FISH AND WILDLIFE TECHNICAL REPORT 30 



Fig. 2. Doppler shift in frequency as the satellite 

 approaches and then moves away from a 

 PTT. The slope of the Doppler curve at the 

 inflection point determines the distance of 

 the animal from the satellite's ground track. 



401.6 



Received 

 Frequency 



401.640 



Satellite 

 Orbit 



Transmitted 

 Frequency f T 



401.6500 MHz 

 lower than f T 



Doppler Curve 



determining the index for each location. 



In January 1988, Argos initiated a new service for wild- 

 life researchers called location class zero (LCO). In this 

 special processing, locations are calculated from as few as 

 two Doppler measurements. For all overpasses in which a 

 location fix from LCO processing is obtained, data appear 

 in files separate from those obtained through normal 

 Argos processing. These locations are generally of lower 

 quality but may still be useful for some wildlife appli- 

 cations. LCO processing also contains records for each 

 normally calculated location and provides the alternate 



Table 1 . Effect of maximum satellite elevation during a 

 pass, and the difference between the assumed and ac- 

 tual platform transmitter terminal (PTT) elevation, on 

 location accuracy of a large, well-insulated transmitter 

 (adapted from French 1986). 



location. Each record also includes a location indicator 

 (LI) index that can be useful in assessing why normal 

 processing failed. Table 3 lists codes used in LCO records 

 to indicate which problems were encountered and their 

 interpretations. 



All data are transferred from various ground stations to 

 the Argos data processing centers in Landover, Maryland, 

 and Toulouse, France, through a network of ground and 

 satellite communication links. Results are distributed to 

 users by way of telephone modem, telex, printouts, 9-track 

 computer tapes, or 1 .2 megabyte floppy diskettes. Argos 

 periodically updates the commands available through its 

 computer system. Fancy et al. (1988) provided a descrip- 

 tion of the process of data acquisition. 



Transmitters monitored by Argos are called platform 

 transmitter terminals (PTT's). All of the PTT's we tested 

 were manufactured by Telonics, Inc., of Mesa, Arizona. A 

 list of other PTT manufacturers appeared in Fancy et al. 

 (1988). Telonics PTT's were first tested on caribou (Pank 

 et al. 1985) and muskoxen (Reynolds 1987) in 1984. 

 These first generation PTT's were replaced by the second 

 generation during April 1985. Additional changes resulted 

 in a third generation of PTT's currently produced by Te- 

 lonics, Inc. (Table 4). Unlike VHP transmitters that usually 

 emit 30-70 signals per minute, PTT's used in the Argos 

 system transmit signals (usually referred to as messages) 

 once every 55-90 s. Most PTT's we used transmitted 

 messages every 60 s. The number of messages received by 

 a satellite will vary depending on the nature of each over- 

 pass, but it rarely exceeds 12. (We recorded a mean of 7.8 



