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.