981695 Sojourneron M arsand Lessons Learned forFuturePlanetary Rovers Brian W ilcox and Tam Nguyen NASA's JetPropulsion Laboratory C opyright© 1997 SocietyofAutom otive Engineers,Inc. ABSTRACT The sitelocations w ere designated by a hum an operator using engineering datacollected during previous On July 4, 1997, the M arsPathfinder spacecraft traversals and end-of-solstereo im ages captured by the successfullylanded on M arsinthe Ares Vallislanding lander IMP (Im ager for M arsPathfinder) cam eras. site and deployed an 11.5-kilogram m icrorover nam ed During the traversalsthe rover autonom ouslyavoided Sojourner.Thismicrorover accom plished itsprimary rock,drop-off,and slope hazards. Itchanged its course mission objectives inthe first 7 days, and continued to toavoidthese hazards and turned back tow ardits goals operatefora totalof83 sols(1 sol= M ars day = 1 Earth w henever the hazards w erenolonger inits w ay. The day + ~24 m ins)untilthe landerlostcom m unication w ith rover used "dead reckoning" counting w heel turns and Earth, probably due tolander batteryfailure. The using on-boardrate sensorsestimate position. Although microrover navigated to m any sites surrounding the the rover telem etryrecorded itsresponses to hum an lander, and conducted various science and technology driver com m ands in detail,the vehicle'sactualpositions experim entsusing itson-boardinstrum ents. were not know n untilexamination of the lander stereo im ages at the end of the sol.Acollection of stereo Inthis paper,the rover navigation perform ance is im ages containing rover tracks allow s reconstruction of analyzed on the basisofreceived rovertelem etry, rover the rover physicaltraversalpaththoughoutthe m ission. uplink com m ands and stereo im ages captured by the Since the primary purpose for a robotic vehicleon lander cam eras. Its physical traversalpathisredraw n anotherplanetistomovepreciselytotargets ofscientific from the stereo im ages containing tracks and is interest,the abilityofthe vehicleto sense and navigateto com pared w iththe rover-recorded path and the driver- precise locations isim portantto gauge. The accuracy of planned path. Implications fornext-generation planetary navigation of Sojourner and itsimplications for future roversare described, including the sub-1-Kg N anorover planetaryroversisthe subjectofthis paper. being builtby NASA to conduct asteroidexploration as partofthe Japanese M U SES-C sam plereturnmission THE ROVER and the large roverw iththe Athena payload w hich w illbe used as partoftheMars sam plereturnprogram . Sojourner(Figures 1 & 2)isasix-w heeled vehicle68cm long,48 cm w ide,and 28 cm high (with17cmground INTRODUCTION clearance). The body is builton the rocker-bogie chassis which,by use ofpassive pivotarms,allow s the vehicleto The Pathfinder spacecraftlanded on M arsonJuly4, maintainanalm ost constantw eightdistribution on each 1997,and the nextday the Sojournerroverrolled dow n a w heel on veryirregular terrain. As a result,Sojourner ram p ontothe surface and began itsexploration of the w as abletotraverse obstacles about1.5timesasbigas Mars environm ent near the lander.Themission called the w heels, since the rear w heelsare abletomaintain for the rover tomovetosites ofinterestnearlyeverysol traction even w hile pushing the frontw heelsintovertical to conduct science and engineering experim ents. steps hard enough to getlifting traction. This consistsof Equipped w ith navigation and articulation sensors and linkages, sixmotorized w heels, and four m otorized- vision cam eras,the rover carried outits dailytraversals steering m echanism s. The vehicle'smaxim um speed is autonom ously based on setsofdriver com m ands sent about 0.7cm/sec. M ore detailsofthe design and from Earth. O ne ofthe technology experim entsplanned implem entationcanbefound in[1],[2],[3]. for the Sojourner m ission w as the reconstruction of the actualpathofthe rover as com pared toit's com m anded The roveris controlled by an Intel-8085 C PU operating at path,so as togive insightforthe design and operation of 2M H z (100KIPS). The on-board m em ory,addressablein futureplanetaryrovers. This paperpresentssomeofthe 16 Kbyte pages,includes 16 Kbyterad-hard PR O M ,176 resultsofthattechnology experim ent. Kbyte EEPROM ,64 Kbyterad-hard R AM and 512 Kbyte R AM . The navigation sensors consist of a rategyro,3 accelerom etersfor sensing the X,Y,and Z axismotion, and 6 w heelencodersforodom etry. A rticulation sensors Abidirectional U H F radio m odem (9600 bits/second) include differential and left and right bogey allow s the vehicletotransm ittelem etry and toreceive potentiom eters. W heel steering and APXS (Alpha- com m ands from earthviathe lander.On-board science Proton X-R ay Spectrom eter) positions are m onitored by instrum entsinclude an Alpha Proton X-Ray 5 potentiom eters. A llmotor currents and the Spectrom eter (APXS) , the W AE (W heel Abrasion tem peratures ofvitalcom ponentsarealso m onitored. Experim ent), and the M AE (Material Adherence Experim ent). The vehicleis pow ered by a 15-wattG aAs solar panel backed up incaseoffailure by a non- rechargeableLithium battery, w hich w as also used for nighttim e APXS operations. The rover is operated on the basisofafixed local coordinatefram e w ithoriginatthe center of the lander base and the X and Y axes pointed toMartian N orth and East (right-hand rule),respectively(Martian N orthis defined by the Lander sun finder). The vehicle'sX,Y positions arecalculated (at~2 H z rate) by integrating its odom eter(average ofthe six w heelencodercounts) w ith the heading changes produced by the rategyro. D ue to the low processor speed and lack of floating point arithm etic, m illimeter (mm) and Binary Angle M easurem ent (BAM ) areusedasdistance and turn Figure 1:The SojournerR over angle unitsrespectively(1 D eg = 182 BAM or 360 D egs = 65,536 BAM ).Whilemoving,the vehicle m onitored its inclination, articulation, contact sensing, m otor and pow ercurrents,and tem peratures tobesurethey did not exceed limit conditions based on risk level settings. Being too close and heading tow ardlander conditions arealso m onitored. The rover periodically sends a heartbeat signal tothe lander at one vehicle-length intervals. Inthe absence of this com m unication signal, the vehicleis autonom ously backed up halfofitslength and a com m unication retrytakes place. The rover motion is com m anded by one of the follow ing com m ands: Turn, M ove, G o to W aypoint,Find R ock , and PositionAPXS. The Turn com m and in general causes the vehicleto Figure 2:The roverassem bly change its heading inplace. The four steered w heels are adjusted intotheir appropriate positions, then the The two frontblack and w hite CCD cameras (768 x 484 vehicle w heelsareturned untilthe desired heading, pixels) provide hazard detection and science/operation indicated by integrating the rategyro,ismet.Incasethe im aging. The rearcolorCCD cameraisusedforscience gyroisdisabled, the odom etryisusedtocalculatethe im aging and APXS target verification. A suiteoffive heading changes; if boththe gyro and odom eter are infrared laser stripe projectors, coupled w iththe front disabled,timing isusedinthe calculation. The TurnTo CCD cameras,provide the proximity sensing and hazard com m and causes the vehicletoturntoaspecific detection capabilityforthe vehicle. Thissystem operates heading,w hilethe Turn By com m and causes the vehicle by locating the im age of the laser stripes on a few toturntoarelative heading. The Turn At com m and selected scan lines ofthe cam eraim ages. D eviations of causes the vehicletoturnsoasto pointtoaspecificX,Y the detected locations from the nom inal flat-terrain position. values indicatethat the terrainis uneven. An array of elevation values iscreated from the stripe-cam era The M ove com m and enables the vehicletomovefor a intercepts. P roximity hazards are detected w hen specified distance, using only odom etry and no hazard elevation differences betw een adjacent pointsinthe avoidance. This"blind m ove" isusefulw hen the terrain array exceed a threshold, or w hen the difference isclearly seen by the operator (inim ages from the betw een the highest and low est point inthe array lander) and the m ove is a shortone. The SetSteering exceeds a threshold. O ther hazards include excessive Position param eter of the M ove com m and determines rollorpitch, or excessive articulation of the chassis, or the arcradius ofthe m ove. contact w ith bum p sensorsonthe front or rear of the vehicle. TheGoto W aypoint com m and causes the vehicleto traverse toaspecified X,Ylocation. The vehicledrives forwardadistance of one w heel radius and stops for laser proximity scanning. A terrain height m ap is constructed internallyfrom the information provided by include the obstacle height m ap provided by the the lasers and C C D im agers. Ifan obstacleis detected proximity and hazardavoidance m echanism forevery6.5 on the left,the vehiclewillturnright,and visa versa. A cm oftraverse. flag issetwhich indicates the direction of the turn, and the vehiclewill continue turning by increm ents untila The health check telem etryprovides a snapshot of the hazard-free zone at least as w ide as the vehicleis currentstatus ofthe vehicle. In addition toalmostallof detected by the laserscanning system . Ifthe clearzone the navigation information,the pow ersupplycurrentand iswider than the vehicleturning circle, then the rover voltage status, individual w heel odom eter readings, drives straight ahead far enough tobring the obstacle com