About the satellite system

The system used is known as Argos and is provided by a French company called CLS - Collecte Localisation Satellites. This system can locate and receive data from so-called platforms anywhere in the world, on land or at sea. A platform consists of a radio transmitter, an antenna, a power supply and, possibly, sensors. Argos currently monitors several thousand platforms. These include moving platforms attached to birds and other animals, sailboats in ocean races, fishing vessels, drifting buoys (to measure ocean currents) and vehicles carrying hazardous materials. Static platforms which have sensors attached are used in remote or inhospitable places to monitor such things as the depth of water in rivers, the amount and type of snow in mountains and the noise coming from volcanoes to give advance warning of an eruption.

Click here to go to the Argos website where there are lots more interesting details about how the Argos system works.

The Argos instruments are flown on board the National Oceanic and Atmospheric Administration (NOAA). At least two of these weather satellites are operational at any time. They are on polar orbits at about 850km above the earth's surface. As the picture shows, each satellite is able to "see" platforms within a circle with a 5000-km diameter.However, the satellites are not stationary. They fly over the north and south poles, orbiting the earth once every 102 minutes. 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) about the polar axis on each revolution. each swath overlaps with the previous one but 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 about up to 14 minutes (10 minutes on average).

Diagrams and information were copied with permission from the Argos User's Manual.

The satellites receive the Argos messages from users' transmitters and relay them to regional receiving stations in real time. They also store them on tape recorders and read out ("dump") all the stored messages every time they pass over one of the three main system ground stations.

Once the data reaches a receiving station it is processed by powerful computers so that meaningful data representing the position of the platform can be sent to users.