12 



FISH AND WILDLIFE TECHNICAL REPORT 30 



0.5-1 Chukchi Sea 



2 3 4 5 6 7 8 9 10 11 12 

 30-day Period 



Fig 7. Location performance indices for PTT's deployed on polar 

 bears: A. Three bears in the Chukchi Sea, showing decreasing 

 performance with time; B. Four bears in the Beaufort Sea, 

 showing variable performance with time. 



than those shown. For example, during tests designed for 

 assessing accuracy and precision of PTT's (see next sec- 

 tion), the location performance index of PTT's was 0.94 

 when placed on fenceposts and buildings but dropped to 

 0.65 when placed on nearby captive caribou. Similarly, 

 Keating (unpublished report) reported a 43% reduction in 

 R s for a PTT deployed on a female bighorn sheep (Ovis 

 canadensis) compared to similar fixed locations. 



The poorer performance of PTT's when placed on 

 animals is likely caused by the proximity of the antenna 

 to the animal's body and the resulting effect on the 

 voltage standing wave ratio (VSWR). The VSWR effect 

 results in reduction of effective radiated power from the 

 antenna. 



Polar bear PTT's were the least efficient of all spe- 

 cies we tested. The antennas of all polar bear PTT's 

 were encased within the collar (as opposed to protruding 



slightly, as in other applications except brown bears). 

 In addition, polar bear PTT's were subjected to con- 

 siderable abuse, inherent with an animal that lives 

 in such a cold climate, moves in and out of icy water, 

 and kills animals that weigh hundreds of pounds. Still, 

 it is possible that the large body mass of both species of 

 bears contributed to the attenuation of the transmitted 

 signal (VSWR effect). 



Projects that experienced relatively high efficiency in 

 obtaining locations also received a higher proportion of 

 better quality locations than did those with low overall 

 efficiency. This result was not surprising, because the 

 quality of the location estimate is positively related to the 

 number of messages. The northern Alaska muskoxen 

 study was the only one in which LQ1 (poorest quality) 

 locations were outnumbered by LQ2 locations (Table 7). 

 Those projects where efficiency was low such as the 

 mule deer study in Idaho and the brown bear study on 

 Kodiak Island also had the highest proportions of the 

 lowest quality (LQ1) locations. 



Precision and Accuracy of Locations 



Some study objectives depend on the system's ability to 

 maintain an acceptably small magnitude of location error. 

 Factors that may have contributed error to Argos's esti- 

 mate included PTT oscillator instability, changes in PTT 

 elevation, animal movement, insufficient number of trans- 

 missions reaching the satellite, and errors in satellite orbit- 

 al data, computational algorithms, or mapping methods. 

 We addressed the following hypotheses concerning loca- 

 tion precision: 



Fluctuating PTT temperature (assumed to affect oscilla- 

 tor stability) reduced precision from that achieved using 

 a PTT at constant temperature. 



The elevation of the satellite as it made its nearest 

 approach to the PTT was related to the precision in the 

 resulting locations. 



Animal PTT's (with short antennas mostly-encased in 

 the collar) did not achieve the level of precision 

 achieved by larger, fixed PTT's with long, external an- 

 tennas (the type used in Argos's own estimates of the 

 system's precision). 



Third-generation PTT's produced locations of greater 

 precision than second-generation models. 



Individual PTT's displayed detectable variation in 

 precision. 



Deployment of PTT's on animals reduced locational 

 precision from that achieved at fixed stations (e.g., roofs 

 of buildings, trees, posts). 



