The Game & Wildlife Conservation Trust use satellite tags to provide location information about the migrating woodcock. The tags weigh 9.5 grams and are mounted on the back of the bird using a leg-loop harness. Each tag consists of a tiny satellite transmitter, a battery and a solar panel to keep the battery charged.
The satellite tag or PTT (Platform Transmitting Terminal) is a small transmitter that sends messages to a dedicated satellite system known as Argos. These polar-orbiting satellites orbit at 850km above the earth’s surface picking up signals, storing them and relaying them back to earth. Receiving dishes on earth then relay the transmitted data to processing centres which then distribute them to the relevant locations. Argos calculates locations using the Doppler effect of the passing transmitter on transmitting frequency 401.650MHz ± 30KHz to calculate the location of the tag.
The signals returned by the satellites contain information on tag identity, strength and clarity of signal, and most importantly the transmission frequency. Each tag emits a signal of a known frequency. The change between the frequency emitted and the frequency received, with the help of some complicated maths, allows the satellite company to calculate how far the signal has travelled.
Each tag will be in contact with several satellites during each transmission period and by calculating the distance from each it is possible to mark the point from which the tag is transmitting. These data are then sent to us by the satellite company and allow us to see where our birds are as a latitude and longitude.
The satellite tag has a duty cycle of 10 hours transmission followed by 48 hours in sleep mode, so that as the bird migrates we receive a regular chain of points every two and a half days. Each position fix is attributed an accuracy according to the number of transmissions and the number of satellites detecting the tag. By joining the points and accounting for their accuracy, we are able to estimate the route the bird has taken.