GNSS Antenna-Human Body Interaction

This paper discusses an investigation of the loss of received signal power associated with operation of L1 GPS antennas in close proximity to the human body. Through the use of a human proxy or phantom body with similar electrical characteristics to that of a human body, signal strength data is collected in 24-hour static tests and repeated for multiple distances between the antenna and the phantom. Through these tests it has been determined that in addition to detuning effects, there is a strong relationship between average received signal power and the distance between the GPS antenna and the body. This second effect is dominant and indicates that some types of GPS antenna operated in close proximity to a user should either be immediately adjacent to the body or at a significant distance from it. INTRODUCTION GNSS based navigation is the foundation of many pedestrian navigation systems. The benefit and cost of GNSS receivers to locate people has increased dramatically. Pedestrian navigation applications include mobile phone users, first responders, health and activity monitoring, consensual tracking (e.g. offender management), recreational use and tracking of military personnel. GNSS navigation systems are commonly available in watches and pods. Some applications contain GPS receivers and antennas in shoes, glasses and jackets (Mexperts 2006, 2010, McClure 2010). Since each application using a GNSS receiver to locate people requires an antenna, the optimal type, size and location on the body is becoming increasingly important. This paper addresses adverse antenna effects when the antenna is placed near or on the human body, specifically in the reactive near field at L1. Using real data collected on a human phantom over prolonged periods, the changes within the antenna are observed as a function of distance from the body. Thus, a performance profile can be generated to quantify the power loss incurred by loading the antenna. Performance of GPS antennas in close proximity to a human body has been theoretically address in Ur Rehman et al (2007) and Kellomaki et al (2007). Both use simulations to provide analysis of antenna detuning effects. Ur Rehman et al (2007) showed a 24.4 MHz shift in the resonance frequency of the antenna when placed 10-40 mm from a simulated human chest. The resonance shift was common at all distances, although the return loss decreased as the antenna was move further away from the chest.