In 1991 we introduced our first Doppler PTT, the 95g PTT-100. This was the PTT that proved it was possible to use the Argos system to track migrating birds by satellite with low power transmitters. Since the debut of the first PTT-100, hybrid circuit technology and advances in microelectronics has allowed us to reduce the size of our PTTs. Rechargeable batteries, charged by solar arrays, allowed us to further reduce the size of the PTTs while extending operational life. By 2001 we were able to reduce the size of the Argos PTT to the point where a GPS unit could be added to it and still have a PTT light enough for a bird to carry. The availability of extremely small components made it possible to offer our smallest PTT yet—the 5g solar PTT introduced in 2009.
The Argos system uses the measured Doppler shift in the radio frequency of transmissions from the PTTs during a satellite pass to calculate the location of the PTT. It does this regardless of the message content of those transmissions. Argos (or Doppler) PTTs transmit basic sensor data; GPS enhanced PTTs transmit highly accurate GPS locations recorded by the PTTs, in addition to basic sensor data, in the message encoded on each transmission.
Doppler PTTs and Argos/GPS PTTs come in both solar and battery powered versions; the Doppler PTT is also available in a battery powered implantable version.