11 Satellite Navigation Systems

1.

The DP INDUCTION COURSE
Module 11
Satellite Navigation System
GNSS (Global Navigation Satellite Systems)
&
Differential systems
Handbook ver. 1
Wednesday, 11 February 2026

2. Global Navigation Satellite Systems

Several systems in use provided by various
countries either alone or in co-operation with
others.
Serve a variety of users including Military,
Security, Aviation, Maritime & numerous Civilian
purposes.
Most commonly quoted and used is the GPS
system provided by the USA
Nb. Intention here is to provide a broad understanding of
its use for DP purposes and whilst it focuses on GPS
is generally applicable for any other systems

3. The GPS System

The GPS (Global Positioning System) is wholly
owned and operated by the US Dept. of Defence
There is no technical or political input from any
other agency or nation
GPS encryption is not within our control
GPS signals may be jammed or “spoofed” by third
parties
GPS was made available to foreign civilian users
and declared fully operational on 17th July 1995

4. The GPS System

SYSTEM CONSISTS OF 3 OPERATIONAL
SEGMENTS
Space segment
Minimum 24 operational satellites
Control segment
Ground Control station
Ground monitor and uplink stations
User Segment
GPS receiver/processor equipment

5.

Typical GPS
display data:
Sercel
NR 203

6. GPS accuracy

GPS positioning accuracy
C/A (Course Acquisition) code +/- 20m
Military P (Precise) code
Accuracy dependant on:
Satellite orbit
Atmospheric path propagation
Clock stability
Multipath signals
Satellite numbers (min 4)
Satellite geometry
Selective Availability
+/- 10m

7. The removal of SA from the GPS transmissions

SA (Selective Availability) is a deliberate
downgrading of GPS data quality for civilian
users, imposed by the US DoD
SA set to zero on 2nd May 2000 at 0400 UTC
US DoD reserves right to re-apply SA
This does NOT give civilian users access to the
P-code transmissions

8.

9. The effects of the removal of SA

May 1st 2000, with SA
100m
May 3rd 2000, without SA
100m

10. SATELLITE GEOMETRY

THE LOWER THE SV
ELEVATION, THE
GREATER THE
HORIZONTAL
RESOLUTION
HDOP = HORIZONTAL
DILUTION OF
PRECISION
N
N
10 DEGREE
ELEVATION
MASK
GPS
SATELLITES
ZENITH
ZENITH
W
E
W
S
S
POOR GEOMETRY
HIGH VALUE OF
HDOP (>5)
E
GOOD GEOMETRY
LOW VALUE OF
HDOP (<3)

11. Differential GPS

12. What is DGPS ?

All satellites are observed from a fixed location Reference
Station
Reference Station location is accurately known, thus
accurate ranges may be CALCULATED
The Reference Station also MEASURES ranges to all
satellites
The Reference station compares MEASURED and
CALCULATED ranges. The differences are the range errors
Corrections for these errors are computed
Transmits DGPS correction signals to users
The user system is able to update its PSRs to increase
accuracy

13. DIFFERENTIAL GPS PRINCIPLE

GPS SVs
DIFFERENTIAL
GPS PRINCIPLE
GPS position calculated at
vessel
GPS position
calculated at Ref
station
Corrected position
computed at vessel
Corrections
computed and
transmitted to
vessel
NOTE: This is a “Single
Reference station” solution

14. The IALA Beacon Network (UK System)

DGPS correction signals available free of charge
Transmission of corrections via MF radio network
Single reference station solutions
Correction update rate 5 - 10 sec
Correction Age of Data (AOD)
Typical accuracies:
15 sec
<200 km 5 - 10m (68% drms)
Operational ranges
by day up to 100km
by night or in poor atmospheric conditions - down to
50km or even zero!
Overall poor reliability. Unsuitable for mainstream DP

15. Network DGPS

An array of Reference stations provides a Network
Each station computes its own corrections
Corrections sent to a central Hub Control station
Corrections multiplexed (compressed into a single signal)
Corrections transmitted by radio/satellite system
Corrections received on board, where a Multi-reference
solution is computed
Real-time QA monitoring provides system warnings

16. Network Differential GPS

GPS SV
GPS signals
received by vessel,
Hub station and all
Reference stations
Reference station
network
Hub or Master
reference station
Network Differential GPS

17. Network Differential GPS

GPS SV
Ref station data transmitted to
the Hub
Network Differential GPS

18. Network Differential GPS

GPS SV
INMARSAT
SV
Multiplexed corrections transmitted
to user vessel
Vessel system computes best
solution related to vessel position
and distances to ref ststions
Network Differential GPS

19. Differential Corrections

Correction data may be transmitted to the user by a
variety of media.
HF/MF
- range 400 - 800 km
Inmarsat link
- worldwide coverage on low power
Spotbeam link
- high power “local” coverage
UHF
- range 40 km (line of sight)
NB. UHF link usually related to a temporary reference station
located on a platform

20. Platform-based UHF reference

GPS SVs
REFERENCE
STATION
LOCATED ON
PLATFORM
DIFFERENTIAL CORRECTIONS
BY UHF TELEMETRY

21. Fugro SeaSTAR Network

Transmission of corrections via Inmarsat or Spotbeam
Wide-area multi-reference DGPS network of over 100
stations
Manned reference Hub/Monitor stations at Aberdeen,
Houston and Perth
Dual-frequency service available (SeaSTAR Plus)
Multi-reference solution computed on board using multiple
reference station corrections (centralised network)
Typical accuracies:
< 500km
- 1 - 3m
<1000 km - 2 - 4m

22. Fugro SeaSTAR Network Low power Inmarsat service

23. DUAL-FREQUENCY DGPS

Intended to counter effects of localised
ionospheric interference (scintillation)
Dual-frequency reference station monitors
L1 and L2
L1 & L2 suffer different delays due to
ionospheric activity
The FUGRO SeaStar Plus is dual-frequency

24. GNSS system problems - Sunspot activity

Magnitude largest in equatorial areas
Activity might affect GPS (and communications)
Magnitude varies with sunspot cycle
Roughly follows an 11 year cycle

25. Ionospheric Disturbances

26. DUAL-FREQUENCY DGPS

SATELLITE
EXTREME
REFRACTION
CAUSED BY
SUNSPOT ACTIVITY
REFRACTION
L1
USER
L2
REFRACTION
L2
THESE ARE
ALL KNOWN
VARIABLES
USER
L1
USER
CORRECTED
REFRACTION
CORRECTED
ENABLING
CORRECTION AT THE
USER LOCATION
REFERENCE
STATION
USER
VESSEL

27. DGPS for DP - problems

Proximity of large structures/vessels
Masking or loss of GPS or correction
signals
Multipath errors
Poor satellite geometry
Sunspot activity
Quality of correction data - AOD and long
distances
Operator induced problems !

28. GPS Multi-path problems

GPS SATELLITES
REFLECTED SIGNALS
MAY COMBINE WITH
DIRECT SIGNALS,
CAUSING FADING AND
SIGNAL LOSS
GPS Multi-path
problems
REFLECTED
SIGNAL
REFLECTION
CAUSES RANGE
JUMP
GPS SIGNALS
BLOCKED BY
STRUCTURE
DP CRANE BARGE

29. Multifix

A solution to a specific problem area
In some vessels, antenna location can be
poor causing shadowing and major
multipath
Multifix combines input from multiple GPS
receivers
Antennae can be widely separated
Position data combined and pooled to
provide PRS input to DP

30.

31.

All hardware contained in a single computer
GPS 1
GPS 2
SIX GPS
RECEIVERS
GPS
DATA
VERIFY
GPS 3
VERIFIED GPS DATA
FOR SPECIFIC
LOCATION
GPS 4
GPS 5
GPS 6
Diff RX 1
THREE
CORRECTION
RECEIVERS
POSITION
OUTPUT
TO DP
REAL-TIME
OFFSET
DATA
Diff RX 2
Diff RX 3
GYRO 1
REAL-TIME
OFFSET
PROGRAM
GYRO 2
THREE
HEADING
AND
ATTITUDE
SENSORS
GYRO 3
APPROXIMATE
POSITION
MRU 1
MRU 2
MRU 3
ROLL, PITCH
HEAVE,
HEADING
DATA
MULTIFIX
POSITIONING
AND QC
SOFTWARE
MULTIFIX
HARDWARE
LAYOUT

32.

GPS ANTENNA
LOCATIONS

33. Multifix operational observations

Position very stable and reliable
Position stability not affected by multipath
and variable obstructions on working vessel
DPOs can effectively manage the system

34. Relative GPS (DARPS)

GPS POSITION
REFERENCE
GPS POSITION
REFERENCE
TURRET-MOORED FPSO
TELEMETRY
DATA LINK
SHUTTLE TANKER
TURRET
FPSO USING THRUSTER
ASSISTED MOORING ON
A WEATHERVANE HEADING
MOORING
LINES
LOADING HOSE
FPSO
OFFTAKE
REFERENCE
POINT
RISERS
SYSTEM ALLOWANCES MADE FOR
ANTENNA/REFERENCE POINT OFFSETS
FINAL DATA GIVES RANGE AND
BEARING ACROSS LOADING HOSE

35. Relative GPS

G-Vec is an Absolute and Relative
position and heading reference

36. DGPS accuracy

Typical overall accuracy: 1 – 3m
Dependent on:
Single or Dual-frequency service
Age of corrections (AOD or “latency”)
Distances from Reference stations
Single or multi-reference station solution
SV geometry (HDOP value)
Presence/absence of ionospheric disturbances

37. IMCA DQI Differential Quality Index

DQI is a factor indicating the quality of the
differential positioning ability
DQI is used by the DP system as an alarm and
rejection criterion

38. GNSS - other satellite-based systems

GLONASS
The “Galileo” system

39. The GLONASS system

Russian system comparable to GPS
No Selective Availability
Satellite orbital inclination of 65° giving improved
coverage in polar regions
Full space segment of 24 satellites
Dual GPS/GLONASS receivers available
Differential GLONASS solutions in place
Maintenance & repair is underway
Improved satellites:
Glonass-M satellites with a predicted life of 7
years (earlier satellites 3 years)
Glonass-K satellites, 10 years

40. GNSS - other satellite-based systems

GLONASS
The “Galileo” system

41. The “Galileo” system

A new satellite-based navigation system fully
equivalent to the GPS system is under
development
Owned and controlled by the EU
4 satellites currently in orbit
Further 6 launches planned for 2014
30 operational satellites planned by 2019
Orbital inclination 56°
Currently under test

42. GNSS - other satellite-based systems

GLONASS
The “Galileo” system
Other countries with GNSS systems
include:
China
- Compass/Beidou
India
- GAGAN (primarily for airspace)
- Quasi-Zenith Satellite System (QZSS)
Japan

43. GNSS - Pros and Cons

ADVANTAGES
Worldwide, 24 hour coverage
Accurate system
Data free of charge
Shipboard equipment cheap and simple to operate
Globally referenced systems
DISADVANTAGES
Stability problems close to large structures
Differential services expensive
AOD or Latency values may be unacceptably high
Ionospheric activity problems