The Argos system is composed of:
- Argos transmitters sending signals to satellites
- Argos instruments on satellites receiving signals from transmitters
- Ground receiving stations to which signals collected by the satellites are downlinked
- Two processing centres to process and distribute collected data to 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 820 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 Wood Hole (USA), these centers process the data and deliver them to users (scientific community, governments, industries...).
Fairbanks receiving station, USA
Wallops Island receiving station, USA
Washington processing centre, USA
Toulouse processing centre, France
Each transmitter is Argos-certified and has a unique identification number matching its transmission electronics. Transmitters periodically send messages containing the following parameters:
- transmission frequency (around 401.650 MHz), which must be stable, as computation of locations is based on measuring the Doppler effect
- repetition period, in other words the time interval between two consecutive messages, which varies from 90 to 200 seconds depending on how the transmitter is being used
- transmitter identification number
- transmitted data
Each message is transmitted in less than one second.
There are currently seven satellites flying Argos instruments able to receive signals from transmitters. Once received by a satellite, Argos messages are:
- stored in the satellite’s onboard recorder and relayed to ground each time it passes over one of the three main receiving stations in Wallops Island (Virginia, USA), Fairbanks (Alaska, USA) and Svalbard (Norway)
- and downlinked in real time to the Argos L-band network of ground receiving stations within range of the satellite
The satellites are in polar orbit at an altitude of around 820 km, passing over the North and South poles on each revolution. The orbit planes turn around the axis of the poles at the same rate as the Earth around the Sun. Each satellite can see all transmitters simultaneously inside a circle of 5,000 km in diameter. The motion of the satellite thus traces out a 5,000-km-wide ground track circling the planet.
- The satellites currently capable of receiving signals from Argos transmitters are:
- NOAA-15, launched 13 May 1998, flying an Argos-2 instrument
- NOAA-18, launched 20 May 2005, flying an Argos-2 instrument
- Metop-A, launched 19 October 2006, flying an Argos-3 instrument
- NOAA-19, launched 6 February 2009, flying an Argos-3 instrument
- Metop-B, launched 17 September 2012, flying an Argos-3 instrument
- SARAL, launched 25 February 2013, flying an Argos-3 instrument
- Metop-C, launched 7 November 2018, flying an Argos-3 instrument
The first next-generation Argos-4 instruments (see photo below) will be launched on Oceansat-3 (ISRO) in January 2020, CDARS (NOAA) late 2021 and Metop-SG 1B (Eumetsat) late 2023. These instruments will offer enhanced performance, with much wider frequency bands and significantly increased processing capacity.
Argos 4 instruments
At the same time, a miniaturized model of the Argos-4 instrument, called Argos-Neo, is also being developed. This model will initially only comprise the receiver/processor part (no transmitter) and will not support high data rates (only signals from 124- and 400-bps transmitters). Argos-Neo will be nearly ten times more compact and ten times lighter than previous generation instruments, as well as reducing power consumption by a factor of more than three. An Argos-Neo flight prototype to be launched on the ANGELS nanosatellite (12U) in October 2019 will be the precursor of the future KINEIS constellation of 20 nanosatellites planned to operate from 2022.
Some 60 stations receive data from satellites in real time and forward them to the processing centres.
There are two categories of stations:
- For regional mode, a network of L-band stations covering a large portion of the globe that receive transmitter data from satellites in real time when they are within range of a station. This network accelerates delivery of data from satellites to users, but does not provide global coverage.
- For global mode, the main receiving stations (generally X-band) retrieve all messages recorded by the satellites from a complete orbit, thus providing global coverage. These three stations are at Wallops Island and Fairbanks in the USA, and Svalbard in Norway. They also receive data in real time.
Two redundant processing centres, one in Woods Hole, USA, and the other in Toulouse, France, process all received data and compute transmitter locations. These global processing centres are tasked with:
- message quality control, by checking signal reception levels, time-tags, transmitter identification numbers, the length of received messages and value of received frequencies (used to compute locations)
- time-tagging of messages in Universal Time (UTC)
- locating transmitters from the instrument’s time/frequency measurements (Doppler effect)
- sorting of messages by transmitter and in chronological order
- data processing
All these results are archived and made available to users.