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GPS and PNT experts react to Ligado decision

Leaders in the GNSS/PNT community are reacting to the Federal Communications Commission (FCC) decision to approve an order to allow Ligado Networks to deploy a low-power nationwide 5G network. The departments of Defense and Transportation have criticized the decision, concerned about interference with GPS signals.

Below are statements we have received from our readers.

Logan Scott

Photo: Logan Scott

Logan Scott

Spectrum regulation is much like land use zoning in that certain services are kept separate so as to avoid disturbing the neighbors. The FCC has in effect allowed Ligado to build an outdoor concert venue next to a monastery and by way of compensation, they offer free earplugs. GPS / GNSS signals are extremely weak and the receivers are extremely sensitive. To give some perspective, by the time they get to the GPS receiver, GPS signals are about a factor of 20 less powerful than cosmic background noise. Ligado’s spectrum is/was licensed for mobile satellite services (MSS) and so was not likely to interfere with GPS.

With their new, and much more valuable license, Ligado now has a legal right to build a terrestrial cellular service. Exhaustive testing over the last 10 years has repeatedly demonstrated that such a system will interfere with high precision GPS/GNSS receivers used in surveying, timing, and earth observation. The DoD has also made strong claims that such a system will cause harm to their systems. In all cases, the effect is much like riding a bicycle at night. You can see fine until someone comes around the corner with the high beams on and blinds you.

The FCC has in effect allowed Ligado to build an outdoor concert venue next to a monastery and by way of compensation, they offer free earplugs.

In their earlier filings, Ligado had asked for permission to transmit at a power level of 1500 Watts. In an amazing piece of legerdemain, they convinced the FCC, but not the DoT or DoD, that by reducing transmit powers to 10 Watts, there would be no harm. This is a stunningly erroneous claim. As you lower the transmit power, you need many more cellular basestations to cover a given area. To use an analogy from my backyard, I can install one high flow sprinkler head to cover the entire yard or a bunch of low flow heads, each covering a small portion. Either way, the grass doesn’t care about anything other than inches of water and I’m gonna get wet if I run across the yard. Ligado’s core argument is equally wet. Nonetheless, it has great appeal to people who don’t understand how cellular systems work.

So, moving forward and assuming the license stands, interference events will become more prevalent and GPS will be deemed “less reliable”. Because interference sources are largely untraceable, blame will rarely attach to Ligado.  I expect that GNSS receiver vendors will incorporate improved filters into receivers and pass the cost along to buyers. Ligado, or more likely whoever they sell the spectrum to, will quickly move to petition for increase transmit powers so as to lower capital costs; after all more basestations cost more. And so, the Visogoths have arrived, 4G in hand with a 5G label.

Mitch Narins

Mitch Narins

Mitch Narins

It is quite concerning when a panel of lawyers make a decision, despite the results of years and years of studies, over the objections of our nation’s technical and operational experts. While I have championed the need for a complementary and resilient PNT system for many, many years, one cannot dispute the worldwide benefits that GPS and other GNSS have brought — from safety, security, and economic perspective.

As noted in the FCC’s press release, “The order also requires Ligado to protect adjacent band incumbents by reporting its base station locations and technical operating parameters to potentially affected government and industry stakeholders prior to commencing operations, continuously monitoring the transmit power of its base station sites, and complying with procedures and actions for responding to credible reports of interference, including rapid shutdown of operations where warranted.” I believe that it is imperative that GPS users, both within the government and in the private sector, understand:

  1. How base station location information will be disseminated, what power levels and radiation patterns will be utilized, and what recourse GPS users will have to object to the proposed location and service;
  2. How the service will be monitored for each base station location, what systems will be used to do the monitoring, and how the monitoring information will be publicly disseminated;
  3. How reports of interference will be filed, how and by whom they will be determined to be “credible,” how and in what timeframe will a “rapid shutdown of operations” be accomplished, and how appropriate mitigations will be identified and implemented.
  4. Given the importance and concern associated with this action, what special oversight mechanisms, involving civilian and military participants, should be put in place to avert potential safety, security, and economic impacts to our nation’s critical infrastructure.

Finally, regardless of whether one agrees or disagrees with the FCC’s order, no one can possibly believe that what has transpired over the last decade is an example of good practice. I hope that somehow a leadership body takes up the task to do an independent lessons learned — we should never have to go through this ever again.

Additional thoughts: In addition to the thoughts I provided previously, I began thinking about one of the PNT cases that concerned aviation in regards to the proposed Ligado service — first responder/medevac helicopters.

While one can model, and even test problems based on the location of facilities where aircraft would be expected to be operating close to the ground, e.g., airports and heliports, one can never be sure where a medevac or other first-responder helicopter may have to set down responding to an accident or incident site. In keeping with the FCC’s own press release, it is unclear how a pilot experiencing problems with GPS (if, in fact, the pilot could recognize the problem) would be able to report the issue and have it mitigated in real time to support a safety or security mission — unless all areas around the ground transmitter were mapped and first responders could request specific ones to be turned off when operations were expected in a know area. Unfortunately, the limits of these operating areas is many times unknown or changing (e..g., forest fires).

It made me think about a demo that a Mitre friend did regarding sUAS and their ability to detect and avoid an aircraft based on receipt of its ADS-B squits. Perhaps these ground transmitters should include ADS-B receivers and automatically go off-air if a helicopter came within a certain distance. I have no doubt that Ligado would not appreciate this idea.

Mitch Narins is the principal consultant and owner of Strategic Synergies, LLC, a technical and management consulting firm that he formed after retiring following over four decades of U.S. government service with the FCC and the Federal Aviation Administration.

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