Stephen R. Steffes

Flight Results from the SHEFEX2 Hybrid Navigation System Experiment

The Hybrid Navigation System was flown on the second SHarp Edge Flight EXperiment sounding rocket mission on June 22, 2012 from Andya Rocket Range in Norway by the German Aerospace Center (Deutsches-Zentrum f�ur Luft- und Raumfahrt, DLR). The on-board navigation algorithm fuses measurements from an IMU, GPS receiver and star tracker with a delayed extended Kalman filter to estimate a navigation solution over time. The in-flight navigation performance is calculated by comparing the navigation solution returned via telemetry to an accurate reconstruction of the trajectory. Trajectory reconstruction combines all available data sent via telemetry, more accurate state transition and measurement models and additional off-line information using the unscented Kalman filter and unscented Rauch-Tung-Striebel backward smoother. The reconstructed trajectory is computed off-line and is much more accurate than the in-flight navigation solution. Comparing the reconstructed trajectory to the telemetry data shows that the system behaved as expected, although it did not meet its performance requirements. The Hybrid Navigation System software is now at TRL 7.

Reconfigurable Hardware-in-the-Loop Test Bench for the SHEFEX2 Hybrid Navigation System Experiment

The Hybrid Navigation System is being developed to achieve the required attitude accu- racy and calculate a navigation solution for the SHEFEX2 mission, which will be launched in February 2012 from Andya Rocket Range in Norway by the German Aerospace Center (Deutsches-Zentrum fur Luft- und Raumfahrt, DLR). This system fuses data from an IMU, GPS receiver, and star tracker using the extended Kalman filter. For testing it needs a re- alistic software- and hardware-in-the-loop test bench that can simulate the expected flight conditions and test the system. The hardware test bench requirements include: real-time simulation progression, accurate synchronization of all components, and providing stimu- lation inputs for the GPS receiver, star tracker and gyro instruments (inside the IMU) representative of the expected flight profile. Another key requirement is that the test bench must be easily reconfigurable by swapping real and modeled instruments. This al- lows the system to be tested with variable levels of complexity, which decreases debugging time by allowing the user to separate or remove the problem points by simply swapping cables. The developed software-in-the-loop simulation consists of the navigation flight code wrapped in a Simulink s-function and fed by several high-fidelity Matlab/Simulink models which simulate the rocket dynamics and system sensors. The hardware-in-the-loop test bench runs the models from the software simulation on a dSPACE real-time simulator, which feeds the modeled sensor outputs to the hybrid navigation systems navigation com- puter. With the help of a Spirent GSS7700 GPS signal generator, a Jenoptik Optical Sky field Simulator, and an Acutronic 3-axis rotation table all synchronized to the dSPACE simulation, the modeled sensors can be replaced with real sensors fed with stimulated in- put. Navigation results from the hardware test bench are compared with results from the software-in-the-loop simulation to ensure that the hardware-in-the-loop simulation is working correctly.

Investigation of the Attitude Error Vector Reference Frame in the INS EKF

The Extended Kalman Filter is used extensively for inertial navigation. If initial attitude errors are small, many authors choose to represent the attitude states as a vector of small angles in the vehicle body frame. Some authors choose to represent this vector in the navigation frame instead, but the corresponding reduction of filter performance in the closed loop filter is not discussed. Performance is regained when switching to an open loop filter, but closed loop filters are widely desired. This paper investigates this performance reduction. To show the effect,Monte Carlo simulation results are shown for several cases with a simplified inertial navigation problem using a closed and open loop filter and attitude states in the body and inertial frames. A qualitative argument is given to explain the effects, which stem from a state propagation model that poorly reflects the true system model for this case. A method is proposed to regain performance by using an estimated inertial frame for the attitude states. This method is only beneficial when the attitude states are measured indirectly via the velocity state equation. Results with this new frame are shown and discussed.

Mars Rendezvous Relative Navigation Implementationfor SINPLEX

In this work the development of the navigation solution for the Mars rendezvous scenario for the SINPLEX system is described. SINPLEX is a miniaturized integrated navigation system comprised of an IMU, laser range finder, navigation camera and star tracker. In this scenario a spacecraft performs a rendezvous and captures a sample container capsule orbiting around Mars while the SINPLEX system maintains an accurate relative navigation solution using only the onboard sensors. This work presents the relative navigation propagation and measurement equations and simulation performance results. Particular emphasis is given to the use of a UD factorized version of the EKF.