DOD participates in the fight against the 5G spectrum proposal and points to risks for GPS

Enlarge / How did we end up here again?

US Marine Corps / DVIDS

The Ministry of Defense has opposed a proposal for the Federal Communications Commission to open the 1 to 2 Gigahertz frequency range – the L-band – for use in 5G mobile networks. The reason: segments of that radio spectrum range are already being used by signals from the Global Positioning System and other military systems.

In a letter to FCC chairman Ajit Pai, defense minister Mark Esper urged Ligado Networks’ rejection of the proposal (formerly known as LightSquared) and said: “There are too many unknowns and the risks are far too great for federal operations let the proposed Ligado system go ahead . This can have a significant negative effect on military operations, both in peacetime and in wartime. “

The FCC has largely destroyed similar opposition from NASA, the US Navy and the National Oceanographic and Atmospheric Administration over another spectrum block in the 24GHz range – used by weather satellites for remote monitoring of water vapor. But comments are still being collected about the Ligado plan for sharing the 1675 to 1680 MHz block of the L band. Pai has supported the plan because that range borders on the existing block of 1670 to 1675 MHz that is already in use for wireless services.

GPS signals use different blocks of the L-band, including a primary channel centered at 1575.42 MHz. GPS uses L-band signals because of their ability to penetrate cloud cover, rain and vegetation. The L-band is also used by the DOD for a number of other purposes, including tactical air navigation, landing assistance telemetry, friend or foe (IFF) signals, and missile range and aircraft telemetry – although the DOD is already moving some of these applications further into the spectrum range. make room for earlier ‘commercial reallocation’.

Ligado’s request to share bandwidth is linked to the company’s planned collaboration with satellite communications company Inmarsat, which would provide satellite coverage for Legato’s 5G network. Inmarsat currently uses the L-band for mobile satellite communication services, including tactical satellite communication that is sold to the military around the 1.5 GHz range.

In an email to Bloomberg, Ligado spokesman Ashley Durmer criticized the DOD’s objections, citing the low power of the proposed 5G signals. “Can it really be true that our military operations are vulnerable to a 10-watt light bulb?” she wrote. “We don’t think so. But that’s what this letter suggests.”

But thanks to the reverse law of squares, signals can be susceptible to low-signal interference, in particular from GPS. This was why the original LightSquared proposal for a blended satellite and 4G network in the range of 1,525 to 1,555 GHz was opposed by DOD and others – GPS signals arriving from space could be overwhelmed by the ground-based broadcasts, as demonstrated by tests conducted in 2011. The tests “showed that there are significant adverse effects on all assessed GPS applications,” said a report from the National Space-Based Positioning, Navigation, and Timing Systems Engineering Forum.