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

2. 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 ?

3. 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 USA Satellite Navigation System
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

5. 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 ~20’200 km
Orbital period of ~12 hours
Repetition of orbits in ~24 hours (23 hours 56 minutes)

7. GPS Signal Structure Carrier frequencies Ranging codes
L MHz
L MHz
Ranging codes
L1: C/A (civil) and P (military)
L2: P (military)
Right Hand Circularly Polarized Signal

8. 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

9. GPS Ground Segment 1 Master Control Station 5 Monitoring Stations
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

10. 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)

11. 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 ?

12. 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

13. EGNOS Partners European Tripartite Group European Space Agency ESA
Part of ARTES 9 program
European Commission
Multimodal users and funding
Eurocontrol
Civil aviation users

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

15. 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

16. 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 ?

17. EGNOS Architecture

18. 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)

19. 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

20. 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

21. Ground Segment: NLES Navigation Land Earth Station
Transmitting the augmentation message to each GEO satellite
EGNOS: 6 (2 per GEO), ESTB: 1

22. 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)

23. 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

24. 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

25. 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 ?

26. EGNOS/ESTB signal Specifications in RTCA MOPS DO229
EGNOS SIS is broadcast on the GPS L1 ( 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

27. 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)

28. 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)

29. Message Type 1 Mask for assignation of the satellites
GPS (PRN 1-37)
GLONASS (PRN 38-61)
SBAS (PRN )
Application of the corrections to the right satellite (maximum 51)

30. 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)

33. 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 ( m2 - ”Not Monitored”)

34. IGPs for the world

35. Ionospheric Delay Computation
Ionospheric Pierce Point (IPP)

36. 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

42. 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

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

44. 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

45. 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 Pseudorange smoothing
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)

48. 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

50. Integrity Mechanism The Protection Levels Compared to Alert Limits
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

51. 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

52. 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