What is a CTD tag?

The CTD tags used by SAVEX are small computers designed and built by the SMRU instrumentation group with sensors which allow them to make several measurements about their immediate environment. They have memory in which they store data, a transmitter, a clock, and batteries, all moulded securely into a resin block. Tags are fixed harmlessly to seals' fur, typically for periods of several months. The tags are not buoyant, and are lost when they drop off.
Our tags do five key things:

1. Take measurements of key environmental parameters: Time of measurement, status (wet or dry), water conductivity, water temperature, depth.
2. Analyse measurements to create profiles of seal behaviour, and ocean temperature and salinity changes with depth. These profiles are used to decide what information to store in the tag's journal.
3. Store journal data until transmission opportunities occur.
4. Transmit stored data to the ARGOS satellite.
5. Monitor and manage the tag's own limted resources of power and memory.

These measurements are stored and transmitted to the ARGOS satellite. A very good explanation is given in the video you find on the right. Please click the play button to see it. If the player does not work, you can get a video here (video, ~12MB)

What’s clever about our tags?

The short answer is, the software.  The program in the tag’s computer has to work independent of any help or support from the moment the tag wakes up on a seal, to months later when it ends its data gathering mission.   It must manage limited battery power, decide when circumstances have changed and new measurements must be made, and detect potential data transmission windows (tags can only transmit at the surface).

The tag must also create digests from actual measurements to reduce the amount of information which must be transmitted.  This is a process which requires the program to do more than simply compress data – it must also study information gathered to determine which parts are likely to be of interest.  In addition the program must try to send representative data packets without confirmation that they have been received.  In practice this requires a disciplined ‘data publication’ routine to optimise the probability that as much as possible of the tag’s precious journal will be received.

All this is a tall order.  The program used in tags is constantly being redeveloped to accommodate new technical advances, and new data requirements.   The current version of the tag control software is over 20,000 lines of code long.  In the future it is hoped to develop systems which can be dynamically updated during data collection missions, however these enhancements will require new satellites allowing scientists to establish two-way communication.