The Argos system is composed of:
- Argos beacons that emit signals towards satellites
- satellites equipped with instruments that receive the beacons emissions
- ground stations that receive the signals collected by the satellites
- two processing center that process and distribute the collected data to the users.
How does the Argos system function:
- About 22,000 beacons are deployed worldwide. Powered by battery or solar energy, they sent data at regular intervals to the Argos instruments on board seven satellites that fly by at 850 km altitude.
- The information collected by the satellites are then downloaded to about sixty receiving stations.
- The receiving stations relay the collected information to the 2 processing centers.
- Located in Toulouse (France) and in Washington DC (USA), these centers process the data and deliver them to users (scientific community, governments, industries...).
The beacons have equipments integrating a certified Argos emitter. Each beacon is characterized by an identification number which is unique and depends on its transmission electronics. A beacon emits periodically a message characterized by the following parameters:
- the emission frequency (around 401.650 MHz), which must be stable, since the computation of the location is based on the Doppler effect measurement.
- the repetition period which is the time interval between two consecutive messages. It varies from 90 to 200 seconds depending on the beacon use,
- the beacon identification number,
- the transmitted data.
The transmission duration of each message is less than one second.
Nowadays, seven satellites are equipped with Argos instruments to receive the signals emitted by the beacons. The Argos messages are received by the satellite then are simultaneously:
- stored in the recorder on board the satellite and transmitted to the ground each time it passes over one of the main three receiving stations: Wallops Island (Virginia, USA), Fairbanks (Alaska, USA) and Svalbard (Norway), or
- transmitted in real time to the ground, and thus can be received by the receiving stations in visibility.
The satellites follow a polar orbit at 850 km altitude: they pass over the North and South poles at each revolution. The orbit planes turn around the poles axis at the same speed than the Earth around the Sun. Each satellite sees simultaneously and at all time all the beacons inside a circle of 5,000 km in diameter. With the movement of the satellite, the ground track of this circle forms a band of 5,000 km wide which wrap around the Earth while passing at the North and South poles.
The satellites nowadays equipped to receive the Argos beacons signals are:
- NOAA-15, launched on May 13, 1998, equipped with an Argos-2 instrument
- NOAA-16, launched on September 21, 2000, equipped with an Argos-2 instrument
- NOAA-18, launched on May 20, 2005, equipped with an Argos-2 instrument
- METOP-A, launched on October 19, 2006, equipped with an Argos-3 instrument
- NOAA-19, launched on February 6, 2009, equipped with an Argos-3 instrument
- METOP-B, launched on September 17, 2012, equipped with an Argos-3 instrument
- SARAL, launched on February 25, 2013, equipped with an Argos-3 instrument
The next instruments to be launched are the last Argos-3 on board Metop-C and the first Argos-4 (of a series of 4 minimum) on board the American Polar Free Flyer satellite, both in 2017.
Nearly 60 stations receive the data sent by the satellites in real time and forward them to the processing centers.
These stations are divided in two categories:
- For the mode called regional, a network of L-band stations covering a large part of the Earth and receiving in real time the data from beacons that are received by the satellite when it is in visibility of the station. This network enables to accelerate the routing of the data received on board to the users but does not ensure the total cover of the Earth.
- For the mode called global, the main receiving stations (generally in X-band) collect all the messages recorded by the satellites along a whole orbit thus give the system its global cover. These three stations are Wallops Island, Fairbanks in the USA and Svalbard in Norway. These stations also receive real time data.
Two redundant processing centers, one in Washington D.C, USA, and the other in Toulouse, France, process all the received data. The computers then proceed to the location and the processing of the received data. The processings realized in one of the global processing centers are:
- the control of the quality of the messages, from the reception level, the time-tag, the identification number of the emitter, the length of received message and the value of the received frequency (used for the computation of the location);
- the time-tagging of the messages in Universal Time (UTC);
- the location of the beacons from the time/frequency measurements performed by the instrument (Doppler effect principle);
- the sorting of the messages by beacon ID (Identifier) and in chronological order;
- the data processing.
All these results are archived and made available to users.