Principles of measurement
Argos instruments locate beacons transmitting from the Earth's surface. Three satellite passes are required to obtain a good fix. More information can be found at Argos: How it works on the CLS web site.
An Argos beacon may also contain sensors that measure temperature, pressure, etc. in its immediate environment.
Animal transmitters are designed by biologists and placed in such a way as to hamper the animal as little as possible (the idea being that the animal should behave normally in order to study its way of life).
The Argos processing centre collects and processes locations and other measurements prior to sending it to the scientists who had either attached a transmitter to an animal or launched a beacon at sea in order to collect the data.