Data Collection and Processing
The Alaska Ecosystem Program Telemetry Research Page provides information on the movements of Steller sea lions acquired through the Argos satellite telemetry system. The information pathway from a satellite tag on a sea lion to this web site involves technology developed over recent decades. Instruments attached to sea lions by NMML researchers are equipped with a pressure sensor and a saltwater conductivity sensor to collect information on diving behavior and time spent on land and at sea. The saltwater conductivity sensor determines when sea lions are on land or when the instrument comes out of the water as the animal surfaces to breathe at sea. The sensor triggers the transmission of a message to NOAA polar orbiting environmental satellites carrying receivers for the Service Argos global data telemetry and geo-positioning system. If a satellite is in view and receives the message or "uplink", the Argos system relays the messages to ground based receiving stations and the data are provided to NMML for analysis and interpretation. Data from active transmitters displayed on the map viewer are sent to NMML automatically every 24 hours, providing near real-time tracking information on sea lion movements.
The geographic locations of satellite tagged sea lions shown on the interactive map viewer are calculated by Service Argos from the doppler shift in the transmitter frequency as the satellite moves toward the transmitter and away from it during a single overpass. A location quality index (defined as the location class or LC) is assigned to each location based on the number of consecutive messages received during a satellite overpass (typically lasting between 8 and 15 minutes) and several other indicators of signal strength. Location symbols of Steller sea lions on the map viewer are coded by LC category and defined in the legend. Argos assigns LC 3, 2, 1, and 0 when a minimum of 4 uplinks are received during an overpass; LC A requires 3 uplinks; and LC B is determined from 2 uplinks. Service Argos estimates the accuracy of LC 3 locations to within 150 m; LC 2 locations to within 350 m; and LC 1 locations, to within 1000 m. The accuracy of LC 0, A and B locations is not guaranteed by Service Argos. Keating (1994) showed greater longitudinal error in Argos locations than in the latitudinal component. Recent research by Hays et al. (2001) and Vincent et al. (2002) suggests that the accuracy of LC A locations may be comparable to LC 1 locations and may have a lower error rate than LC 0 locations, while the class B locations are clearly less accurate than all other categories. LC B locations are not provided on the interactive map viewer due to the high error rate documented for locations of this quality.
The interactive map viewer on the Alaska Ecosystem Program Telemetry Research Page is designed for interested users to observe the location of satellite tagged sea lions in near real time and to explore movements of previously tracked animals. The map viewer is not appropriate for use as an analysis tool and is not intended as such. Argos satellite telemetry data are commonly filtered during analysis to remove low quality or "erroneous locations". For example, if a large number of locations are obtained from the Argos system, less accurate location classes can simply be discarded (e.g. Boyd 1998). An alternative approach is to apply a filtering algorithm that removes locations that exceed a maximum speed of travel between adjacent locations (e.g. McConnell et al. 1992). NMML researchers use both of these approaches in the analysis of satellite location data, however the location data provided on the map viewer have not been filtered beyond the removal of LC B locations and thus erroneous locations likely exist in these "raw data" files. In addition, the daily downloads of Argos location data may include duplicate locations or omitted data due to system malfunctions that are corrected in monthly data summaries provided to NMML.
Citations
Boyd, I.L., D.J. McCafferty, K. Reid, R. Taylor and T.R. Walker. 1998. Dispersal of male and female Antarctic fur seals (Arctocephalus gazella). Canadian Journal of Fisheries and Aquatic Sciences 55:845-852.
Hays, G.C., S. Akesson, B.J. Godley, P. Luschi and P. Santidrian. 2001. The implications of location accuracy for the interpretation of satellite tracking data. Animal Behavior 61:1035-1040.
Keating, K.A. 1994. An alternative index of satellite telemetry location error. Journal of Wildlife Management 58:414-421.
McConnell, B.J., C. Chambers and M.A. Fedak. 1992. Foraging ecology of southern elephant seals in relation to the bathymetry and productivity of the southern ocean. Antarctic Science 4:393-398.
Vincent, C., B.J. McConnell, V. Ridoux, and M.A. Fedak. 2002. Assessment of argos location error from satellite tags deployed on captive gray seals. Marine Mammal Science 18:156-166.