Anchor-free localization using round-trip delay measurements for martian swarm exploration

Robotic swarms are promising technique to explore infrastructure-less environments. Many researches in robotic swarm focus on the swarm control and assume an external source for the swarm location information. In this paper, we investigate the radio-based anchor-free localization problem for a robotic swarm. Formation is estimated collectively with only inter-agent distance measurements using the round-trip delay (RTD) technique. Fundamental limits, such as the lower bound of anchor-free localization, tracking and ranging are derived. We further investigate the connectivity and ranging accuracy trade-off with realistic radio resource characteristics. The local Cramér-Rao Bound (CRB) and posterior Cramér-Rao Bound (PCRB) approximations are used to calculate the equivalent ranging variance (ERV). The ERV is used for distributed assessing the reliability of neighbor´s location information and low complexity distributed anchor-free localization algorithms design. ERV-aided distributed Gauss-Newton algorithm (ERV-DGN) and ERV-aided distributed particle filter algorithm (ERV-DPF) are proposed to achieve robust anchor-free localization. Overlooking the ambiguity is a limitation of localization CRB. This problem can be avoided by controlling the number of simultaneous RTD links from the hearability range. The performance of the ERV-DPF with real measurement data shows a sub-meter accuracy level for anchor-free localization. Hence, accurate anchor-free localization for robotic swarm using radio RTD measurements is applicable and promising.