Attitude Performance Comparison Between Turret-mounted and Airframe-mounted INS Installations

摘要

There has recently been an increased interest in developing Intelligence Surveillance and Reconnaissance and Persistent Surveillance capabilities from unmanned aerial vehicles (UAVs). System developers responding to this need are faced with UAV restrictions in payload weight, volume and power and forced into a design tradeoff decision relating to number and placement of onboard inertial measurement units (IMUs). While it may be optimal to have two IMUs on board, one placed on the airframe to optimize navigation and the other attached to the imager platform within the turret to maximize performance in image registration, the more likely solution will force the developer to choose between one or the other. At issue, then, is the need to either de-couple platform gimbal angles, or else add them to the Inertial Navigation System (INS) state solution depending on which of the two IMU placement configurations is adopted. Measurements of the additional error in attitude estimation associated with numerically de-coupling (and adding) the turret gimbal angles have been obtained from dedicated flight tests aboard a small aircraft. Instrumentation included two identical BEI Systron Donner C-MIGITS III IMUs, one mounted on the airframe and the other next to a calibrated imaging system on a Atlantic Positioning Systems SPS-500 positioner. Controls for the test included a Post Processing Kinematic GPS on the airframe and surveyed ground targets. Flight geometries included straight, level-flight segments and orbits around fixed aim points. Measurements of camera position and attitude were obtained from each of the INS state solutions during these flights and referenced to ground-truth to obtain comparative performance estimations. Not surprisingly, the performance differences are most pronounced during periods of high winds when the 2σ-error distribution was 0.35 milliradian for the turret-mounted configuration and an estimated 0.50 milliradian for the airframe-mounted configuration.