A real-time system consists of control software, the hardware controlled by the software, and the environment that physically interacts with the controlled hardware. Simulating control software with simplistic assumptions of the environment characteristics is not sufficient for meaningful real-value simulation. The hybrid systems development methods with their simulation environment are a good solution that provides meaningful real-value co-simulation of the target system and its execution environment. It also gives the results that might be gathered when using on-line test-beds. The problem with most of such methods is that the integration and simulation take place too late because they are done after the target system and its environment are fully developed. This paper shows a co-development method, and its supporting environment, which facilitates the repetitive testing of on-going system specifications.