Table 2 Data relay

The tags are certified to communicate with six polar orbiting satellites operated by Argos [27]. Communications between the tag and the satellites are subject to strict rules. Information is sent in units, or messages (termed “uplinks”), which may be up to 960 msec in duration and require a minimum of 40 sec between attempted transmissions. Two or more complete uplinks are required for the system to compute a location. Uplinks have a fixed length: 28 bits are used to identify the instrument. A further 248 bits are available to represent the behavioural and hydrographic data.

The combination of this transmission regime, the brief and infrequent surfacing behaviour of air-breathing marine animals, the fact that satellites will not always be available, the lack of acknowledgement of receipt of a message and the tags’ energy constraints all demand complex data collection software, extreme data compression and transmission strategies that maximise the information that is sent using a small amount of energy. As a result, the software and processing routines implemented on-board CTD-SRDLs have been developed over many years to try to maximise the amount of biologically and hydrographically relevant information that can be sent, while using the lowest possible bandwidth.

The critical factor in terms of effective use of energy is the balance between the rate of data collection and the transmission of the data (Fig. 2). Routine sampling of the instrument’s sensors requires little energy. It should be noted however, that even when longevity of the tag is forfeited for higher resolution data, there is still an upper limit to the throughput and rate of information transfer this tag is capable of.