Manual on sea level measurement and ... - unesdoc - Unesco

Sea Level Measurement and InterpretationLEOs can be further sub-divided into Big LEO and LittleLEO categories. Big LEOs will offer voice, fax, telex,paging and data capability, whereas little LEOs willoffer data capability only, either on a real-time directreadout (‘bent pipe’) basis, or as a store-and-forwardservice. Since the satellite footprint decreases in sizeas the orbit gets lower, LEO and MEO systems requirelarger constellations than GEO satellites in order toachieve global coverage and avoid data delays. Lowerpower is, however, generally required for LEO and MEOsatellite communication because of the shorter averagedistance between transmitter and satellite. Some systemsimplement several high-gain antennas to generate‘spot beams’ and so reduce the requirement of themobile to have a complex antenna and/or high outputpower. A key feature of several MSS currently underdevelopment will be their inter-operability with existingpublic switched telephone and cellular networks, usinga dual-mode handset.From a technical point of view, some systems do offersignificantly enhanced capabilities compared to existingmethods. Potential advantages include two-waycommunication, more timely observations, and greaterdata rates and volumes. Some systems may also proveto be considerably less expensive than existing channels,although this is as yet unclear. Table 5.1 containsa list of the known satellite systems and their currentstatus.The Global Telecommunication System (GTS), hithertoused for the transmission of meteorological data andinformation also has its place in GLOSS and is thereforeconsidered in a separate section (5.3.3).Table 5.1 A list of the known satellite systems and their current status.System Status* Orbit type Message type Terminal size CommentsAPRIZESAT Operational Little LEO Data: TBD Handheld 7 4 nanosatellites in orbit, 2-waycommunications, directed at assettrackingARGOS Operational Little LEO Data: 32 bytes Handheld 1 Various enhancements, including2-way messaging, are scheduledECCO(CCIGlobal)On hold LEO Voice/data Handheld TBD 12 equatorial satellites planned by2003. Status questionable – mergedwith ICO-Teledesic GlobalELLIPSO LicensedOn holdBig LEO Voice/data Handheld TBD 17 satellites in highly elliptical orbits,serving major land masses. Statusquestionable – merged with ICO-Teledesic GlobalEYESAT Experimental Little LEO Data: 60 bytes Handheld1 satellite 1995, principally for radio5 amateursLicensed Little LEO Data: TBD TBD 6 satellites for utility metering (aimedE-SAT On holdat continental USA only initially)FAISAT LicensedOn holdLittle LEO Data:128 bytesHandheld 10 38 satellites 2000+ Test satellitelaunched 1997GEMNETCancelled(pre-op)Little LEO Data: nomaximumLaptop 10 1st satellite 1995: launch failure36 satellites by ???GLOBALSTAROperational Big LEO Voice/data:no maximumHandheld 1 48 satellites + spares (constellationcomplete); two-way (voice) andreal-time transmission. Limited coveragedue to lack of ground stations.Financial difficulties.GOES,METEOSAT,MTSATOperational GEO Data: variousoptionsLaptop 10 4 satellites; directional antenna desirableNOAA–ESA–MTSAT (Japanesemeteorological) satellites.GONETS-D Pre-operationalLittle LEO Data Handheld TBD 8 satellites in orbit, 36 more planned36IOC <strong>Manual</strong>s and Guides No 14 vol IV