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Augmentation Testing. Introduction. Problem… SPS GPS positioning accuracy is ~10 meters because of uncertainty in the satellite clocks, ephemerides and atmospheric delays Currently GPS does not provide enough integrity for safety of life (SOL) requirements

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augmentation testing

Augmentation Testing

Proprietary & Confidential—Page 1

  • Problem…
    • SPS GPS positioning accuracy is ~10 meters because of uncertainty in the satellite clocks, ephemerides and atmospheric delays
    • Currently GPS does not provide enough integrity for safety of life (SOL) requirements
    • Thus, GPS isn’t accurate enough and doesn’t have enough integrity for some applications, such as automatically landing Unmanned Aerial Vehicles (UAVs), docking ships, precision aircraft approach and landings, etc.
  • Solution…
    • Improve the GPS positioning accuracy and system integrity!
    • How?
      • Using Differential GPS and Augmentation techniques and methods

Proprietary & Confidential—Page 2

augmentation systems
Augmentation Systems
  • Augmentation means that the GPS signals are augmented by additional information and signals to improve Integrity and Accuracy
  • Why augment GPS?
    • GPS has very low inherent integrity
      • Data message is fixed length and inflexible
      • System warnings about faulty satellites can take 10’s of minutes to be incorporated
      • Unsuitable for Safety-of-Life applications, such as landing commercial airliners
    • GPS L1 accuracy is compromised by system errors
      • Ephemeris inaccuracy, Troposphere/Ionosphere effects

Proprietary & Confidential—Page 3

augmentation systems cont
Augmentation Systems (cont.)
  • What augmentation systems are available?
    • Ground Based Augmentation Systems (GBAS)
      • Examples: DGPS, LAAS, JPALS, GRAS, RTK, pseudolites
      • Typically local or regional, distances 10 – 100km
      • Provide a rapid response to local users for increased integrity
      • Continuous, real-time monitoring and broadcast of corrections
      • Typical position accuracy improved to < 3 meters (with carrier-phase measurements accuracy improved to centimeters)
    • Space Based Augmentation Systems (SBAS)
      • Examples: WAAS, EGNOS, MSAS, GAGAN
      • Provides corrections over a broader region
      • Possible to inform the users within 6 seconds on problems that occur with the GPS system for improved integrity
      • Typical position accuracy improved to < 3 meters

Proprietary & Confidential—Page 4

differential gps dgps
Differential GPS (DGPS)

GPS Satellites

Xuser, Yuser, Zuser

PR and ΔPR Corrections

Xmeas, Ymeas, Zmeas

PR Error

GPS Receiver

Xactual, Yactual, Zactual

Measured – Actual = Error

Proprietary & Confidential—Page 5

dgps details
DGPS Details
  • Both user and reference receiver measurements are temporally and spatially correlated
    • Thus, DGPS techniques can effectively remove the uncertainties of satellite clock and ephemeris
    • Atmosphere may be correlated depending upon the ionosphere activity and locations within the troposphere
      • Correlations improve as the user gets closer to the reference
      • Range of corrections can be up to 100 km
    • Any multipath is uncorrelated
  • Corrections are typically applied in real-time, but susceptible to transmission latencies which can affect performance
  • DGPS can also be used for providing carrier-phase measurements in real-time for centimeter accuracy to support Real-Time Kinematic (RTK) testing

Proprietary & Confidential—Page 6

  • Based on the DGPS concept of a system at a known location providing corrections to a user to augment the users Global Navigation Satellite System (GNSS) navigation solutions
  • The receivers have to be capable of receiving and applying these corrections
  • What corrections? How often? What format? ….
  • Thankfully there are some standards!
    • RTCM (RTCM SC-104)
    • LAAS (DO-224)

Proprietary & Confidential—Page 7

  • Radio Technical Committee for Maritime (RTCM) SC-104 standard
    • Specifies the code and carrier correction data messages for DGPS applications
  • RTCM messages are commonly used by DGPS and RTK systems for transmitting correction data to users
  • There are different ‘Types’ of RTCM messages, some common ones are:
    • Type 1: Code differential corrections
    • Type 3: Reference station information
    • Type 9: Similar to Type 1
    • Types 18-19: RTK Carrier corrections
    • Types 20-21: RTK PR corrections

Proprietary & Confidential—Page 8

rtcm example from simgen
RTCM Example (from SimGEN)

Measured *

- Truth =


Range Rate correction

Carrier Phase correction

* Note: This value is corrected for declared clock errors (af0, af1, af2)

Proprietary & Confidential—Page 9

  • Local Area Augmentation System (LAAS) is another application of DGPS defined in Interface Control Document (ICD) RTCA/DO-246B
  • Ground-based system, intended for use at airfields being developed by the FAA to ultimately permit Category III GPS precision approach and landings
  • Uses GPS monitoring stations and a broadcast station for complete airport coverage of accurate DGPS corrections and integrity information to aircraft within 20-30 miles of the airport

GPS Satellites

Monitoring Stations

LAAS Broadcast Station

Proprietary & Confidential—Page 10

laas vhf broadcast
LAAS VHF Broadcast
  • Similar to the RTCM standard, LAAS utilizes up to 5 message types for augmenting GPS around the airport
  • The primary message types are:
    • Type 1: Differential corrections data, time of validity, error estimates, etc.
    • Type 2: GBAS related data, numbers of monitor stations installed, tropospheric data, monitor station locations, effective range for valid operation
    • Type 4: Final approach data, airport ID, approach category, runway number/letter, threshold height, glide path angle
    • All messages have CRC error detection

VHF Data Broadcast

Proprietary & Confidential—Page 11

  • Provides satellite and ionosphere corrections over a wide region for increasing GNSS navigation accuracy and integrity
    • Examples are:
      • Wide Area Augmentation System (WAAS) – North America (Operational 2003)
      • European Geostationary Navigation Overlay Service (EGNOS) – Europe (Pre-Operational)
      • MTSAT Satellite-based Augmentation System (MSAS) – Asia (Operational 2007)
  • The requirements for the receiver to use these messages are specified in DO-229C (Minimum Operational Performance Standards for Global Positioning System / Wide Area Augmentation System Airborne Equipment) by RTCA, Inc.
  • Before WAAS, the U.S. National Airspace System (NAS) did not have the potential to provide horizontal and vertical navigation for approach operations for all users at all locations…SBAS makes this possible


Proprietary & Confidential—Page 12

sbas cont
SBAS (cont.)
  • Consists of monitoring stations and geostationary satellites located over the desired region
    • Monitoring stations observe and measure GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere
  • The computed corrected differential messages get compiled into a 250bps Navigation data message on the ground and transmitted to the SBAS satellites for relaying the message to users
    • Transmitted to users on L1 (1575.42 MHz)
    • Future provisions may support L5 (1176.45 MHz) for increased accuracy


Proprietary & Confidential—Page 13

sbas cont1
SBAS (cont.)
  • Primary objectives are to determine the following for users to improve accuracy and integrity:
    • Integrity information, able to respond within 6 seconds of detection of an anomalous GPS satellite
    • Fast-changing component due to the satellite clock error
    • Slow-changing component due to the satellite ephemeris
    • Slow-changing component due to the ionosphere propagation delays for a set of points corresponding to a latitude/longitude grid

As a result, the FAA can guarantee both Horizontal and Vertical Protection Levels (HPL and VPL) for aircraft on their CATI approaches, which require a Localizer Performance with Vertical guidance (LPV) of 200 feet


Proprietary & Confidential—Page 14

sbas cont2
SBAS (cont.)
  • Wide Area Augmentation System (WAAS) – North America
    • 38 monitoring stations
    • 2 geostationary satellites
      • PRN135 - Intelsat Galexy XV 133°W
      • PRN138 - TeleSat Anik F1R 107.3°W
  • European Geostationary Navigation Overlay Service (EGNOS) - Europe
    • 33 monitoring stations
    • 3 geostationary satellites
      • PRN120 - Inmarsat-3-F2/AOR-E 15.5°W
      • PRN124 - Artemis 21.5°W
      • PRN126 - Inmarsat-3-F5/IOR-W 25°W
  • MTSAT Satellite-based Augmentation System (MSAS) – Asia
    • 8 monitoring stations
    • 2 geostationary satellites
      • PRN129 – MTSAT-1R 140°E
      • PRN137 – MTSAT-2 145°E


Proprietary & Confidential—Page 15

spirent s augmentation capability
Spirent’s Augmentation Capability
  • SBAS
    • WAAS, EGNOS and MSAS simulation capability supported
    • All SBAS signals are based on ICD RTCA/DO-229C November 28th 2001: Minimum operational performance standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment
  • GBAS
    • RTCM correction messages are supported for DGPS and RTK applications
    • LAAS simulation is based on GNSS Based Precision Approach Local Area Augmentation System (LAAS) Signal-in-Space Interface Control Document (ICD) RTCA/DO-246B
      • GSS4150 Very High Frequency (VHF) transmitter for RF transmission of LAAS messages

Proprietary & Confidential—Page 16

simulating dgps

Spirent generated DGPS corrections (RS-232)

  • NMEA 0183
  • RTCM SC-104
Simulating DGPS
  • With a single RF output, Spirent supports generation of RTCM DGPS and RTK corrections
  • The NMEA Output capability supports generation of the DGPS RTCM message types 1,3, and 9
  • The RTCM definition file supports RTCM DGPS and RTK message types 1, 3, 9,16,18-22, and 58
  • The corrections can be logged to a file or output in real-time via RS-232

Receiver Under Test


DGPS corrections

User DGPS System

Proprietary & Confidential—Page 17

simulating dgps cont

Receiver Under Test

Simulating DGPS (cont.)
  • Multiple RF outputs (from either a single chassis or multiple) support system level hardware in the loop testing
  • Permits testing with receiver(s) under test AND the DGPS/RTK base station receivers for generation of the necessary DGPS/RTK corrections

Receiver Under Test



DGPS/RTK corrections

DGPS Receiver

Proprietary & Confidential—Page 18

simulating laas
Simulating LAAS
  • LAAS simulation is supported either on RS-232 or transmitted on VHF using the GSS4150
  • Users can control:
    • Define message options (Identifier, Delays, Data errors, Format Rate)
    • Set broadcast frequency (108 to 117.975MHz), signal power profile
    • Differential Corrections
    • Reference Station specifics
    • Data errors and modification

Receiver Under Test





Proprietary & Confidential—Page 19

simulating sbas
Simulating SBAS
  • If enabled, SBAS can be transmitted on both L1 and L5 frequencies
  • Similar to the common GPS Constellation editor
  • Supports definition and control of:
    • Monitoring stations
    • Message output control, interval, sequence, etc.
    • Navigation data errors
    • Navigation data modification
    • Signal control, latencies, covariance, degradation, etc.
    • SBAS satellite PRNs, locations and motion
    • Ionosphere errors and reference grid data

Proprietary & Confidential—Page 20

sbas constellation file
SBAS Constellation file
  • Separate files for each constellation type
    • Allows for concurrent SBAS type simulation
  • All 3 SBAS constellations use the same editor screens
    • Similar editor to the GPS/GLONASS/Galileo Constellation editors
  • Each SBAS satellite occupies a single hardware channel, just like each GPS, GLONASS and Galileo SV

Proprietary & Confidential—Page 21

sbas monitoring station control
SBAS Monitoring Station control
  • Specify location of monitoring stations
    • Normally one would be in the region of the simulated vehicle

Proprietary & Confidential—Page 22

sbas message output control
SBAS Message Output Control
  • Used to define the output interval rate and priority on a per message basis
  • Mixed correction message has no rate
    • This message provides the fast/long term correction data
    • If enabled the determination of transmission is automatic

Proprietary & Confidential—Page 23

sbas message output control1
SBAS Message Output Control
  • Table shows priority order if multiple messages scheduled for same time
  • Low priority messages may be interleaved with high
  • If no message is scheduled, a Null message is sent
  • Thought and consideration needs to be given to message timing
    • If intervals are too short and required data is large there may be insufficient time to broadcast the data in the specified interval
      • Scenario then halts with an error

Proprietary & Confidential—Page 24

covariance matrix
Covariance Matrix
  • Defines the clock-ephemeris covariance matrix values as broadcast in message type 28 (MT28)
  • Different data for each SV can be entered
  • If arbitrarily defined, or if it becomes invalid with time, Spirent recommends to only use the data to verify receiver decoding of the message
  • By default to mitigate this, all the diagonal terms in the matrix are set to the same value and the off-diagonal terms are set to zero.
    • This makes the degradation term independent of satellite and user position.
  • Selecting automatic SimGEN calculates the covariance data based on the relative ground station and satellite locations.
    • Note: at least four ground stations are required
  • Refer to the WAAS ICD for more detail!

Proprietary & Confidential—Page 25

user range error
User Range Error
  • Allows user to control User Differential Range Error Indicator (UDREI) values broadcast in the fast correction message
  • Users can either use the grid (manual) or have SimGEN generate the UDREI values (automatic)
    • If Manual, the UDREI values are defined for the relative locations of the monitoring stations, thus the UDREI values get worse the farther away (NOTE: SimGEN’s default should be updated to be more accurate)
    • To enable Automatic, the covariance matrix has to be automatic. The user defines the system wide limit for SBAS pseudorange measurement accuracy. Entering an optimistic value for Measurement accuracy lets you reduce the total number of ground station locations while giving “good” UDREI values.

Proprietary & Confidential—Page 26

sbas general options per satellite
SBAS General Options Per Satellite
  • PRN assignment 120-138
  • URA value as per Message Type 9 (MT9)
  • Base Power level offset (relative to –130dBm)
  • MT9 message clock offset and drift terms
  • Selecting the ‘Use nav. Data file’ option specifies that the entire message stream (including CRC and FEC) for the selected satellite is defined by the contents of a user generated ASCII file

Proprietary & Confidential—Page 27

sbas satellite motion almanac
SBAS Satellite Motion Almanac
  • Specify the geostationary position for each satellite in the SBAS constellation type
  • Remember to have an SBAS satellite covering the area of the vehicle
  • Take care when using velocity, acceleration and jerk for each SBAS satellite to exercise the appropriate Nav Message data
    • The effects are cumulative with time and can become very large
  • The real SBAS satellite locations may change over time, thus they may be different from SimGEN’s default locations

Proprietary & Confidential—Page 28

ionosphere grid selection
Ionosphere Grid Selection
  • SBAS satellites broadcast Ionospheric Delay Correction data using a reference grid covering the full surface of the Earth
  • The grid is broken down into Bands
    • Bands 0 to 8 include North and South hemisphere latitude values for a limited range of longitude (40 degrees) on a Mercator projection map, giving 1808 points in total.
    • Bands 9 and 10 define a total of 384 points in the North and South Polar Regions.
    • Note: Certain grid points are common between Bands 0 to 8 and Bands 9 and 10
  • Spirent recommends using the ‘Full’ mode for defining the reference grid for each SBAS satellite
    • This mode provides increased fidelity over the abridged mode

Proprietary & Confidential—Page 29

ionosphere grid selection1
Ionosphere Grid Selection
  • In practice, SBAS satellites only broadcast correction data for the Bands and grid points in the geographical area of the SBAS service provider.
  • Message type 18 (MT18) provides a Band mask that defines the grid points in use for each Band being broadcast
  • Message type 26 (MT26) provides the Ionospheric Delay Corrections (e.g. vertical delay values) that relate to the points defined by each valid Band mask.
    • Calculated using the GPS (Klobuchar) Ionospheric model at the valid grid points
    • They are consistent with the delays applied in the simulated GPS signals and change with time

Proprietary & Confidential—Page 30

ionosphere error data
Ionosphere Error Data
  • Specify Grid Ionospheric Vertical Error Indicator (GIVEI) values in Iono Correction Message Type 26 (MT26)
    • You can also use the GIVEI values to introduce an element of error into the correction data
    • This dialog is similar to that for defining UDREI values

Proprietary & Confidential—Page 31