02-10-2012, 04:58 PM
SOLAR BASED NAVIGATION FOR ROBOTIC EXPLORERS
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Introduction
Humans continually seek out new places, searching for the unknown and the undiscovered.
Explorers have survived inhospitable conditions and unproven equipment to pursue knowledge.
Today, capable and adaptable robots can explore areas too dangerous or costly for humans to visit,
such as planets and moons, ocean floors, polar regions, and other remote earthly locations. As an
extension of the human presence, robotic explorers must provide as much information as possible
about the region being explored, reacting intelligently to changing conditions and remaining alive
and active for extended periods of time. One primary environmental variable, the interaction of
the sun and terrain over time, creates both difficulties and opportunities for robotic explorers. Onboard
intelligence enabling the prediction of shadows and solar power generation provides the
abilities needed for a new type of robotic navigation -- solar navigation -- which produces selfsufficient
robots capable of prolonged operation in remote locations.
RESOURCE CREATION
Reasoning about actions and their effect on resources is crucial in remote locations. On-board
resources are restricted by size and weight allowances for any robot launched into space or placed
into other remote areas, due to limited financial budgets and physical constraints. Successfully
accomplishing a mission, therefore, requires careful use of the available resources. Robotic
explorers must evaluate potential tasks not only in terms of the resources they will expend, but
also of the resources they can gain. Knowledge of resource usage and production is thus of vital
importance, potentially making the difference between success and failure of a mission.
The most critical exploration resource is power, since without power, a robot cannot perform
tasks or record imagery or even communicate. Without nearby humans to easily refuel robots or
replace batteries, power generation becomes crucial for extended missions. For the class of
remote, exploring robots in the inner solar system, solar power is the mode of choice [17, 18].
The use of non-renewable batteries or fuel cells alone is not practical for long missions, due to the
enormous weight and volume that would be needed to transport the required amount. Nuclear
power is currently infeasible, due to cultural resistance and lack of technological readiness.
Despite the drawbacks of solar power, such as inefficiencies due to material composition, dust
storms or clouds, solar power is a prime power source in the inner solar system, ranging from the
sun to the asteroid belt. By intelligently planning the collection of solar power during the course
of its mission, a robot can minimize its power storage requirements, which directly translates to
reduced weight, complexity, and cost.
Going beyond the use of planning to conserve and allocate resources, this thesis addresses the
issue of generating new resources. Solar power, and even wind power, can be harnessed by robots
as they explore their surroundings. By considering its own potential to generate and expend
power, a robotic explorer can extend its lifetime and accomplishments.
ENVIRONMENTAL KNOWLEDGE
To generate solar power, several factors need to be considered: the visibility and strength of
sunlight, terrain shadowing, the solar panel configuration, and the changing orientation of those
Chapter I Introduction
Kimberly Shillcutt 3
panels with respect to the sun. The motion of the sun over time in a particular location combined
with terrain maps will indicate whether or not the sun will be visible at a given location, and at
which angle the sunlight will be incident on the panels. With this knowledge, an intelligent
planner can determine the amount of power which can be generated. The importance of solar and
terrain knowledge, along with timing, can be seen in Figure 1, showing several snapshots of the
shadows appearing within a hypothetical polar crater at 80 S on Earth. Twenty four hours of
daylight are available during this summer period, but selecting the best location to explore
requires knowledge of the extensive shadowing.
POLAR REGIONS
Polar regions are prime locations for robotic explorers, whether on the Earth, the moon, Mars,
or other bodies. The underlying reason for this interest is the sun. For many of these bodies, the
poles have the potential to harbor water ice and other volatiles, permanently shielded from the
low-lying sun by craters and other terrain features [10, 32, 58]. For the moon, given the small
angle between its spin axis and the ecliptic normal, the sun elevation varies from only about 1.6°
above the horizon to 1.6° below at the poles [44]. With the sun always so low on the horizon,
small rises in the terrain cause extended shadows, and some regions may be permanently
shadowed year-long. In these regions, ice can collect over millions of years, and never sublimate.
Figure 2 indicates some potential cold traps, shown in white, derived from Earth-based radar
measurements of the terrain [58].