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GPS could be deemed ‘less reliable’ if FCC’s Ligado decision stands

May 27, 2020  - By
Photo: Logan Scott

Logan Scott

Spectrum regulation is much like land-use zoning: certain services are kept separate to avoid disturbing the neighbors. The U.S. Federal Communications Commission (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 was licensed for mobile satellite services (MSS) and so was not likely to interfere with GPS.

“The U.S. Federal Communications Commission (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.”

With its new, and much more valuable license, Ligado now has a legal right to build a terrestrial cellular service. Exhaustive testing over the past 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 Department of Defense (DOD) has also made strong claims that such a system will cause harm to its 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.

In its 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 Department of Transportation (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 base stations 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 going to 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 it sells the spectrum to, will quickly move to petition for increased transmit powers to lower capital costs; after all, more base stations cost more. And so, the Visigoths have arrived, 4G in hand with a 5G label.

Logan Scott is founder and owner of Logan Scott Consulting,

About the Author:

Senior Editor Tracy Cozzens joined GPS World magazine in 2006. She also is editor of GPS World’s newsletters and the sister website Geospatial Solutions. She has worked in government, for non-profits, and in corporate communications, editing a variety of publications for audiences ranging from federal government contractors to teachers.

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