Growing Geostationary Orbit Traffic Not Seen Raising Big Congestion Worries

Unlike LEO, what is in GSO "stays there," said Tim Flohrer, European Space Agency Space Debris Office head. But satellites in GSO don't stay exactly on the equator, as orbital forces push them out, so they build up an inclination that has them drifting in and out of GSO, he said. There are ways to estimate the carrying capacity of GSO, but the challenge is getting consensus on the way to measure it, he said.

GSO's future carrying capacity depends on variables including future launch traffic and how tracking capabilities over time are better honed and allow orbital slots to get smaller, Flohrer said. He said compliance with regulatory guidance will mean GSO traffic remains a less critical issue than LEO congestion. GSO compliance with operational guidelines is 85%-90%, "much better than in LEO," he said.

The low collision risk in GSO means ample room for more satellites, said Jean-Luc Froeliger, Intelsat senior vice president-space systems. The "box" of space satellites licensed to operate in that orbit translates to about 50 square miles, and as long as satellites stay in those boxes there's no risk of collision, he said in an interview. Coordination among satellites means sometimes as many as four or five will collocate within one of those orbital slot boxes. "We could have thousands of satellites in [geostationary orbit] if properly managed without risk of collision," he said. "It's big. Risk of collision there is minimal."

The bigger concern is RF interference, with the 2-degree spacing rule meaning no satellites using the same spectrum can operate within 2 degrees of one another, Froeliger said. Given the limits on the number of frequencies available, that puts caps on GSO satellite numbers. The relative saturation of satellites using the C, Ka and Ku bands has driven operators to higher-frequency Q and V bands, though higher frequencies face more atmospheric interference, he said.

Satellites will voluntarily move out of their boxes when moving from one orbital slot to another, Froeliger said. Involuntarily moving out of those boxes is rare, usually due to a satellite anomaly, he said.

With the geostationary arc being about 260,000 km in circumference, "in reality there is a lot of 'space' up there," emailed Greg Autry, director-Arizona State University's Thunderbird Initiative for Space Leadership, Policy and Business. The ability to pack more satellites into the arc depends on their operations and how good they are at station keeping, or staying a fixed distance from another object, he said. Two slots per degree of the arc would mean 720 satellites, but there has been research indicating 10 satellites per degree, or 1,800, would be feasible, he said. "If the satellites are really good at maneuvering and have sufficient propellant to keep away from each other and use tight laser beam transmissions, you could get a lot more," he said.

The current moderate growth of GSO traffic means years before crowding becomes a concern, said David Arnas, Purdue University aeronautics and astronautics assistant professor. It's impossible to know with 100% precision where a GSO satellite is since they're outside of GPS, Arnas said. But satellite operators regularly cooperate to allow multiple satellites in individual orbital slots, he said. The carrying capacity limit of GSO will depend largely on such future cooperation, he said. He said emerging technologies allowing better control, like low-thrust engines, could allow satellites to orbit tens of kilometers apart, upping the carrying capacity of the GSO.