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Neal Fedora

2012 SPIRENT FEDERAL GPS TRAINING CONFERENCE Scenario Generation 2: Building a Realistic Test Environment. Neal Fedora. The Environment. Atmosphere. Example Environments. Spirent Communications Plc Paignton, United Kingdom. Example Environments (cont.). Changing environments

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Neal Fedora

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  1. 2012 SPIRENT FEDERALGPS TRAINING CONFERENCE Scenario Generation 2: Building a Realistic Test Environment Neal Fedora

  2. The Environment Atmosphere

  3. Example Environments • Spirent Communications Plc • Paignton, United Kingdom

  4. Example Environments (cont.) • Changing environments • Tunnels, Buildings, Houses

  5. Example Environments (cont.)

  6. The Test Environment • Atmosphere • Ionosphere and Troposphere effects • Vehicle Effects • Is it static or dynamic? • Where is the antenna located? • Environment Effects • Are there buildings, mountains, hangers, etc. that are obscuring the sky? • Is multipath expected? If so, how much of an effect?

  7. How does SimGEN Model the Environment? • Atmosphere • Ionosphere and troposphere models • Obscurations • Antenna Patterns • Terrain Obscuration model • Vertical Plane model • Land Mobile Multipath • Turn satellites “off”

  8. How does SimGEN Model the Environment? (cont.) • Multipath • Land Mobile Multipath • Vertical Plane model • Reflection Pattern model • Various other types • Ground reflections • Fixed offset • Doppler offset • Legendre • Polynomial • Sinusoidal • Embedded

  9. The Atmosphere • The two layers of the atmosphere that affect GPS the most are the Troposphere and the Ionosphere • Troposphere: Stable with little variation, this is a non-dispersive medium, therefore different wavelengths are delayed the same amount • Ionosphere: Greater variation from sun-night, seasons, solar cycles, etc., this is a dispersive medium • Since a dispersive medium, it refracts the L1 (1575.42 MHz), L2 (1227.60 MHz) and L5 (1176.45 MHz) carriers differently, allowing iono delay measurements L1 L2 L5

  10. The Atmosphere • The amount of refractive delay pseudorange error is dependent upon the path the LOS signal travels through the atmosphere to the receiver • At the horizon it travels through more of the atmosphere than at zenith (directly above), thus low elevation satellites typically exhibit more error on their pseudoranges

  11. The Atmosphere (cont.) • SimGEN uses models to simulate the troposphere and ionosphere delays • There are several models available within SimGEN to model the Troposphere • STANAG (SimGEN’s Default) • NATO Standard Agreement STANAG 4294 Issue 1 • Permits the user to specify the Surface Refractivity Index (Ns) at mean sea level - Range 220 to 420 (defaults to STANAG value 324.8) • Bean and Dutton Model 2 (BD2) • Model 2 by Bean, B.R. and Dutton, E.J., Radio Meteorology, Dover Publications, New York, 1966 • RTCA-1996 (RTCA96) • Tropospheric correction reference model as defined in document RTCA/DO-229 January 16, 1996 • RTCA-1998 (RTCA98) • Tropospheric correction reference model as defined in document RTCA/DO-229 June 8, 1998 • RTCA-2006 (RTCA06) • Tropospheric correction reference model as defined in document RTCA/DO-229 December 13, 2006

  12. The Atmosphere (cont.) • The default SimGEN global ionosphere model is the Klobuchar model defined in ICD-GPS-200, which uses coefficients of amplitude and periodicity of the ionosphere vertical delay • They are used by SimGEN in both the navigation data and for modeling the ionospheric delays on the RF output • Broadcast Klobuchar Data: http://www.ngs.noaa.gov/CORS/data.shtml • Improved Klobuchar Data: http://www.aiub.unibe.ch/content/research/gnss/code___research/igs/klobuchar_style_ionospheric_coefficients/index_eng.html

  13. The Atmosphere (cont.) • Besides SimGEN’s models, users have the ability to incorporate their own atmosphere models and effects • “MOD” commands can be used as a User Command file for full control of code, carrier delays and signal levels for all satellites and frequencies • User Defined Data files permit custom troposphere and ionosphere models among other powerful features • Uses equation: D + Rt + 0.5At2 • D = Delay, R = Rate, A = Acceleration

  14. Obscurations • Depending upon the antenna location, vehicle orientation and environment, there may exist obscurations that block the satellites line-of-sight (LOS) signal • This may be from the vehicle, environment or surrounding objects • SimGEN supports various methods of implementing obscurations in the scenario • Turning satellites on/off • Antenna patterns and antenna switching • Terrain obscuration • Vertical plane model • A careful analysis of the intended environment and the fidelity of the test requirements is necessary to choose the best method

  15. Antenna Patterns • Antenna patterns present an easy way to model both environmental and vehicle obscurations • Using antenna patterns constrains the obscuration to the vehicle and can be switched for modeling various obscuration effects • In the antenna pattern editor the user can model an obscuration by inputting the maximum attenuation (+46dB)

  16. Antenna Patterns (cont.) • Antenna Switching • Used to model loss of signal when entering tunnel, or moving away from a launch pad, hanger or aircraft • An example of a munition antenna switching is shown below

  17. Terrain Obscuration Model • Model assumes an omni-directional view and creates a randomized, repeatable environment based on the user input parameters • Applies to Land Vehicle and Aircraft models only • The Terrain Obscuration Command File allows the user to specify switching of the predefined models throughout the scenario relative to the Distance travelled in the scenario

  18. Vertical Plane Model • The vertical plane file permits the user to specify obscurations of a given height and distance parallel to the vehicle • Multiple vertical planes are supported for sequentially switching of obscurations • Applied at time into run Action Time Vertical Planes Command List

  19. Obscurations from Receiver Data • Most receivers output a message that contains information on the satellites that are being used in the receivers navigation solution • This may be the common NMEA $GSV message or another unique message

  20. Obscurations from Receiver Data (cont.) • The GPS receiver can log the observed C/No for the satellites in view, typically a ‘SV’ message (GISV, GSV, etc) • Knowing the received C/No and satellites in view, the user can use this information to emulate the actual received signals from the environment and antenna • Either remotely or through a command file, the user can use the receivers SV log to turn on/off satellites and/or adjust the transmitted signal power levels to replicate the C/No • SimGEN provides a tool within SimPROCESS that generates this User Command file from a NMEA GSV log file • Using receiver data for adjusting the satellite signals is dependent upon the date, time and location for when the receiver observed the satellites

  21. Multipath • Depending upon the reflective surfaces and environment, the received multipath at the antenna may have unique fading and amplitude characteristics • For instance glass and other smooth surfaces reflect signals much better than rough textured surfaces like brick, which exhibit more diffuse multipath • The following types of multipath models are available in SimGEN • Ground reflection • Fixed offset • Doppler offset • Vertical plane • Reflection pattern (great for vehicle multipath) • Land Mobile Multipath • Legendre • Polynomial • Sinusoidal • Embedded Multipath (8000 only) Multipath Echoes Assigned in SimGEN

  22. Adding Multipath to the Scenario • Multipath signals occupy signal generator channels (except for Embedded Multipath) • Adding multipath signals can be carried out either in real-time during the simulation or prior to running • This is performed by the following methods • Prior to the run Automatically with Land Mobile Multipath Model • Manually in real-time with Channel Assignment window • Prior to the run with Channel Assignment window • Prior to the run via a User Actions file

  23. Multipath Types • Multipath signals are typically affected in both phase and amplitude • The received multipath echo may have some additional signal loss due to freespace loss, changing in polarization, direct or diffuse, etc. • In addition, there is an additional delay associated with the multipath echo because of the increased range off the reflected surface • SimGEN’s multipath models use signal delay and loss for defining the multipath echoes in the simulation • Each model is different, with each requiring different user inputs Select Desired Model User Inputs

  24. Ground Reflection Easy model to use Can select more than one echo for each satellite User defines just the signal loss (dB) of the multipath signal SimGEN will calculate the delay IF a ground reflection exists Fixed Offset Another easy model Can select more than one echo for each satellite User defines both signal delay and loss Can be applied for any satellite being simulated Multipath Types (cont.)

  25. Vertical Plane When using vertical planes in the scenario, the user can select which satellites are to have reflections off of these surfaces User defines the signal loss, carrier doppler and phase and if a reflection is present, then SimGEN will accurate represent the associated delay for the multipath echo Land Mobile Multipath Automatically applies proven Rician and Rayleigh fading models to direct LOS and multipath signals defined by mask editor Applies to static and land vehicles The mask editor allows the user to construct various environments by defining the obscuration's and expected multipath regions Multipath Types (cont.)

  26. Embedded Multipath (8000 Series Only) • Implemented to exploit the capabilities within the GSS8000 simulator • Embedded multipath is a unique method of simulating satellite echoes without occupying an additional simulator output channel • Embedded multipath is only accessed under the User Actions File menu and must be defined prior to running the simulation • This feature permits specifying echoes on each frequency, for each satellite and the associated echo attenuation, delay in step sizes of 2.442 meters and carrier phase lead • Fixed for the duration of the simulation

  27. Putting It All Together…. ~20 meters ~120 meters

  28. Putting It All Together…. ~20 meters ~120 meters

  29. Putting It All Together…. • Logged receiver data can be used for simulating the observed satellite signals for a given date, time and vehicle effects • Vehicle trajectory may be generated from waypoints, SimGENmotion model commands or 6-DOF trajectory file • Incorporate obscurations of the buildings by using vertical planes • If expected, add multipath with Fixed Offset or Vertical Plane • The Land Mobile Multipath Model may also be used for modeling obscurations and multipath when using the static or land vehicle motion models • Use antenna patterns and SimGEN’s antenna switching capability to incorporate the vehicle effects

  30. Summary • SimGEN provides various methods of characterizing the atmosphere, obscuration's and multipath effects in your testing • The modeledenvironment can be as simple (SimGEN’s atmosphere and Terrain obscuration models for example) or as complex (UDD and multipath models for example) as you’d like to make it • A good understanding of your intended conditions is essential for modeling a representative environment in SimGEN • What obscuration will that building or wing exhibit on my antenna? • What type of multipath would I expect to see from that building, road, etc.? • Several of these topics will be introduced and demonstrated in the corresponding workshop following this presentation

  31. Any Questions? Thank you for your attention and time

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