TRACKING TECHNOLOGY

Transmitters
Satellite transmitters are frequently referred to as PTTs (Platform Transmitter Terminal). The PTTS we have used (Northstar Science and Technology, LLC) on peregrine falcons for the past 2 years are solar powered. These units transmit a signal every 60 seconds as long as they have enough light to power the transmission. The number of signals recorded by orbiting satellites from each PTT per day is variable but ranges to a high of 20 and depends on the position of satellites, the weather, and day length. PTTs weigh only 20 grams and are attached using a backpack mount. PTTs are custom fit to each bird using a neoprene harness.

Satellites
Every day, weather satellites governed by the US National Oceanic and Atmospheric Administration circle the earth in a polar orbit. Four of these satellites have ARGOS instruments on board capable of receiving ultra-high frequency (UHF) messages from transmission platforms on earth. As each of these satellites comes above the horizon they begin to receive messages from transmission stations. Each message contains a code identifying the source and other data depending on the configuration of the transmission device. The satellite will continue to receive messages from the station until it disappears over the opposite horizon. Such satellite passes last between 2 and 12 minutes depending on how high the satellite travels above the horizon. Satellites store all data received and transmit this information to NASA's Goddard Space Flight Center where it is analyzed and forwarded to researchers.

Falcon Locations
The location of falcons carrying PTTS is estimated from multipled signals received by a

satellite using a phenomenon known as the Doppler Shift. This phenomenon may be perceived when a speeding train or car moves toward you and then passes by - the sound suddenly changes from high-pitched to low-pitched. Because the satellite is moving rapidly (about 26,640 km/hr), the frequency of signals received from a stationary platform vary according to the satellite's position. The ARGOS computers make use of this frequency change and use a complex series of formulas to estimate the PTT's position. The error associated with the estimate is strongly influenced by the number of signals received during a satellite pass. In general, the more signals the satellite receives during a single pass the more accurate the estimated location.

ARGOS refers to the position accuracy as location Class or "LC". An LC of 3 is the best and indicates that 95 times out of 100 the true location will fall within a radius of 150 m around the predicted location. An LC of 2 indicates that the 95% confidence interval is 350 m and an LC of 1 has a radius of 1000 m. Four good signals must be received by the satellite during a single pass in order for the location accuracy to be estimated.