J. David Grossman
A 21st Century Spectrum Plan Should Protect GPS
January 7, 2019 By: J. David Grossman, Executive Director, GPS Innovation Alliance
Demand for mobile broadband continues to grow. By one account, monthly data usage on smartphones will increase by nearly 7 times over the next five years. Logically, this means policymakers and regulators are leaving no stone (or in this case ‘band’) unturned when it comes to finding spectrum suitable for these high-speed services.
Finding ways to increase the efficiency of spectrum usage is not only good public policy but also supports the deployment of innovative new services that benefit consumers and businesses alike. The GPS Innovation Alliance (GPSIA) strongly supports such efforts which will lead to expanded deployment of broadband and ultimately connectivity for all Americans. But decisions regarding the repurposing of spectrum are fundamentally complex and must be considered while weighing a multitude of factors, including any technical or engineering challenges.
One of these considerations is whether inherent technical differences between two spectrum-dependent services, located in close proximity to each other, prevent their ability to co-exist. GPS satellites orbit more than 12,000 miles above the earth and rely on solar panels to generate the power needed to send GPS signals back to the ground. As a result of the limited power that the solar panels provide, GPS satellites transmit with no more power than a 50-watt light bulb, and signals arrive to your GPS-enabled device at a power level that is less than a millionth of a billionth of a watt. Compare that to a terrestrial-based communications network transmitting a signal that can literally be billions of times stronger.
These divergent power levels make coordination between these systems exceedingly difficult due to the fact that GPS signals as received on earth are below the thermal noise floor (the level of noise occurring naturally and apart from manmade sources). While some incorrectly suggest that “poor” receiver design is to blame, the truth is that GPS receivers have been designed to perform an extraordinary engineering feat by extracting the signals from the noise and delivering a signal to the end user that is continuously available, accurate, reliable and resilient.
Adding to this challenge, unlike interference between mobile communications networks, where the user can observe the results of interference in dropped calls or poor call quality, the positional accuracy of a GPS device can be degraded by interfering noise in a way that is not detectable, can mislead users about their location, and, in the case of automated guidance applications, cause poor performance or outright malfunctions. In extreme cases of interference, where a GPS receiver “loses lock” on available GPS satellites altogether, the user is left with no means of determining location until the interference is abated.
The good news is that a path forward already exists to provide the hundreds of megahertz needed to deploy next generation 5G mobile broadband networks while protecting critical infrastructure and safety-of-life systems that are dependent on GPS. Just as municipalities use zoning rules to keep a loud manufacturing plant away from a residential neighborhood, a “zoning” approach to spectrum management groups similar services together, ensuring that high powered spectrum users are kept away from dissimilar services like GPS that require a “quiet neighborhood.” Such a straightforward approach avoids the Federal Communications Commission (FCC) having to engage in extensive rule making and standards development to balance the interests of dissimilar spectrum uses in every spectrum “border” area.
In cases where utilizing this zoning approach is not possible, enhanced predictability in spectrum use can be had by abiding by the “1 decibel (dB) standard.” Because GPS operates below the noise floor, this measurement has had a long and well-established history in both international and domestic regulatory proceedings as a way of assessing whether a potential new terrestrial service, adjacent to GPS or another Global Navigation Satellite System (GNSS) causes a 1 dB degradation in a receiver’s carrier-to-noise ratio, or a 25% increase in the noise floor in GNSS bands. Use of a defined change in the noise floor (1 dB) provides a readily identifiable and predictable metric that all interested parties can take into account now and in the future when determining compatibility with GPS operations.
One thing is certain: demand for spectrum will continue to increase. Recognizing this reality, just a few months ago, the White House issued a Presidential Memorandum calling for the development of a sustainable spectrum strategy. Importantly, the plan promises to “take appropriate measures to sustain the radiofrequency environment in which critical United States infrastructure and space systems operate." Given the vital role that GPS plays in our economy and daily lives, this is both a laudable and an essential goal.
The National Telecommunications and Information Administration (NTIA) is now working to fulfill this vision and has asked for public comment on several key issues, including “protecting U.S. space assets from RF interference.” Across the globe, billions of GPS devices are being used to support everything from military operations to precision agriculture, aviation and the electric power grid. This translates into millions of jobs and billions of dollars in economic benefits. What if we could no longer count on GPS to be there when we need it? Providing protections for GPS as part of NTIA’s plan can help ensure we never have to face such a scenario.
 Mobile data traffic growth outlook, Ericsson (June 2018), available at: https://www.ericsson.com/en/mobility-report/reports/june-2018/mobile-data-traffic-growth-outlook
 Presidential Memorandum on Developing a Sustainable Spectrum Strategy for America’s Future, White House (October 25, 2018), available at: https://www.whitehouse.gov/presidential-actions/presidential-memorandum-developing-sustainable-spectrum-strategy-americas-future/.
 Developing a Sustainable Spectrum Strategy for America’s Future, National Telecommunications and Information Administration (December 21, 2018), available at: https://www.ntia.doc.gov/files/ntia/publications/2018-27690_3.pdf