Development and Analysis of SHEFEX-2 Hybrid Navigation System Experiment

Kurzfassung

There is need in Europe for an accurate navigation system for Earth-based aerospace applications with high dynamics, such as launch and atmospheric entry vehicles. The Hybrid Navigation System (HNS) was developed in response to this need. This system fuses data from an IMU, a Global Position System (GPS) receiver, and a star tracker (STR) using an extended Kalman filter. Data fusion allows the user to combine the strengths of each instrument to compensate for their individual weaknesses. Combining the data from different sensors in this way reduces the effect of measurement errors from each sensor and increases navigation accuracy compared to using an uncoupled approach. This work applies modern navigation and computing methods to a real life problem and verifies these methods with flight data. Its flight on the SHEFEX2 sounding rocket mission validates the software- and hardware-in-the loop simulations, the real-time computing methods and the navigation algorithm. This is useful for future navigation missions which may base their algorithms on this work and for other electronic systems which may use these techniques for their own simulations. The novel navigation algorithm consists of a high-rate and a low-rate task running in parallel and passing data back and forth. The high-rate task processes the IMU data and integrates the strapdown navigation equations as soon as new IMU data is available (400Hz). It is also responsible for computing the linearized state transition model, calibrating clocks and correcting the whole states. The low-rate task propagates and updates the filter at 1Hz using data sent from the high-rate task and the sensors and corrects the error state vector. Together with some changes necessary for running on a real-time embedded system, this algorithm was implemented in C code and run on the HNS navigation computer. The HNS is a complex system with many devices working together in parallel. As in any mission, thoroughly testing the software and hardware is very important. To fully test the system a realistic software-in-the-loop simulator and hardware-in-the-loop test bench were developed which could simulate the expected flight conditions and test the system. Additionally, various real-world tests were done to test aspects of the system that could not otherwise be tested by simulation. These included a road test and a night sky test. SHEFEX-2 was launched on June 22, 2012 at 19:18 UTC from And{\o}ya Rocket Range in Norway by DLR. The flight results are presented here to show the performance of the individual sensors and the overall navigation system. An accurate off-line reconstruction of the trajectory is described, which is compared against the HNS flight data as an overall measure of the system's performance. The trajectory reconstruction method and navigation flight performance are explained in detail. The presented flight results show that the HNS performed very well and, except for the STR, the mission was a complete success. The SHEFEX-2 flight was the first flight of this system, which brings the HNS software and several hardware components to TRL 7 and paves the way for the HNS to be used as a mission critical subsystem in future missions.