"sensor" data was discarded. The error rate by period is shown in Table 4. 



TABLE 4. Error rate, by period, for the 1990 6-hour summary periods. 



The highest error rates occurred from mid-day to midnight (Periods 4 and 1) and the 

 lowest rate occurred from sunrise to mid-day (Period 3). The overall error rate of 19% is 

 somewhat higher than that experienced by other Argos ocean users (15%) possibly 

 because their buoys have stronger signals, larger batteries and larger antennae. The 

 National Data Buoy Center (NDBC) has experienced error rates averaging 15% (Ray 

 Partridge, pers. comm.). The NDBC performed direct monitoring tests using a local user 

 terminal (LUT) and determined that most of the errors are induced after earth stations 

 have received the downlinked messages from the satellites. Each re-transmission of the 

 message adds additional risk of creating errors. 



In 1990, the new ST-6 transmitters had a software problem resulting in drift of the 

 internal clock by two hours in a six week period for PTTs #823 and #839 (Figures 1 1 

 and 12). This drift resulted in fewer successful uplinks during the fourth transmission 

 period (noon to dusk). Figure 13 illustrates the total number of orbits (passes) for each 

 period and each PTT while Figure 14 illustrates the number of transmissions per period. 



The number of transmissions per minute and their relationship to periods of the 

 day are shown in Figures 15 and 16. PTTs with the largest sample size (N = 126 for 

 PTT #823 in Figure 15; N = 119 for PTT #839 in Figure 16) show a similar overall 

 average rate of transmission but have different patterns in their relative distribution 

 between periods. Because the transmitters had a minimum transmission rate of 40 to 52 

 seconds, the maximum number of transmissions per minute was 1.5 to 1. 1, respectively. 



We expected the number of transmissions/period to reflect the same pattern as 

 the number of passes per period, but Period 1 (dusk to midnight) had the lowest number 

 of transmissions despite having the highest number of passes. This may have been due 

 to less surface resting. The high number of transmissions per minute reported during 

 Periods 2 and 4 for PTT #840 (Figure 16) indicate that the animal was at the surface for 

 long periods, which was confirmed by the discrete dive data (see "Surface Resting"). 

 Thus, animal behavior can affect the number of transmissions received, the probability of 

 locations and location accuracy. 



28 



