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Multi-Modal Image Registration and Mapping for Titan Balloons
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| Over a three-year effort, the research proposed here will facilitate registration of Cassini synthetic aperture radar (SAR) and
visible/infrared (VIR) imagery and address needs of a future Titan balloon mission by developing and characterizing the performance
of algorithms for: - Autonomous registration of SAR and VIR imagery from flyby, orbital, and balloon platforms to improve science
image registration and to provide global balloon position estimates; - Autonomous topographic mapping, orthoimage generation, and
incremental balloon motion estimation with imagery from the balloon to improve image registration, data compression, and balloon
navigation. We will also show that these algorithms are amenable to onboard implementation with space-qualifiable computing
hardware that is already in development based on field-programmable gate arrays (FPGAs). Experience from Cassini shows the
scientific value of accurate registration of SAR and VIR imagery. Registration error of 0.2 to 0.5 pixels is desirable, but may not be
guaranteed by spacecraft position and attitude knowledge. Half pixel or better registration error has been demonstrated in limited
experiments on automatic registration of SAR and VIR imagery from Earth orbiting satellites, using algorithms developed for
matching multi-modal medical imagery. Balloon position knowledge is required for a number of reasons, including guidance to desired
sampling sites. Radiometric navigation can give accuracy around 100 meters (m) for a balloon, but with latency of many hours, which
limits its value for real-time guidance. Celestial navigation based on sensing directions to the sun and Saturn could operate in real-time,
but with error of multiple kilometers. Image registration has the potential to provide real-time balloon position knowledge with error
comparable to the image resolution; i.e., approximately 500 m for Cassini flyby data and 100 m for a future orbiter. Regions of Titan
seen to date generally have low relief, but mountains about 1 km high do exist. Creating topographic maps onboard the balloon and
creating orthoimage mosaics from the maps for registration to orbital imagery will improve registration and balloon position
estimation. Substantial steps have already been taken toward demonstrating feasibility of doing this onboard, in real-time, with
flight-qualifiable FPGAs. Doing these functions onboard will enable several other benefits, including: - Providing topographic maps for
science of the image swath seen by the balloon at resolutions ten times better than possible from orbit; - Reducing downlink data
volume requirements by roughly a factor of two by doing mapping onboard instead of on the ground; - Detecting terrain hazards as a
byproduct of real-time onboard mapping; - Detecting changes in real-time, for example to detect active cryovolcanism, as a byproduct
of real-time balloon position knowledge at meter scale derived from the mapping process. JPL is developing a high performance, low
power, space computing capability based on FPGAs as a coprocessor to standard rad-hard computing architectures. We have already
implemented some parts of these algorithms in real-time in such FPGAs, which gives us confidence that the entire set is amenable to
real-time, onboard implementation. Doing this research now will make the results available in time for a potential Titan Explorer
technology decision gate early in the next decade. This work can also benefit a Venus balloon mission and small bodies missions. Our
team has developed prototypes of the proposed capabilities and inserted onboard computer vision capabilities in previous missions,
particularly the Mars Exploration Rovers. The team includes leading participants in remote sensing of Titan and deep space navigation
to ensure relevance of the work and application of the results. |
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