The Washington Post recently ran an article on the growing dangers of space debris in Earth’s orbit. It highlighted how proposed mega-constellations of satellites could add to the existing levels of debris, threatening satellites and the benefits they provide to people on Earth. Concerns over debris can be hyperbolic at times, but space debris is a problem, and junk does threaten satellites and spacecraft. However, the conversation around mega-constellations can miss another form of crowdedness and one of the main limiting factors for large constellations: radio frequency interference.
The satellite industry is the largest part of the space economy, valued at around 62% of the total space economy in 2016. That economic contribution, however, requires that satellites be in communication with Earth. Most satellites rely on the radio frequency (RF) spectrum to communicate with each other or the ground, and those frequencies are quickly becoming crowded.
While part of the wider electromagnetic spectrum, radio waves are particularly useful for space-to-ground transmissions. The atmosphere absorbs large chunks of the rest of the electromagnetic spectrum, but radio waves get through. Radio waves, however, are a finite resource.
This has led to concerns that proposals to launch hundreds or thousands of new satellites will create frequency problems. Of particular concern is whether and how these constellations of satellites would interfere with signals from satellites in geosynchronous or medium-earth orbits. Geosynchronous (GSO) and medium-earth orbit (MEO) systems are a critical part of American space capabilities, both for national security and the economy. GSO systems provide persistent remote imaging and serve as communications nodes. In MEO, GPS satellites provide precision, navigation, and timing services.
The problem of spectrum has been increasingly relevant, particularly as demand across industries has grown. In the early 2010s, satellite communication company LightSquared (now Ligado Networks) proposed a 4G cellular communications network. The Federal Communications Commission (FCC) shut down the proposal, however, after concerns that it would degrade GPS access. The rebranded Ligado Networks is trying to move forward with a new proposal, but that too is getting pushback.
As more and more satellites are launched, spectrum problems will be exacerbated. Technical solutions could solve some spectrum issues, but are complex and require significant coordination between satellite companies and governments. For example, the sensitivity of GSO and MEO systems to interference from new satellites has increased as these systems incorporate newer technologies and higher capacity satellites.
The issue is also highly complex on the policy side. Globally, the International Telecommunication Union (ITU) allocates frequencies among its designated regions. Domestically, the FCC handles allocation of radio-frequency spectrum for non-government users, but the National Telecommunications and Information Administration handles government users. American proposals for utilizing spectrum have to work within an international framework—in which revisions to the ITU treaty can take years of planning, significant resources, and expansive lobbying—while balancing competing domestic priorities, complicating an already complex issue. This complexity also bleeds into space-based spectrum allocation issues. Efforts to free up terrestrial spectrum for broadband use, for example, are raising concerns that vital satellites uses could be affected, particularly if spectrum reserved for satellites is opened up.
There aren’t easy answers to the spectrum problem. Ensuring that thousands of satellites can communicate with the ground will require a mix of technological innovation, wisdom in domestic spectrum allocation, and global diplomacy. Parts of the problem will have to be addressed through government policy, but some will require industry to develop and adhere to best practices. If the spectrum allocation problem is not addressed—and soon—the debate over mega-constellations may become moot. The benefits of large constellations, then, may never take off.