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A Description of the Rover Sojourner

Rover in the Stowed Position

The Microrover Flight Experiment (MFEX) is a NASA OACT flight experiment for autonomous mobile vehicle technologies, whose primary mission is to determine microrover performance in the poorly understood planetary terrain of Mars. The microrover is planned to be delivered, integrated with the Mars Pathfinder (MPF) lander, and land on Mars on July 4, 1997 following a seven-month cruise through interplanetary space. After landing, the microrover is deployed from the lander and begins a nominal 7 sol (approximately 7 day) mission to conduct technology experiments such as determine wheel-soil interactions, autonomous navigation and hazard avoidance capabilities, and engineering characterizations (thermal control, power generation performance, etc.). In addition, the microrover carries an alpha proton x-ray spectrometer (APXS) which when deployed on rocks and soil will provide element composition. Lastly, to enhance the engineering data return of the MPF mission, the microrover will image the lander to assist in status/damage assessment.

The microrover is a 6-wheeled vehicle of a rocker bogie design which allows the traverse of obstacles a wheel diameter (13cm) 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 0.4m/min.

The microrover is powered by a 0.22sqm solar panel comprised of 13 strings of 18, 5.5mil GaAs cells each. The solar panel is backed up by 9 LiSOCL2 D-cell sized primary batteries, providing up to 150W-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 16W. The normal driving power requirement for the microrover is 10W.

Microrover components not designed to survive ambient Mars temperatures (-110degC 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 -40degC and +40degC during a Martian day.

Control is provided by an integrated set of computing and power distribution electronics. The computer is an 80C85 rated at 100Kips which uses, in a 16Kbyte page swapping fashion, 176Kbytes of PROM and 576Kbytes 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.

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 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 transfered 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 overlayed on the image of the vehicle, measures the location and heading of the vehicle. This information is also transfered 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.

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