Error Properties of Argos Satellite Telemetry Locations Using Least Squares and Kalman Filtering
Citation: Boyd JD, Brightsmith DJ (
Error Properties of Argos Satellite Telemetry Locations Using Least Squares and Kalman Filtering
Janice D. Boyd 0
Donald J. Brightsmith 0
Mark S. Boyce, University of Alberta, Canada
0 1 The Parrot Fund, College Station, Texas, United States of America, 2 Schubot Exotic Bird Health Center of the Department of Veterinary Pathobiology, Texas A&M University, College Station , Texas , United States of America
Study of animal movements is key for understanding their ecology and facilitating their conservation. The Argos satellite system is a valuable tool for tracking species which move long distances, inhabit remote areas, and are otherwise difficult to track with traditional VHF telemetry and are not suitable for GPS systems. Previous research has raised doubts about the magnitude of position errors quoted by the satellite service provider CLS. In addition, no peer-reviewed publications have evaluated the usefulness of the CLS supplied error ellipses nor the accuracy of the new Kalman filtering (KF) processing method. Using transmitters hung from towers and trees in southeastern Peru, we show the Argos error ellipses generally contain ,25% of the true locations and therefore do not adequately describe the true location errors. We also find that KF processing does not significantly increase location accuracy. The errors for both LS and KF processing methods were found to be lognormally distributed, which has important repercussions for error calculation, statistical analysis, and data interpretation. In brief, ''good'' positions (location codes 3, 2, 1, A) are accurate to about 2 km, while 0 and B locations are accurate to about 5-10 km. However, due to the lognormal distribution of the errors, larger outliers are to be expected in all location codes and need to be accounted for in the user's data processing. We evaluate five different empirical error estimates and find that 68% lognormal error ellipses provided the most useful error estimates. Longitude errors are larger than latitude errors by a factor of 2 to 3, supporting the use of elliptical error ellipses. Numerous studies over the past 15 years have also found fault with the CLS-claimed error estimates yet CLS has failed to correct their misleading information. We hope this will be reversed in the near future.
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Competing Interests: The authors have declared that no competing interests exist.
Introduction
Documenting animal movements is key for understanding
species home ranges, migration patterns, resource tracking, and is
vital for developing realistic conservation plans. Remote tracking
of animals began in the late 1950s with VHF radio telemetry and
this technique is still frequently used to track terrestrial animals
over relatively short distances [1]. However, tracking of
wideranging animals and intercontinental migrants was not possible
until the 1970s with the development of 511 kg Argos system
PTTs (platform terminal transmitters) for tracking large mammals
[2]. The second generation of smaller, lighter satellite transmitters
appeared in the mid to late 1980s and weighed as little as 110
150 g [3]. By the late 1990s PTTS as light as 30 g became
available [3], with further size and weight reductions limited by
current battery and solar cell technology. For some telemetry
applications the newer GPS (Global Positioning System) has
replaced VHF and Argos. An important advantage of both Argos
and GPS is that, unlike short-range VHF telemetry, the satellites
are placed in orbits that allow positions to be obtained from every
location on earth, allowing studies of wide ranging and migratory
animals in inaccessible regions both terrestrial and marine.
However, the use of GPS is limited by the need to download
stored positions or the need for a data relay system to transmit
positions to a distant user often via VHF or Argos. Present
GPSbased systems with data relay are generally not light enough for
deployment on animals weighing ,1000 g (i.e., maximum , 30 g,
using the maximum 3% of body weight rule of thumb) for more
than several days or a few weeks largely because of power
limitations [4], [5], [6], [7], [8], [9], [10].
Quantifying location error is a key component of all telemetry
studies as it allows users to realistically analyze and interpret their
data (e.g., [11], [12], [13]). CLS, the French entity that operates
the Argos system (hereafter referred to as Argos) provides
theoretical estimates of the errors of its positions and sends users
estimates of these errors with each computed location.
Unfortunately, Argos does not make clear if these error estimates refer to
the precision (reproducibility) or the accuracy (deviation from true
location) of the positions. Given that most users probably find
accuracy estimates most useful they likely assume that this is what
error estimates refer to.
The primary error descriptor for Argos locations is the location
code or location class (LC) [14]. The LC is base (...truncated)