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1 INTRODUCTION
Audio watermarking (AW) corresponds to digital
information imperceptibly embedded into the audio
signal.AWformaritimeterrestrialradiotelephonyis
inspiredfirstofallbytheabilityofimplementationan
automatic identification in the voice telephone
communication. Applied to the existing analog
radiotelephony in all frequency bands (VHF, MF,
HF), the watermarking system could overcome
existinglimitations, enhanceefficiency and clearness
of radiotelephone messaging while ultimately
decreasingsocalled“humanfactor”.
In the maritime and aeronautical VHF services
analogue channels with frequency/phase and
amplitudemodulationcorrespondinglyareutilized.
For the meanwhile the identification of the sea
vesselsisrealized
bymeansofverbalcallingofship’s
call sign or numerical identification. However on
accountofdifferentreasonssuchverbalidentification
maybeabsent,transmittedwithdelay,orunderstood
with errors. This problem, applied to VHF
communication is illustrated in Fig. 1. Motor vessel
“Arcona”transmits a certain messageto all
stations.
Butoneofthereceivingvesselsmissedthenameand
call sign of the transmitting ship, and another ship
interpreted the name of transmitting ship as
“Gargona”instead“Arcona”.
It is obvious that false, incorrectly interpreted or
delayed verbal identification negatively affects
maritime navigation. Automatic identification could
avoidmisidentification
andcallsignconfusion.
Radio Regulation claims obligatory identification
for every radio transmission. In a present maritime
terrestrialradiotelephonyidentificationiscompletely
dependent upon operator. This situation takes place
onthebackgroundofhightechnologyinstrumentsis
beingimplementedinthemaritimebranch.
Stealthy Information Transmission in the Terrestrial
GMDSS Radiotelephone Communication
A.V.Shishkin&V.M.Koshevoy
OdessaNationalMaritimeAcademy,Ukraine
ABSTRACT:Audiowatermarking(AW)technologyisconsideredforstealthyinformationembeddingdirectly
intoaudio signal.Inherent inanalog radiotelephone channel interferences against watermarks areanalyzed.
Robust encoding/decoding algorithms are presented and appropriate project of AW system named as
AutomaticRadioTelephoneIdentificationSystem(ARTIS)is
proposed.Experimentalresultsforthepractical
VHF radio channel are presented. Relating on processing complexity the designed system enables
imperceptiblytransmitdataonarateupto260bit/secinthestandardVHFradiochannel.ARTISprovidesthe
full compatibility with the existing radio installation, and doesn’t require replacement of standard
VHF
transceivers and operational procedures. Besides, automatic identification the system may be used in the
specialapplications,forexample,underthethreatofterroristattack;generallycontributestonavigationsafety
andinformationsecurity.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 7
Number 4
December 2013
DOI:10.12716/1001.07.04.09
542
It is well known an Automatic Transmitter
IdentificationSystem(ATIS)[1]formarineVHFradio
that is used and mandated on inland waterways in
Europe for identifying the transmitting vessel. In
ATIS the identity of the vessel is sent digitally
immediately after the shipʹs radio operator has
finished talking
and releases push‐to‐talk (PTT)
button. Identification is performed by appending a
shortdatamessageinDigital SelectiveCalling(DSC)
format. The main drawback of ATIS is post‐report
transmissionofidentificationdata.
In COMSAR proposal [2] the necessity of
automatic identification is grounded and quite
reasonably noted that
the identification should be
done immediately after pressing the PTT button on
the contrary of ATIS releasing PTT. Another
shortcoming of ATIS is principle limitation for
Medium/High frequency (MF/HF) applications. This
limitationresultsfromthesmallbitrate(100bit/sec)in
comparisontoVHFDSCrate(1200bit/sec).
In restricted navigation environment
the
immediate and clear automatic identification is
extremely necessary. Automatic identification would
excludethehumanfactorandincreaseanefficiencyof
VHFradiocommunicationandmaritimesafetyinthe
whole.
Figure1. Automatic identification in the VHF maritime
radiotelephony
Only verbal identification doesn’t protect against
illegal radio transmission. Illegal transmissions are
especiallyharmful ontheVHFdistresschannel16.Of
course, unauthorized transmissions are performed
anonymously. Reliable automatic identification of
such transmissions could avoid the violation of
radiotelephoneregulation.
Another advantage of automatic identification
follows from the ability of digital
information
inputting to another ships’ navigational and
information systems, for example Electronic Chart
Display and Information System (ECDIS). ECDIS
makes visualization of neighboring vessels in the
range of VHF radio (i. e. approximately 30 nautical
miles).Howeverthetransmittingvesselbynomeans
is marked in an electronic map. Automatic
identificationwouldimplementthevesselmarkingat
electronicchartdisplay.
One more application of AW is a covered
information transmission in the special applications
(forexample,facingthethreatofterroristaggression).
ItisessentialthatAWbasedidentificationdoesn’t
require altering an existing radio installation and
operational procedures. To introduce
AW‐
identification function only new telephone receiver
withthe embedded processorat the transmitter side
andprocessorwithmini‐displayswitchedtocommon
audiooutputatthereceiversidehavetobemounted.
Automatic identification starts right away PTT
pressing and runs continuously during all
transmitting period independently from voice
signal
occurrence. No additional time and frequency
channelrecoursesarerequired.
AW‐identification provides the full compatibility
withtheexistingtransceiversandmakespossibilityof
step‐wiseimplementation.
SimilarapplicationofAWmaybeimplementedin
the aeronautical (118…136) MHz mobile service. In
paper [3] watermarking based on speech unvoiced
phonemes
recognizingandreplacingthembycertain
noiselikesequenceisproposed.
Maritime AW based identification is proposed in
[4],[5].Inthesepaperswatermarkingprocessdoesn’t
requireanyrecognizingalgorithmsandbasedonlyon
statisticalsignalproperties.
In this paper we present stealthy AW forming
algorithm based on signal energy
saving and
complete algorithmic ARTIS scheme for maritime
application.
2 INFORMATIONEMBEDDINGALGORITHM
2.1 Audiowatermarkingsystemanditscharacteristics
AW for maritime analog radiotelephony has some
important features. First, watermarks are inaudible.
Second, watermark technology doesn’t demand any
additionaltimeandfrequencyresourcesbeyondthat
used in the basic telephone
channel. Thirdly,
watermarks besides identification may convey also
another digital. Finally, watermarking doesn’t
demandalterationofstandard radioinstallation and
operational procedures. Moreover, looking forward
towardsdigitalcommunications,voicewatermarking
straightawayinthemicrophonecouldalreadysolve
identificationproblem.
Watermarking system is characterized by some
competitive properties: fidelity, data
payload and
robustness [6]. Fidelity defines an audible similarity
betweenanoriginalvoicemessageandwatermarked
message after information embedding. Data payload
referstonumberofbitsawatermarkencodeswithina
unitoftime.Robustnessistheabilityofwatermarksto
survivethevoicesignalprocessingoperations.