Lighter-than-air vehicles such as hot-air balloons have been proposed for exploring Saturn’s moon Titan, as well as other bodies with significant atmospheres. For these vehicles to navigate effectively, it is critical to incorporate the effects of surrounding wind fields, especially as these winds will likely be strong relative to the control authority of the vehicle. Predictive models of these wind fields are available, and previous research has considered problems of planning paths subject to these predicted forces. However, such previous work has considered the wind fields as known a priori, whereas in practical applications, the actual wind vector field is not known exactly and may deviate significantly from the wind velocities estimated by the model.

A probabilistic 3D path-planning algorithm was developed for balloons to use uncertain wind models to generate time-efficient paths. The nominal goal of the algorithm is to determine what altitude and what horizontal actuation, if any is available on the vehicle, to use to reach a particular goal location in the least expected time, utilizing advantageous winds. The solution also enables one to quickly evaluate the expected time-to-goal from any other location and to avoid regions of large uncertainty. This method is designed for balloons in wind fields but may be generalized for any buoyant vehicle operating in a vector field.

To prepare the planning problem, the uncertainty in the wind field is modeled. Then, the problem of reaching a particular goal location is formulated as a Markov decision process (MDP) using a discretized space approach. Solving the MDP provides a policy of what actuation option (how much buoyancy change and, if applicable, horizontal actuation) should be selected at any given location to minimize the expected time-to-goal. The results provide expected time-togoal values from any given location on the globe in addition to the action policy.

This stochastic approach can also provide insights not accessible by deterministic methods; for example, one can evaluate variability and risk associated with different scenarios, rather than only viewing the expected outcome.

The resulting path-planning tool is a general-purpose guidance algorithm that can be applied to exploration balloons on any moon/planet with atmosphere, including Titan, Mars, Venus, and gas giants, provided the wind field models are available. The algorithm is particularly useful for mission planning and trade studies because it not only delivers the optimal expected path, but also provides insights into the variability and risk associated with different mission scenarios (e.g., under different wind variability or vehicle capabilities). Finally, these techniques may be useful for other variably buoyant vehicles operating in strong vector fields, such as underwater vehicles in ocean currents, which may have additional scientific or military significance.

This work was done by Michael Wolf, James C. Blackmore, and Yoshiaki Kuwata of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO-47111

NASA Tech Briefs Magazine

This article first appeared in the August, 2011 issue of NASA Tech Briefs Magazine.

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