## 2 Materials and Methods
### Procedures  
The study was conducted during the Spanish Beach Handball Tour held in Orihuela-Alicante (region of south-east Spain). A maximum of six matches per team were played in two days. A total of 46 observations were included in the study. Some matches reached the third set (shoot out), but this additional set was not considered in the analysis. A global positioning system (GPS, Spi Pro X, GPSports, Canberra, Australia) was used on each player's back to analyze their physical demands during the match, in stable atmospheric and signal reception conditions. Positional variables (distance and speed) were registered at a sampling frequency of 15 Hz. On the other hand, acceleration was recorded at 100 Hz through a triaxial accelerometer. A heart rate (HR) band was also used by each player (Polar Electro, Kempele, Finland). A standardized warm-up was applied before the test with running, mobility, and high-intensity activities [33]. This system consists of three segments: (a) satellite signal reception and location system (spatial); (b) Earth calibration station (control); and (c) GPS receiver (user). The space segment is made up of a network of at least 24 satellites in Medium Earth Orbit (MEO) that emit the signals (30 operational), of which at least four are required for three-dimensional navigation, which includes latitude, longitude and altitude (in the case of the present investigation, information was received from up to 12 satellites) [34]. This navigation system is made possible by the creation of a constellation of GPS satellites. The GPS satellite constellation has satellites in six equally spaced orbital planes (Earth Centered Inertial coordinates--ECI); it also indicates the exact position of the satellites in these orbital planes. The uneven offset of the satellites in each plane is designed to minimize the effect of satellite disruption [35; 36]. The control segment is made up of a set of high-precision stations located on the ground. This segment has several objectives: (1) To keep each satellite in its proper orbit through small and infrequent commanded maneuvers; (2) To perform corrections and settings to the satellite clocks and payload as required; (3) To track the GPS satellites and generate and upload the navigation data to each of the GPS satellites; and (4) To manage major relocations in the event of satellite failure to minimize impact [35; 36]. However, one problem identified in this segment is the overdetermined system of equations to estimate the location. The skip to the output of the location data from the GPS receivers and the sudden acceleration to very high speeds lead to a large error in the determination of the receiver's position. Some methods such as the least squares(LS) method that has been presented to solve navigation equations so far generally have low accuracy and high error [37]. On the other hand, an alternative solution was presented by [PERSON] et al. [38]; these authors proposed the method of forming a formed subsystem using an auxiliary system obtained from the original system by reducing the columns, providing a more precise estimate of the Rational Polynomial Coefficients (RPCs) compared to the Least-Squares Method (LSM) and Least Absolute Deviations (LAD) methods. In this sense, the authors [PERSON] and [PERSON] [39] conclude that both GPS technology and computer-based tracking systems involve systematic errors, overestimating the distance travelled. Nevertheless, such errors are relatively small and predictable, so it is considered that the use of any of these technologies to monitor athletes' movements should not be prevented [32]. The above-mentioned problem occurs at very high speeds reached by, e.g., aeroplanes [37]. The user segment corresponds to the GPS receivers that receive and decode the signals transmitted by the satellites (Figure 1).