581
1 INTRODUCTION
Thebasicmethodofincreasingthenavigationsafety
isto ensure precision shippositioning by setting up
navigation marking systems, including radio
navigation systems accessibleto allvessels
[Czaplewski, K., 2018; Czaplewski, K., Goward, D.,
2016; Jang, W.S. et al., 2018; Specht, C. et al., 2016].
The Differential
Global Positioning System (DGPS)
system in its marine version is the basic positioning
system for coastal navigation and it is also widely
used in hydrography, both marine and inland
[Stateczny, A. et al., 2018], meeting all categories of
requirements laid down in the International
Hydrographic Organization (IHO) S44 standard
[IHO, 2008]. Moreover, DGPS system also has other
applications,includingoffshoreactivities[Baptista,P.
etal.,2008].
At the same time, methods of positioning
improvement are applied both by such technical
solutions as alternative positioning systems [Kelner,
J.M.etal.,2016;Sadowski,J.,Stefański,J.,2017],radar
positioning [Stateczny, A.
et al., 2019] and multi
GNSS (Global Navigation Satellite System) solutions
[Specht, C. et al., 2019a; Yang, C. et al., 2016] using
Kalmanfiltering[Xinchun,Z.etal.,2016].
Unlike various alternative positioning solutions
mentionedhere,themarineDGPSsystemdominated
the vessel positioning process due to the following
operational
features:
Polish DGPS System: 1995-2018
Studies of Reference
Station Operating Zones
C.Specht,M.Specht&P.S.Dąbrowski
GdyniaMaritimeUniversity,Gdynia,Poland
ABSTRACT: The operating zone of a radio navigation system is one of its main operating features. It
determinesthesizeofawaterbodyinwhichthesystemcanbeused,whileguaranteeingvessels’navigation
safety.
TheDGPSsystemintheLF/MFrangeisnow
thebasicpositioningsystemincoastalwatersaroundtheworld,
whichguaranteesnotonlymetrepositioningaccuracy,butitisalsotheonlyonetoprovidenavigatorswith
signalsonpositioningreliability.
ThispaperdescribesandsummarisesovertwentyyearsofstudiesdealingwiththeoperatingzoneofthePolish
DGPS
referencestationnetwork.Thispaperisthefifthinaseriesofpublicationswhoseaimwas topresentin
detailtheprocessofinstallation,testingandlongtermevaluationofthenavigationalparametersofthePolish
DGPS system, launched in 1995. This paper includes the theoretical foundations of determination of
the
DziwnówandRozewieDGPSreferencestationoperatingzonesintheyears19952018.Moreover,itpresents
the measurement results for the signal levels and the results of their analyses, which determine the station
operatingzones.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 13
Number 3
September 2019
DOI:10.12716/1001.13.03.13
582
referencestationsareplacedatthesamelocations
whereradiobeaconsusedtooperate,whichmade
it possible to use the existing transmission
infrastructure,
atypicalrangeofareferencestationisapprox.100
km,whichmakesitpossibletocovermostwaters
ofcoastalstates,
thepositioningaccuracyofapprox.1m(p=0.95)
guarantees high navigation safety for vessels
approaching ports, which is competitive against
other commonly accessible systems [Specht, C. et
al.,2019b],
the ability of the DGPS system to transmit
positioningreliabilitysignalsisuniqueamongall
thepositioningsystemsworldwide.
Being a combined system, the marine DGPS
system requires that the operating zone should be
definedasaspatialproductoftheGlobalPositioning
System(GPS)operatingzoneandtheDGPS
reference
station operating zone. The GPS system operating
zone is by definition regarded as global, but due to
the dynamics of the satellite constellation system
movementandavariablenumberofsatellitesvisible
above the required topocentric altitude, it can be
alternativelydefinedasafunctionoftime[Specht,C.
et
al.,2015].Inthiscase,itacquiresaspatialtemporal
dimensionratherthanbeingonlywaterbodyrelated.
The availability of individual satellites in an area
[Specht, C., Dąbrowski, P., 2017], their average
number in a global approach and their geometric
configuration, and in pa rticular their constellation
within a
specific period of time, can be predicted
analytically.
Thefollowingaretheproposedprecisedefinitions
ofbothterms:
a GPS operating zone is the percentage of time
relative to its predefined interval, during which
the required number of satellites is above the
required topocentric altitude, thereby enabling
users to determine position with a permissible
error,atanypointonorabovetheEarth,
a DGPSreferencestationoperatingzoneisaspace
referred to geographic coordinates of the DGPS
reference station (
, , h), in which telemetric
transmission can be received with required
reliability.
Thispaperpresentsthetheoreticalfoundationsof
the DGPS operating zone determination and shows
thefindingsofresearchonthePolishDGPSsystemin
theyears19952018.
2 THEDGPSSYSTEMOPERATINGZONE
THEORETICALFOUNDATIONS
Sending radio
signals, associated directly with the
systemrangeandcoverage,dependsonanumberof
factors and circumstances. Propagation conditions
vary depending on the place, time and frequency
range. However, although the radio wave
propagation theory is well developed, calculation
resultsbasedonformulasandtheoreticalcurvesoften
deviate from reality
[Enge, P. et al., 1992]. This is
caused by an insufficiently strict approach to highly
complex propagation phenomena, their insufficient
explanation and practically unavoidable
simplifications. Therefore, theoretical considerations
regarding the coverage of an area by a radio
navigation system must be verified practically, and
theplanneddeploymentofaDGPSreference
station
shouldtakeintoaccounttherealresultsofoperating
systemsofthistype.
Characteristic features of 300 kHz radio waves
include different propagation conditions during the
dayandnight,whichiscausedbytheoccurrenceof
thesurfacewaveandtheskywaveatnight.Theycan
cause undesired interference
(Figure 1), resulting in
even signal disappearance. The DGPSrelated
experience so far shows that the effect of this
interference is not significant (over distances under
consideration)[Specht,C., 1997], although obviously
it has to be considered at boundary ranges and its
occurrencemustbepredictedevenduringthedayin
winter [Huuhka, E., Lehtoranta, V., 1995]. For this
reason, radio wave propagation at this frequency
shouldbeconsideredmainlyfromthepointof view
of the surface wave propagation, particularly taking
intoaccounttheeffectofthegroundandinterference.
Figure1. Thesignalstrength‐E[dBμ] (blue) of a Hoburg
DGPSreferencestationandthemeanpositioningerror‐M
(p=0.95)(red)asafunctionoftime.Recording:24051995
11:50 UTC 25051995 04:50 UTC, Gdynia, measurement
frequency:1minute[Specht,C.,1997].
It is noteworthy that interference of the surface
wave and skywave at night results in fluctuation of
the SignaltoNoise Ratio (SNR), which causes
changes in the Bit Error Rate (BER) and, ultimately,
damages Radio Technical Commission for Maritime
(RTCM) messages. This results in increasing of the
Age of Corrections
(AoC) and, further, mean
positioningerror (M).Accordingtotherequirements
for telemetric transmission of the DGPS system, the
minimumSNRis7dB,withBERof10
3
.Itshouldbe
acceptedthatapartfromtherequiredsignalstrength
(34 dBμ), the required SNR value is another factor
which determines the DGPS reference station
operatingzone.
ADGPSreferencestationtransmitssignalswithin
the range of: 283.5325 kHz with MinimumShift
Keying(MSK),atatransmission
speedof(R):50,100,
200bd.Inpractice,itisacceptedthattheradiosignal
at distances larger than 70 Nm from a broadcasting
antennaneartheEarthsurfaceisafieldconsistingof
twotypesofwaves:surfaceandionospheric.Signals
sentfromaDGPS referencestationbasically reach
a
useronasurfacewave.Thesignalstrengthdepends
mainly on the radio beacon transmitter power, the
receiverstation distance and conductivity of the
groundabovewhichtheradiowavepropagates.The
received signal strength for the ground of the ideal
conductivitycanbeexpressedwiththeformula:
583
5
10 10
310
20 log 10 log
1000
P
E
d







, (1)
where:
E
signalstrength[dBμ],
P radiatedpower[kW],
d distancefromtransmitter[km].
This equation also describes approximate
propagationconditionsabovethemediumofvariable
conductivity,butitistrueonlyforsmallranges.With
respecttotherealconditions,thesignalstrengthcan
becalculatedfromthefollowingformula:
5
0
310 P
EF
d


, (2)
where:
0
E
signalstrength[μV/m],
F
dampingfactor[].
The value of
F
for routes of nonuniform
conductivity is calculated by the Millington method
taking into account the electric properties of
individualsectionsoftheroute.Ifarouteconsistsof
twosectionsofdifferentelectricproperties,then:






11 2 2
21
21 12
,,
,,
,,
Fd F d
F
Fd Fd
Fd Fd




, (3)
where:

11
,
F
d
dampingfactorcalculatedforauniform
routesectionofconductivity
1
andlength
1
d [],
d sumofdistances
1
d
and
2
d
[m].
The damping factor for routes of uniform
conductivity can be calculated from Burrows graphs
orfromanapproximateformula:
2
20.3
20.6
d
dd
x
F
x
x


, (4)
where:


22
22
'1 60
'60
d
d
x





, (5)
assumingthat:
d
x
distancenumber[],
d
distancefromthestation[m],
'
relativepermittivity[ ],
wavelength[m],
groundconductivity[mS/m].
Another method of the surface wave signal
strength determination uses Comité Consultatif
InternationaldesRadiocommunications(CCIR)
curves.
SkywaveisanothersignalwhichreachesaDGPS
receiver, especially at night. A radio wave from the
DGPS station transmitter antenna reaches the
ionosphere layer D, from which it
is reflected and
interfereswiththesurfacewave,creatingionospheric
interference.TheconditionoftheionosphericlayerD
is a function of: time of day and time of year,
geomagneticlatitude.Duringtheday, the sunisthe
basicionizationsource,creatinganeffectivereflection
surfaceat70km(thesun
atitszenith)and75km(at
sundown). Solar ionization disappears at night and
theeffectivereflectionsurfaceincreasesto8090km.
Thesignalstrengthinashortverticalantennacan
bedescribedas:
52
310 cos
s
jtr
sky
PRDFF
E
s

, (6)
where:
s
ky
E
signalstrengthofskywave[μV/m],
anglebetweenthedirectionofpropagationand
thehorizon[rad],
s
R
ionosphericreflectioncoefficient[],
j
D
ionosphericconditioncoefficient[],
t
F
a coefficient characterising the transmitter
antenna[],
r
F
acoefficientcharacterisingthereceiverantenna
[],
s
totallengthofskywave[km].
Assuming the spherical shape of the Earth, the
valueofscanbeexpressedas:
 
1
2
2
2
22cos
2
EEEE
sRhRRRh




, (7)
where:
R
EEarthradius[km],
heffectivereflectionaltitude[km],
central angle between the transmitter and the
receiver[rad],
therefore:


2sin
2
cos
E
Rh
s




. (8)
Figure 2 presents signal strength curves. The
signalstrengthforthesurfacewaveandskywaveasa
functionofthetimeofdayandtimeofyear,distance
from the reference station for the frequency of 300
kHzandEPRof1kW.
Figure2. The signal strength for the surface wave and
skywaveasafunctionofthetimeofdayandtimeofyear,
distancefromthereferencestationforthefrequencyof300
kHz and EPR of 1 kW. The red line denotes the signal
strengthforthesurfacewave[IALA,
1996].