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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 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 ?


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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


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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


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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)


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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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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)


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UDREI

  • Can be transmitted in

    • MT 2-5 (normal case)

    • MT 6 (all UDREIs, case of an alarm)


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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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Ionospheric Delay Computation

  • Ionospheric Pierce Point (IPP)


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Ionospheric Delay Computation

  • Interpolation and slant delay computation


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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


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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


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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)


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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,)


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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 ?


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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


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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


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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 < 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


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