Therefore, location within the beam pattern does not appear to influence detection as much as distance. Twenty-two radar detections were recorded for PTT locations that were calculated to be above or below the radar’s beam pattern (Table 1). Most of these were near the edge of the beam pattern
where they might have been detected because the antenna’s check details sensitivity is not a sharp cut-off. Birds that were below the beam pattern and not detected were significantly (t=2.55, P<0.01, d.f.=92) farther below the beam than birds that were detected (means: 74 vs. 43 m). Likewise, birds that were above the beam and were not detected were significantly (t=2.71, P=0.01, d.f.=8) farther above the beam than those that were detected (means: 441 m versus 121 m). Apparent detections of a PTT-equipped bird calculated to be outside the beam might also have been the result of several birds flying in close proximity and one or more others of the group flying at an altitude within the antenna's coverage pattern. This scenario is highly likely
Alvelestat nmr based on the behavior of both species of vultures when soaring. Comparisons between the speeds and directions recorded by the GPS-PTT tags and the individual radar Tracks can be misleading. In some cases, the match between the PTT speed and heading and the radar’s values is poor because the birds were soaring and circling (as determined from their radar tracks); in other individual cases there is a reasonable match. The closeness of the values depends on exactly when the values are recorded relative to one another. On the radar side, a contributing factor is that the antenna requires 2.5 s for each revolution. The time difference would be Cytidine deaminase twice that if a bird was not detected
on each scan. On the satellite side, there is delay from when GPS unit turns on until it obtains the fix from sufficient satellites. This typically requires no more than several seconds because the birds are above the trees and other structures that might block view of the sky. Spurious values can be generated by GPS measurement and produce errors when the bird is moving slowly over the ground (Hurford, 2009). These errors also could contribute to the poor correlations, especially in heading. Some speed values, as well as headings, change rapidly for a track from a circling bird because the speeds are ground speeds. When a circling bird turned into the wind, we noted that its ground speed decreased 10 m s−1 or more. Occasionally, a GPS reported 0 m s−1 speed but simultaneously recorded an altitude up to 475 m. We examined 21 records with 0 m s−1 in which the bird was calculated to be within the radar’s beam (Table S1). In 13 cases (62%), the radar data corresponded to the GPS location report, with associated speeds of 5–15 m s−1 calculated by the radar.