Abstract:
Relative navigation is paramount in space missions that involve rendezvousing
between two spacecraft. It demands accurate and continuous estimation of the six
degree-of-freedom relative pose, as this stage involves close-proximity-fast-reaction
operations that can last up to five orbits. This has been routinely achieved thanks to
active sensors such as lidar, but their large size, cost, power and limited operational
range remain a stumbling block for en masse on-board integration. With the onset
of faster processing units, lighter and cheaper passive optical sensors are emerging as
the suitable alternative for autonomous rendezvous in combination with computer
vision algorithms. Current vision-based solutions, however, are limited by adverse
illumination conditions such as solar glare, shadowing, and eclipse. These effects are
exacerbated when the target does not hold cooperative markers to accommodate the
estimation process and is incapable of controlling its rotational state.
This thesis explores novel model-based methods that exploit sequences of monoc ular images acquired by an on-board camera to accurately carry out spacecraft
relative pose estimation for non-cooperative close-range rendezvous with a known
artificial target. The proposed solutions tackle the current challenges of imaging in
the visible spectrum and investigate the contribution of the long wavelength infrared
(or “thermal”) band towards a combined multimodal approach.
As part of the research, a visible-thermal synthetic dataset of a rendezvous
approach with the defunct satellite Envisat is generated from the ground up using a
realistic orbital camera simulator. From the rendered trajectories, the performance
of several state-of-the-art feature detectors and descriptors is first evaluated for
both modalities in a tailored scenario for short and wide baseline image processing
transforms. Multiple combinations, including the pairing of algorithms with their
non-native counterparts, are tested. Computational runtimes are assessed in an
embedded hardware board.
From the insight gained, a method to estimate the pose on the visible band is
derived from minimising geometric constraints between online local point and edge
contour features matched to keyframes generated offline from a 3D model of the
target. The combination of both feature types is demonstrated to achieve a pose
solution for a tumbling target using a sparse set of training images, bypassing the
need for hardware-accelerated real-time renderings of the model.
The proposed algorithm is then augmented with an extended Kalman filter
which processes each feature-induced minimisation output as individual pseudo measurements, fusing them to estimate the relative pose and velocity states at
each time-step. Both the minimisation and filtering are established using Lie group
formalisms, allowing for the covariance of the solution computed by the former to be automatically incorporated as measurement noise in the latter, providing
an automatic weighing of each feature type directly related to the quality of the
matches. The predicted states are then used to search for new feature matches in the
subsequent time-step. Furthermore, a method to derive a coarse viewpoint estimate
to initialise the nominal algorithm is developed based on probabilistic modelling of
the target’s shape. The robustness of the complete approach is demonstrated for
several synthetic and laboratory test cases involving two types of target undergoing
extreme illumination conditions.
Lastly, an innovative deep learning-based framework is developed by processing
the features extracted by a convolutional front-end with long short-term memory cells,
thus proposing the first deep recurrent convolutional neural network for spacecraft
pose estimation. The framework is used to compare the performance achieved by
visible-only and multimodal input sequences, where the addition of the thermal band
is shown to greatly improve the performance during sunlit sequences. Potential
limitations of this modality are also identified, such as when the target’s thermal
signature is comparable to Earth’s during eclipse.