Instrument Host Overview
========================
The Microrover Flight Experiment (MFEX), also known as the 'Mars
Pathfinder Rover' and 'Sojourner', is a NASA Office of Advanced
Concepts and Technology (OACT) flight experiment for autonomous mobile
vehicle technologies.  Its primary mission was to determine microrover
performance in the poorly understood planetary terrain of Mars. The
microrover was delivered, integrated with the Mars Pathfinder (MPF)
Lander, to Mars on July 4, 1997.  After landing, the microrover was
deployed from the lander and began a nominal 7 sol mission, which was
later extended, to conduct technology experiments such as determining
wheel-soil interactions, autonomous navigation and hazard avoidance
capabilities, and engineering characterizations (thermal control,
power generation performance, etc.).  In addition, the microrover
carried an Alpha Proton X-ray Spectrometer (APXS) which when deployed
on rocks and soil provided element composition.  Lastly, the
microrover imaged the lander to assist in status and damage
assessment.

Mechanical, Thermal, and Mobility Subsystem
-------------------------------------------
The microrover is a 6-wheeled vehicle of a rocker bogie design which
allows the traverse of obstacles a wheel diameter (13 cm) in size.
Each wheel is independently actuated and geared (2000:1) providing
superior climbing capability in soft sand. The front and rear wheels
are independently steerable, providing the capability for the vehicle
to turn in place. The vehicle has a top speed of 0.4 m/min.

Microrover components not designed to survive ambient Mars
temperatures (-110 degrees Celsius during a Martian night) are
contained in the warm electronics box (WEB). The WEB is insulated,
coated with high and low emissivity paints, and heated with a
combination of 3, 1W RHUs, resistive heating under computer control
during the day and waste heat produced by the electronics. This design
allows the WEB to maintain components between -40 and +40 degrees
Celsius during a Martian day.

Control and Navigation Subsystem
---------------------------------
Control is provided by an integrated set of computing and power
distribution electronics. The computer is an 80C85 rated at 100 Kips
which uses, in a 16 Kbyte page swapping fashion, 176 Kbytes of PROM
and 576 Kbytes of RAM. The computer performs I/O to some 70 sensor
channels and services such devices as the cameras, modem, motors and
experiment electronics.

Vehicle motion control is accomplished through the on/off switching of
the drive or steering motors.  An average of motor encoder (drive) or
potentiometer (steering) readings determines when to switch off the
motors.  When motors are off, the computer conducts a proximity and
hazard detection function, using its laser striping and camera system
to determine the presence of obstacles in its path.  The vehicle is
steered autonomously to avoid obstacles but continues to achieve the
commanded goal location.  While stopped, the computer also updates its
measurement of distance traveled and heading using the averaged
odometry and on-board gyro.  This provides an estimate of progress to
the goal location.

Power Generation and Distribution
---------------------------------
The microrover is powered by a 0.22 square meter solar panel comprised
of 13 strings of 18, 5.5 mil GaAs cells each. The solar panel is
backed up by 9 LiSOCL2 D-cell sized primary batteries, providing up to
150 W-hr. This combined panel/batteries system allows microrover power
users to draw up to 30W of peak power (mid-sol) while the peak panel
production is 16 W. The normal driving power requirement for the
microrover is 10 W.

Radio Communications
--------------------
Command and telemetry is provided by modems on the microrover and
lander.  The microrover is the link commander of this 1/2 duplex, UHF
system.  During the day, the microrover regularly requests
transmission of any commands sent from Earth and stored on the lander.
When commands are not available, the microrover transmits any
telemetry collected during the last interval between communication
sessions.  The telemetry received by the lander from the microrover is
stored and forwarded to the earth the same as any lander telemetry.
In addition, this communication system is used to provide a
'heartbeat' signal during vehicle driving.  While stopped, the
microrover sends a signal to the lander.  Once acknowledged by the
lander, the microrover proceeds to the next stopping point along its
traverse.

Commands for the microrover are generated and analysis of telemetry is
performed at the microrover control station, a Silicon Graphics
workstation which is a part of the MPF ground control operation.  At
the end of each sol of microrover traverse, the camera system on the
lander takes a stereo image of the vehicle in the terrain.  Those
images, portions of a terrain panorama and supporting images from the
microrover cameras, are displayed at the control station. The operator
is able to designate on the display points in the terrain which will
serve as goal locations for the microrover traverse.  The coordinates
of these points are transferred into a file containing the commands
for execution by the microrover on the next sol.  In addition, the
operator can use a model which, when overlaid on the image of the
vehicle, measures the location and heading of the vehicle.  This
information is also transferred into the command file to be sent to
the microrover on the next sol to correct any navigation errors.  This
command file is incorporated into the lander command stream and is
sent by the MPF ground control to the lander, earmarked for
transmission to the microrover upon request.

For more information on the Microrover Flight Experiment, see the
papers [MATIJEVIC1994] and [MATIJEVICETAL1997A].

Copyright 1996, Jet Propulsion Laboratory, California Institute of
Technology and the National Aeronautics and Space Administration.