Hybrid Navigation System for the SHEFEX II Mission
In October 2005 the first experiment 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 faceted surfaces and sharp edges. The next generation experiment, SHEFEX II, was launched in 2012 again from Andøya Rocket Range. This mission focused on hypersonic flight control using controllable canard fins and included experiments for new thermal protection system concepts.
Accurate control of the vehicle requires precise knowledge of the angle of attack and the side slip angle. Both of which are derived from the flight path and vehicle attitude. A hybrid navigation system supplied this information by combining measurements from several sensors. An inertial measurement unit (IMU) was used to capture high dynamics, but its measurements are subject to instrument errors that can cause the error in the navigation solution to grow exponentially if not corrected. Therefore a Global Positioning System (GPS) receiver provided position and velocity measurements with bounded errors but only at a low rate. The strengths of the two instruments were combined to provide a high rate navigation solution with better accuracy than can be achieved by using only each individual instrument.
To increase the attitude accuracy, an experimental star tracker was also integrated together will the IMU and the 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 II was a low cost, low accuracy sensor designed and built in-house. It is based on a commercial off-the-shelf camera and computer components with a special baffle and shutter release mechanism. The on-board software uses a chain of image processing, star centroiding and star identification techniques to calculate an estimated attitude.
SHEFEX II launched on June 22nd, 2012 at 19:18 UTC from Andøya Rocket Range in Norway. The flight lasted about 485 seconds until the telemetry link was lost. During this time the Hybrid Navigation System (HNS) performed excellent. The successful real-time navigation solution at high altitudes and high velocities from IMU and GPS measurements could be demonstrated with this flight of the HNS.