By Rebecca Morelle
Science reporter, BBC News
For the past two months a Danish-Greenlandic team has been tracking walruses as they migrate from Greenland.
Argos uses a number satellite platforms
Transmitters attached to the walruses have been beaming up information to a satellite system called Argos, which has allowed the researchers to follow the tusked beasts' progress from the comfort of their offices.
But this project is just one of a vast array of scientific studies that has taken advantage of Argos since it was set up nearly 30 years ago.
"It is a pioneer satellite system," said Christian Ortega who is in charge of Argos science applications.
Argos was established in 1978, a collaboration between the French space agency (Cnes), the US space agency (Nasa) and the US National Oceanic and Atmospheric Administration (Noaa).
It was originally intended as a scientific tool for collecting and relaying meteorological and oceanographic data, but with its location tracking properties, scientists quickly realised it could do much more.
"It all started with a huge programme where 200 drifting buoys were deployed around the Antarctic Ocean," explained Mr Ortega.
Ocean-based transmitters have provided a wealth of information
"The idea was to collect data - atmospheric pressure and sea surface temperature - from the buoys and to locate them. But at the same time, the buoys were drifting, and because Argos could locate their positions, the scientists also found out they could start to compute the direction and the speeds of the currents."
The work kick-started a raft of similar projects, and scientific data by way of Argos has greatly contributed to scientists' understanding of the oceans, helped meteorologists to predict the weather and has contributed to climate models.
Peter Challenor, head of ocean observation and climate at the UK's National Oceanography Centre, said: "Argos has really changed the way we do things.
"It is there working away behind the scenes, but over the years it has really helped to give us the big picture for the oceans and climate."
The successes garnered by oceanographers and meteorologists quickly alerted biologists to the Argos's potential, and in the early 1980s scientists began fixing transmitters to animals.
Scientists began tagging large animals, such as turtles
The transmitters allowed biologists to find out where wild animals were roaming, which in the case of some creatures - especially those that travel by sea, air or difficult geographical terrains - had been impossible up to that point.
The studies took place on larger animals, such as dolphins, turtles and sharks, but as the technology advanced, transmitters were miniaturised allowing much smaller animals to be tracked.
This has had dramatic implications for marine mammal research.
Bernie McConnell, from the UK National Environment Research Council's Sea Mammal Research Unit at the University of St Andrews, said: "One of the biggest advances has been to miniaturise the Argos transmitters and combine them with intelligent data loggers.
"Essentially, this allows us to observe animals' behaviour, and their immediate environment, underwater."
More than 4,200 animals are tracked monthly by Argos, providing key data about migration routes, flight altitudes and breeding grounds.
And now biologists and oceanographers are collaborating.
"What is interesting is that some biologists are equipping animals with oceanographic instruments," explained Mr Ortega.
"You have elephant seals that can sense the temperature and salinity, and they are providing data from areas that are difficult for oceanographers to reach."
The Argos system itself comprises six satellites, which follow polar orbits at an altitude of about 850km (530 miles), 50 terrestrial receiving stations and two data processing centres.
Unlike the Global Positioning System (GPS) that needs a minimum of three satellites to be in range to pinpoint an object's location, Argos requires just one satellite to "see" a transmitter to do this.
"The Argos system was designed as a very, very simple and robust system," Mr Ortega told the BBC News website.
As a satellite passes overhead, it picks up data from a transmitter, which is continuously sending out messages in short pulses. During the 10 minutes or so that the satellite is in range, it will measure the frequency of each message it receives.
However, an effect known as the Doppler shift means that each frequency the satellite receives is slightly different - as it moves towards a transmitter, it records a higher frequency, as it moves past it, it records a lower frequency.
Using these frequency changes, together with the satellite's speed, position and the original frequency that the transmitter beamed out, the transmitter's position can be calculated.
Researchers are able to pick up the results via email, websites or they can even extract it via a "virtual globe" system such as Google Earth.
As technology advances, scientists are looking to relay ever-increasing amounts of data from their transmitters, and now Argos tags can be equipped with GPS to guarantee pin-point positioning or GSM to take advantage of the mobile networks on land.
To deal with the increasing data volumes and more sophisticated uses, a more advanced Argos relay unit was launched in October 2006 on Metop, Europe┐s most sophisticated climate and weather platform to date.
The new satellite brings two-way communication and a higher data rate channel to the Argos flotilla.
Cnes is now working on fourth-generation satellites to further boost Argos's capabilities.
Animals like birds need small and light transmitters
But, in future years the system may face challenges from the private sector.
Satellite phone companies, such as Iridium, are being used in place of Argos by some oceanographers because of the increased bandwidth that it provides.
And some believe the competition may be no bad thing for a system that has run largely unchallenged for several decades.
However, until the transmitters can rival the tiny size of Argos tags, it seems that for the time-being, those wishing to track animals will remain with Argos.
Mr Ortega said: "Argos will stay online at least until 2015, and at the moment, there is nothing really quite like it.
"In some ways, it is surprising to see that it is still so attractive and working so well after 30 years, but this could be because there is so much simplicity behind its design."
1. Transmitters on animals or objects relay pulses of data
2. Passing satellite collects data and measures signals' frequencies
3. Satellite relays data to terrestrial receiving stations
4. Processing centre processes data and determines positions
5. Researchers view information via email, website or 'virtual globe'