Hybrid Navigation System for the SHEFEX-2 Mission
In October 2005 the first version of DLR’s hypersonic SHarp Edge Flight EXperiment (SHEFEX) program was successfully launched from the Andøya Rocket Range in northern Norway. The mission investigated new shapes for future launch or reentry vehicles using a reentry vehicle body with facetted surfaces and sharp edges. The next generation experiment, SHEFEX-2, will launch in 2010. This mission will focus on hypersonic flight control using controllable canard fins and include experiments for new thermal protection system concepts.
Accurate control of the vehicle requires precise knowledge of angle of attack and side slip angle. Both of which are derived from the flight path and vehicle attitude. A hybrid navigation system supplies this information by combining measurements from several sensors. An inertial measurement unit (IMU) is used to capture high dynamics, but it is subject to instrument errors that can cause the error in the navigation solution to grow exponentially if not corrected. A Global Positioning System (GPS) receiver provides position and velocity measurements with bounded errors but only at a low rate. The strengths from the two instruments are combined to provide a high rate navigation solution with better accuracy than can be achieved by using only one of the two separate instruments.
To provide further attitude accuracy, an experimental star tracker will also be integrated together will the IMU and GPS. A star tracker measures the attitude of the vehicle with respect to the stars with bounded errors and at a low rate. The star tracker used for SHEFEX-2 will be a low cost, low accuracy sensor designed and built in-house. It is based on off-the-shelf camera and computer components with a special baffle and shutter release mechanism. The onboard software uses a chain of image processing, star centroiding and star identification techniques to calculate an estimated attitude.