Validity of the Catapult ClearSky T6 Local Positioning System for Team Sports Specific Drills, in Indoor Conditions

Luteberget L.S., Spencer M., Gilgien M.

Aim: The aim of the present study was to determine the validity of position, distance
traveled and instantaneous speed of team sport players as measured by a commercially
available local positioning system (LPS) during indoor use. In addition, the study
investigated how the placement of the field of play relative to the anchor nodes and
walls of the building affected the validity of the system.
Method: The LPS (Catapult ClearSky T6, Catapult Sports, Australia) and the reference
system [Qualisys Oqus, Qualisys AB, Sweden, (infra-red camera system)] were installed
around the field of play to capture the athletes’ motion. Athletes completed five tasks, all
designed to imitate team-sports movements. The same protocol was completed in two
sessions, one with an assumed optimal geometrical setup of the LPS (optimal condition),
and once with a sub-optimal geometrical setup of the LPS (sub-optimal condition). Raw
two-dimensional position data were extracted from both the LPS and the reference
system for accuracy assessment. Position, distance and speed were compared.
Results: The mean difference between the LPS and reference system for all position
estimations was 0.21 ± 0.13 m (n = 30,166) in the optimal setup, and 1.79 ± 7.61 m
(n = 22,799) in the sub-optimal setup. The average difference in distance was below
2% for all tasks in the optimal condition, while it was below 30% in the sub-optimal
condition. Instantaneous speed showed the largest differences between the LPS and
reference system of all variables, both in the optimal (≥35%) and sub-optimal condition
(≥74%). The differences between the LPS and reference system in instantaneous speed
were speed dependent, showing increased differences with increasing speed.
Discussion: Measures of position, distance, and average speed from the LPS show
low errors, and can be used confidently in time-motion analyses for indoor team sports.
The calculation of instantaneous speed from LPS raw data is not valid. To enhance
instantaneous speed calculation the application of appropriate filtering techniques to
enhance the validity of such data should be investigated. For all measures, the placement
of anchor nodes and the field of play relative to the walls of the building influence LPS
output to a large degree.