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Blog posts after 1 Feb 2018 about Steppe eagles tracked from Oman can be found at the Steppe eagle blog

Friday, February 20, 2015

Tutorial on the Technology

This blog post will be a short tutorial on the technology used to track the vultures.

The transmitters we use are "GPS PTTs" (Global Positioning System Platform Transmitter Terminals), and essentially they use two sets of satellites to locate the bird and to transmit data on its position and other information about the transmitter (battery status, activity of the bird, etc.) back to earth (us).  These two satellite processes are somewhat independent, but also integrated.  When working properly what happens is this:

The GPS receiver in the PTT calculates on a regular basis its location using the GPS system of satellites and stores that data on board the PTT.  Every two days the transmitter then turns on for a period of 10 hours and uploads that data via the Argos system of satellites, which then transmits the data to a ground station, which then forwards the data to us. The transmission of data via the Argos system also contains the other information about the tag or from sensors in the tag (when you hear the transmission it sounds like a fax).  The transmission signal sent by the PTT has not only those data, but has a stable signal pulse that is used by the Argos system to calculate an estimate of the PTT's position using geometry, the Doppler effect and information on the location, trajectory and speed of the satellites.

While this all sounds a bit complicated, the main tangible result is that for every bird we are tracking we get two sets of location data; GPS data and Argos data.  While the GPS data is typically more accurate, it takes more energy to incorporate the GPS technology, so for smaller birds Argos-only locations are possible.  This is not a problem for Egyptian vultures, and so we get both.

Below are examples of the Argos data and GPS data for the same bird over the same period of time, so that you can see the difference.  However, you can also see the similarities, which highlight that this bird has been making regular visits to the rubbish dump at Al Amerat and has recently be near Al Hajar.

GPS locations of 143580 during 30 January - 19 February 2015.
Argos locations of 143580 during 30 January -19 February 2015

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