PNT (positioning, navigation and timing) threats are at an all-time high, creating unprecedented operational risk to defense information systems, communications networks and DoD cloud data centers without defense-in-depth PNT capabilities. GPS/GNSS provides critical PNT services for all kinds of communications and networks. These signals, however, are weak and vulnerable to attacks. From jamming to complex asynchronous and synchronous spoofing, these strikes are becoming more frequent and more sophisticated. Now there is even the risk of catastrophic GPS/GNSS satellite space attacks by rogue countries using laser-ranging technology. Having a backup source to GNSS (or even multiple backup sources depending on the risk appetite of the end user) has become a matter of national security driven by the Federal EO 13905, the DHS Resilient PNT Conformance Framework and the IEEE P1259 Resilient PNT Standard for User Equipment in progress. This session will discuss the concept of GPS/GNSS-backup-as-a-service (GBaaS) as a response to the national security threat. GBaaS is based on a resilient and assured PNT clock, which uses an advanced zero-trust multisource PNT framework. Derived from ITU-T G.8272.1 and defined as an enhanced primary reference time clock (ePRTC), GBaaS utilizes a stable frequency optical cesium atomic clock to back up GPS/GNSS. Other possible sources, such as NIST, eLoran and LEO, can be combined with the GBaaS concept to enhance the resiliency level of the clock system.