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EGNOS Training Course. EGNOS Demonstration in China . O. Perrin, Tianjin, 2 December 2003. 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 ?. Contents.

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egnos training course
EGNOS Training Course

EGNOS Demonstration in China

O. Perrin, Tianjin, 2 December 2003

contents
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 ?
contents3
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 ?
gps basics
GPS Basics
  • NAVSTAR GPS
    • Navigation Signal Timing and Ranging, Global Positioning System
  • USA Satellite Navigation System
  • Developed in the 60’s
    • Merge of Transit and Timation projects
  • Military system made available free of charge to the civil user community
gps space segment
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
gps orbits
GPS orbits
  • Medium Earth Orbits (MEO)
  • 6 orbital planes, inclination 55 degrees
  • 4 operational plus 1 spare per plane
  • Altitude of ~20’200 km
  • Orbital period of ~12 hours
  • Repetition of orbits in ~24 hours (23 hours 56 minutes)
gps signal structure
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
c a ranging code
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
gps ground segment
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
gps user segment receivers
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)
contents11
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 ?
egnos background
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
egnos partners
EGNOS Partners
  • European Tripartite Group
  • European Space Agency ESA
    • Part of ARTES 9 program
  • European Commission
    • Multimodal users and funding
  • Eurocontrol
    • Civil aviation users
egnos schedule
EGNOS Schedule
  • Critical Design Review
    • January 2002
    • EGNOS design frozen
  • Operational Readiness Review
    • 2004
    • Technical validation of EGNOS
    • Start of initial operations
and the estb
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
contents16
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 ?
ground segment rims
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)
ground segment mcc
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
what is the cpf computing
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
ground segment nles
Ground Segment: NLES
  • Navigation Land Earth Station
  • Transmitting the augmentation message to each GEO satellite
  • EGNOS: 6 (2 per GEO), ESTB: 1
space segment
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)
user segment
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
what is egnos providing
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
contents25
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 ?
egnos estb signal
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
message structure
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)
message type 0
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)
message type 1
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)
fast corrections
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)
udrei
UDREI
  • Can be transmitted in
    • MT 2-5 (normal case)
    • MT 6 (all UDREIs, case of an alarm)
long term corrections
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)
ionospheric corrections
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”)
ionospheric delay computation
Ionospheric Delay Computation
  • Ionospheric Pierce Point (IPP)
ionospheric delay computation36
Ionospheric Delay Computation
  • Interpolation and slant delay computation
degradation parameters
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
geo navigation message
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
geo almanacs message
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)
sbas network time
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,)
sbas service message
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
clock ephemeris covariance
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
additional messages
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
tropospheric corrections
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
contents45
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 ?
position computation
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
measurement model
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)
solving the equations
Solving the equations
  • Linearisation of the equation system
  • Least Square Adjustment using a weight matrix
integrity mechanism
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
integrity mechanism50
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 < AL  integrity is assured
    • PL  AL  integrity can not be assured
protection levels
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

reference documents
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
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