Satellites are highly specialised wireless transmitter and receiver devices placed in orbit around the earth. They can relay hundreds of signals simultaneously, to and from devices on the earth, from simple digital data messages to complex television programmes. For communication they use frequency bands that are in the electromagnetic spectrum - from audible long wave lengths, through radio waves, to short wave lengths such as radio active radiation.
Satellite communication technologies are used in a wide range of applications from radio and television broadcasting to broadband and multi-media services, fixed position telephony (landlines) and mobile communications. Another key application is environmental monitoring, or remote sensing.
One such system, known as ARGOS, is a satellite-based system which collects environmental and location data and delivers it to users worldwide. Through ARGOS researchers are able to collect valuable data which is helping scientists gain a clearer picture of the Earth’s environment and natural resources.
The ARGOS-based satellite system was set up after agreement was reached in 1978 between the French Space Agency (CNES), the US National Oceanic and Atmospheric Administration (NOAA) and the US National Aeronautics and the Space Administration (NASA). The Japanese space agency (NASDA) and the European Meteorological Satellite Organization (EUMETSAT) later joined in 2001. It is operated by the French company Collecte Localisation Satellites (CLS) and is scheduled to operate until at least 2010. There are currently over 10,000 users of this system worldwide with 20% of users tracking terrestrial animals, marine animals and birds. Although ARGOS is perhaps the more widely used system in the environmental sector, Inmarsat satellites are also used by researchers to obtain location-based data.
The ARGOS monitoring system consists of three main elements.
Firstly, a Platform Transmitter Terminals (PTT), a small device attached to the specimen the user wishes to track. The PTT transmits location data coded into signals in the microwave part of the radio spectrum to the ARGOS satellite. PTTs are configured by size, weight, power consumption and housing according to their application and the size of the species or object being tracked. They have been adapted for applications as diverse as bird tracking through to monitoring ice floats.
Second is the space segment. Argos instruments are flown on board the National Oceanic and Atmospheric Administration (NOAA) and Polar Orbiting Environmental Satellites (POES). At least two of these satellites are operational at one time, and each simultaneously sees all transmitters within a 5000 km diameter circle. Each time an ARGOS satellite passes over a transmitter it collects the signal information and accurately measures their frequency. The transmitter is usually visible to the satellite for between 8-15 minutes as it passes overhead.
Thirdly, the final element is the ground segment. The signal information and measured frequencies from the PTTs are relayed by ARGOS to two main processing centres in France and USA. This data is then forwarded on to the researcher via what is known as the “Automatic Distribution Service” (ADS). The ADS can send the data via e-mail, a fax, a data transmission network or by public network within 15 minutes of the satellite download. Archived data is available on tape, floppy disk or CD and is sent to the user once a month.
Interestingly, ARGOS is a two-way system, although it wasn't always. Originally just one-way, satellites only collected data messages from the PTTs. From 2000, with the launch of a new satellite - ADEOS-II - Argos users can now send short messages to their PTT via the web. Typical messages could order the transmitter to switch on or off, or modify the sensor-sampling rate. Not all PTTs, however, support two-way communication
Useful websites
National Oceanic and Atmospheric Administration (NOAA) - http://www.noaa.gov
US National Aeronautics and the Space Administration (NASA) – http://www.nasa.gov
Argos System - http://www.argosinc.com
Collecte Localisation Satellites (CLS) – http://www.cls.fr/welcome_en.html