\section{Related Research}
  \label{sec:lit}

Researchers have advanced a spectrum of mobile robot
concepts suited for exploring remote, natural environments, such as other planets.
In this section we review relevant mobile robots 
that have been built and operated.

In general, the variety of related research in this area has 
(i) produced mechanisms with high mobility but without high autonomy,
(ii) researched autonomous units but without mission capabilities, and 
(iii) developed autonomous systems with mission capabilities but only in
structured environments.
To our knowledge, there are no robot systems other than the Ambler that combine
high mobility, high autonomy, reliability and mission
capabilities  for use in unstructured, remote, natural environments.

Sustained effort at NASA's Jet Propulsion Laboratory has developed 
a family of rovers including the
Surveyor Lunar Rover Vehicle, Robby, Rocky, Go-For,
and now the MESUR/Pathfinder microrover. 
Using these wheeled mechanisms as testbeds,
JPL researchers have developed control architectures --- including 
Computer-Aided Remote Driving \cite{Wilcox87british},
behavior control \cite{Gat91icraAlfa,Gat91icra},
Semi-Autonomous Navigation (SAN) \cite{Wilcox92icra},
and mission-specific hybrids \cite{stone93icar} --- for sharing
mission control between human operators and robots.
Using these control architectures, various vehicles have successfully
demonstrated navigation in desert settings.
Still, the degree of autonomy is limited even with SAN,
which requires more
computation and look-ahead planning than the other control architectures.

Researchers at LAAS have developed a complete system that provides for
mission planning and supervision from a ground station, and for on-board
mission interpretation and execution \cite{chatila93icar}.
They have demonstrated the system with a six-wheeled rover called ADAM in
an experimental testbed called EDEN.
The demonstration involved execution of a ``GO-TO(Landmark)'' task.
Although the degree of autonomy is high, the system has yet to be
demonstrated in extreme terrain for long durations.

Researchers at MIT have argued that small rovers on the order of 1 to 2 kg
are suitable for planetary exploration \cite{Brooks89british}
and lunar base construction \cite{Brooks90iros}.
They have developed the Attila family of small six-legged walkers,
and are striving to make the robots totally autonomous units 
using the subsumption architecture and ``behavior languages'' 
\cite{Angle90iros}.
Although the degree of autonomy inherent in the approach is high, 
the potential for efficiency appears to be low:
local behaviors may cause the rover to take suboptimal actions, causing the rover to 
transfer substantial energy to the terrain by repeatedly bumping into obstacles.
Worse, unanticipated interactions between behaviors may cause the rover to take 
actions that are hazardous to its well-being.
Researchers advocating behavior-based rovers point out that it is feasible to send 
many such rovers, and thus it is not important if some do not survive 
\cite{Brooks89british}.  However, it is entirely possible that each rover will exhibit 
the same behavioral problems, causing all to fail.  Further, the capability of 
performing meaningfully complex missions in realistic settings is yet to be 
demonstrated.  

Other researchers have built and operated prototype planetary rovers,
including the Quadra-Rhomb \cite{hirose93icar},
the Marsokhod \cite{Friedman91,Kemurdjian92icra},
and the Walking Beam \cite{Chun89}.
To date, these research efforts have concentrated primarily on mechanical
design and real-time control, 
and have not yet addressed or achieved autonomous performance.

Mobile roboticists have developed a number of integrated systems
that exhibit (or potentially exhibit) relatively high degrees of autonomy.
Examples of indoor robots with autonomous capabilities include
Hilare \cite{Chatila85icra}, the longest-running mobile robot project,
and many others.
Examples of outdoor robots with autonomous capabilities include
the Autonomous Land Vehicle \cite{Turk88},
the Navlab \cite{Thorpe90book},
and VaMoRs \cite{Dickmanns92pami}.
These robots typically operate in environments relatively rich in structure
(e.g., well-defined hallway corridors, or roadways with lane markings)
compared to the surface of the Moon or Mars.

Researchers have developed a variety of walking mechanisms with the high
mobility required by planetary rovers.
These walkers include
the Adaptive Suspension Vehicle \cite{Song89book},
Titan I through Titan VI \cite{Hirose84,Hirose91},
hopping machines \cite{Hodgins91,Raibert86},
the Odex \cite{Russell83},
the Recus \cite{Ishino83},
and the Aquabot \cite{Ishino83}.
Many of these efforts emphasize electro-mechanical design and performance.
Thus, these robots tend to be either teleoperated or operate under
supervisory control, so the degree of autonomy achieved is low.
