Integrity of Satellite Navigation in the Arctic TODD WALTER & TYLER REID STANFORD UNIVERSITY APRIL 2018
Satellite Based Augmentation Systems (SBAS) in 2018 2
SBAS Networks in 2021? 3
What is Meant by Integrity? Integrity for aviation encompasses several aspects Flags to indicate whether service the service is safe to use An upper bound on position errors An associated probability for this bound A maximum time to notify users if the bound becomes invalid Example requirements for aircraft on precision approach Real-time upper bound must be below 35 m The probability that the error is greater than the real time bound must be less than 0.00001% (10-7 ) per approach The user must be notified within six seconds of any unsafe change 4
Threats and Threat Models Threats lead to inaccuracies in the positioning Threat models limit the extent of the threats Provide description of threat, magnitude of possible effects, and upper limits on the likelihood of occurrence Descriptions for both nominal and faulted conditions Models must be comprehensive Must span all feared events Other threats must be sufficiently rare such that they can be neglected System must mitigate full threat space to sufficient probability
GNSS Error Sources 1-Clock and Ephemeris 2-Signal Deformation 3-Code-Carrier Incoherence 4-Inter-Frequency Bias Nominal Narrow fault Wide fault Orbit/clock estimation and prediction inaccuracy Nominal differences in signals due to RF components and waveform distortion Incoherence in generated code and carrier signals Delay differences in satellite payload signal paths at different frequencies Includes clock runoffs, bad ephemeris, unflagged maneuvers Failures in satellite payload signal generation components Failures in satellite payload signal generation components Failures in satellite payload signal generation components Erroneous Earth Orientation Parameter (EOP), ground-inherent failures N/A N/A N/A 5-Satellite Antenna Bias Look-angle dependent biases caused at satellite antennas Failures in satellite antenna components 6-Ionosphere Incorrectly modeled ionospheric delay Large ionospheric deviations due to disturbed ionosphere N/A Multiple large ionospheric deviations due to disturbed ionosphere 7-Troposphere Incorrectly modeled tropospheric delay N/A N/A 8-Receiver Noise and Multipath Nominal noise and multipath errors Receiver fault or strong multipath reflection Receiver fault or multiple strong multipath reflections
Increase relative to minimum Satellite Clock and Orbit Error Uncertainty Uncertainty increases away from monitoring stations SBAS bounds this error using MT27 or MT28 7
Undersampled Ionospheric Threat Condition Small scale features may not be adequately sampled by monitoring stations Simplified broadcast model has limited fidelity 8
WAAS Measurements WAAS did not sample the largest iono delays at this time Threat model dictates further protection against gaps in observation 9
Dual Frequency GNSS The addition of a second civil frequency to GPS allows users to directly and robustly estimate ionospheric delay Eliminates the direct delay error Undersampled errors are eliminated Multipath errors are amplified by this approach Galileo and other constellations also are dual frequency Ionospheric scintillation remains as a potential outage source Not a factor at mid-latitude Significant in both equatorial and polar regions 10
WAAS Scintillation Outages Multi-directional Scintillation affects polar, mid-latitude, and equatorial regions differently Courtesy: Eric Altshuler Sequoia Research Pointing South 11
Limits to Coverage in the Arctic The reference network sets the limits on coverage Clock/orbit and ionospheric uncertainty grows as user move outwards None of the SBAS reference networks span the Arctic Dual frequency improves coverage but does not solve all problems Satellites do not reach as high elevation angles in the Arctic Creates weaker geometries to support vertical positioning The SBAS satellite are in geostationary orbits Are too low to be reliably tracked above 76 N 12
Current SBAS Coverage Arctic Circle Summer Sea Ice Extent 13
Current SBAS + SDCM, BDSBAS, & KASS Arctic Circle 14
Dual Frequency Multi-Constellation SBAS GPS + BeiDou GPS + GLONASS GPS + Galileo Arctic Circle 15
Summary Significant effort is required to provide integrity on all forms of GNSS derived positioning Understanding and mitigating of all likely threats The reference networks through satellite clock/ephemeris and ionospheric uncertainty determine the limits where service is available Expanded networks increase the coverage region Dual frequency virtually eliminates ionospheric uncertainty SBAS delivery through GEO satellites also limits arctic coverage Other satellite orbital types will be available in the future Scintillation is an important concern in the Arctic But primarily for service availability and continuity, rather than integrity 16
Backups 17
Mean Daily L2 Cycle Slips in Hawaii Black line is 28 day average daily cycle slips Correlated with solar activity Courtesy: Eric Altshuler 18
Mean Daily L2 Cycle Slips for Alaskan Auroral Region Black line is 28 day average daily cycle slips Green line is detrended Ap data Cycle Slip Activity never drops completely to zero Courtesy: Eric Altshuler 19
Percentage of Days Affected by L1 Scintillation Scintillation affects the polar region over more time and larger areas than for other regions Can be mitigated by receiver tracking loop improvements Courtesy: Eric Altshuler 20