The satellite telemetry system
   

Satellite telemetry

SMRU has designed SRDL's (Satellite Relay Data Loggers) in shapes and sizes appropriate to a variety of species. In addition to batteries and a radio transmitter for communicating with the Argos satellites, all SRDL's contain a depth sensor, a submergence sensor and a real-time clock. Some designs also include a swimming speed sensor and/or a water temperature sensor.

The SRDL also contains a microprocessor which can read the current value provided by any of the sensors. It can also switch the SRDL into 'sleep' mode to save energy. As a minimum, the microprocessor must be programmed to collect readings from the sensors and to construct a 256-bit message from that data. The message can be transmitted if the submergence sensor is indicating "dry" and the minimum inter-message interval (specified by Argos) has elapsed.

Limitations on battery power and data bandwidth have led us to develop a highly-configurable program in an attempt to extract as much information as possible from the small number of bits that we are able to receive from the SRDL. This section describes how this program works, and how the information it sends is retrieved and interpreted back at the lab.