Maintaining large-scale satellite constellations using a logistical approach

Today, constellations of large-scale communication satellites, also known as megaconstellations, are becoming increasingly popular. OneWeb launched the first batch of satellites from a first constellation of 650 satellites in February 2019 and SpaceX also launched the first batch of its constellation of 12,000 satellites in May 2019. On July 8, Amazon also filed a request with the FCC satellite constellation with 3,236 satellites. The realization of the global Internet service by satellite should allow to change the deal.

However, the unprecedented size of these mega-symbols also brings many challenges, some of which are hidden and unexplored. Researchers at the University of Illinois at Urbana-Champaign have identified a critical hidden challenge regarding the replacement of broken satellites in megacastellations and have come up with a unique solution with methods of inventory control.

"Maintaining these large-scale mega-stacks efficiently is far more complex than traditional space systems – in fact, it's more and more like a land logistics problem that FedEx or UPS have been working on, so we've addressed this maintenance issue." Mega The idea of ​​land logistics, which proves to be not only unique and interesting, but also very appropriate in this context, "said Koki Ho, assistant professor at the Department of Aerospace Engineering at the University of Italy.

The challenge described by Ho is to efficiently exchange a new satellite for another that breaks down. For carriers, broken satellites mean disrupted Internet communications and Internet services, resulting in disgruntled customers and lost revenue.

"Deploying a large-scale constellation is a problem, but maintaining it is another potentially more difficult problem," Ho said. "When satellites stop, it's important to quickly deliver parts companies need a continuous service to ensure global coverage.To do this we need to have enough spare parts in orbit.The question is how can we think of a way smarter to use as few satellites as possible to meet the space requirements? "

In previous satellite constellations, Ho stated that this was not a problem because the scale was small enough that sophisticated methods for calculating the number of spare parts needed were not necessary; it was enough to have some spare parts by orbital plane. But with a constellation of hundreds of satellites, the strategy will not work. In addition, the new small satellites are cheaper but have a relatively high failure rate, so you need a lot more spare parts in every orbital plane, which is inefficient.

"Our idea is to use a multi-level inventory control method in land logistics and apply it to the context of orbital mechanics," said Ho. "In our solution, another lower orbit at the actual orbit, which we call the parking orbit, becomes an intermediate satellite warehouse.A small number of reserve satellites are in the current orbital plane for immediate replacement, while a Larger stock of replacement satellites waiting in the parking orbit. Those of the orbital aircraft meet an immediate need, spare parts in the parking orbit can replenish the orbit real. "

The research also takes advantage of the J2 effect of the orbital plane, caused by the obliquity of the Earth, to provide spare parts. The Earth is not a perfect sphere, Ho explained, and as this is not a perfect sphere, the orbital plane will move.

"This offset rate of an orbital plane is different depending on the altitude," Ho said. "So, when we have a parking orbit located at an altitude lower than that of the constellation 's. their orbital offset speed is different.The mathematical model we have created takes into account this change of velocity and the plane closest to the satellite to be replaced.The method makes it possible to determine which orbital plane is the first that will correspond At the plan that has a request and determine if this plan is actually endowed with spare parts, have spare parts, then we are waiting for the next plane, "Ho said.

Ho added that this method also removes the costly urgency of launching a replacement satellite.

"With this warehouse strategy, when a satellite is failing, there is already a stock of available stock to replace it.When the stock goes below a threshold, you can use more This takes advantage of the batch launch effect It is cheaper to send a rocket with a group of satellites than to launch them separately. "

Ho thinks this new method of supply solves a timely problem.

"People are talking a lot about these megacostellations but they have not thought enough about some of the new challenges that they bring," Ho said. "The use of an approach of Unique warehouse was an effective solution to solve this complex problem. "

The document "Optimal Strategy for the Satellite Constellation Reserve Using Multilevel Inventory Control" is written by Pauline Jakob and Koki Ho of Columbia University and Seiichi Shimizu, Shoji Yoshikawa of Mitsubishi Electric Corporation, Amagasaki in Japan. It appears in the Journal of spaceships and rockets.