scintillation can impair the performance of phase tracking loops in GNSS receivers by introducing deep amplitude fades and abrupt phase changes in a signal. A statistical model has been developed that accurately recreates these effects by shaping the complex spectrum rather than treating phase and amplitude individually. Generated scintillation histories have been incorporated into the output of a GPS signal simulator so that any compatible receiver can be evaluated without modification. Such a hardware-in-the-loop approach provides a controlled test environment and the ability to characterize receiver performance statistically by running many experiments. It expands the range of possible test conditions beyond those available during field testing. The method is simple to implement, and its value has been demonstrated by a variety of tests applied to four different receivers.Item Development and Field Testing of a DSP-Based Dual-Frequency Software GPS Receiver(2009) O'Hanlon, Brady W.; Psiaki, Mark L.; Kintner, Paul M.; Humphreys, Todd E.A real-time software GPS receiver for the L1 C/A and L2 C codes has been implemented on a Digital Signal Processor (DSP) and tested in both scintillating and nonscintillating environments. This receiver is being developed as a low-cost space weather instrument with improved tracking robustness in comparison to a traditional semi-codeless dual-frequency receiver and with flexibility in its choices of signal tracking algorithms and data outputs. The receiver is capable of continuous background signal acquisition and utilizes the L1 C/A code to assist in acquisition of the L2 C signal. Efficient on-the-fly generation of oversampled PRN code replicas for the L2 CM and CL codes, which are required for real-time software radio signal processing, has been implemented to ensure a manageable requirement for memory. Bit-wise parallel correlation techniques have been implemented to reduce the number of operations needed for correlation. The receiver currently tracks both the L2 CL and CM codes for the purpose of calculating TEC. Results are presented based on data generated by a signal simulator, on real data taken in Ithaca, NY (42.44 N, 76.48W), and on real data taken during ionospheric scintillation in Natal, Brazil (5.8S, 35.2W) in January 2009. Position and velocity solution accuracy is evaluated using both real and simulated data.Item Advances in GNSS Equipment(2010) Humphreys, Todd E.Item Opportunistic Frequency Stability Transfer for Extending the Coherence Time of GNSS Receiver Clocks(2010) Wesson, Kyle D.; Pesyna, Kenneth M. Jr; Bhatti, Jahshan A.; Humphreys, Todd E.A framework is presented for exploiting the frequency stability of non-GNSS signals to extend the coherence time of inexpensive GNSS receiver clocks. This is accomplished by leveraging stable ambient radio frequency signals, called “signals of opportunity,” to compensate for the frequency instability of the reference oscillators typically used in inexpensive handheld GNSS receivers. Adequate compensation for this frequency instability permits the long coherent integration intervals required to acquire and track GNSS signals with low carrier-to-noise ratios. The goal of this work is to push the use of GNSS deeper indoors or into environments where GNSS may be subject to interference.Item Frontiers in Radionavigation(2010) Humphreys, Todd E.Item The GPS Assimilator: a Method for Upgrading Existing GPS User Equipment to Improve Accuracy, Robustness, and Resistance to Spoofing(2010) Humphreys, Todd E.; Bhatti, Jahshan A.; Ledvina, BrentItem GPS Spoofing Detection System(2010) Psiaki, Mark; O'Hanlon, Brady; Humphreys, Todd E.; Bhatti, Jahshan A.A real-time method for detecting GPS spoofing in a narrow-bandwidth civilian GPS receiver is being developed. It is needed in order to detect malicious spoofed signals that seek to deceive a C/A-code civilian GPS receiver regarding its position or time. The ability to detect a spoofing attack is important to the reliability of systems ranging from cell-phone towers, the power grid, and commercial fishing monitors. The spoofing detector mixes and accumulates base-band quadrature channel samples from two receivers, one a secure reference receiver, and the other the defended User Equipment (UE) receiver. The resulting statistic detects the presence or absence of the encrypted P(Y) code that should be present in both signals in the absence of spoofing.Item Indoor GPS: Tightly Coupled Opportunistic Navigation(2010) Pesyna, Ken; Wesson, Kyle; Bhatti, Jahshan A.; Humphreys, ToddItem Riding out the Rough Spots: Scintillation-Robust GNSS Carrier Tracking(2010) Humphreys, Todd E.Item Radionavigation Integrity and Security(2010) Humphreys, Todd E.Item The GPS Assimilator: a Method for Upgrading Existing GPS User Equipment to Improve Accuracy, Robustness, and Resistance to Spoofing(2010-09) Humphreys, Todd; Bhatti, Jahshan; Ledvina, BrentA conceptual method is presented for upgrading existing GPS user equipment, without requiring hardware or software modifications to the equipment, to improve the equipment’s position, velocity, and time (PVT) accuracy, to increase its PVT robustness in weak-signal or jammed environments, and to protect the equipment from counterfeit GPS signals (GPS spoofing). The method is embodied in a device called the GPS Assimilator that couples to the radio frequency (RF) input of an existing GPS receiver. The Assimilator extracts navigation and timing information from RF signals in its environment—including non-GNSS signals—and from direct baseband aiding provided, for example, by an inertial navigation system, a frequency reference, or the GPS user. The Assimilator optimally fuses the collective navigation and timing information to produce a PVT solution which, by virtue of the diverse navigation and timing sources on which it is based, is highly accurate and inherently robust to GPS signal obstruction and jamming. The Assimilator embeds the PVT solution in a synthesized set of GPS signals and injects these into the RF input of a target GPS receiver for which an accurate and robust PVT solution is desired. A prototype software-defined Assimilator device is presented with three example applications.Item Opportunistic Frequency Stability Transfer for Extending the Coherence Time of GNSS Receiver Clocks(Institute of Navigation, 2010-09) Wesson, Kyle D.; Pesyna, Jr., Kenneth M.; Bhatti, Jahshan A.; Humphreys, Todd E.A framework is presented for exploiting the frequency stability of non-GNSS signals to extend the coherence time of inexpensive GNSS receiver clocks. This is accomplished by leveraging stable ambient radio frequency signals, called “signals of opportunity,” to compensate for the frequency instability of the reference oscillators typically used in inexpensive handheld GNSS receivers. Adequate compensation for this frequency instability permits the long coherent integration intervals required to acquire and track GNSS signals with low carrier-to-noise ratios. The goal of this work is to push the use of GNSS deeper indoors or into environments where GNSS may be subject to interference.Item Opportunistic Frequency Stability Transfer for Extending the Coherence Time of GNSS Receiver Clocks(2010-09-24) Wesson, Kyle D.; Pesyna, Kenneth M. Jr; Bhatti, Jahshan A.; Humphreys, Todd E.