Validation of an indoor real-time location system for tracking sheep

Precision livestock technologies such as remote sensors are increasingly
used to monitor the health, behavior, and welfare of livestock. We aimed
to evaluate the performance of a commercially available ultra-wideband
real-time location system (UWB RTLS) for tracking the 2D spatial locations
and distances traveled by meat-breed ewes and lambs in an indoor barn.
First, we assessed static performance by attaching the sensors to
stationary posts and arranging them in a 1 x 1 m grid throughout the barn
(29.0 x 11.8 m) for a total of 285 locations. At each post location, the
sensors were placed at approximate ewe (0.9 m) and lamb (0.3 m) wither
height. The precise 2D locations of each post were recorded using a laser
tape measurer and used as the ground truth for comparison to the RTLS’
recorded x and y coordinates. Secondly, we conducted a dynamic validation
test to evaluate the positional error and percent error of distances
traveled while the sensors were worn by six free-roaming ewes and their
singleton lambs. The ground truth locations of each sheep were recorded
from video frames every second over 15 minutes and compared to the RTLS
data. Overall static and dynamic error was 0.39 ± 0.20 m (mean ± SD) and
0.53 ± 0.31 m, respectively. Static error was lower in sensors positioned
at lamb height than at ewe height, but the opposite pattern was true for
dynamic error. Error was higher in pens further from the master anchor.
Ground truth and RTLS distances traveled were positively correlated but
the RTLS overestimated distances by 54% on average. In conclusion, the UWB
RTLS can acquire precise location estimates that are suitable for a range
of scientific and practical applications, but distance estimates should be
adjusted to account for overestimation.