In 1987 CAST produced a laboratory test system that allowed for the stimulation of the navigation software in the target processor. This laboratory tool was the predecessor to the Global Positioning System Tester (GPST) that was introduced in 1989. The GPST is a complete laboratory test system that includes a GPS simulator, a complete suite of avionics sensor models and most of the modern avionics interfaces i.e. 1553, 3910, IMU, ARINC etc. The original GPS simulator that was used in the GPST was manufactured by Stanford Telecommunications located in San Jose, California. We also developed an advanced interference generator (Jammer) capability.