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[Ashmore Trip 1] A short note on data-loggers

May 5, 2013

[This is the second in a series of ‘Ashmore Trip 1’ posts, essentially diary entries. Internet was unavailable at the Ashmore Reef, so they are being presented one-day-per-day now that I am back in Melbourne.]

We’re taking two different types of location-tracking devices with us to Ashmore, and I think the differences between them are big enough that it’s worth giving a few details on what they do, and how they differ.

The first type is a non-transmitting GPS logger: once every minute, it looks up its location by GPS, records that position, and goes back to sleep. These devices are accurate to within about ten metres, and retail for around AU$80 (though if you’re buying them in bulk, you can get discounts). Because GPS is a relatively battery-intensive way to find your location, and batteries are heavy, these are on the heavier end of the bird logger spectrum – we won’t attach them to anything lighter than a Lesser Frigatebird. They have a relatively short deployment time, with a battery life of around nine days.

(size comparison: Australian 20c and 5c coins)

They look like *this*

Because these devices do not transmit, we will need to re-capture birds and take the loggers off to get the data back.

The other kind of logger we’re taking is far more expensive, but (in my opinion) far cooler. They’re GLS loggers, manufactured for the British Antarctic Survey. They are tiny, ultra-lightweight, and have a battery life of around three years. They record locations in a completely different way to the GPS loggers – each unit is simply a battery, a timekeeper, a light-sensitive device, a tiny thermometer, an immersion sensor, and a tiny chip to record data to. That’s it.

Size comparisons: Australian 20c and 5c pieces, and a standard (much-used) pencil.

They look like *this*

In order to get position data from these beasties, you look at the time of the day when the light-sensor was detecting light. Relative to a known start location, you can tell a lot about your location by looking at when it gets light, and how light it is at certain times of day. For instance, as you move West or East, your dawn and dusk times become later or earlier, respectively. As you move towards the Equator, your dawn happens more quickly, and vice-versa as you move away from it. The thermometer is used as an add-on to this process, making the estimation of the location slightly but usefully more accurate.

The trade-off of using this kind of device is that the accuracy isn’t high: it’ll tell you the position of a bird within about two hundred kilometres, not a few metres. It’s great for telling you that the bird is ‘off the coast of South America over winter’, but won’t tell you that its foraging path *exactly* matched *this* particular fishing vessel, like you could get from GPS.

On this trip, we’ll catch birds and deploy GPS to get a handle on where the foraging areas are for birds on the reef during April. When we remove the GPS loggers, we’ll replace some of them with the tiny GLS ones, and check back in future years to find out where else they go.


From → Science, Scientist

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