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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. Argos will also be flown on board the MetOp satellites of the EUMETSAT (European Organization for the Exploitation of Meteorlogical Satellites) by mid 2006.

The POES satellites see the North and South Poles on each orbital revolution. Their orbital planes rotate around the polar axis at the same rate as the Earth around 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 100 minutes.

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 25° west (2800 km at the Equator) around the polar axis at each revolution. This results in overlap between successive swaths. 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 or 56 for 4 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 about up to 14 minutes (10 minutes on average).