Due to the development of new radar technology for advanced driver-assistance systems (ADAS) and automated driving (AD) applications, testing radars for real world conditions is highly desirable. However, testing autonomous driving functions on public roads can be dangerous and the tests results are not always reproducible. In this paper, we present a novel Over-the-Air (OTA) Hardware-In-the-Loop (HIL) radar target simulator for testing radar systems. The complete simulation setup including hardware and software implementations will be presented in this article. We illustrate a test procedure by creating Euro NCAP scenarios, and explain the benefits and importance of realtime HIL testing of automotive radars.