Aircrafts, civilian, transport or military, all require reliable attitude and heading information for control, high up in the sky where there is no visual reference available to understand whether or not the aircraft is flying level. This information is further utilized by the pilot to navigate and stabilize the aircraft during the flight operations. The attitude and heading information is displayed on the Primary Flight Display (PFD)/Multi-function Display (MFD)/Independent Standby Instrumentation System (ISIS) to provide visual aid to the pilot about the orientation and heading information. An aircraft generally has a 2nd or 3rd level of redundancy for INS-GPS to make system fault-tolerant. This redundancy being achieved by introducing alternate INS-GPS/AHRS to take care of failure cases in the emergency situations. The Attitude and Heading Reference System (AHRS) is preferred as a redundant system in the avionics configuration in the aircraft for standby purpose. The AHRS consists of a cluster of 3-axis accelerometer, 3-axis gyroscope, and 3-axis magnetometer. The AHRS is used to provide additional means to get the attitude (Roll, Pitch), acceleration, angular rates and heading parameters of the aircraft.
Aeron’s AHRS, OCT2-AN5300S, is an Airworthy AHRS designed and developed for an in service aircraft. The unit has been designed as per DO-254 standards and qualified as per MIL-STD-461E, MIL-STD-810G, and DO-178B, Level B+ with 100% MC/DC coverage. The system has been accorded the CEMILAC (An Indian Airworthy Certification Agency) certification. The AHRS gives attitude and magnetic heading information of the aircraft. It comes with a standard RS-422 serial interface. It consists of MEMS-IMU with a built-in magnetometer. It also accepts data of external airspeed and external magnetometer data. The heart of the system is Extended Kalman Filter (EKF) based estimator which fuses information taken from inertial sensors (accelerometers), rotational sensors (gyroscopes), internal and external magnetometers and external aiding sensors like (GNSS/IRNSS velocity, true airspeed, magnetometer, etc.) in order to continuously estimate attitude and heading of the aircraft. It has multiple levels of sensor redundancy to handle failure cases. The AHRS comes with a user-friendly compass swing tool for calibration of magnetometer. It also facilitates in-situ programming of the Operational Flight Program (OFP) software. The OCT2-AN5300S has been successfully tested for entire flight envelope and in highly dynamic (2g, 3g and 4g) manoeuvres.
The AHRS, OCT2 AN5300S can be used as a secondary system for attitude and heading information as a stand by instrument. It can be interfaced with Attitude Indicators, integrated Secondary Flight Displays (iSFD) to provide an alternate means for real time attitude and heading awareness.
