Orbital configuration and maintenance of nanosatellite constellations through electric propulsion

Abstract

South Africa is in the process of developing a low Earth orbit (LEO) nanosatellite constellation to monitor activity in its exclusive economic zone (EEZ) within the continental shelf to facilitate marine domain awareness. This project investigates how electric propulsion can be utilised in nanosatellites to form, maintain, alter or change the orbit of a constellation of nanosatellites for a three- to five-year lifespan. The constellation is assumed to be deployed from the International Space Station (ISS) in a cluster that will be spread out to form a constellation. With limited propellant on-board, altitude manoeuvres will be investigated that use atmospheric drag, rather than propellant, to form the initial constellation, and then use the on-board propulsion to maintain the desired constellation configuration. The study also determines whether there will be enough propellant left to de-orbit the constellation at the end of the mission to mitigate orbital debris. A literature review covered possible constellation configurations and orbital mechanics to determine the most suitable constellation to meet the mission requirements, as well as an in-depth study of electric propulsion technologies. Once the feasibility of a constellation configuration and maintenance was confirmed, the study then determined through a link budget whether communication would be possible between the constellation and the ground segment. The research was simulation based, and made extensive use of commercially available orbital dynamics simulation packages, such as STK. The constellation formation and maintenance assume deployment from the International Space Station (ISS) in a 6x2 Walker-delta configuration, deploying a cluster of six satellites per plane. A ΔV budget is a crucial factor in determining whether electric propulsion presents a feasible option to carry out the mission and what the manoeuvre limitations will be for the mission lifespan. The research shows that through electric propulsion it is possible to form an evenly spaced out constellation in roughly 17.2 days with a total ΔV of 9.97 m/s and at a cost of 8.12 g of propellant (from a total of 100 g propellant assumed). To maintain this constellation for five years, it will take an estimated ΔV of 8.18 m/s by utilising the on-board propulsion twice, using a total of 6.7 g of propellant. After the constellation formation and five-year maintenance simulations, enough propellant would be available to de-orbit the constellation. De-orbiting will take between three and six months and an estimated ΔV of 50 m/s with an anti-burn from the propulsion in the opposite direction of flight. This would consume 40.2_g of propellant, which means there would be ample propulsion left for additional manoeuvres during the lifespan of five years. Communication between the ground and the constellation would be possible on the VHF uplink, S-Band downlink and UHF up- and downlink bandwidth, each having an acceptable link margin for communication.