EGNOS Training Course

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 ?

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 • 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 • 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 • 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 • 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 • 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) • 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)

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

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 ? • 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

EGNOS Architecture

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

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 • 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 ? • 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

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

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 • 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”)

IGPs for the world

Ionospheric Delay Computation • Ionospheric Pierce Point (IPP)

Ionospheric Delay Computation • Interpolation and slant delay computation

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 • 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 • 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 • 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 • 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 • 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 • 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 • 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

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

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 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

EGNOS Training Course