Inclusion of rotor blade modulation in hardware-in-the-loop testing of RF systems

RF systems onboard rotorcrafts are susceptible to a periodic variation in both the magnitude and phase of the received signals due to the rotation of the rotor blades. This is often referred to as rotor blade modulation (RBM). As one might imagine, RBM has the potential to degrade a given RF system; therefore, RBM must be accounted for when characterizing the performance of any RF system onboard a rotorcraft. One can use field tests or hardware-in-the-loop (HITL) testing to characterize a particular RF system. Typically field tests are very expensive, and therefore it is beneficial to use HITL testing. In this paper we present three approaches that can be used to include RBM in HITL testing of RF systems. The first two approaches make no approximations and thus provide an exact model of the RBM. The main drawback of these two approaches is that they are not suitable for implementation in HITL testing. Thus we have developed the "approximate simulator," which can be used to include RBM in HITL testing of RF systems. The accuracy of the approximate simulator is demonstrated by comparing the approximated signal received by an antenna mounted on a model rotorcraft with the measured signal received by the antenna. The results show that a very accurate model of the RBM is obtained using the approximate simulator. Furthermore this accuracy is achieved with implementation costs that are far less than that of the two exact approaches.