EGNOS Training Course

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EGNOS Training Course

• EGNOS Demonstration in China

O. Perrin, Tianjin, 2 December 2003
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Contents

• Short GPS Refresher
• What exactly is this EGNOS Project ?
• How does EGNOS work ?
• What is EGNOS transmitting ?
• What is the user computing with the EGNOS signal ?

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Contents

• Short GPS Refresher
• What exactly is this EGNOS Project ?
• How does EGNOS work ?
• What is EGNOS transmitting ?
• What is the user computing with the EGNOS signal ?

4
GPS Basics

• NAVSTAR GPS
• Navigation Signal Timing and Ranging, Global
  Positioning System
• USA Satellite Navigation System
• Developed in the 60s
• Merge of Transit and Timation projects
• Military system made available free of charge to
  the civil user community

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GPS Space Segment

• Currently 28 operational satellites
• Block I not available any more
• Currently block II and IIA satellites only
• Currently launched satellites IIR (in the future
  IIR-M with new L2C and M codes)
• Evolutions block IIF (L5) and GPS III
• Fitted with atomic clocks (Rubidium or Caesium)
  for stable frequency reference

6
GPS orbits

• Medium Earth Orbits (MEO)
• 6 orbital planes, inclination 55 degrees
• 4 operational plus 1 spare per plane
• Altitude of 20200 km
• Orbital period of 12 hours
• Repetition of orbits in 24 hours (23 hours 56
  minutes)

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GPS Signal Structure

• Carrier frequencies
• L1 1575.42 MHz
• L2 1227.60 MHz
• Ranging codes
• L1 C/A (civil) and P (military)
• L2 P (military)
• Right Hand Circularly Polarized Signal

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C/A ranging code

• Pseudo Random Noise (PRN) to identify the
  satellites (CDMA)
• Navigation data
• 50 bps
• Satellite ephemeris
• Satellite almanacs (whole constellation)
• Satellite health status
• UTC information
• Ionospheric parameters
• Satellite clock correction

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GPS Ground Segment

• 1 Master Control Station
• Located in Colorado Springs, USA
• 5 Monitoring Stations
• Hawaii, Ascension Island, Diego Garcia,
  Kwajalein, and Colorado Springs
• However, constant tracking of all satellites is
  not achieved
• One of the reasons for lack of integrity
• Need for an augmentation for safety-of-life users

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GPS User Segment (receivers)

• Measure the travel time of the signal and
  multiply it by the speed of light (one-way
  ranging)
• Computation of 3D position by triangulation
• Rx clock is usually not an atomic clock
• Rx clock offset is an additional unknown
• Rx measure pseudo-distances
• 4 satellites are needed to compute a position (3
  coordinates plus receiver clock)

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Contents

• Short GPS Refresher
• What exactly is this EGNOS Project ?
• How does EGNOS work ?
• What is EGNOS transmitting ?
• What is the user computing with the EGNOS signal ?

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EGNOS Background

• European Geostationary Navigation Overlay Service
• Global Navigation Satellite System of the 1st
  generation (GNSS-1)
• Augmentation of the existing GPS (US) and GLONASS
  (Russia) constellations
• Project launched in 1998
• Service for safety-of-life users

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EGNOS Partners

• European Tripartite Group
• European Space Agency ESA
• Part of ARTES 9 program
• European Commission
• Multimodal users and funding
• Eurocontrol
• Civil aviation users

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EGNOS Schedule

• Critical Design Review
• January 2002
• EGNOS design frozen
• Operational Readiness Review
• 2004
• Technical validation of EGNOS
• Start of initial operations

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And the ESTB ?

• EGNOS System Test Bed
• Prototype system of EGNOS available since early
  2000
• Reduced system
• Allows users to gain experience by tests and
  demonstrations
• Allows testing of expansion capability
• System used for the Chinese tests

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Contents

• Short GPS Refresher
• What exactly is this EGNOS Project ?
• How does EGNOS work ?
• What is EGNOS transmitting ?
• What is the user computing with the EGNOS signal ?

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EGNOS Architecture
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Ground Segment RIMS

• Ranging and Integrity Monitoring Stations
• Channels A and B for redundancy
• Some stations have a channel C
• Equipped with an L1/L2 receiver and atomic clock
  for precise timing
• Track GPS, GLONASS and GEO
• EGNOS 34, ESTB 12 ( 3 China)

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Ground Segment MCC

• Master Control Centres
• Central Processing Facility (CPF)
• Automatic processing of raw data coming from RIMS
• Independent check of measurements of RIMS A by
  RIMS B
• Central Control Facility (CCF)
• Monitoring and control of EGNOS
• EGNOS 4, ESTB 1

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What is the CPF computing ?

• Integrity Information
• For each satellite monitored
• Differential Corrections
• Pseudo-range corrections
• Orbit and clock corrections
• Ionospheric Corrections
• Single layer ionospheric model for L1

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Ground Segment NLES

• Navigation Land Earth Station
• Transmitting the augmentation message to each GEO
  satellite
• EGNOS 6 (2 per GEO), ESTB 1

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Space Segment

• Existing GPS and GLONASS
• 3 Geostationary Satellites
• Inmarsat AOR-E (PRN 120)
• Inmarsat IOR-W (PRN 126)
• Artemis (PRN 124)
• Broadcasting an augmentation signal on GPS
  frequency L1
• EGNOS 3 GEOs, ESTB 1 (IOR, 131)

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User Segment

• Any user equipped with a GPS receiver with
  firmware able to process SBAS data (EGNOS is
  broadcast on L1)
• Mainly navigation applications
• Civil aviation
• Road transports
• Maritime
• Rail

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What is EGNOS providing ?

• Improved availability
• The GEOs can be used as additional ranging
  sources (GPS-like)
• Improved accuracy
• Thanks to differential corrections
• Improved integrity
• Thanks to real-time monitoring (6s TTA)
• Improved continuity

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Contents

• Short GPS Refresher
• What exactly is this EGNOS Project ?
• How does EGNOS work ?
• What is EGNOS transmitting ?
• What is the user computing with the EGNOS signal ?

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EGNOS/ESTB signal

• Specifications in RTCA MOPS DO229
• EGNOS SIS is broadcast on the GPS L1 (1575.42
  MHz)
• GEOs use GPS-like PRN code (ESTB IOR, PRN 131)
• Data rate 250 bits per second
• 5 times faster than GPS data rate
• Forward Error Correction code

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Message Structure

• 1 message 250 bits 1 second
• 250-bit message structure
• 8-bit message preamble (for data acquisition
  purposes)
• 6-bit message type identifier (0 63)
• 212-bit message data
• 24-bit message parity (Cyclic Redundancy Check)

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Message Type 0

• Do not use the GEO for safety applications
• Transmitted every time there is a major problem
  and the system is completely unavailable
• Transmitted during testing phases
• In ESTB, MT0 contains pseudorange corrections (1
  MT 2 in each MT 0 for bandwidth saving reasons)

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Message Type 1

• Mask for assignation of the satellites
• GPS (PRN 1-37)
• GLONASS (PRN 38-61)
• SBAS (PRN 120-138)
• Application of the corrections to the right
  satellite (maximum 51)

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Fast Corrections

• Correction of the fast changing errors (S/A)
• Pseudorange correction for each satellite
• MT 2-5 Fast Corrections for 13 satellites
• Fast correction to be applied to the pseudorange
• Integrity User Differential Range Error
  Indicator (UDREI) (quality of the pseudorange
  after the application of corrections)
• Referring to UDRE (upper bound on the pseudorange
  error after application of the fast corrections,
  with 99.9 probability)

31
UDREI

• Can be transmitted in
• MT 2-5 (normal case)
• MT 6 (all UDREIs, case of an alarm)

32
Long Term Corrections

• Corrections for slow varying errors (satellite
  position, satellite clock)
• MT 25 Long-term Satellite Error Corrections
• Satellite position correction (3 parameters)
• Satellite velocity correction (3 parameters)
• Satellite clock correction (2 parameters)
• If no velocity information 4 satellites otherwise
  only 2 satellites
• MT 24 Mixed Fast Corrections / Long-term
  Satellite Error Corrections (not ESTB)

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Ionospheric Corrections

• MT 18 ionospheric mask
• Ionospheric Grid Points (IGP) mask
• 1808 IGPs (11 bands) all around the world at an
  altitude of 350 km (pre-defined)
• MT 26 L1 ionospheric corrections
• Vertical delay estimate for 15 IGPs (imaginary
  satellite exactly above the IGP, 90 elevation)
• Integrity Grid Ionospheric Vertical Error
  Indicator (GIVEI) (0-15) is also transmitted
• Refers to GIVE (0.0084 m2 - Not Monitored)

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IGPs for the world
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Ionospheric Delay Computation

• Ionospheric Pierce Point (IPP)

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Ionospheric Delay Computation

• Interpolation and slant delay computation

37
Degradation parameters

• In case the user misses one or more messages
• MT 7 Fast Corrections Degradation
• UDRE degradation
• How quick the corrections change
• MT 10 Degradation Factors
• 15 parameters to evaluate the degradation of
  long-term and ionospheric corrections

38
GEO Navigation Message

• MT 9 GEO Ranging Function Parameters (Ephemeris)
  for 1 GEO
• GEO satellite position (X, Y, Z)
• GEO satellite velocity (VX, VY, VZ)
• GEO satellite acceleration (aX, aY, aZ)
• GEO clock offset aGf0 and drift aGf1

39
GEO Almanacs Message

• MT 17 GEO Satellite Almanacs for 3 GEOs
• PRN code number
• Health and status (Ranging, Corrections,
  Integrity)
• Service provider (WAAS, EGNOS, MSAS)
• GEO satellite position (almanac)
• GEO satellite velocity (almanac)

40
SBAS Network Time

• MT 12 SBAS Network Time / UTC Offset Parameters
• UTC parameters to relate EGNOS time to UTC time
  (offset, drift, leap seconds)
• Time information (GPS week number, GPS TOW,)

41
SBAS Service Message

• MT 27 SBAS Service Message
• 1 to 5 Regions can be defined
• Increase UDRE values in selected regions in order
  to guarantee integrity
• New definition (DO229C) implies a triangular or
  rectangular shape region
• In China, ESTB uses the DO229A definition, which
  creates a circular region

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Clock-ephemeris Covariance

• MT 28 Covariance matrix (10 terms)
• Expansion of UDRE as a function of the user
  location
• Provides increased availability inside the
  service area and increased integrity outside
• MT 27 and MT 28 cannot be used together
• Optional message not broadcast by ESTB

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Additional Messages

• MT 62
• Internal Test Message
• Meaningless content
• Not used in ESTB
• MT 63
• Null Message Type
• Filler message if no other message available

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Tropospheric Corrections

• Local phenomenon
• Not sent as part of the EGNOS SIS
• Tropospheric Correction depends from
• Receiver altitude
• Pressure, temperature, humidity
• Day of year
• Latitude
• General model to determine these parameters

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Contents

• Short GPS Refresher
• What exactly is this EGNOS Project ?
• How does EGNOS work ?
• What is EGNOS transmitting ?
• What is the user computing with the EGNOS signal ?

46
Position Computation

• Satellite selection process
• Choice of satellites with SBAS corrections
• If not enough, choice of other satellites
• If no solution is possible with SBAS, Pegasus
  does not compute a solution
• Pseudorange smoothing
• Smoothing filter using carrier phase measurements
  (before corrections)
• Pseudorange correction

47
Measurement Model

• 3D distance equation
• 4 unknowns
• User position (Xu, Yu, Zu)
• Receiver clock offset (DT)
• The user needs to observe at least four
  satellites (same as GPS)

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Solving the equations

• Linearisation of the equation system
• Least Square Adjustment using a weight matrix

49
Integrity Mechanism

• Integrity is the measure of the trust that can be
  placed in the correctness of the information
  supplied by the system
• It protects the user against misleading or wrong
  information
• Integrity has to be assessed by each user,
  depending on the requirement of his application

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Integrity Mechanism

• The Protection Levels
• Depend on the user and satellites position
  (geometry)
• Computed by the user receiver based on
  information sent by EGNOS
• Compared to Alert Limits
• Alert Limits are fixed for a particular type of
  operation
• PL lt AL ? integrity is assured
• PL ? AL ? integrity can not be assured

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Protection Levels

• Protection Levels
• Horizontal and Vertical Protection Levels
• Bound on position error at the 10-7 level
• Multiplication of estimated errors
• Computed using the projection matrix

HPL
VPL
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Reference Documents

• SBAS Specifications
• Radio Technical Commission for Aeronautics
  (RTCA) Minimum Operational Performance Standards
  (MOPS) DO229 C
• Detailed Implementation of MOPS
• Pegasus Technical Note (TN)
• Provided under Pegasus/Documentation