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, LS Consulting

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.

John Fischer, Orolia

John Fisher. (Photo: Orolia)

At Orolia, we support the position of the DOT, DHS and DOD and the measures they are taking to protect the GNSS band. However, interference is a fact of life in this band as the GNSS signals are very weak, so additional measures must be taken for resiliency, especially for critical infrastructure. Adversarial interference is much worse than this, so a range of solutions are possible: from the simple – for example, a horizon blocking antenna that shields energy from a nearby tower – to full multi-layer defenses that include RF filtering, advanced DSP filtering/detection, alternative PNT from STL signals, atomic clock and IMU backup, and CRPA antennas. No one solution covers every case, but the problem can be managed.

Mitch Narins, Strategic Synergies

Mitch Narins

It is quite concerning when a panel of lawyers makes a decision, despite the results of years and years of studies, and over the objections of our nation’s technical and operational Position, Navigation and Timing (PNT) 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 perspectives.

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; and
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.

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 in response 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 such a  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 all ground transmitters were mapped and first responders could request specific ones to be turned off when operations were expected in a known area.  Unfortunately, the limits of these operating areas are, many times, unknown or rapidly changing (e.g., forest fires) and any delay in reaction to unforeseen events could prove detrimental to safety, security, and economic wellbeing.

Mitre conducted a test wherein small Unmanned Aerial Systems (sUAS) could 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 comes within a certain distance.  I have no doubt that Ligado would not appreciate this solution …

I would hope that those who disagree with this decision will seek out a means to correct this through either the legislative or judicial branches or both.  Personally, I cannot see how this either “promotes the general welfare” or “provides for the common defense” (I didn’t have to go to law school to come to those conclusions).  I also believe that (1) the ever growing importance and criticality of PNT services should provoke the Congress to modify the makeup of the FCC to include PNT expertise at the very top; and (2) the FCC Office of Chief Engineer needs to become an independent point of expertise akin to the Office of Inspector General to preclude undue pressure and potential political “bending” of technical analyses.

Finally, regardless of whether one agrees or disagrees with the FCC’s order, no one can possibly believe that this decade-long tale of woe is an example of governmental “best practice.”  As it took the energy and significant resources from so many more important PNT initiatives, I hope that a quality/leadership body (perhaps GAO and/or a group of IGs) take up the task to do an independent “lessons learned” assessment of this decade-long tale of woe needs to be carefully examined, as it took the energy and significant resources from so many more important PNT initiatives  — we should never have to go through this ever again.

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, US Navy/Marine Corps, and the Federal Aviation Administration.

Alison Brown

Allison Brown

GPS user equipment needs to be designed to be more resilient to interference. The threat to GPS has been well understood for many years but agencies and vendors have been slow to respond to improve national resiliency against jamming threats. The President signed into law the National GPS Timing Resilience and Security Act in 2018 and on 2/12/2020 signed an Executive Order on Strengthening National Resilience through Responsible Use of Positioning, Navigation, and Timing Services recognizing “the disruption or manipulation of these services has the potential to adversely affect the national and economic security of the United States.” The onus is now on vendors and agencies to develop resilient solutions that can operate in the presence of intentional and unintentional interference. These solutions would also mitigate any potential impact from the use of adjacent bands by operators such as Ligado to deploy 5G and Internet of Things services.

Ellen Hall

Ellen Hall

Overall, the GPS industry should be encouraged to do more to protect themselves from harmful interference. While it is too late for today’s receivers, the next generation needs to introduce more resilience. I agree with the House Armed Services Committee Chairman in saying, “While I strongly support development of the world’s most robust, safe and secure network, using L-band spectrum in such close proximity to critical GPS, as Ligado’s proposal requires, carries an unacceptable risk….”

The FCC said the approval order included stringent conditions aimed at ensuring that GPS would be vulnerable to harmful interference. It seems the only way to prevent harmful interference, would be to permanently fund an oversight commission to monitor Ligado’s system. I believe only Congress can appropriate funding for this effort, therefore the FCC’s decision should be a recommendation to Congress to implement. It is not enough to leave it to a five-person committee to “encourage” that “stringent conditions” are implemented. Enforcement is crucial. Critical infrastructure should be regulated to ensure that we can cope better with “noisy neighbors.”