Companies have produced GPS simulators containing modules or modes intended to address issues surrounding space applications, covering the domains of launch, ascent, orbit, reentry, and landing. These have met with mixed success at best, largely due to a problem of market size for space based applications, compared to the much larger terrestrial based GPS receiver market. The generation of specialized signals and hardware that implement these specific functions is therefore often relegated to a black box, undocumented implementation within the simulator. While it is possible to utilize such a GPS simulator for pre-launch testing, the use is far from optimal, and when specific errors occur, it is frequently impossible to receive timely support from the simulator vendor. Consequently, a need exists for a GPS simulator technology which can be transparent and open, and which is designed from the point of inception for space applications. The innovation proposed is the use of a COTS software defined ratio with an open software framework that specifically caters to space application HWIL testers. The proposed innovation results in a multi-purpose signal simulator that can be used for pre-launch testing for spacecraft or other applications such as signal of opportunity navigation using a low-cost SDR approach.