Blog Editor’s Note: The author is President of the RNT Foundation.
“Astounded,” shocked,” “and “dismayed” were some of the adjectives used by a vast array of experts to describe the Federal Communications Commission’s (FCC) April decision to approve an application by Ligado Networks.
Government testing had shown that Ligado’s operations would interfere with reception of Global Positioning System (GPS) signals for many users. The application had been strenuously opposed by the executive branch, particularly the departments of Defense and Transportation, and myriad industry and citizen groups. Before the decision was announced most had considered the issue safely dead.
Yet the FCC approved the application in the face of this overwhelming opposition. By way of explanation, the commission said their technical experts had concluded that interference with GPS signals was unlikely. And if it did occur, it would be very minor. Unfortunately, they offered no analysis or technical evidence to support the claim.
This left many scratching their heads as to how government experts at the FCC could come up with findings so different from government experts at the Departments of Defense and Transportation.
There is a growing school of thought that the answer to “how could they do that?” is not a matter of realpolitik or commercial scheming. Rather it lies in the fundamentally different ways communications and navigation systems use the airwaves. And that the communications engineers at the Federal Communications Commission unconsciously used inappropriate criteria in their analysis.
This seems to be the thinking behind a provision in the Senate version of the 2021 National Defense Authorization Act. It directs tasking “…the National Academies of Science and Engineering for an independent technical review of the order to provide additional technical evaluation to review Ligado’s and DOD’s approaches to testing.”
“GPS and communications systems are entirely different,” says Dr. Brad Parkinson of Stanford, the original chief architect of the Global Positioning System. “Communications is about strong signals with an unknown message pushing through the background noise so the receiver can read the ones and zeros. GPS receivers look for a known, but very weak messages down within the background noise. They precisely measure the time between the ones and zeros. Typically, GPS receivers resolve time of arrival to 1 billionth of a second or better.”
Differences Between RF Comms and RF Nav – Two Different Worlds
|Two-Way Radio Communications||One-Way Satellite Radionavigaton|
|Receiver Looks for||Unknown message,
Strong signal pushing through noise
|Known message, Weak signal down in the noise|
|Receiver must||Decide if signal is 1 or 0||Precisely time the transition between 1 and 0|
|Errors, gaps in transmission||Occasional errors are often tolerable||Usually makes receiver inaccurate or drop signal and fail|
|Acquiring/ reacquiring usable signal||Same as following signal||Harder than following signal|
|User Growth||More users require more spectrum||Infinite users for same fixed spectrum|
|Criticality||Important. Failures can be corrected by retransmission.||Safety of Life in many applications. No do-overs.|
|Business Model||Commercial – fee or ad based||Public good provided free by government. Use encouraged.|
|Signal strength||Typically, more than 100 times background noise||Without additional interference, about 1% of background noise:
1/10th of 1 millionth of 1 billionth of a watt
|Resilient Navigation and Timing Foundation – Reproduction Authorized|
Another important difference is that some interference is tolerable in communications. A bit of static, or even a momentary break, often does not prevent the message from getting through. And in two-way communications, like a cell phone call, one party can just ask for the message to be repeated.
No so in navigation. Long experience has shown that one of the first effects of interference with many navigation receivers is hazardously misleading information. The receivers “wander off” before they cease giving information altogether. And once an interference source knocks it offline, a GPS receiver will often not be able to function again until the source is long gone.
GPS advocates like to point out, “There aren’t any do-overs in navigation. No ‘say again’ or ‘please repeat that.’”
And while both communications and navigation are very important, navigation has a greater and more immediate impact on safety of life in far more instances. Disruptions in navigation signals threaten all forms of transportation, much of it at high speed. Examples documented in the last 18 months have included a passenger aircraft nearly hitting a mountain and the crash of a large drone that could have injured or killed someone on the ground.
This impact on safety of life has compelled the Department of Transportation (DoT) to include a safety buffer when evaluating systems that might interfere with GPS signals. This helps ensure navigation is never impacted. This safety buffer is much like weight limits established for vehicles crossing a bridge. The limits help ensure the bridge never approaches its breaking point.
The criteria used by the engineers at the FCC seems more like what they might have used when evaluating possible interference with a communications system. “A little” interference could be acceptable.
A little interference in your car radio could mean a some static while you are jamming to the oldies.
In tests, interference with GPS has driven some cars off the road.
To be fair, most of the FCC tasking and work since it was established in 1934 has focused on communications issues. Spectrum management has likely been viewed as a tool in support of that mission, rather than a goal in and of its own.
Yet in 2020 spectrum is increasingly used by far more applications and in diverse ways that are often very difficult to call “communications.” GPS, radar, advanced sensors, space-based earth observation, radar altimeters for aircraft, intelligent and autonomous transportation systems, WiFi, weather forecasting – all use spectrum and/ or depend upon a stable and predictable spectrum environment. And as technologies continue to evolve this will only increase.
Perhaps it is time to re-charter the FCC as the Federal Spectrum Commission. It would undoubtedly still have to make hard, perhaps controversial, decisions. But perhaps it would be more likely to fully appreciate the needs of all radio frequency users, and publish technically convincing analyses to support their decisions and reassure stakeholders.
Mr. Dana A. Goward is the President of the non-profit Resilient Navigation and Timing Foundation