A Generic Interface Standard for Inertial Measurement Units

Laboratory testing of integrated Inertial Navigation System/ Global Positioning System (INS/GPS) systems is an important part of acceptance testing for military (and civil) navigation equipment. However, it is not practical to provide an INS with a stimulation representative of an aircraft trajectory in the mechanical laboratory. Therefore, the Inertial Measurement Unit (IMU) containing the INS' gyros and accelerometers measurements must be replaced with simulated IMU outputs. Previously, this has required the use of proprietary information on the system under test and bespoke test interfaces. Work was conducted to develop a standard interface that could be used to allow testing of the integrated system in a laboratory through the generation of simulated delta-velocity and delta-theta measurements as if from the system accelerometers and gyroscopes and fed, along with synchronised simulated GPS radio frequency signals, to the unit under test [1]. This interface was subsequently built and tested to demonstrate its viability and accuracy and this was reported to the ION [1],[2]. The idea has been adopted by NATO as STANAG 4572 and will be a requirement on many future procurements. It is expected that there will be government initiatives to promote the use of the interface in the near future. It has been suggested previously [3] that this form of interface could also be used as the standard output from IMUs, not just for testing but for passing of information round a data bus to a navigation processor and/or any other equipment requiring the input. This is particularly important for lower cost applications where the IMU is packaged as a separate unit rather than an integral part of the INS. This paper investigates that proposal further. Applications of this were mentioned in earlier papers [4],[5]. A study has been conducted on the most commonly used IMUs, including the latest MEMS devices, and their interfaces to suggest a common interface for use in all future tactical grade applications of IMUs. The result is one interface that if adopted could simplify integration of systems with IMUs and the potential for easy upgrade or change of IMUs. The testing of the systems receiving the IMU inputs will also be simplified. This interface was then compared with the STANAG 4572 defined interface and suggested changes given to arrive at a common standard for both integrated INS/GPS systems and IMU driven ones. For information a diagram of the STANAG 4572 implementation is attached at the end of the paper, Figure 2.