Swarm Communication in Space - Evaluating Ad-Hoc Routing Protocols for In-Situ Space Exploration Networks

Kurzfassung

Upcoming space exploration missions targeted on visiting extraterrestrial worlds like the surface of other planets and moons in our solar system demand new technologies due to more complex mission designs. The utilization of multiple robotic units during such missions enables the investigation of a broad surface area. By establishing communication channels between all the robotic units, a swarm of agents is created capable of jointly executing scientific tasks. These will include the collection of sensor data which is distributed within the swarm agents and sensor measurements, time synchronization, and localization information will be exchanged between the agents. The German Aerospace Center (DLR) has designed a wireless communication system that is suitable for space exploration missions of this type where autonomous robots jointly explore unknown terrain on extraterrestrial worlds. The system enables data transmission as well as localization of the swarm agents by implementing a Physical Layer (PHY) and Medium Access Control (MAC) for unit-to-unit communication in the context of an ad-hoc network during in-situ space exploration missions. To allow the communication of swarm agents which are not in direct communication range, routing protocols are needed to relay packets of other agents. This work presents a network simulation environment focused on in-situ space exploration missions for the evaluation of existing ad-hoc routing protocols. The simulation utilizes a wireless MAC using a Time Division Multiple Access (TDMA) channel access function implemented in this work resembling the DLR MAC on the network’s nodes. Elevation data of the Moon’s surface is taken into account for radio-propagation modeling in the simulation. Within this simulation environment 11 different experiment designs are implemented to evaluate the performance differences of the Dynamic MANET On-demand (DYMO), Destination-Sequenced Distance-Vector (DSDV), and Greedy Perimeter Stateless Routing (GPSR) protocols in operation on the TDMA MAC and a standard IEEE 802.11g MAC. Performance metrics are defined to compare the routing protocols utilizing the different MACs. The evaluation reveals observations like less stable operation of all routing protocols on the TDMA MAC and a bias introduced to the route establishment of the protocols by the builtin ordering. Further, the differences observed in operation of the individual routing protocols is discussed in details, such as DSDV always being capable of establishing the shortest route to its destination whereas the other routing protocols chose longer routes. Especially DYMO preferred staying on longer routes if the shortest route was not initially available. It is concluded that the DLR MAC will benefit from applications and routing protocols being aware of the TDMA scheme and incorporating the TDMA cycles into their operation to avoid additional waiting times or biases.