## 5 Discussion
### Accuracy and Validity of LPS Systems  
One of the first LPM systems (45-Hz; 19 antennae) to be tested [33] revealed that, in static conditions, the average positional error was 1 cm, while in dynamic conditions the LPM underestimated distances for almost all courses varying from 0 (sprinting straight) to 29 cm (combined course while walking). Using a similar frequency (45-Hz of the Immotio) and smaller number of antennae (N = 12), a mean absolute error of all position estimations was found of 0.234 m [22]. The LPS of Catapult ClearSky T6 was tested by two studies [38; 39] in indoor conditions, despite using different methodological approaches considering that in the study by [PERSON] et al. [39] there were 18 antennae and a 10-Hz sampling frequency while in the study by [PERSON] et al. [38] there were 16 antennae and a 20-Hz sampling frequency. In both studies [38; 39], the LPS was compared with retroreflective-marker-based systems (VICON and Qualisys). In the study by [PERSON] et al. [39], the comparisons in linear locomotor activities revealed mean differences between ClearSky and Vicon with bias between 0.2 and 2.3%. The mean differences between systems in the total distance, mean and peak speed and mean and peak accelerations ranged from 0.2 to 12%; however, for the case of mean and peak decelerations differences reached 84% [39]. In the other study testing ClearSky vs. Qualisys conducted by [PERSON] et al. [38], mean differences were found for all position estimations of 0.21 m (in optimal conditions) and 1.79 m (in suboptimal conditions). For comparisons of distances, the mean differences were 0.31 m (for optimal conditions) and 11.42 m (for the suboptimal conditions) while instantaneous speed had mean differences between 34.8 and 39.2% in optimal conditions and 74.4 and 90.8% in suboptimal conditions [38]. Summarizing the evidence of both studies relative to ClearSky, validity is acceptable for measuring position, distance, speed and acceleration, although instantaneous speed and decelerations are not accurate enough due to large differences obtained in comparison to gold-standard methods. In addition, testing a LPS (Kineson One, version 1.0) using 12 antennae and a 20 Hz sampling rate, the study conducted by [PERSON] et al. [31] revealed typical error of estimation (TEE) for criterion variables within a circuit between 0.1 and 1.9. In the same study, the LPS system was also compared with a 10- and 18-Hz GPS system [31]. Overall, better validity values of LPS were found for determining distances covered and sprint mechanical properties, although the LPS system presented more outliers due to measurement errors compared to the 10-Hz GPS [31]. The NBN23 LPS system (Nothing But Net model) was also tested for its validity [30]. The system consisted of 12 antennae and frequencies between 9 and 50-Hz. The mean absolute error for distance variables varied between 0.10 m (in walking) and 0.18 m (in running), suggesting good values of validity [30]. For the case of time variables, mean absolute error varied between 0.2 s (at walking) and 0.14s (at walking). Time presented moderate to very high correlations [30]. The NBN23 LPS system revealed validity for monitoring distance and running time, although intensity affects the accuracy of the system [30]. Validation of an LPM system using glass fiber technology was also conducted using 19 antennae and 45 Hz [33]. Comparing average course speed to the average actual course speed, correlations were found (r = 0.71 to 0.97). Despite that, a systematic error of LPM was found in lower speed compared to actual speed [33]. Differences between LPM and actual speed were between -1.3 and 3.9% [33]. Finally, the TEE revealed a clear increasing tendency following the increase in the speed, thus TEE at low speed was more stable and less variable than in sprinting conditions [33]. Considering the values, it was possible to determine the validity of the system for measuring distance and speed [33].