Transmitter

Kiwisat 101 Argos PTT

The transmitter or Platform Transmitter Terminal (PTT) was supplied by Sirtrack.  Sirtrack is a New Zealand company specialising in the design and manufacture of telemetry for wildlife applications.  It was designed and customized by Sirtrack and researchers in Queensland specifically for tracking estuarine crocodiles.  It is powered by a single lithium C cell and to obtain 12 months life, a duty cycle of 24 hours on/96 hours off has been set.  This equates to being on for approximately 35 hours per week.  With a repetition rate of 45 seconds, a life of around 450 days will be achieved, which approximates to a year after taking into account cell capacity variations.  A VHF transmitter was also attached to the PTT so the device can be found using conventional radio tracking if it becomes faulty.  It is also fitted with a Saltwater Switch and a Haulout Timer set to 4 hours.

The Argos system consists of five polar orbiting satellites equipped with receivers centred at 450.650MHz, considered to be Ultra High Frequency.  At altitudes of around 740-850 km these satellites have visibility circles of around 5000km, receiving signals from PTT’s.  The locations are calculated using the Doppler shift on PTT transmissions.  The average duration of transmitter visibility each time the satellite is overhead is 10 minutes, during which time a minimum of two PTT transmissions must be received to enable a location to be calculated.   Because these satellites are polar orbiting, the number of daily passes increase the closer a PTT is to the poles, as opposed to the equator.  The proposed research area is relatively close to the equator, so the PTT may only be visible to the satellite area 9-12 times per day as opposed to the polar maximum of 42 times per day.  Two main ground receiving stations interrogate the satellites once per orbit, which prompts the satellite to send data to separate regional stations, where the data is distributed automatically or on demand. 

Showing the visibility circle of an Argos satellite

Diagram describing the Doppler shift