Fig. 3

Left North–South spherical distance, \(S_{\mathrm {NS}}\), i.e., the distance between consecutive positions (along-track) of Swarm Alpha. Right East–West spherical distance, \(S_{\mathrm {EW}}\), i.e., the distance between adjacent positions (cross-track) of Swarm Alpha and Charlie. \(S_{\mathrm {NS}}\) and \(S_{\mathrm {EW}}\) are calculated based on Swarm Alpha and Charlie positions between April 2014 and March 2016