
24
eventsmayleadtopositioningerrors.Satellite‐Based
Augmentation Systems (SBAS) are designed to
augment the navigation system constellations by
broadcasting additional signals from geostationary
(GEO) satellites. EGNOS (European Geostationary
Navigation Overlay Service) is the European SBAS
providing an augmentation service to the Global
PositioningSystem(GPS).
EGNOShasbeen
designedtobroadcastaGPS‐like
rangingsignalinEuropewithembeddedcorrections,
providing improved performances over GPS. With
EGNOS, all navigation receivers will benefit from
enhanced accuracy, availability and continuity over
GPS.
TheEGNOScoverageareaisWesternEurope,but
could be readily extended to include other regions
within
the broadcast area of the geostationary
satellites,such asAfrica. EGNOSisthefirstelement
of the European satellite‐navigation strategy and a
major stepping‐stone towards Galileo, Europeʹs own
globalsatellitenavigationsystemforthefuture.
In addition to EGNOS, there are other SBAS
around the world with similar
characteristics and
compatible among them. Figure 1 presents the
coverageofthedifferentSBASsystemsintheworld.
Figure1.ExistingandplannedSBASsystems
The main objective of the EGNOS Open Service
(EGNOSOS)istoimprovetheachievablepositioning
accuracybycorrectingseveralerrorsourcesaffecting
theGPSsignals.Thecorrectionsfreelytransmittedby
EGNOSgeostationarysatellitescontributetomitigate
the ranging error sources related to satellite clocks,
satellitepositionandionosphericeffects.The
EGNOS
OS minimum accuracy is specified in the following
table[1].
Table1.EGNOSOSHorizontalandVerticalAccuracy
_______________________________________________
Accuracy DefinitionValue
_______________________________________________
Horizontal Correspondstoa95%confidence 3m
boundofthe2‐dimensionalposition
errorinthehorizontallocalplanefor
theWorstUserLocation
Vertical Correspondstoa95%confidence 4m
boundofthe1‐dimensionalunsigned
positionerrorinthelocalverticalaxis
fortheWorstUser
Location
_______________________________________________
Accuracy Definition Value
2 ONBOARDGNSSRECEIVERS‐SOLAS
CONVENTION
To have a picture of what kind of navigation
equipment can be found onboard vessels, SOLAS
Convention [2] must be consulted. The SOLAS
Conventionisconsideredasthemostimportantofall
international treaties concerning the safety of
merchantships.ChapterVwithinSOLAS
Convention
deals with safety of navigation; it identifies
navigationsafetyserviceswhichshouldbeprovided
by Contracting Governments and sets forth
operational provisions applicable in general to all
shipsonallvoyages.OfspecialinterestisRegulation
19 within chapter V, which establishes the carriage
requirementsforshipbornenavigationalsystemsand
equipment.
2.1 SatelliteNavigationEquipment
AccordingtothatRegulation,allshipsirrespectiveof
size are required to be fitted with a GNSS receiver.
This will probably be a GPS receiver using the US
GlobalPositioningSystemwhichmayormaynotbe
equippedtoprovidedifferentialcorrections,sincethe
carriage
of a DGPS receiver or an SBAS enabled
receiver is not mandatory. The question is: Does a
simpleGPSreceiverfulfiltheIMOrequirementsinall
navigationphases?
Themostcommonsystemusedasprimarymeans
of navigation is GNSS, however currently available
GNSS do not fulfil IMO requirements in regards
to
accuracy and integrity in all the navigation phases.
AccordingtoResolutionA.915(22)[3]onthe“Revised
maritimepolicyandrequirements forafutureGlobal
Navigation Satellite System (GNSS)” GPS (and
GLONASS) has been recognized as a component of
theWorldWideRadionavigationSystem
1
(WWRNS)
for navigational use in waters other than harbour
entrancesandapproachesandrestrictedwaters.That
is,GPSalone,withoutaugmentationisnotenoughin
thesesituations.
A.915(22) recognises that differential corrections
canenhanceaccuracy(inlimitedgeographicareas)to
10 m or less (95%) and also offer external integrity
monitoring. In this sense, this Resolution mentions
the following techniques that can improve the
accuracy and/or integrity of GPS and GLONASS by
augmentation
2
:
Differential correctionsignals fromstationsusing
the appropriate maritime radionavigation
frequency band between 283.5 and 325 kHz for
localaugmentation.
Craftorreceiverautonomousintegritymonitoring.
Integrated receiverscombiningsignals fromGPS,
GLONASS, LORAN‐C and/or Chayka (a Russian
1
InMay2016,theIMOrecognisedalsoGalileoaspartof
theWorldWideRadioNavigationSystem.
2
TotakeintoaccountthatResolutionA.915(22)wasadopt‐
edon29November2001andconsequentlyitisnotupdat‐
ed.Someofthesystemshavebeendecommissioned,forin‐
stanceLORAN‐C,andtechnologicaladvancesarenot
considered.