GPS World

Tag: MATLAB

  • Research Online: Laser localization system implementation, UAS sense and avoid integrity

    Implementation of a Laser Localization System

    By Aidan F. Browne and David Vutetakis, The University of North Carolina at Charlotte.
    Presented at IEEE/ION PLANS 2016 in Savannah, Georgia.

    A novel laser-beacon localization system has been developed that has applications in positioning and navigation of mobile ground or aerial vehicles where other forms of localization are absent (such as GPS). The system allows for accurate position determination within an area of interest with reasonable accuracy.

    The overall operation of the system is accomplished using only two external co-located beacons and a single on-board detector to perform pseudo-triangulation. The two beacons are spaced two meters apart, and continuously scan the area of interest in a sweeping fashion. As a beacon sweeps across the area of interest, its instantaneous angle is encoded in the pulse frequency of its emitted laser beam using a unique range of frequencies. A rotating detector on the vehicle is continually scanning over a 360-degree arc; it captures and decodes received beacon information in combination with its own relative angle at time of receipt.

    The system has been successfully modeled in MATLAB to evaluate its effectiveness in terms of spatial localization accuracy under thousands of scenarios as well as to analyze the effects of the error parameter variations.

    A prototype of the system has been realized using stepper motors, TTL-modulated 4.5 milliwatt line-generating lasers and a transimpedance amplified photodetector. Initial system testing has been promising with consistent results, indicating that the assumed error levels for the model were reasonable. Testing is underway to validate the results of the model and demonstrate the feasibility of the system.

    UAS Sense and Avoid Integrity

    By Michael B. Jamoom, Mathieu Joerger, and Boris Pervan, Illinois Institute of Technology
    Presented at IEEE/ION PLANS 2016 in Savannah, Georgia.

    Sense and avoid (SAA) concepts and methods can be tools for certification authorities to set potential requirements for integrating unmanned aircraft systems (UAS) into the National Airspace System.

    One new method seeks to ensure the safety of SAA functions for UAS in the presence of multiple intruders. Integrity and continuity are used as quantifiable safety performance metrics, and are addressed though determination of the probability of data mis-associations for multiple intruders. A miss-association occurs when the system incorrectly associates one intruder’s measurement with another intruder’s trajectory. Incorrect intruder associations are hazardously misleading information, impacting integrity. Likewise, a detected mis-association can result in a break in the continuity of the SAA operation.

    A sensitivity analysis is performed based on two two-intruder encounters. The resulting impact of mis-associations between multiple intruders on integrity and continuity is quantified for a nominal composite SAA sensor.

  • Carrier-Phase Anomalies Detected on SVN-48

    By Brady O’Hanlon, Mark L. Psiaki, Paul M. Kintner Jr., and Steven P. Powell

    Anomalous behavior of the L1 C/A-code carrier phase has been detected on PRN07/SVN-48. The anomalies are sudden step-like changes of phase by about 10 degrees/5 millimeters. These steps are followed by negative steps of the same magnitude that restore the original phase time history. These anomalous square pulses have been observed with durations as short as 0.1 seconds and as long as 600 seconds. They can occur about once a minute or be absent for hours.

    These anomalies could be of consequence for some GNSS applications. For precise monitoring of differential total electron content (TEC), the magnitude of this anomaly is the same order as the signals of interest. Precise point positioning (PPP) systems seek to achieve CDGPS accuracy without direct double-differencing. The lack of double-differencing would allow any L1 C/A carrier phase anomaly to directly affect the PPP solution.

    This behavior was detected when testing a dual-frequency software receiver that processes the GPS civilian signals on L1 and L2. The anomaly was first noted when calculating carrier-phase-based TEC:

    where bTEC is a bias term that occurs in the phase-based calculation. Figure 1 shows a plot of the resulting TEC, after removal of its mean value, with six square-edged pulses that range in duration from 0.1 to 590 seconds, with the first a short one at t = 48 seconds. The last pulse starts at 710 seconds and ends at 1300 seconds. In all cases, the anomaly consists of a positive step change in TEC followed some time later by a negative step change of identical magnitude. Step magnitudes in the range 0.04 to 0.07 TEC units have been observed.

    Figure 1. Square pulses on phase-based TEC due to L1 C/A carrier phase anomalies.

    Tests were performed to ascertain whether the anomalies were caused by the L1 signal, the L2 signal, or a combination of the two. Additional tests ruled out receiver malfunction as the cause of the anomalies.

    Observation of detrended L1 and L2 carrier-phase time histories quickly revealed that the anomalies occur on the L1 carrier phase. The detrended L1 C/A carrier phase shows square-edged pulses corresponding to times, magnitudes, and signs of the TEC anomalies, but the detrended L2C carrier-phase plots show no such pulses. Figure 2 shows a typical detrended L1 C/A beat carrier-phase anomaly.


    Figure 2. A typical detrended L1 C/A beat carrier-phase anomaly.

    Extensive tests checked whether the anomalies may have been caused by the receiver. They were initially discovered using a digital storage receiver of raw RF front-end samples followed by off-line software receiver processing. Such carrier-phase anomalies could result from signal glitches in the RF front-end’s mixing chain, from data recording anomalies in the RF front-end samples, or from errors in the software receiver code. The former two possibilities were ruled out by two means. One was to process signals from other satellites for the same RF samples. Mixing problems or data sample problems would cause similar anomalies on all GPS signals, but other GPS signals were found to be free of anomalies. Additional tests used simultaneous data collection by two digital storage receivers spaced 700 meters apart and using different RF front-end hardware. Both receivers showed identical anomalies at identical times.

    Software receiver code errors were ruled out by employing two independent sets of receiver processing code, one developed in MATLAB, the other in C. These two pieces of software were developed independently by different individuals and run independently by their developers. Both showed identical anomalies.

    A final check used a different receiver, the NovAtel GSV4004B. Figure 2 plots its detrended L1 C/A carrier phase along with that of the C-based Cornell software receiver. Both show the same anomaly. Thus, the anomalies appear to be caused by the SVN-48 transmitter.

    All observations were made from roof-mounted antennas in Ithaca, New York. The anomalies were first observed on March 24, 2010 and were observed again on April 1, 5, 7, and 29, and as late as May 13th. For one period of several hours on May 11, no anomalies occurred. Other Block IIR-M satellites have been monitored briefly, but without finding any similar anomalies to date: SVNs 58, 55, 57, 49, and 50.