Electromagnetic measurement systems

Electromagnetic systems (EMS) find the unknown positions of the measurement transponders by means of time-of-flight of the electromagnetic waves – radio waves – travelling from the transponder to the base stations (Stelzer, 2004). EMS provide large capture volumes (see Figure 2), but are less accurate than OMS: each EMS in the chart has a lower accuracy than the worst performing optoelectronic system. Unlike an OMS, no line-of-sight is necessary to find the positions of the transponders; also the human body is transparent for the field applied (Schepers & Veltink, 2010). Limitations of the system, related to the experimental set-up, are the sensitivity for ferromagnetic material in the environment, which decrease the accuracy of the data (Day, Dumas, & Murdoch, 1998); moreover, when the distance between the base station and the transponder is increased, noise increases and the quality of the signal decreases (Day et al., 1998; Schuler, Bey, Shearn, & Butler, 2005). EMS generally have low sample frequencies which is a drawback for sports analysis. The frequencies are lowered when using multiple markers.

Of the EMS systems, the GPS-GLONASS dual frequency system shows a promising range-accuracy combination: 0.04m accuracy in a range of 15000 m2. GNSS are satellite navigation systems of which GPS, GLONASS and GALILEO are examples. Satellites transmit data containing information on the location of the satellite and the global time. Since all satellites have a different position, the time it takes for the data to reach the receiver is different, which gives the option of determining the distance of the satellites. If the receiver gets the information from four satellites, the position in 3D can be estimated, although height information is determined 2 to 3 times worse than horizontal displacement (Berber, Ustun, & Yetkin, 2012). Note that in the graph, all GNSS systems are differential GNSS systems, which have an additional GNSS receiver as static base station within 5 km of the test site. The measurement of the satellite signals of the base station is combined with the measurements of the mobile GNSS to increase accuracy.

Drawback of GNSS systems are the cost, weight, and dimensions of the GNSS receivers and antenna. The GNSS system cannot be used indoors and is also sensitive to occlusions and weather outside. The accuracy of a GNSS system is dependent on its specifications; for example, (low cost) single frequency GNSS units are of substantially lower accuracy (up to 4 m) than high cost dual frequency units (up to 0.04 m), especially under poor conditions (Duffield, Reid, Baker, & Spratford, 2010; Tan, Wilson, & Lowe, 2008). The high-end dual frequency units are however more bulky.

Contrary to GNSS, all other EMS systems can be used indoors, since they utilize local base stations instead of satellite signals. LPM (Local Position Measurement) consists of base stations, positioned throughout the area, and transponders, worn by the subjects. The main base station first sends a trigger to each transponder, whereupon each transmitter sends tagged electromagnetic waves to all other base stations. The same as for GNSS, at least four base stations need to receive a signal to determine the 3D position of the transponder via time-of-flight. The system functions both indoors and outdoors. The accuracy of the system presented in the chart is 0.23 m for a dynamic situation (23 km/h) in an area of 3840 m2 .

Comparable to the working principle of LPM, but less accurate, is the WASP system (Wireless Ad-hoc System for Positioning); WASP uses tags and anchor nodes, placed at fixed positions, to track participants in 2D. The accuracy that can be achieved is dependent upon the venue, varying from 0.25 m in indoor sporting venues to a couple of meters when operating through multiple walls(Hedley et al., 2010). In sport studies, accuracies between 0.48-0.7 m were found at an indoor basketball field (420 m2) (Hedley, Sathyan, & MacKintosh, 2011; Sathyan, Shuttleworth, Hedley, & Davids, 2012). The accuracy is also limited by the bandwidth of the transmitted radio signal.
RFID is a wireless non-contact system which uses electromagnetic waves and electromagnetic fields to transfer data from a tag attached to an object, to the RFID reader. There are two sort of tags: active tags, which actively emit radio waves, and passive tags, which can be read only over short ranges since they are powered and read via magnetic fields (induction). Passive tags practically have no lifetime, since they do not require any power from batteries (Shirehjini, Yassine, & Shirmohammadi, 2012). The RFID carpet of Shirehjini et al. (2012) consists of passive tags and reported accuracies of 0.17 m in a 5.4 m2 area(Shirehjini et al., 2012). Ubisense is a commercially available system, originally designed for enterprises to track assets and personnel, that uses the active RFID technology. In sports, the system was tested at an indoor basketball field (420 m2), reporting an accuracy of 0.19 m (Perrat, Smith, Mason, Rhodes, & Goosey-Tolfrey, 2015; Rhodes, Mason, Perrat, Smith, & Goosey-Tolfrey, 2014).
Factors such as attenuation, cross paths of signals and interference from other RFID tags, RFID readers, and different RF devices can affect the communication between the tags and RFID readers (Ting, Kwok, Tsang, & Ho, 2011).

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