automatic collection and transmission of ... - E-Library - WMO
automatic collection and transmission of ... - E-Library - WMO
automatic collection and transmission of ... - E-Library - WMO
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TRANSMISSION OF HYDROLOGICAL OBSERVATIONS 17much smaller proportion <strong>of</strong> technicians versus number <strong>of</strong> measuring stations, eventhough this latter personnel would have to be very highly qualified.These are some <strong>of</strong> the more important criteria to beplanning process but each individual project will have its owncareful attention should be given to all the alternatives withfits before any final decision is made.considered inparticularitiestheir costs <strong>and</strong>the<strong>and</strong>bene-When designing a system for the <strong>automatic</strong> <strong>transmission</strong> <strong>of</strong> measured data, thethree main components to develop are:(a)(b)(c)Measuring <strong>and</strong> encoding equipment;The <strong>transmission</strong> link;Receiving, decoding <strong>and</strong> analysing.While developing each <strong>of</strong> these units separately, it is necessary to considerthe three together in the design stage. This is essential as the special characteristics<strong>of</strong> anyone <strong>of</strong> these components can have serious Gonsequences on the decisionsregarding the others.The type <strong>of</strong> installation for measuring <strong>and</strong> transmitting the data will dependgreatly on the parameter or parameters to be measured especially as regards theirvariability with time <strong>and</strong> space, the climate in the area which would affect the choice<strong>of</strong> sensor, power supply etc. <strong>and</strong> the type <strong>of</strong> <strong>transmission</strong> to be used witb considera~tion given to the distances to be covered.2.4 Transmission linksThe type <strong>of</strong> <strong>transmission</strong> link used is determined by the frequenQY bondrequirements <strong>and</strong> economics. Availability locally <strong>of</strong> anyone <strong>of</strong> the alternate choicesis a constraint. Possible choices for <strong>transmission</strong> links include: dedicated l<strong>and</strong>lines, commercial telephone or telegraph lines, direct radio links, satellite re<strong>transmission</strong>systems.2.4.1 Dedicated l<strong>and</strong> linesThese are perhaps the easiest to install when relatively short distances areinvolved <strong>and</strong> no commercial lines already exist. L<strong>and</strong> lines are typically able totransmit frequencies <strong>of</strong> up to 3 000 Hertz* without special techniques. Time division<strong>and</strong> frequency multiplexing** can be used to provide more economic use <strong>of</strong> the tronsmissionline.* Hertz is a unit <strong>of</strong> frequency.** The process <strong>of</strong> using one <strong>transmission</strong> line to transmit several measurements iscalled multiplexing.
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