The Northrup Grumman Space Technologies High Efficiency Cryocooler (NGST HEC) was designed to support a 10 Watt cooling load at 95 Kelvin while rejecting heat to an effective sink interface temperature of 300 Kelvin. This design is an example of the pulse tube with inertance tube variant of the Stirling thermodynamic cycle whose compressor section uses dual opposed pistons to minimize vibration imparted to any cooling load through the cold end. The Air Force Research Laboratory has characterized the extended performance envelope of this refrigeration system, including its off nominal design point performance and efficiency, its response to transient loading and rejection temperatures, and its cool down performance from ambient. In order to assess this system's long term ability to support extended continuous duty space missions, this cryocooler has been running continuously for over two years, as part of a five year study on whether significant degradation in performance can be measured over that time. Finally, comparison of this cryocooler to other similar space qualifiable refrigeration systems has been made.