Mars Pathfinder
Frequently Asked Questions

Sojourner Rover

I read the lander/rover has "a cooling fan" for its computer components, is this fan similar to the one in my machine at home keeping my CPU "cool"?

This fan was added to the SIM rover (the rover you see in the MarsRoom) to enable continuous operation under room temperature and outdoors (Los Angeles) conditions. The Warm Electronics Box (WEB) which houses the CPU and other electronics on all versions of the rover is an excellent insulator designed to trap heat generated by the electronics. This trapped heat soaks out during the night on Mars so that through this passive thermal device (insulation), the electronics are maintained within the WEB between +40degC to -40degC while externally the rover on Mars experiences a temperature range of 0degC to -110degC. The WEB on the SIM rover was designed just like the Mars rover (Sojourner) WEB. Hence there is a need for support cooling when the external temperature range in the Marsroom or outdoors is between +40degC to + 10degC.

--Jake Matijevic, Microrover Flight Experiment Manager

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Sojourner can range 500m from the lander before communication (as determined by bit error rate) degrades. This distance was confirmed in tests conducted with engineering models of the modem for both the rover and lander. The actual performance on Mars may be affected by factors such as temperature and terrain. We expect to conduct simple experiments during the mission to help measure the performance of the communication system.

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The footprint of the rover was determined by the size of the vehicle which could be safely stowed on the lander petal. Within this constraint, the rocker-bogie system for Sojourner was adapted from the system implemented on Rocky 4 (an early technology demonstration vehicle). That system (6 evenly spaced wheels, allowing for more than a wheel diameter of obstacle traversal, nearly symmetrical in capability for forward/backward motions, unusual rock configuration needed for hangup, etc.) met all requirements for the Sojourner vehicle mission. Pivot joint stops were determined by potential for interference with other portions of the vehicle structure (solar panel, wire bundles, etc.).

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The basic principles would apply for a lunar mission. Some adjustments could be made in wheel size to ensure proper ground pressure for traversing over soft lunar soil. Other mobility factors could change based on the actual mission requirements.

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The lander and rover carry 9600baud radio modems. With allowance for the data transfer protocols, the effective data rate between these systems is 2400bps. The communication range of the rover has been measured in tests with engineering models of the modem at 500m. At that range, the link degrades (as determined by bit error rate) for reliable communication. After 10m, the rover is effectively beyond the imaging range of the lander. These images from the lander are used by ground-based operators to plan rover operations. Beyond 10m, rover images will be used to plan traverses.

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The rover can perform all planned operations (e.g., driving, communicating, imaging) under solar panel power alone. In this condition (without batteries) some care must be taken to schedule rover operations at times of the day where solar panel production exceeds planned utilization to allow adequate margins to avoid 'brown-outs'. In addition, terrain which creates the potential for array shading must be treated as hazards for the vehicle.

Of course, night time wakeups will no longer be possible once batteries are depleted. Measurements from our science instrument (the APXS) must be scheduled during the day (rather than at night). This would mean some rover days dedicated to APXS measurements which can take up to 10 hours to gather.

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The hazard detection system on board the rover identifies an obstacle as traversible. Three settings are stored on board at this time and can be selected by command. For obstacle heigth, these values are: 6cm, 8cm and 13cm (= a wheel diameter). Based on the selected parameter, an obstacle measured at or above the setting is considered non-traversible. The rover automatically avoids non-traversible obstacles, turning away and attempting to find a path to the objective of the traverse which is free of such obstacles.

Additional measurements are taken from sensors such as the bogie potentiometers, accelerometers, etc and are factored into the determination of hazard conditions. All are evaluated in determining if a path is safe for traverse.

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Certainly NASA (and other) missions have flown with rechargeable batteries for many years. However, at the time of the design of the Sojourner rover, a low mass/low complexity (in terms of electronics, operating temperature)/low cost solution for rechargeable batteries was not available. In addition, the 7 sol primary/30 sol extended mission requirement could be met by the primary batteries selected. Recent advances in battery technology would make us rethink this design for a future rover - particularly one with a longer life requirement.

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The efficiency of the miniature radioisotope power source is less than 10%; that is, a 1W heating radioisotope source would produce less than 0.1W of electrical power. A significant amount of radioisotope material would be required to meet the nightime power needs of the rover which range from 1W to 6W depending on the operation.

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During its mission the Pathfinder rover deploys its science instrument, the Alpha Proton X-ray Spectrometer (APXS), onto rocks or soil on the surface of Mars. Data collected by this instrument is transmitted from the rover to the lander and then to earth. Scientists on earth can determine from this data the elemental composition of the rocks or soil. Images of rocks and soil taken by the Imager for Mars Pathfinder (IMP) also contribute data which assists in identifying composition and minerology of the surface in the vicinity of the lander. Finally, the movement of the rover across the surface is measured (e.g., torque output from the wheels) and images will be taken of wheel tracks both from the rover and lander. This data will allow determination of the mechanical properties of the soil (e.g., cohesion, shear strength).

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Nothing prevents the Pathfinder lander or 'Sojourner' rover solar cells from being obscured by Martian dust. However, it is currently unknown the degree to which such dust collects, obscures or otherwise impacts solar array production. The rover carries an experiment which measures the dust collected in the vicinity of the solar panel and determines the degree to which such dust reduces solar panel production. Measurements taken from this experiment during the course of the mission will help characterize the impact of dust on solar panels and assist us in designing future Mars missions.

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At the time of the design of the 'Sojourner' rover, a low mass/low complexity (in terms of electronics, operating temperature)/low cost solution for rechargeable batteries was not available. Also, the 7 sol primary/ 30 sol extended mission requirement for the rover could be met by the primary batteries selected. Recent advances in battery technology would make us rethink this design for a future rover - particularly one with a longer life requirement.

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The capabilities of the 'Sojourner' rover do not include arighting itself if it is turned over on its solar panel (or back). The hazard avoidance and mobility capabilities of the vehicle are designed to prevent (during traverses) an 'overtipping' condition. Although the speed of the winds on Mars is considerable (estimated to 6m/sec during daytime or 13mi/hr to gusts of 30m/sec during dust storms or 67mi/hr) the atmosphere is quite thin (approximately 7mb at the surface of Mars versus 1000mb at sea level on Earth). There is insufficient atmosphere even in gusting winds to 'pick up' the rover.

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The 'Sojourner' rover cannot tilt its solar panel to maximize panel exposure to the sun. The solar panel is oversized to account for tilts associated with driving across the surface.

The panel surface is quite fragile as each solar cell is mounted below a clear cover glass. Although a mechanism as described might 'upright' an overturned rover, the damage inflicted by the overturning event would be considerable.

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The 'round-trip' light time from earth to Mars is approximately 20 minutes at the time Pathfinder lands on the surface of Mars. Processing delays at the rover, at the lander and on Earth add time between a command request and telemetry response.

Recognizing that this delay would not allow ground operators at Earth to control the rover in 'real-time' on Mars, the designers of the 'Sojourner' rover included an on-board hazard detection and avoidance system, a navigation system and fault protection software to protect the rover during traverses. The hazard detection system (a combination of laser-ranging and imaging along with tilt, turn and vehicle configuration sensors) allows the rover to autonomously detect obstacles in its path and turn to avoid them. The navigation system on-board the rover monitors the avoidance maneuver, eventually directing the rover to complete its commanded traverse after the rover has driven past the obstacle. The fault protection software monitors the vehicle state (e.g., power generation, thermal control) to ensure that no other condition prevents the vehicle from safely completing its traverse.

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Beyond 30 sols (if not before), a functioning 'Sojourner' rover will be directed to explore beyond the range of the imaging system on the lander (approximately 10m) if not beyond the range of the communication system (perhaps 500m). In this extended mission, the rover will be asked to image the terrain, take measurements with the Alpha Proton X-ray Spectrometer of soil and rocks, and take engineering measurements which together help in the characterization of the landing site.

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The rover motors are geared to a 2000:1 ratio to provide the torque necessary for driving over obstacles and sandy terrain. The speed of the rover is 0.4m/min, which is computed based on a wheel motor which turns at 2000RPM resulting in a single wheel rotation.

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What happens to the rover if the communication between the rover and the lander fails (for instance out of range)? Is the rover lost or can it navigate without communication and move closer to the lander?

Good question. There are several answers, depending on when and for how long lander-rover communications fails.

Whenever the rover is driving, it periodically stops and sends a "heartbeat" message to the lander. If the lander responds, communications is normal, and the rover continues its traverse. If the rover doesn't get a response, it backs up to the last location at which it successfully communicated, and tries again. This should take care of problems caused by driving unexpectedly out of range. (It's very unlikely that we would drive out of communications range during our primary mission, because the range of our radio is several hundred meters. In the primary mission, we will probably operate within 10 meters or so of the lander, keeping the rover in good view from the lander's camera. Later in the mission we may get more bold.)

Under some circumstances the lander may have turned off its radio modem during a period when the rover is attempting to communicate. This may occur overnight when the rover wakes up to do a health check or read out the results of its APXS instrument. If the lander does not respond to the rover, then the rover will temporarily buffer its data into non-volatile memory. As soon as communications is reestablished (probably the next morning) the rover will transmit all of its buffered telemetry to the lander.

If the communications failure is more permanent (more than a day), the rover will eventually activate one of its on-board contingency sequences. The contingency sequences enable the rover to perform a version of its mission on its own. The rover attempts to send telemetry back to the lander, under the assumption that the communications problem is only from the lander to the rover, and that the lander may still be listening. (If the lander is still receiving, then we still get telemetry on the ground. If not, it doesn't hurt to try.) The rover will drive to preset locations, deploy and operate the APXS instrument, perform other experiments, and take pictures. If communications betweem rover and lander starts working again, the rover will accept the new command sequence from the lander and exit contingency mode.

During the rover's "extended" mission, we could decide to drive the rover out of communications range. If so, we would temporarily disable the "heartbeat" function, and design the traverse sequence to bring the rover back into range before the sequence ends. The contingency sequence in the extended mission would command the rover to drive in the direction of the lander.

We have carefully examined the types of problems that could occur in rover-lander communications, and designed the rover to recover from these problems under most circumstances.

--Andrew Mishkin, Sojourner Rover Mission Operations Engineer

What type(s) of CPU does the rover have? How fast is it? How much memory does it have? What other storage devices?

Does the software on the Rover include a model of the Rover (for things like movement, orientation, power management, ...) or are these things handled seperately at a lower conceptual level?

How does the Rover know its location and orientation? What type of model does it assume it lives in (i.e., flat glid, flat hexgrid, flat 2 dimensional, 3 dimensional, etc)?

The 'Sojourner' rover has only a single CPU used in its operation. The computer is an 80C85 with a 2MHz clock rated at 100KIPS. It can address 64K of memory. The computer uses, in a 16Kbyte page swapping fashion, the memory provided in 4 different chip types:

Size (Kbytes)	Type	Function
16	PROM, Harris 6617	Boot code and 'Rover-Lite' backup code
64	RAM, IBM 2586	Main memory
176	5, SEEQ 28C256 32Kbyte chips	Programs, patches and nonvolatile data storage
512	Micron MT1008 RAM	Temporary data storage

At boot up or upon reset the computer begins execution from the radiation hardened PROM. The programming stored in PROM loads programs into the radiation hardened RAM from non-volatile RAM. Program execution proceeds from the RAM. As commands are executed, other programming in non-volatile RAM is required and then swapped into the RAM for execution. To prevent excessive thrashing, some programs are executed from non-volatile RAM.

The software on the rover does not have explicit kinematic, dynamic or functional models. However, the software does contain tables, constants and equations which are derived from experience in performance with the vehicle and embody predictions (for example) for use in power and thermal management.

The rover maintains an estimate of distance (from the lander) and orientation. As it drives, it updates distance by tracking wheel revolutions using encoders which count motor shaft revolutions. An average of the values derived from the six wheel encoder measurements is used to determine distance traveled. A gyro is used to measure changes in orientation. Distance and orientation estimates are referenced to a coordinate system centered at the lander. Once per day, the lander cameras image the rover. Measurements derived from this image provide an update which is given to the rover as part of its daily command load. The on-board rover estimators update the estimate in distance and orientation during traverses between daily commanded updates.

There are no assumptions concerning terrain modeled within the rover software. By the use of accelerometers, one aligned on each axis, the rover knows its orientation to the gravity vector on the surface. All other measurements and checks of position of rover components (e.g., bogies, wheels, etc.) are developed from sensor measurements.

--Jake Matijevic

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Lander opens petals completely. Petal on which the rover sits happens to end up perched atop a rock/boulder, up above the ground. It seems that in this situation, the rover would not be able to leave its petal and get (safely) to the ground. I do notice there are ramps that lead off the petal, but are there situations envisioned where the rover cannot leave the lander? If so, how seriously does this reduce the amount of science that can be done?

Like you, we have envisioned situations where it is all but impossible for the rover to get safely off of its petal. Although we tried, we could not design it to work in all possible cases. The good news is that we believe it to be very unlikely to get into the kind of situation you describe. We believe that for the following reasons:

1. The geometry of the inflated airbags tends to push the lander away from large objects - as the bags roll, they tend to "find" low flat areas somewhat free of the largest boulders. In our tests in the Mars Yard at JPL we found it quite difficult to get the lander (inside the partially inflated airbags) to sit right next to a large boulder. We had to "cheat" by forcing that situation manually.
2. Even when we did manage to get the lander to tilt way up so that the rover petal was in the air (held by its petal actuator in a sort of "iron cross"), we found that we could lower the petal below the "flat" position to reduce the distance between the petal and the surrounding ground. Of course, this tends to lift up the base "petal", but once the rover has made it down the ramp we could raise that petal back up.
3. The rover ramps are a meter long and are flexible enough at the point where they "hinge" so that they will go as far down (steep) as it can be reached. The rover designers would like to avoid ramp inclines greater than 35 degrees which puts the top edge of the petal no more than about 0.57 m (or a little below 2 feet) above the ground. Considering the density of 0.5 m rocks is very low, this is quite respectable. (They tell me that they might consider letting the rover drive down a 45 deg ramp, but they would have to hold their breath!)

If for some reason the rover fails to get off of the ramp, the consequences for the mission are somewhat damped. The image data obtained by the IMP camera and the weather data obtained by the ASI/MET would still provide a lot of science data for future researchers to mull over for many years to come. But it would be a sad loss to both the rover designers (who would very much like to prove the concept of robotic roving vehicles on Mars) and to the geologists who would like to get a better understanding of the elemental composition of Martian rocks.

--Rob Manning, Mars Pathfinder Chief Flight Engineer

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