## 4 Discussion and Implication  
This study examined the validity of a GNSS device to measure acceleration during different accelerating and decelerating movements, resembling those commonly observed in team sports. The main findings indicate that the GNSS device had an acceptable validity for acceleration data when the raw speed data were smoothed with a fourth-order (zero lag) low pass Butterworth filter with a cut-off frequency of 2 Hz. A secondary finding was that the type of movement performed influenced the validity. The raw GNSS data showed the largest deviation from the Vicon data with a RMSE of 2.24 m\(\cdot\)s\({}^{-2}\). Acceleration metrics derived from raw data should therefore be interpreted with caution. When using threshold-based metrics with raw data, an athlete's movement profile will be overestimated compared with their \"true\" profile. Practitioners using raw acceleration data should be aware that the reported acceleration values and threshold-based metrics could be overestimated. On the other hand, the manufacturer smoothed data were more accurate than the raw data, but still had a larger deviation from the Vicon data compared to the most accurate smoothing technique. Practitioners using manufacturer smoothed acceleration data should be mindful that the acceleration data could deviate from the true value by 1.21 m\(\cdot\)s\({}^{-2}\). When solely using manufacturer smoothed data, the accuracy of the acceleration data will stay consistent and should not impact the decisions made by practitioners. However, problems will arise when using raw data in combination with the manufacturer smoothed data. As the deviations from the Vicon data were different between the manufacturer smoothed data and raw data, using both data types together could cause inaccurate data. For example, if using raw data to set a high acceleration threshold (e.g., 80% of max acceleration), this threshold could be overestimated by 2.24 m\(\cdot\)s\({}^{-2}\). When this threshold is then applied to the manufacturer smoothed acceleration data, which has a stronger smoothing (smaller deviation from the truth compared to the raw data), the data will almost never reach the threshold. Practitioners should also be aware that the manufacturer smoothed acceleration data, exported by different GNSS manufacturers, may not be comparable because they may be have been processed using different smoothing techniques. Using a smoothing technique improved the GNSS validity, with a 2 Hz Butterworth filter cut-off frequency showing the best level of agreement, accuracy, and precision across all smoothing techniques and levels assessed. The Butterworth filter potentially performed best due to its ability to effectively reduce high-frequency noise while preserving the underlying low-frequency components (which typically correspond to the actual movement) [30]. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Movement**} & \multicolumn{2}{c}{**Mean Bias \(\pm\) SD**} & \multirow{2}{*}{\
\begin{tabular}{c} **95\% LoA** \\ **(m\(\cdot\)s\({}^{-2}\))** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **RMSE** \\ **(m\(\cdot\)s\({}^{-2}\))** \\ \end{tabular} } \\  & \multicolumn{1}{c}{} & & \\ \hline COD180 & \(0.00\pm 0.72\) & \(-1.41\) to \(1.41\) & \(0.72\) \\ COD90 & \(-0.02\pm 0.58\) & \(-1.12\) to \(1.16\) & \(0.58\) \\ JumpA & \(0.00\pm 1.05\) & \(-2.06\) to \(2.06\) & \(1.05\) \\ JumpD & \(0.00\pm 1.22\) & \(-2.39\) to \(2.39\) & \(1.21\) \\ Linear & \(0.03\pm 0.52\) & \(-0.99\) to \(1.05\) & \(0.52\) \\ Stop & \(0.01\pm 0.52\) & \(-1.01\) to \(1.03\) & \(0.52\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison (GNSS-Vicon) of GNSS and Vicon acceleration data of six different accelerating and decelerating movements. GNSS acceleration data smoothed with a fourth-order (zero lag) low pass Butterworth filter with a cut-off frequency of 2 Hz compared to the Vicon data. This is particularly useful in athlete tracking where high-frequency noise often arises from sensor imperfections or external interference [15].