Radar-Updated Inertial Landing Navigation With Online Initialization

This article focuses on the employment of an inertial measurement unit (IMU) and a radar altimeter and velocimeter as navigation sensors for Mars landing navigation. At the beginning of the parachute descent phase, high dynamic oscillatory motion may saturate the gyroscope and produce large attitude estimation errors, thereby producing a high landing risk. To address this problem, an online navigation initialization algorithm is presented by combining data from the IMU and the radar. By defining a new inertial frame, attitude, and Mars-related velocity are conveniently initialized. A key contribution of this article is the computation of gravitational acceleration using IMU and velocimeter's measurements for the approximation of the nadir vector. Given the nadir vector, the inertial position is initialized in conjunction with the altimeter's measurement, the gravitational acceleration is modeled such that the inertial navigation equation can be propagated and the propagated altitude and velocity can be corrected by radar radar-derived measurements.