By Andy Kwarteng
While we are waiting for the birds to migrate (further) a bit more information on the system we are using.
The Argos satellite system (www.argos-system.org) which is operated by Collecte Localisation Satellites (CLS) has been used to collect information on animal movements since its inception in 1978. There are currently six Argos instruments onboard NOAA and METOP polar orbiting satellites. Acquisition of such worldwide data helps scientists to improve their understanding of animal behavior, ocean information and forecasting. In this study, we are using the Argos system to monitor the movement of 10 bar-tailed godwits which were caught at Barr Al Hikman, Sultanate of Oman, and were fitted with 5 gram Platform Terminal Transmitters (PTTs).
The duty or charge/transmission cycle of the PTTs is programmed ON (transmit) for 10 hours and then turn OFF (recharge) for 25 hours. While the PTT is in the ON phase it sends a signal to the Argos satellites once every minute. If the charge falls below about 3.6 volts, it will stop transmitting, so sometimes the ON hours will be less than 10. However, the unit will stay on its original temporal cycle. The PTTs connect with a satellite (i.e., messages received by satellite during an overpass) about four or five times each duty cycle. The number of locations we get depends on a lot of factors (e.g., satellites are within a receiving distance when units are transmitting). Locations derived from satellites are assigned location class (LC) values depending on the estimation of the location error. The following are the best to worst LC and their associated location errors: 3 (<250 m), 2 (25-500 m), 1 (500-1500 m) and 0 (>1500 m). Accuracy cannot be estimated from less than four messages which are assigned LC values A and B. Locations which are considered invalid by Argos are assigned LC value Z. The presence of low-quality location resulting from several potential factors may necessitate the exclusion of implausible locations.