“…Currently there are 31 operational GPS satellites (GPSWorld, 2015), Glonass has 24 operational satellites (Dvorkin et al, 2012;Krasovskii, 2010;Nikitin, 2008), Galileo has six operational satellites (Cai et al, 2014;Lekkerkerk, 2014b;Mastracci and Fromm, 2004) and BeiDou has 15 operational satellites (Chen et al, 2009;Yuan, 2014).…”