LROC-PANGU-GAN: Closing the Simulation Gap in Learning Crater Segmentation with Planetary Simulators

It is critical for probes landing on foreign planetary bodies to be able to robustly identify and avoid hazards - as, for example, steep cliffs or deep craters can pose significant risks to a probe's landing and operational success. Recent applications of deep learning to this problem show promising results. These models are, however, often learned with explicit supervision over annotated datasets. These human-labelled crater databases, such as from the Lunar Reconnaissance Orbiter Camera (LROC), may lack in consistency and quality, undermining model performance - as incomplete and/or inaccurate labels introduce noise into the supervisory signal, which encourages the model to learn incorrect associations and results in the model making unreliable predictions. Physics-based simulators, such as the Planet and Asteroid Natural Scene Generation Utility, have, in contrast, perfect ground truth, as the internal state that they use to render scenes is known with exactness. However, they introduce a serious simulation-to-real domain gap - because of fundamental differences between the simulated environment and the real-world arising from modelling assumptions, unaccounted for physical interactions, environmental variability, etc. Therefore, models trained on their outputs suffer when deployed in the face of realism they have not encountered in their training data distributions. In this paper, we therefore introduce a system to close this "realism" gap while retaining label fidelity. We train a CycleGAN model to synthesise LROC from Planet and Asteroid Natural Scene Generation Utility (PANGU) images. We show that these improve the training of a downstream crater segmentation network, with segmentation performance on a test set of real LROC images improved as compared to using only simulated PANGU images.