The aim of so-called range-only target motion analysis (ROTMA) is to estimate the trajectory of a target by a single platform collecting range-only measurements. We focus on the observability analysis when the target is in constant-velocity motion. More precisely, the observability conditions are established when the observer's trajectory is composed of one or several legs. We establish a link between the Fisher information matrix (FIM), and the observability, after having proven the similarity of this matrix with the FIM encountered in bearingsonly target motion analysis (BOTMA). We compare observability statuses in BOTMA and ROTMA all along the paper.