"The Department of Defense (DoD) has depended on different space-based assets as critical components to battlefield operations. As technology has evolved, space-based assets have become the eyes and ears of the military. Unfortunately, these assets have become one of the most vulnerable U.S. infrastructures. GPS [global positioning system] capabilities are great and adversaries may be quick to deny or degrade them. Currently, there is not an alternative to GPS for ground, air, sea, or space operations. The methods of operating with a degradation or loss of GPS are contingent on the type of operation, whether it is land, sea, air, or space; as the current backup alternatives to GPS can only support one operation at the time. For this reason, it is critical for the joint force to have at least one backup alternative. At this time, the only alternative appears to be space-based, as a space-based alternative can provide simultaneous global coverage more affordably and accurately on PNT [position, navigation, and timing] to the entire joint force. The most feasible potential space-based alternative to GPS is the existing commercial Iridium system, a sixty-six satellite communication constellation in low-earth orbit, under a program known as the high integrity GPS or iGPS. This constellation would be outfitted to transmit GPS signals to achieve benefits similar to GPS. The Congressional Budget Office recommends that all GPS receivers operate with an additional signal from the U.S. Iridium constellation frequencies and vice versa. If a full space outage happens, a Rapidly Deployable satellite system must be ready to use immediately."
Defense Technical Information Center (DTIC): https://discover.dtic.mil/