Reconstruction of the 3D Orientation of a Spinning Sounding Rocket by Detecting the Earths Horizon and Sun on Onboard Camera Images
Creators: name:  B. BRAUN 
name:   J. BARF 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT DYNAMICS
GNC
AVIONICS
ESTIMATION
DYNAMICS RECONSTRUCTION

description: The paper presents a method for reconstructing the three-dimensional orientation of a sounding rocket along its trajectory from the Earths horizon and the Sun which are detected on the images of an onboard camera, without using gyroscope measurements. The camera was installed on the one-staged, spinning PMWE FIONA sounding rocket which was launched from Andoya Space Center in October 2021. The camera was looking sideways and had been intended to observe the unfolding of the drogue and main chutes at the final descent. Because lift-off took place on October 1st at 10:03:18 UTC, the Sun was favorably elevated such that it passed just through the center of the image with every revolution of the spinning rocket during the ascent and coasting phase. Furthermore, since the sounding rocket was in principle upright until the beginning of atmospheric re-entry, the Earths horizon was continuously visible on the camera images. The sounding rocket was equipped with a GoPro Hero3 camera which took images with a rate of 50Hz and a resolution of 1920px1080px. Hyperbolae and ellipses, respectively, are fitted to the detected Earths horizon and Sun ellipse curves, from which the direction vectors to the center of the Earth and to the Sun are derived. By observing these two non-parallel vectors once in an Earth-fixed navigation frame and once in a vehicle-fixed frame, the orientation is estimated with the Triad algorithm in the moments when the Earths horizon and the Sun are simultaneously visible on the camera image. In periods in which only the Earth\'s horizon is visible, the rotational degree of freedom (DoF) around the direction vector to the center of the Earth cannot be determined, only the two remaining ones. The rotational DoF about the direction vector to the center of the Earth that is only discontinuously observable is reconstructed by interpolation with time. To do this easily, a special Earth-fixed coordinate system is introduced, the x-axis of which is aligned with the direction vector to the center of the Earth. The accuracy is expected to be 1 deg (1?) about all axes. It is beneficial that the Earth and Sun direction vectors are almost orthogonal. The development and flight testing of these algorithms is part of MORABAs ongoing efforts to develop a new, more robust and less expensive integrated navigation and attitude/heading reference system for fast spinning sounding rockets. Up until now, MORABA has used an unaided inertial platform as navigation system on the spinning sounding rockets. This platform contains a gimbal on which the gyroscopes and accelerometers are installed and which is rotatable about the roll axis such that the inertial sensors are isolated from the full spin rate. Because the mechanics consisting of bearings, actuators and slip rings is exposed to high loads and vibration during the propelled flight phase, the device is sensitive to damage. The new design uses a strapdown inertial measurement unit without any mechanical parts. Since the roll axis gyroscope now experiences the full spin rate, the orientation error of the inertial navigation solution about the roll axis increases rapidly due to the scaling factor error of the gyroscope. To overcome this deficiency, the inertial navigation system has to be aided by GNSS position and velocity measurements as well as orientation measurements within an integrated navigation system. Alternative orientation aiding methods, which, for example, use GNSS signals from several antennae, are concurrently studied.