The GRClok-1500 (GPS/GNSS & Rubidium Clock) is a smart, low cost, compact and fully integrated GPS/GNSS receiver & rubidium oscillator. It uses the adaptive SmarTiming+ technology, disciplining the GPS/GNSS reference at 1ns resolution, providing a host of complex time and frequency features in one package solution, while achieving state-of-the-art performance, reliability and extended lifetime. Its holdover (GPS/GNSS-unlocked or free-run) performance features <2μs/48 hours or <7μs/week with a temperature profile of within +/-2°C from its prior GPS/GNSS-locked temperature condition.