Monday, October 11, 2010

First Locations for Sinbad and Sheba, 11 Oct 2010

Well, here are the first locations received from Sheba. Don't be deceived by the locations being out over the sea and on the mainland, Sheba has most certainly not left Fahal yet. However, this map shows characteristics of the data we will be receiving from both Sheba and Sinbad when they do leave, and so you might as well learn a bit about satellite tracking before they start to migrate.

The first thing to note is that the locations given are not GPS locations and are not as accurate as GPS locations. The locations are estimates made from the Argos System of satellites that are orbiting the earth from pole to pole. These satellites listen for the transmission from the radiotag (signal is a fixed frequency) and then calculate the location using the Doppler shift in the frequency. The accuracy of the location estimates depend upon the quality of the transmission heard by the satellite and the number of times the satellite hears the transmitter before disappearing over the horizon. In the best cases, accuracy is within 150 m, but in poor situations the location estimate may be out by a couple of km. In the map you can see the "spread" of locations, but like I said, Sheba is probably sitting tight on Fahal

Actually, that is just the short version of what is really happening. If you really want to know then ask your questions by sending us a comment, and one of us will try to answer them.

You may be asking: Why, if GPS is more accurate, aren't these tags GPS transmitters? The simple answer to that is that GPS transmitters are too heavy. Obviously, a bird that has to fly needs to carry a light transmitter that will not potentially impede its flight. The smallest satellite radio tags are Argos tags (the ones we are using weigh 9.5 g), and the smallest satellite GPS tags are over 20 g. If we had fitted tiny Sinbad or Sheba with a tag > 20 g, then they might have had a hard time flying, which would have affected hunting and migration.

The picture above is the map for Sinbad, and if those locations were 100% accurate then Sinbad would have been flying over Faisal's house!... Trust me. He hasn't...yet!

A few more points for you to consider or remember for later when you look at maps...
  • The bare rock of Fahal is probably negatively affecting the signal by reflecting it .
  • Error is greater from east to west than north to south

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