A lesson in interpreting the data

First of all, the five birds we fitted with satellite tags earlier this month are all alive and moving around.  We can be almost 100% sure that this is the case because each tag has an activity sensor that indicates whether the the tag has been moving.  Since we just have to wait until the birds start on migration (which should be in the next month or so), we might as well use the time constructively, and learn how to interpret the tracking data...

Below is a map of all the locations of all the birds taken over a 10 day period ending on 24 October.  There are many more locations than what you see because many locations overlay one another, and so do not show up on the map (Hint:  If you click on the map, it should open in a new window and be larger and easier to see.).

One of the first things one notices is that not all locations are over Fahal Island, where the birds were reared. Although this implies that the birds actually moved sometimes large distances (about 40 km in one case) this is most probably not the case in every instance, and here is why....

These tags are very small (9.5g) to meet the constraints of tracking such a small bird, and solar powered. Because GPS tracking devices use lots of energy and so need rechargeable batteries, small transmitters like these use another method to estimate the location of the tag, and this method is not as consistently accurate as the GPS system.  This is how it is done:  

The tag transmits a signal that has two parts.  One part is a very stable pulse which the satellites use to estimate the location of the tag.  The second part transmits data like temperature and activity. The satellites that are passing overhead use the Doppler shift in the frequency of the stable part of the transmission to calculate the location of the tag, and the quality of the transmission and number of satellites involved are used to determine the likely accuracy of the location, its Location Class or LC.  Thus, for every location there is a likely error in accuracy, but that error is estimated.  In the best cases (LC3) the nominal accuracy is 125 m. LC2 has a 250 m accuracy, LC1 has a 750 m accuracy, and LC0 has an unmeasurable accuracy that is > 750 m (The location in the above map far to the west has an LC=0).

Although this sounds confusing the main thing to remember is that these inaccuracies exist and that they mean that data from these sorts of transmitters are best used to look at long distance movements, like migration between Oman and Madagascar.

Below is the map of locations for a single bird (130393) during 14-24 October.