GNSS I Principle and Concept - SIRAJ

G N S S I P r i n c i p l e a n d C o n c e p t 

Anne-Laure Vogel – Egis Avia 

Rabat, 17-10-2011

GPS GLONASS GNSS Operations Comparison 

Table of Contents 

Core constellations 

GPS 

GLONASS 

GNSS 

Concept 

ABAS 

SBAS 

GBAS 

Operational aspects 

System Comparison 

Conclusion 

Conclusion

GPS GLONASS GNSS Operations Comparison Conclusion 

Principle Error Sources 

GPS Global Positioning System 

Broadcast- 

Receiving 

Antenna 

Monitor 

Master Center 

5 US Ground Stations 

navigational update: 

- Synchronization of satellites atomic 

clocks 

- Adjustment of the ephemeris of each 

satellite's internal orbital model 

ICAO 

Constellation 

24 satellites (nominal) 

Six orbital planes, four 

satellites per plane 

The receiver 

computes position 

and time 

L2 

1227.6 MHz 

P(Y)-Code 

L1 

1575.42 

MHz 

C/A Code


Principle Error Sources ICAO 

GPS Principle 

1. Distance measurement 

(pseudo-range) between user 

and satellite based on the 

difference of time 

2. Knowledge of the satellite 

position at the time of 

transmission 

3. Determination of navigation 

solution 

4. Errors computation 

3 

4 

1 

2



GPS: Error Sources 

Pseudo-range measurements may be affected by: 

different type of errors: 

Atmospheric propagation 

(ionosphere, troposphere) 

Multipath




Pseudo-range measurements may be affected by: 

different type of errors: 

95% of the time, 

position within 

a radius of 45 m 

Ephemeris and clock errors 

Satellite failures 

Selective Availability (artificial falsification of the time in the signal, 

obsolete � turned off in 2000) 

With SA ON With SA OFF 

95% of the time, 

position within a 

radius of 6.3 m




GPS Error source GPS error (meters) 

Satellite Clock 2 

Ephemeris 2.5 

Receiver 1 

Ionospheric 

Propagation 

5 

Tropospheric 

Propagation 

0.5 

Multipath 1 

Ionosphere is the main error contributor for GPS L1 only civil users


Principle Error Sources 

ICAO 

GPS: ICAO GNSS Concept 

GPS alone is sufficient in terms of 

acurracy but NOT integrity from Enroute 

to NPA 

Different 

augmentations 

have been 

designed by 

ICAO to 

minimize GPS 

errors and 

provide 

integrity for 

different phases 

of flight 

Signal In Space 

requirements 

(Annex 10)


1 2 3 4 

GLONASS GLObal NAvigation Satellite System 

GLONASS achieved 100% coverage of Russia's territory by 

2010 

The full orbital constellation of 24 satellites was restored, 

enabling full global coverage in October 2011 

4 more satellites to be launched before the end of 2011


Concept ABAS SBAS 

GNSS: Global Navigation Satellite System 

Core 

Constellations 

GPS 

GLONASS 

GNSS I 

GBAS 

ABAS 

Airborne Based 

Augmentation 

System 

SBAS 

Satellite Based 

Augmentation 

System 

GBAS 

Ground Based 

Augmentation 

System 

RAIM 

AAIM RAIM 

EGNOS 

WAAS 

MSAS 

… 

GBAS / LAAS


Concept ABAS SBAS GBAS 

ABAS Augmentation: RAIM & AAIM 

Receiver Autonomous Integrity Monitoring (RAIM): 

Uses redundant measures of GNSS pseudo-ranges 

RAIM availability allows aviation operations from en route to NPA – current 

RAIM techniques do not support integrity monitoring for vertical guidance 

during approaches 

Widely implemented from general aviation to commercial aviation - INS may be 

used in complement to improve continuity of service. 

Two basic functions (receiver dependant): 

Fault Detection (FD) 

Fault Detection and Exclusion (FDE) 

Aircraft Autonomous Integrity Monitoring (AAIM): 

Uses INS filtering techniques to control the integrity of GNSS pseudo-ranges 

AAIM availability allows aviation operations from en route to NPA – current 

AAIM techniques do not support integrity monitoring for vertical guidance 

during approaches 

Implemented on higher end commercial aviation only. 

Supports Fault Detection and Exclusion (FDE) with an increased availability wrt 

RAIM



ABAS 

SBAS GBAS 

ABAS Augmentation: RAIM performance 

With 24 satellites, in average: 

For receivers that cannot take advantage of SA being discontinued, 

RAIM availability is 99.99 % for En-route and 99.7 % for NPA 

FDE availability is 99.8 % for En-route and 89.5 % for NPA 

For receivers that can take advantage of SA having been 

discontinued (e.g. SBAS receivers), with 24 satellites, in average: 

RAIM availability is 100 % for En-route and 99.998 % for NPA 

FDE availability is 99.92 % for En-route and 99.1 % for NPA



SBAS Augmentation: Principle 

Central 

Control 

Facility 

ABAS 

SBAS 

Uplink 

Stations 

GBAS 

GPS Satellites 

Master 

Stations 

Network of ground 

reference stations 

Geostationnary 

satellites


Concept ABAS SBAS GBAS 

SBAS Augmentation: Principle 

SBAS provides, via geostationary satellites: 

integrity 

differential correction information 

GPS-like ranging signal 

SBAS comprises: 

a) a network of ground reference stations that monitor satellite signals; 

b) master stations that collect and process reference station data and 

generate SBAS messages; 

c) uplink stations that send the messages to geostationary satellites; and, 

d) transponders on these satellites that broadcast the SBAS messages on 

the GPS frequency.



ABAS SBAS 

GBAS Augmentation: Principle 

© EUROCONTROL 

GBAS



ABAS SBAS 

GBAS 

GBAS Augmentation 

The GBAS ground installation monitors GPS and/or GLONASS signals at an 

aerodrome and broadcasts locally relevant: 

integrity monitoring for ranging sources 

pseudorange corrections 

provide GBAS related data 

approach data (final approach segment) via a VHF Data Broadcast (VDB) to 

aircraft within a nominal range of 23NM. 

When an SBAS GEO is available, GBAS can provide differential corrections for 

the GEO ranging signal. 

A single GBAS ground installation may provide guidance 

for: 

up to 49 precision approaches within its VDB coverage, 

serving several runways and possibly several aerodromes, 

an unlimited number of users (aircraft) within its 

coverage volume.


Integrity Today Future 

GNSS: Integrity Concept 

NPA and 

APV Baro-VNAV 

APV SBAS 

50 m 

to 

35 m 

(LPV 200ft) 

Even if it is the same APV concept, 

40 m 

� APV SBAS is very similar to a Cat I approach 

APV SBAS PA Cat I 

PA Cat I 

10 to 15 m 

� APV Baro VNAV is actually a NPA with vertical guidance 

556 m (0.3NM) 

NPA and APV Baro-VNAV 

?? m


Integrity 

GNSS Approach and Landing Operations: 

Today 

OPERATION En 

route 

ABAS 

SBAS 

GBAS 

Today Future 

Terminal NPA APV I PA 

CAT I 

PA 

CAT II 

PA 

CAT III 

Airport 

Surface 

Building aviation navigation operations requires: 

A signal in space supporting the target phase of flight requirements 

(Accuracy, Integrity, Continuity of service, Availablity) 

An airborne system certified and operationally approved for the 

target phase of flight 

Procedures assuring other aircraft protection and obstacle clearance


Integrity Today Future 

GNSS Approach and Landing Operations: 

In the future 

Getting benefice from other constellations (Galileo for example), other usable 

frequencies (L5 for example), the following operations could be envisaged using 

augmentations: 

OPERATION En 

route 

ABAS 

SBAS 

GBAS 

Terminal NPA APV I PA 

CAT I 

Existing GBAS standards support Cat I operations 

PA 

CAT II 

PA 

CAT III 

Airport 

Surface 

Future standards will cover Cat II/III operations: 

Studies are on-going for middle term implementation with current mono-frequency 

GPS (concept GAST-D) 

In the long term, GBAS Cat II/III will be based on multi-frequencies / multiconstellations


Systems comparison (non exhaustive) 

System Advantages Drawbacks 

GNSS GPS / 

GLONASS 

�Can be used 

anywhere 

�Free system 

ABAS �Easy 

implementation 

�Different 

architectures 

exist 

�GPS is not compliant with ICAO 

requirements for civil aviation operations 

�Managed by 1 state (Selective 

Availability for example for GPS) 

�For the moment can only be used to 

reach NPA performance 

�Difficulty to predict performances for all 

users


Systems comparison (non exhaustive) 

System Advantages Drawbacks 

GNSS SBAS �Improve GPS signals (accuracy, integrity, 

availability and continuity) 

�Vertical guidance 

�Quasi CAT I can be reached (LPV 200 ft) 

�No local infrastructure on airports 

GBAS �Improve GPS signals (accuracy, integrity, 

availability and continuity) 

�Precision approaches (CAT I for the moment, 

CAT II/III under standardisation) 

�Improved airport capacity in low visibility 

conditions (no sensitive areas) 

�Single installation for multiple runways/airports 

precision approaches 

�Reduced frequency contention (1 frequency for 

VDB emission only) 

�It took a long time 

for users to show their 

interest particularly in 

Europe 

�CAT I suffers from 

high competition 

from ILS CAT I and 

SBAS: no 

development plan for 

GBAS CAT I 

�CAT II / III 

approaches not 

available for the 

moment


Conclusion 

Development and improvement of GNSS is still on-going to 

have better performances, safer procedures, better capacity… 

GNSS more and more used for the different phases of flight 

(en route, terminal, approach and landing) 

CAT II/III operations can only be reached with conventional 

means for the moment 

GNSS is a key enabler for Performance Based Navigation 

(PBN) implementation

GNSS I Principle and Concept - SIRAJ

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