p3 egnos lesson china

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 ?

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 ?

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)

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 ?

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

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 ?

EGNOS Architecture: 

EGNOS Architecture

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

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 ?

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”)

IGPs for the world: 

IGPs for the world

Ionospheric Delay Computation: 

Ionospheric Delay Computation Ionospheric Pierce Point (IPP)

Ionospheric Delay Computation: 

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

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 ?

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 Mechanism: 

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