389

1 INTRODUCTION

Over‐confidence in the modern positioning systems

(especially GNSS) can lead to dangerous situations

whichinturncanleadtoaccidents.Forthisreasonit

isworthtohaveatooltodeterminetheaccuracyand

integrity of positioning data, in addition to the raw

point‐based position. Sat

ellite‐based augmentation

systems (SBAS) such as theEuropean Geostationary

Navigation Overlay Service (EGNOS) provide

ranging signals transmitted by geostationary

satellites. Wide area differential corrections and

additionalparametersaimtoguaranteethe integrity

of the GNSS. Data from EGNOS can be obtained

either via the satellite link, or from ground  systems

ropagating these data online via Internet, such as

Sisnet[7].Thisallows almostanyone to accessthese

datawithoutsophisticatedequipment.

2 PARSINGOFTHEEGNOSMESSAGESS

TheprocessofEGNOS messages parsing consists of

several stages. Data block format of the EGNOS

messageframeispresentedintheFigure1[1].

The beginning of the message frames received

from the sat

ellite can be recognized by the cyclic

prefix:53(16),9A(16)and C6(16), but when data are

receivedas.emsfile(inpostprocessing)eachframeis

precededbyanadditionalsatellitenumber,dateand

timeofregistration[7].

Figure1.EGNOSdatablockformat

After receiving a frame the checksum calculated by

CRC-24Q algorithm should be checked. CRC-24Q is

Conception of EGNOS Integrity Data Utilization in ENC

Systems

M.Bilewski&P.Zalewski

aritimeUniversityofSzczecin,Szczecin,Poland

ABSTRACT: The paper presents the processing of EGNOS integrity data received either directly from the

satellitenetworkortheSisnetinternetrelay.RelationshipbetweentheindividualframesinEGNOSmessages

hasbeen described.Usefulnessofthese datawasindicatedby thethreeexamples. In thefirstexample data

formEGNOSwasusedtocompareint

egrityparametersandthecalculatedpositionerror.Infurtherexamples

thesimulatedpositionwitherrorandcalculatedintegrityparameterswereimplementedtogether.Theresults

werepresentedintwoways:withuseofdedicatedprogramwhichpresentsonlyshipcontours(theonenot

coveringGNSSuncertaintyandtheonecoveringthi

suncertaintycalledMarineVesselProtectionArea)andas

alayerinexistingECDIS.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 10

Number 3

September 2016

DOI:10.12716/1001.10.03.02

390

Cyclic Redundancy Check parity in the 24 bit version

developed by Qualcomm Corporation. After calculating the

checksum, quick method based on the hash table can be

used [8]. In the case of an invalid checksum message is

rejected. In the next stage it is checked whether messages

necessary to parse according to the tree presented in the

Figure 2 were previously received.

The arrows in the Figure 2 indicate the flow of

information. Messages required are connected by the

continuous lines and the messages preferred to parse are

connected by the dashed lines.

The data is then stored in a database to enable intuitive

access. Separate process was designed for reading from this

database and checking if data were obtained correctly and

their ages are valid.



PRNMask

(1)

IonosphericGrid

PointMask

(18)

LongtermSatellite

ErrorCorrections

(25)

FastCorrections

(2‐5)

Integrity

Information

(6)

IonosphericDelay

Corrections

(26)

FastCorrection

DegradationFactor

(7)

GEONavigation

(9)

SBASService

Message

(27)

Degradation

Parameters

(10)

SBASNetworkTime

(12)

GEOSatelite

Alamanacs

(17)

Clock‐Ephemeris

CovarianceMatrix

Message

(28)

InternalTest

Message

(62)

Mixed:

FastCorrections

and

LongtermSatellite

ErrorCorrections

(24)

Figure2.RelationshipbetweenmessagetypesinEGNOS

To extract specific data from .ems files the

dedicatedprogramʺparser_egnosʺwaswritteninC#

on the .Net platform 4.5. The individual data series

aregroupedintoclassesbythisprogramandstored.

Figure3showstherelationshipbetweentheseclasses.

3 COMPARISONOFINTEGRITYPARAMETERS

ANDTHECALCULATEDPOSITIONERROR



Thedatawascollectedduringrealexperimentcarried

out on board of the vessel. The position error was

calculatedasadifferencebetweenobtainedpositions

from GNSS and RTK, which was treated as a

reference position. Integrity parameters were

gathered from the Sisnet database for the

correspondingperiod. The results

ofthecomparison

are presented in the Figure 4 which shows slightly

correlation.



Figure3.Codemapof“parser_egnos”

391



Figure 4. Example of integrity data usage. Estimated

accuracy from EGNOS (

GIVE



) and measured position

error(differencebetweenGNSSandRTKpositions)inone

day

4 DETERMININGTHEMVPA

The concept of Marine Vessel Protection Area

(MVPA)determinationwithEGNOSaugmentedGPS

aspositioningsourceisbasedontheassumptionthat

thevesselisa2‐dimensional(2D)spatialobjectandas

suchispresentedin electronic chart system (ECDIS)

integratedintoFullMissionBridgeSimulator

(FMBS).

The navigator has to monitor the position of the

vesselcontourrelativetothedeclaredsafetyisobaths

(navigationdangers).As aresult,the vesselposition

cannotbetreatedasapoint[6].Theerrorinantenna

positionwillpropagatetothefinalMVPAaccording

totheGauss’sErrorPropagation

Law.Thestatistical

evaluation of this propagation effect has been

presentedin[3]andfullyelaboratedin[4].

For each waterline point the uncertainty ellipse

wasdeterminedtakingintoaccount:

 varianceofGNSSpositionintheEastdirection,

 varianceofGNSSpositionintheNorthdirection,

 GNSSposition

East‐Northcovariance,

 variances of waterline points relative to antenna

(inship’sbodyfixedcoordinateframe),

 varianceofship’sheading,

 coverage k‐factor corresponding  to the expected

confidencelevel.



Figure5. Selection of the two MVPA points (red colour)

from the error ellipse attached to the original point of

waterline(bluecolor).

Estimated error of each ship’s contour point

involvestheerrorsoftwojointlydistributedvariables

of x

Nand yE coordinates. From every ellipse formed

this way the two points are selected to create an

MVPA polyline in order to maximize the possible

surface area. Method of determining these points is

shown on the Figure 5. More details about

determiningMVPApresentedin[5,6].

5 GRAPHICALINTERPRETATIONOFMVPA

Figure

6 shows the developed application which

presents actual shape and position of MVPA with

referencetotheship’swaterlinewithoutuncertainty/

integrity data included in ship’s body fixed

coordinatesystem.InNMEAstandardthepositionin

navigationsystemsistransmittedeverytwoseconds.

Buttons ‘slower’ and ‘faster’,  in section A,

 allows to

increase and decrease frequency of generated

simulated position and calculated MVPA, shown in

section H. Section B is used to switch between two

typesofimaging.Mode1issemi‐transparenthistory

of simulated position, MVPA and solid waterline

(Figure 7). Mode 2 is solid representation of these

three polylines (Figure 6). Section C contains three

checkboxes to select the displayed polylines.

Coverage factor (k) is presented in section D, and it

canbechangedinsectionG.SectionsEandFpresent

headingand position:original (real) and simulated

respectively. Impact of the coverage factor value is



presentedintheFigures8aand8b.



Figure6. The interface of application illustrating the

waterlinesandMVPA

392



Figure7. The interface of application illustrating the

drawingmode



a)



b)

Figure8.Applicationscreenshotsillustratingthewaterlines

andMVPAfortwovaluesoftheparameterk.

6 THELAYEROFMVPAAPPLIEDINECDIS

TheECDISgraphicaluserinterface(presentedinFig.

9)ismadeupofsixmaincomponents:thechartarea,

menu bar, status panel, function buttons,

alphanumericpanelandinformationpanel.Thechart

areapresentstheENCchartinawaytheuserchose

it

according to his requirements. Menu bar contains a

few panels where application settings could be

changedbyuser.ThefirstonecalledʺSettingsʺallows

tochoosethewaytheapplicationisexchangingdata

with other devices. It can be realized via appointed

COM port or via net by choosing

the one of two

available protocols, UDP and TCP/IP. PanelʺShipʺ

definesthepositionsystemantenna  coordinates and

showsthevesselʹsgeneralparticulars.PanelʺChartsʺ

presentsthelistofactuallyinstalledENCchartswith

itsstatus.ThelastʺNavigationʺpanelcontainsthelist

of custom layer which can be switched

 on and off.

The status panel shows the information regarding

current scale and cursor geographical coordinates.

Information panel shows the data depending on

modetheECDISisworkinginparticularmoment.It

couldbeinformationregardingENCchart,symbols,

route,waypoint,MOB,andtheEGNOSdata.

During the tests the

calculated ship’s waterline

wasaddedtoECDIS‐thisisshowninFig10.ECDIS

interface was also enriched by three alarms: HAL

Exceeded, TAL Exceeded and MVPA Trespassed.

HAL(Horizontal AlarmLimit)Exceededisactivated

ifcircular protection level HPL (derived solely from

GNSS / SBAS) exceeds the Horizontal Alert Limit



HAL set for a specific marine area and operation.

Time Alarm Limit (TAL) establish maximum age of

the message in the EGNOS system, above which an

TALExceededalarmisactivated.MVPATrespassed

393

isanalarmthatisactivatedwhenanotherobjectwill

appearinsidetheMVPA.



Figure9.ECDISinterfacewithMVPAandEGNOSintegrity

layer (A‐chart area, B‐menu bar, C‐status panel, D‐

function buttons, E‐alphanumeric panel, F‐information

panel,G–layerselection,H–alarmpanel)



Figure 10. ECDIS with simulated waterline (red), original

waterline(black)andMVPA(blue)

7 CONCLUSIONS

Results of the comparison of selected EGNOS

integrity parameter (ionospheric propagation

variance) and the calculated position error didn’t

provethecorrelationbetweenthem,butitprovedthat

themodelproposedin[6]shouldnotconsiderσ

GIVEas

themostsignificantintegrityparameter.Generallythe

mathematicalmodelofMarineVesselProtectionArea

(MVPA)basedonEGNOSprovidedintegritydatais

an innovative solution of marine navigation safety

evaluation. The display of MVPA on the ECDIS

shouldmakeiteasierandsafertomaneuvertheship.

Expansion of

the waterline can be particularly

suitable in the ports. It should illustrate navigators

how uncertain may be the data presented by GNSS

devices.Itshowsalsothatinproximityofotherships

orobjects,specialattentionshouldbepa idtovisualor

radar observations and that you cannot absolutely

trustECDIS

presentationbasedsolelyonGNSS.

REFERENCES

[1]RTCA, Inc. SC‐159, RTCA DO‐229D Minimum

Operational Performance Standards (MOPS)for Global

Positioning System / Satellite‐Based Augmentation

SystemAirborneEquipment,2006,Change1,2013.

[2]WolfJ.K.,BlakeneyR.D.II,AnExactEvaluationofthe

Probability of Undetected Error for Certain Shortened

Binary CRC Codes,  MILCOM

 ’88 Conference

Proceedings,Vol.1,1988,WashingtonDC, USA, paper

15‐2,pp.287‐292.

[3]Zalewski P., Tomczak A., Method of Probabilistic

EvaluationofShip’sContourInclusiveAreaforaPilot

Navigation System, 2

 International Congress of Seas 

and Oceans, 20‐24 September 2005, Szczecin –

Świnoujście,Poland,pp.229‐239. 

[4]Zalewski P., Systemy autonomiczne w procesie oceny

bezpieczeństwa jednostek pływających na akwenie

ograniczonym, 2013, Szczecin, Poland, Wydawnictwo

NaukoweAkademiiMorskiejwSzczecinie.

[5]Zalewski P., Presentation of Satellite

 Based

Augmentation System integrity data in an Electronic

ChartSystemdisplay,ScientificJournalsoftheMaritime

UniversityofSzczecin,2015,Szczecin,Poland,45(117),

pp.150‐156.

[6]Zalewski P., Gucma L., Born A., Urbanska K., Schlüter

S.,PorettaM.,AssumptionsofFullMissionShipBridge

Simulation Including EGNOS, IAIN

2015 CONGRESS

PROCEEDINGS, International Association of Institutes

of Navigation World Congress 2015, 20.‐23.10.2015,

Prag,Tschechien,pp.32‐39.

[7]http://www.egnos‐pro.esa.int/sisnet/, Accessed: 17 July

2016.

[8]http://ask.okbase.net/topic/2014010716_13100_39068800

8.htm,Accessed:17July2016.