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Simulating GNSS Anomalies

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  1. Simulating GNSS Anomalies Paul Crampton March 2012

  2. How can GNSS Signals get messed up? • In the Control Segment • Finger trouble by the operator • In the Space Segment • Uplink Failures • Satellite Failures • Clocks • Signal Transmission Path (e.g. SVN-49)

  3. How can GNSS Signals get messed up? (2) • In the User Segment • Atmosphere (e.g. Ionospheric Scintillation) • Environment • Terrain Obscuration • Multipath • Interference • Receiver • Antenna • Algorithms • Ground Station Updates (No Blame Implied!) • Clock

  4. Modeling the Mess-Ups • SimGEN’s internal models • Data Errors/Data Modification • Track Errors • Undeclared Satellite Clock Errors • Ionospheric and Tropospheric models • Terrain Obscuration Models • Multipath Models • Interference Signal Simulation • Antenna Effects • G Sensitivity of the receiver clocks

  5. Modeling the Mess-Ups (2) • SimGEN’s new “Cornell Ionospheric Scintillation Model” • SimGEN’s UCD File Capability • Ideal for “other” Unmodeled effects e.g. SVN-49

  6. Modification (MOD) Command • Some tests require more control of the signal characteristics then what is provided “out of the box” by SimGEN • These tests may require modeling: • Ionospheric scintillation • Code/Carrier divergence • Diffuse or user specified multipath • User specified Ionospheric/Tropospheric models • Any other unmodeled effects that need unique control of each satellite code, carrier and amplitude • So for where SimGEN’s default models may not be sufficient, the user can generate their own models using the “MOD” command

  7. MOD Command (cont.) • The MOD command is a SimREMOTE command that can be sent to SimGEN remotely or scripted locally on the SimGEN PC • Using the MOD command, the user has the powerful capability of overlaying user defined code, carrier and amplitude contributions on top of SimGEN’s calculated values • These contributions can be defined for: • Each frequency (L1, L2,L5) • Each satellite • Resolution/transmit rates at up to 100Hz • Multipath echoes • It is important to note that the MOD commands are continuously applied until updated by the user

  8. MOD Command Format • Similar to other SimREMOTE commands, the MOD command has: • Timestamp for specifying the time of applicable for the data in the MOD command • Specifications for which vehicle, antenna and signal type (GPS, GLONASS, etc.) to apply the modifications • Which satellite, channel, multipath echo and frequency to apply the modifications • The modifications to signal power level, carrier and code offsets • Format • <timestamp>, MOD, <veh_ant>, <signal_type>, <svid_chan_num>, <multi_index>, <mode>, <all_flag>, <freq>, <all_freq>, <sig_level>, <carr_offset>, <code_offset>

  9. MOD Command Example • So we’ve shown the format of the MOD command, so let’s review an example… • Example • 0 00:00:10.00, MOD, v1_a1, gps, 14, 0, 0, 0, 0, 1, 3.2, 10.7, 10.65 • Timestamp, apply the modification 10 seconds into the scenario • Apply for vehicle 1, antenna 1 and GPS signal types • 14 (ID) = SVID or Channel 14 • 0 (Multi_Index) = Incident Signal, 1+ represents the “reflection” • 0 (Mode) = SVID, 1 indicates channel number • 0 (All_Flag) = Just for this SVID, 1 indicates all SVIDs/Channels • 0 (Freq) = L1 (for GPS)

  10. MOD Command Example (2) • 0 00:00:10.00, MOD, v1_a1, gps, 14, 0, 0, 0, 0, 1, 3.2, 10.7, 10.65 • Apply the modifications: • 1 (All_Freq) = On all frequencies, 0 indicates just the specified freq. • 3.2 (Sig_Level) = dB increase in signal level • 10.7 (Carr_Offset) = m increase in carrier range • 10.65 (Code_Offset) = m increase in code range

  11. Incorporating MOD Commands into SimGEN • User Command File • On the Scenario Tree – Options Branch • From SimGEN V2.70 on • User Command File tied to the scenario • ASCII text file that uses extension .UCD

  12. User Command File • We’ve introduced the MOD command and shown how that can also be a User Command File, so what is it and what else can it be used for? • The User Command File (*.UCD) is an ASCII text file that contains some commands, such as the MOD command, that are to be used throughout the scenario • In addition to using it for scripted MOD commands, the User Command File allows other SimREMOTE commands to be scripted and used during the scenario • These include for instance: • Turning satellites on/off, maybe to model obscurations • Modifying signal power levels, maybe to model effects on the line of sight such as foliage or building materials • Adding multipath signals

  13. Incorporating UCD Files into SimGEN • As mentioned previously, the User Command Files are incorporated into the scenario in SimGEN under the Options scenario tree shown to the right • It has been available since SimGEN V2.70 • Previously only available via “remote command file” which held no scenario association • Must be enabled in order for SimGEN to apply the commands in the User Command File • It is an ASCII text file that uses the extension .UCD for designating it as a User Command File

  14. Additional User Command File Information • When using commands, the User Command File should NOT contain certain commands such as: • SC, RU, AR, TR, TIME, UTC_OFFSET (that make reference to the scenario) • VEH_ , ANT_ , SIG_ (that make data requests) • Motion commands should be referenced in the .umt file • The User Command File cannot be edited via a SimGEN GUI • Refer to the SimREMOTE manual for ascii syntax of the commands

  15. User Command File Example • Since the User Command File permits various commands to be used during a scenario, an example of a *.ucd text file may be: • 0 00:00:05,POW_ON,v1_a1,1,1,0,0,0 • Turn ON satellite SVID 1 at 5 seconds into the simulation • 0 00:00:30.00,MOD,v1_a1,gps,31,0,0,0,0,1,5,5,5 • At simulation time 30 seconds, apply a power offset of 5dB and code/carrier offsets of 5m to SVID 31

  16. Proprietary & Confidential—Page 16 The Atmosphere (Ionosphere) • Ionosphere • Extends from about 50km to 1000km • Caused by the suns radiation • Changes widely between day and night • Typically stable in temperate zones with more variation near the equator and magnetic poles • Variability from day to day and year to year • Solar activity e.g. sunspots, 11year cycle (next peak around 2013) • Geomagnetic disturbances • Seasons (axial tilt towards the sun) • Composed of ions and free electrons, defined by the total electron content (TEC) • TEC – the number of electrons in a tube of 1 m² cross section extending from the receiver to the satellite

  17. What is Ionospheric Scintillation? • The Sun has an 11 year Sun Spot Cycle • The next maximum is in May 2013 • Last maximum saw outages of GPS lasting 10’s of minutes • In the build-up to the maximum the Ionosphere surrounding the earth intensifies and thickens • Solar flares result in high energy RF bursts which can disrupt GPS signals • Ionospheric and Magnetic storms will also disrupt GPS signals • Ionospheric irregularities cause shift in phase of received GPS signals • Ionospheric irregularities also cause fluctuation of received signal level • Irregularities can fluctuate rapidly and periods of fluctuation can last several hours • Irregularities are localized to a portion of the ionosphere – to the receiver, some satellites will be affected while others will be unaffected

  18. Effect on Receivers • Phase disturbance • Signal Level reduction • Loss of lock • Both resulting in “cycle slips”

  19. Using UCD Files to Model Ionospheric Scintillation • 00:01:39.870,MOD,v1_a1,gps,29,0,0,0,0,1,-0.803,0.030,0.0000 00:01:39.970,MOD,v1_a1,gps,29,0,0,0,0,1,-0.717,0.030,0.0000 00:01:40.070,MOD,v1_a1,gps,29,0,0,0,0,1,-0.845,0.030,0.0000 00:01:40.170,MOD,v1_a1,gps,29,0,0,0,0,1,-0.953,0.031,0.0000 00:01:40.270,MOD,v1_a1,gps,29,0,0,0,0,1,-0.916,0.032,0.0000

  20. Frequency of Ionospheric Scintillation at Solar Maximum

  21. Cornell Ionospheric Scintillation Model • “GNSS and Ionospheric Scintillation – How to Survive the Next Solar Maximum” • July/August 2009 • Paul M KintnerJr, Cornell University, • Todd Humphreys, University of Texas at Austin, • Joanna Hinks, Cornell University • IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 3, NO. 4, AUGUST 2009 “Simulating Ionosphere-Induced Scintillation for Testing GPS Receiver Phase Tracking Loops” • Todd E. Humphreys, Mark L. Psiaki, Joanna C. Hinks, Brady O’Hanlon, and Paul M. Kintner, Jr

  22. Cornell Ionospheric Scintillation Model

  23. Operational Scenario for SVN-49 Testing • Spirent Federal Website • • Test Case Scenario to demonstrate the effect • Realistic as possible • SVN-49 (PRN-01) set healthy and exhibiting the anomaly • Scenario runs for 3 hours and 20 minutes (roughly) • Covers elevation angles from 90 degrees to 0 degrees • L1 and L2 • Fixed Multipath to introduce the 38ns code offset • UCD files introduce the amplitude variation with elevation angle • UCD files introduce the carrier phase inversion

  24. Summary • A great deal of SimGEN’s functionality is devoted to modeling the unfortunate things that can happen to GNSS signals • Models cover • Control Segment problems • e.g. Nav Data Errors and Modifications • Space Segment problems • e.g. Satellite Clock drift • User Segment problems • e.g. Ionospheric Scintillation • e.g. Receiver Clock G sensitivity • Unmodeled effects can be simulated by introducing .UCD files

  25. Thank You Paul Crampton