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Satellites The argos instruments are flown on board the National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellites (POES). At least two satellites are operational at any time. Launches are scheduled through 2010. From around the turn of the century Argos instruments will also be flown on satellites operated by the Japanese space agency NASDA and the European Meteorological Satellite organization, Eumetsat.
The satellites receive the Argos messages from users' transmitters and relay them to ground
in real time. They also store them on tape recorders and read them out ("dump") the messages
every time they pass over one of the three main system ground stations:
The POES satellites see the North and South Poles on each orbital revolution. Their orbital planes rotate about the polar axis at the same rate as the Earth about the Sun, or one complete revolution per year. Each orbital revolution transects the equatorial plane at fixed local solar times. Therefore, each satellite passes within visibility of any given transmitter at almost the same local time each day. The time taken to complete a revolution around the Earth is approximately 102 minutes.
Receiving Stations At any given time, each satellite simultaneously "sees" all transmitters within an approximate 5000-kilometer-diameter "footprint," or visibility circle. As the satellite proceeds in orbit, the visibility circle sweeps a 5000 kilometer swath around the Earth, covering both poles. Due to the Earth's rotation, the swath shifts 250 miles west (2800 km at the Equator) about the polar axis on each revolution. This results in overlap between successive swaths (see below). Since overlap increases with latitude, the number of daily passes over a transmitter also increases with latitude. At the poles, the satellites see each transmitter on every pass, a total of roughly 28 times a day for two satellites.
The duration of transmitter visibility by the satellite (or of the pass duration over the transmitter) is the "window" during which the satellite can receive messages from the transmitter. It lasts up to 14 minutes (10 minutes on average). The three meain ground stations receive all messages recorded by the satellite during a complete orbital revolution, providing, complete global coverage. The stations are Wallops Island, Virginia, USA; Fairbanks, Alaska, USA; and Lannion, France. Regional receiving stations receive transmitter data from the satellites in real time whenever the satellite is within visibility (see below). The main ground stations also serve as regional receiving stations.
Regional receiving stations operate in Hawaii, USA; Casey, Antarctica; and Darwin, Melbourne and Perth, Australia. More regional stations are planned.
Example of Argos form and what it means:
First number shown: ID number
01936 Date: 05.11.97 12:48:39 LC: 3 IQ: 50 |
