https://arstechnica.com/tech-policy/2019/11/dod-joins-fight-against-5g-spectrum-proposal-citing-risks-to-gps/

The Department of Defense has weighed in against a proposal before the Federal Communications Commission to open the 1 to 2 Gigahertz frequency range—the L band—for use in 5G cellular networks. The reason: segments of that range of radio spectrum are already used by Global Positioning System signals and other military systems.

In a letter to FCC Chairman Ajit Pai, Secretary of Defense Mark Esper pressed for the rejection of the proposal by Ligado Networks (formerly known as Lightspeed), saying, "There are too many unknowns and the risks are far too great to federal operations to allow Ligado’s proposed system to proceed... This could have a significant negative impact on military operations, both in peacetime and war."