How Argos system works?
| Argos is a unique worldwide location and data collection system dedicated to studying and protecting the environment. |
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Argos PTT transmitters
Any equipment integrating an Argos-certified transmitter is referred to as a ‘platform’. Powered by batteries or solar energy, the Argos transmitters upload short duration messages (of less than one second) to Argos instruments on satellites that pass overhead at an altitude of 850 km.
Each platform is characterized by an identification number specific to its transmitter. A platform transmits periodic messages characterized by the following parameters :
- Transmission Frequency (401.650 MHz ± 30 kHz, which must be stable as the location is computed on the basis of Doppler effect measurements, the Repetition period, which is the interval of time between two consecutive message dispatches, varying between 90 and 200 seconds according to the extent to which the platform is used, the platform identification number, and the volume of data collected.
- The transmission of each message takes less than one second.
Satellites
Polar orbiting satellites collecting data: flying at an orbit of 850 km above the earth pick up the signals and store them on-board and relay them in real-time back to earth.
The satellites are on a polar orbit at an altitude of 850 km
The satellites see the North and South Poles on each orbital revolution. The orbit planes revolve around the polar axis at the same speed as the Earth around the Sun, i.e. one revolution a 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, approximately 14 times per day per satellite.
The period during which the satellite can receive messages from a platform is equivalent to the time during which the platform is within its visibility. On average this is 10 minutes.
Argos messages are received by the satellite simultaneously. They are stored on the onboard recorder and retransmitted to the ground each time the satellite passes over one of the three main receiving stations based on Wallops Island (Virginia, United States), Fairbanks (Alaska, United States), and Svalbard (Norway), or they are retransmitted to the ground to regional reception stations in the satellites’s field of view.
Receiving stations
Nearly 70 stations receive real time data from the satellites and retransmit them to processing centers. This distribution network provides worldwide coverage.
The three main receiving stations, Wallops Island and Fairbanks in the United States and Svalbard in Norway, collect all the messages recorded by the satellites during an orbit, thus providing coverage of the entire Earth.
Data received by the satellites are retransmitted to regional stations in real time if the station is within satellite visibility. The main receiving stations also receive data in real time.
There are two categories of stations :
- For the ‘regional’ mode, a network of L-band stations covers a large part of the Earth and receives the beacon data in real time from the satellite when it is in range of a station. The beacons upload their data to the satellite when it passes overhead. This network enables faster routing of the data received on board to the users but does not allow full coverage of the Earth.
- For the ‘global’ mode, the main receiving stations (generally in X-band) collect all the messages recorded by the satellites along a full orbit thus giving the system global coverage. These three stations at Wallops Island, Fairbanks in the USA and Svalbard in Norway, also receive data in real-time.
Processing centers
There are two global Argos processing centers, one located near Toulouse in Southwestern France, at the CLS headquater, and the other near Washington, DC, USA. Once the data arrive at a processing center, locations are automatically calculated and information made available to users.
In the two processing centers, designed for full redundancy, the computers calculate locations and process the received data.
The following processing is carried out at the global processing centers:
- Verification of message quality, reception level, time-tagging, transmitter identification number, sensor message lengths and receiver frequency value (to compute the location)
- Message time-tagging in coordinated universal time (UTC)
- Message classification by platform and by chronological order
- Data processing
All these results are stored and made available to Argos users.
Argos users around the world receive data directly in their office or on-site, depending on their choice (by email, fax, web connection, cd-rom, or directly on mapping software). Once the data are received, they are often shared with the scientific community or distributed to governments or industries that use the data as important management tools.
