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Sea Level Measurement and InterpretationSTARSYS Cancelled Little LEO Data: 27 bytes Handheld 2 12 satellites 1998+(pre-op)multiple messages24 satellites 2000+TELEDESIC LicensedOn holdBig LEO Broadband 288 LEOs planned, now reduced to30 MEOsFCC licence granted, merged withnew ICOTEMISAT Experimental Little LEO Data 7 satellites planned for environmentaldata relay. 1 satellite launched1993.THURAYA Operational GEO Voice/data Handheld 1 multiple-spot beam satellite inorbit (over Middle East); 1 plannedVITASAT Pre-operationalLittle LEO Data 2 satellites in orbit,2 more plannedVSAT Pre-operationalLittle LEO Data 2 satellites in orbit,2 more plannedWEST PlannedOn holdMEO Broadband 9 satellites plannedThe status of each system in Table 5.1 is categorized accordingto seven groups:• Planned: Little is known about the system, except aname, notional type, and services to be offered. Mostlynot licensed, although some may be.• Licensed: System has been licensed by a national orinternational regulatory agency (in most cases the FCC),but no satellites have been launched.• Experimental: System has one or more satellites in orbitfor experimental purposes (not usually part of the finalconstellation). Includes new systems planning to useexisting satellites.• Pre-operational: System is in process of launching, orhas launched, its constellation, but is not yet offeringfull services. Some limited evaluation service may beavailable.• Operational: System has full or nearly full constellationin place and is offering readily available service to externalusers (not necessarily commercial).• Cancelled: System has been cancelled, either beforesatellites launched (pre-op.) or after (post-op.).• On hold: No progress reported or scheduled.5.2 Choice of a SystemSelection of a communication system for sensor realtime(RT) or near-real-time (NRT) data transmissionis always a compromise among a number of constraints.The principal factors guiding decision in theadoption of a system are:• data rate, data-rate profile in different operationalmodes (if more than one)• power availability (power from mains or autonomous/self-powered)• guarantee of data transmission (private networkor shared data line)• location, availability of telecommunication infrastructure(satellites in field of view)• land or marine application (fixed or moving)• availability of funding.Satellite communication systems at data-transmissionrates of kbits/s and Mbits/s are operating in theL-band (1–2 GHz), the C-band (4–8 GHz)or the Ku(10–18 GHz)/Ka(18–40 GHz) band.For marine applications, L-band systems are currentlythe best choice. Satellite cell phones are operatingtypically in the L-band and may be used for data transferneeds of a few kbits/s. The data-transmission rateon the L-band is much more bandwidth-limited, butsome systems allow for more than 100kbits/s. Antennadirectionality is less critical and even non-directionalantennas with sufficient beam width (eg +/–60°) areworkable, though at lower data-transmission rates (i.e.from a few kbits/s to some 10 kbits/s).The higher the frequency the easier it is to transmitlarge data sets at reasonable antenna sizes. However,attenuation by rain is stronger at higher frequencies,therefore Ka transmission from space has so farnot been very common. Ku has also hitherto beenless favoured in countries with heavy rainfall, but isbecoming more used nowadays.One of the key issues with any communication systeminvolving data is the data capacity. Many satellite sys-38IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and Interpretationtems have a limited capacity during any one transmission.Telephone links, by and large, have an adequatebandwidth for most foreseeable applications, especiallywith the new ADSL–Broadband facilities thatare being introduced. The latter may be somewhatlimited in its spatial coverage at present, but it is fairto say that the communications industry is one of thefastest growing areas of commercial activity and consequentlycoverage may be greatly increased in theforeseeable future.Two-way communications with a tide gauge can beadvantageous. It can be used to update software orcalibration values at the station, to interrogate thesystem for faults, to change the sampling rate and tocarry out many house-keeping functions that wouldotherwise wait for a site visit. This allows the systemto be flexible and improves overall reliability.In adopting a communication system for a tide gaugeinstallation, one consideration has to be its reliabilityunder severe environmental conditions. For example, fortsunami warning, some of the tide gauges may have tobe positioned in a tectonically active region to provide anacceptable early warning. In the event of an earthquake,the first losses are often the PSTN network, mobiletelephone links as well as electrical power. Under suchcircumstances, satellite links may be the only option.Additionally, some form of uninterruptible power supply(UPS) is necessary. This often takes the form of a batteryback-up system with an adequate reserve capacity ofseveral hours.A number of manufacturers, including tide gauge anddata logger manufacturers, produce relatively inexpensiveready-to-use communications systems suitable fortide gauges. For a list, see the websites given on thePSMSL website: http://www.pol.ac.uk/psmsl.5.3 Data Transmission SystemsFor the last decade or more, tide gauge installationshave used the satellite systems of ARGOS, GOES,Meteosat, MTSAT and INMARSAT for data transmission.More recently, other, newer possibilities are beingexploited or considered for exploitation: GLOBALSTAR,INMARSAT/BGAN, IRIDIUM, ORBCOMM and VSAT.Characteristics of each system in terms of the costof hardware, bandwidth and latitude coverage differsignificantly.5.3.1 Systems already well establishedARGOS (www.argos-system.org) operates worldwideusing polar orbiting satellites with an orbital periodof about 100 minutes. A platform transmitter terminal(PTT), with a data bandwidth capacity of 256bits per satellite pass, is located at the gauge and,depending on location, the delay in data reception bythe user may be several hours. Data are available tousers through the Argos Global Processing Centres atToulouse, France, and Largo, Fla., USA. The numberof accessible satellite passes per day is latitude-dependent,varying from about 7 at the equator to 28 at thepoles. Users of ARGOS for tide gauge data acquisitioninclude GRGS in France which will be able to provideadvice to potential users.GOES-E (USA), GOES-W (USA) (www.goes.noaa.gov.),METEOSAT (Europe) (www.esa.int/SPECIALS/MSG/; www.cnes.fr), and MTSAT (Japan) (www.fas.org/spp/guide/japan/earth/gms/) form a network of geostationary satellitesoffering overlapping longitudinal coverage. Latitudecover is limited to about 75° because of their equatorialorbit position. Each data collection platform (DCP) locatedat the gauge is allocated fixed time slots during which649 bytes of data can be transmitted to a satellite. Up toone time slot every six minutes can be allocated to eachDCP, so that, if necessary, data could be available to userswithin this time frame. Previous problems with clock drifthave been eliminated by including GPS receivers in eachDCP. Users of these systems include POL in the UK andNOAA and the University of Hawaii Sea Level Center inthe USA. Data sent via the geostationary meteorologicalsatellites (GOES, Meteosat, MTSAT) is usually passed on tothe Global Telecommunication System (GTS) of the WMO(see section 5.3.3 & 5.3.4). Information about how toapply for DCP transmission slots can be found at:GOES: http://noaasis.noaa.gov/DCS/METEOSAT: http://www.eumetsat.int/MTSAT: http://www.jma.go.jp/jma/jma-eng/satellite/dcs.html.INMARSAT Standard-C (www.inmarsat.com) also usesa network of geostationary satellites giving worldwidecoverage except for latitudes above 75°. This systemallows two-way data communication in near real timeat a rate of 600 bits/s, with a data message up to about8 kbytes. Tide gauge users of INMARSAT in the pastinclude the Australian Hydrographic Service.5.3.2 Systems now being applied or consideredfor application in the transmission of sea leveldataThere has been a major increase in the uptakeof broadband services globally and more specificallyat even remote islands that form the basis ofPOL’s sea level measurement network in the SouthAtlantic. POL has sites at Ascension Island, St. HelenaIsland, the Falkland Islands and Tristan da Cunha.Leased lines, offering continuous, high-speed internetaccess are available on all these islands exceptTristan da Cunha.POL has developed instrumentation that can take theIOC <strong>Manual</strong>s and Guides No 14 vol IV39
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