Opportunity number (English: Opportunity), also called opportunity number orMars Exploration Rover -B (MER-B) is a surface rover that carried out a Mars exploration mission in 2004;It isNASAOne of the two Mars Exploration Rover missions.It was launched from Earth in 2003 and landed at 05:05 UTC (about 13:15 local time) on January 25, 2004Meridian Plateau, almost three weeks after Sisters Spirit landed in another place.
On February 13, 2019, NASA announced that it would stop trying to contact the Opportunity rover, and the Opportunity mission officially ended.Previously, Opportunity may have low power failure, task clock failure and other failures. The project team has been trying to restore contact with Opportunity, but ultimately failed.The design life is only 90Mars DayThe Opportunity rover, which is planned to travel 1km, greatly exceeds its design life. The actual travel distance is more than 45km, and the exploration time is up to 15 years.[4]
Opportunity (English: Opportunity), also known as Opportunity or Mars Exploration Rover - B (MER-B), is a surface rover that carried out a Mars exploration mission in 2004;It is one of two vehicles of NASA Mars Exploration Rover mission.It was launched from Earth in 2003, and landed on Meridian Plateau at 05:05 UTC (about 13:15 local time) on January 25, 2004, almost on SistersSpiritThree weeks after landing in another place.Opportunity has continuously and effectively operated for 30 times longer than the original design (90 days);As the solar panel is cleaned, it can continue to perform a lot of geological analysis and surface mapping of Martian rocks.The focus of the mission included completing the mission of 90 Mars days, discovering the first meteorite heat shield rock on Mars (in the Meridian Plateau), and studying the Victoria impact crater for more than two years.Opportunity survived the sandstorm in 2007, and is now heading for the struggle crater.be locatedPasadenaJet Propulsion Laboratory (JPL)California Institute of TechnologyA subordinate department ofMars Exploration Rover Plan.
target
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Opportunity Mars Rover[6]
The scientific objectives of the Mars exploration program are as follows:
Search for features of rocks and soils to find out if there has ever been water flow in the past.In particular, the samples sought will include minerals affected by water precipitation, evaporation, sedimentary cementation and hydrothermal activity.
Measure the distribution area and composition of minerals, rocks and soil around the landing site.
Measure what geological processes cause local rock formations and how they affect chemical processes.These processes include water, wind erosion, precipitation, hydrothermal process, volcanic activity and impact of meteorite.
The instruments on the Mars Reconnaissance Orbiter will classify and confirm these observations on the surface;Opportunity will help determine whether the instruments used by the orbiter to observe Mars are accurate and effective.
Search for iron bearing minerals and identify and quantify some relatively specific minerals that contain water or form in water, such as iron bearing carbonate.
Classify the minerals and components of rocks and soils and analyze their formation.
Look for possible evidence of the environmental conditions contained in the geology when liquid water existed before.
Assess whether the environment on Mars is conducive to life.
In the next two decades, NASA will continue to lead missions to study whether life ever existed on Mars;The search operation is carried out together with the analysis of whether the Martian environment is suitable for life.Life, as we know, must have water to survive, so the history of the existence of water on Mars has always been controversial for finding the possibility of living on Mars at that time.Although the Mars Exploration Rover Program has no ability to directly find evidence of the existence of life, it provides very important information on whether the natural environment in the history of Mars is habitable[1]。
Design structure
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The protective cover with the national flag is made from the relics of the twin towers of the World Trade Center[6]
Opportunity is a six wheeled, solar powered vehicle with a height of 1.5 meters, a width of 2.3 meters, a length of 1.6 meters and a weight of 180 kilograms.The six wheels have zigzag protruding lines (rocker bogie) to adapt to the terrain. Each wheel has its own motor. The car body is loaded at the front and rear ends to keep itself safe within a 30 degree tilt range.The maximum speed is 5 mm/s (2 inches/s), although the average speed is only one fifth of the maximum speed.Both Opportunity and its sister, Spirit, carried pieces of metal from the World Trade Center in New York, which were rebuilt into shields to protect the cables on the drilling machine.
Solar panelsThe array can generate about 140 watts of power per Martian dayLithium ion batteryStore power and use it at night for nearly 4 hours.The computer on the vehicle body of Opportunity uses a 20MHZ RAD6000a central processor128MBDRAM, 3MBEEPROMAnd 256MBflash memory。Its body operating temperature is between − 40 ° C and 40 ° C, and the electric heater can supportRadioisotope thermal motorBasic temperature control is also provided.One gold film and one layersilicon dioxideAerogelInsulate.
The communication between Opportunity and the Earth is conducted by a low gain antenna at a low transmission speed, and there is also aHigh gain antennaAlso communicate.Low gain antennas are also used to transmit data to orbiters orbiting Mars.
The revised scientific/engineering instruments include:
Panoramic camera - used to investigate the structure, color, mineralogy and organization of local rock formations.
Navigation camera - large field of view but low resolution and black and white, used for navigation and walking.
Micro thermal emission spectrometer (Mini TES) - to identify possible rocks and soils at close range and determine the cause of action.
Hazard avoidance cameras (Hazcams) - two B&W cameras with 120 degree field of view, providing additional information on the vehicle to display the surrounding environment.
The robot arm on the Opportunity car body includes the following instruments:
The M ö ssbauer spectrometer MIMOS II - is used to observe the mineralogy of iron bearing rocks and soils in the field.
Alpha Particle X-ray Spectrometer - used to observe the analysis of large elements that form rocks and soils in the field.
Magnet - used to collect magnetic sand particles.
Microscope imager - used to obtain high-resolution photos of rock and soil observed in the field.
Rock friction tool - used to reveal new material composition for investigation by vehicle instruments.
The camera will provide 1024 * 1024 pixel photos, and the data will be compressed, stored and transferred in ICER later.
Positioning method
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Radio measurement, control and positioning
The Mars rovers Spirit and Opportunity use their radio systems to communicate directly with the Earth tracking station or with the Mars orbiter, and determine the position of the Mars rover in the Mars inertial reference system according to the Doppler frequency shift of the radio signal.Through repeated measurement, control and positioning, the positioning accuracy of the rover in the inertial reference system can reach 1~10m.The position of the rover determined by radio measurement and control can be converted to the satellite fixed reference system with a conversion accuracy of ± 250m.
After the Spirit Mars rover landed, the navigation team of the Jet Propulsion Laboratory (JPL) of the United States jointly processed the two-way Doppler signals directly obtained from the Earth observation and control stations of the second to fourth Mars days and the two-way Doppler signals obtained from the Spirit and Odyssey orbiters at the two communication windows to obtain the landing vehicle position (i.e. the initial position of the Mars rover).The same method was used for the positioning of the Opportunity lander.The software update and upload mission was carried out at the 94th to 98th Mars Day positions of Spirit and the 75th to 78th Mars Day positions of Opportunity, and the radio measurement, control and positioning method was used to locate the rover again at these two positions.The landing point and software update position are also located by means of ground object recognition and plane triangulation in satellite images.Converting the radio TT&C positioning position to the satellite fixed reference system and comparing it with the position based on satellite image positioning, it is found that the difference between the two positioning methods Spirit and Opportunity is about 370m and 135m respectively, which indicates that both methods are accurate and reliable,The difference between the positions obtained by the two methods is mainly due to the error caused by the conversion between the inertial reference system and the satellite fixed reference system.Since then, the radio measurement, control and positioning method has also been used in a few key positions where rovers have stayed for a long time.
The advantage of radio measurement and control positioning method is global absolute positioning, but the disadvantage is that it can not achieve real-time autonomous positioning.It should be used in combination with other methods and eliminate the error caused by coordinate conversion of inertial reference system and satellite fixed reference system.It is suitable for lander positioning and positioning where the rover stays for a long time[2]。
Dead reckoning
Dead Reckoning is based on odometer and Inertial Measurement Unit (IMU) to calculate the position and attitude of a Mars rover. It does not rely on external environmental information, and is a real-time autonomous positioning method on the vehicle.The IMU used by the Spirit and Opportunity Mars rovers is the Litton LN-200 type, whose attitude and position are calculated and updated by the Surface Attitude Position and Pointing (SAPP) software at the frequency of 8Hz. The attitude update is measured by the three-axis accelerometer and three-axis gyroscope, and the position is calculated jointly by the IMU and odometer revolutions.The design accuracy of the SAPP software to obtain the position of the rover is 10% of the driving distance, that is, the cumulative positioning error at the driving distance of 100m does not exceed 10m.
The advantages of dead reckoning method are low power consumption, strong autonomy, simple calculation and relatively low cost. The disadvantage is that IMU drift over time and wheel slip will cause large errors in long-distance navigation and positioning. For example, serious skidding occurred when climbing the Columbia Mountain in the Spirit landing area, Eagle Crater and Endurance Crater in the Opportunity landing area,When climbing Columbia Mountain, 125% of the wheels skidded once (the instruction was to drive forward, but the actual skidding was to the rear).According to its inherent advantages and disadvantages, the track push algorithm, as the basic real-time positioning method on the vehicle, will still be widely used. When it is possible to apply other methods with higher accuracy, its positioning error will be corrected regularly or irregularly.
Azimuth determination by solar image
The Spirit and Opportunity rovers also use their Pancam as a sun sensor to obtain the sun image, determine the position of the sun's centroid in the image, and calculate theSolar azimuthAnd altitude angle, and then calculate the solar azimuth and altitude angle using the solar ephemeris and solar time, calculate the absolute azimuth angle of the Mars rover relative to the true north direction through the relationship between the two groups of attitudes, and correct the cumulative error of azimuth angle caused by IMU drift over time.According to the test on the earth, the accuracy of determining the azimuth with the sun image is ± 3 °.This method cannot be used for rover positioning alone. It should be combined with dead reckoning method in future rover and rover exploration missions to improve the accuracy of azimuth measurement irregularly.
Visual range
Visual Odometry (VO.The basic process of the VO algorithm developed by JPL is:
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Use F ö rstner operator to extract feature points on the first stereo pair;
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The correlation coefficient method is used to match the feature points on the first stereo pair, and the double quadratic interpolation method is used to locate the matching position to the sub-pixel, and the 3D coordinates of the feature points that are successfully matched are calculated;
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According to the position and attitude of the second stereo image pair obtained from track estimation, these 3D points are projected into the second stereo relative, and the correlation coefficient method is used to match to achieve the tracking of feature points and calculate new 3D coordinates;
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The RANSAC method is used to eliminate the gross errors of matching and tracking in the process of calculating the six degree of freedom rigid transformation of two groups of three-dimensional points. Finally, the maximum likelihood estimation is used to calculate the position and attitude changes of the second stereo image pair relative to the first stereo image pair, and then the position and attitude changes of the rover between the front and rear positions are obtained;
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Repeat the above process for the newly acquired stereo pair to update the position and attitude of the rover. If the number of feature points extracted and tracked is insufficient or the final position and attitude estimation does not converge, the position and attitude obtained by dead reckoning will not be updated.
In order to ensure a large overlap between the front and rear stereo image pairs and a small change in the target shape, the shooting distance between adjacent image pairs should not exceed 75 cm, and the change in photography azimuth should not exceed 18 °.Due to the speed limitation of the computer on the Mars rover, it takes nearly 3 minutes to acquire and process a stereo image pair and update the position and attitude. The speed is too slow, so VO cannot be used for the full journey of Spirit and Opportunity, but is used for local positioning (generally less than 15m) on some short distance critical paths, such as when the wheels are expected to slip and when approaching the designated scientific goals.
JPL has conducted several experiments on the earth to verify the effectiveness and accuracy of the visual range measurement method under the Mars environment.In a typical test of "Martian Courtyard", the rover traveled for 24m, and the VO positioning error was less than 2.5%;In a 29m driving test of "Johnson Valley", the positioning error generated by VO is less than 1.5%.According to the report of JPL researchers, during the more than one year from landing to March 5, 2005, Spirit traveled 184 Mars days, of which 52 Mars days used visual range, and the convergence success rate was 97%;Opportunity has 172 Mars days to travel, 75 of which have applied visual range, with a success rate of 95%.The failure of VO is generally due to insufficient feature points, too small distribution range of feature points, and the influence of the shadow of the rover itself.
In a word, the advantages of the visual range measurement methods used by the Spirit and Opportunity rovers are strong autonomy and high accuracy, which can correct the positioning error of the dead reckoning method when the wheel slips and IMU drifts;Its disadvantage is that the calculation speed is slow, it can only be used for local positioning, and its success depends on terrain features.The disadvantage of slow VO speed is partly due to the limitation of computing power of the Mars rover computer. The computing power of the future Mars rover or lunar rover computer should be much stronger.In view of the VO failure in the case of poor terrain features, new algorithms should be developed and combined with other positioning methods to overcome it.With the improvement of computer capability and algorithm, the future VO positioning method can achieve fast, full range positioning and is expected to be widely used.
Adjustment positioning by beam method
ohio state university Drawing andGeographic Information System LabThe developed positioning method based on bundle adjustment (BA) is to connect the images taken by the navigation camera and panoramic camera at different camera stations to form an image network. Through the photogrammetric bundle adjustment of the image network, the image position and azimuth parameters as well as the accuracy and consistency of the ground point position are improved, so as to achieve the long-distance high-precision positioning of the rover.The joint field test with JPL in the Silver Lake Desert of California shows that the positioning accuracy of the combined beam adjustment using landing images and ground rover images is 0.1%, and the positioning accuracy of the beam adjustment using only the rover images is 0.2%.The advantage of the beam adjustment positioning method is that it does not require short distance continuous photography, and it can conduct global positioning on the entire path of the rover, with high positioning accuracy. The disadvantage is that it has not yet reached full automation, and needs to be calculated on the earth.
The adjustment and positioning of the Spirit rover by the beam method has been continuous since the landing point, and BA has corrected the large cumulative errors caused by the dead reckoning method in wheel slip and IMU drift.For example, from the 154th to 670th Mars days, Spirit climbed to the top of Husband Mountain from the foot of the mountain and began to go downhill. Compared with the BA positioning results, the cumulative positioning error of dead reckoning is 67.9m, accounting for 3.7% of the driving distance of 1.85km, of whichMaximum relative error10.5% (cumulative error 56.6m when driving 540.6m).Since there is no GPS on Mars to provide accurate ground live data, it is impossible to accurately evaluate the absolute positioning accuracy of the rover.However, in the 1-m resolution satellite image of MOCNA (Mars Orbital Camera, Narrow Angle) released on January 3, 2005, most of the ruts from the landing point to Columbia Mountain can be seen.By comparing the driving route observed in this satellite image with the driving route obtained by beam adjustment positioning, it is found that the difference at the end of the route is 12m, which is about 0.4% of the driving distance of 3.08km.This error indirectly expresses the accuracy of BA positioning, which also includes the error of satellite image processing.
From the landing of Opportunity in the Eagle Crater to the 62nd Martian Day, the positioning processing based on beam adjustment corrected the positioning error of up to 21% caused by wheel slip.Since then, the continuous positioning method based on BA cannot be implemented because there are almost no obvious features such as rocks on the surface of Mars in this area, and the driving distance is too long and no images are taken.In places where obvious features (such as meteorite craters) can be observed, use the orthophoto image generated by the rover image to compare with the satellite image map to locate the Opportunity rover.The beam adjustment positioning method should play an important role in the future lunar rover and Mars rover exploration missions.This method needs to be further enhanced to improve the degree of automation. One of the keys is to automatically select the connection points between adjacent camera stations to form an image area network.In the past two years,Ohio State UniversityJointly developed with JPL the long-distance Mars rover positioning technology based on BA and VO integration, which significantly improved the degree of automation while maintaining high accuracy, and was tested in Yinhu Desert.
Comparison between ground image and high-resolution satellite image
Although the positioning accuracy of VO and BA methods is far higher than that of dead reckoning methods, they still inevitably have error accumulation in long distance positioning.Even if the positioning accuracy of BA is 0.2%, the cumulative error of 20m will be generated for the 10km driving route.High resolution satellite images can be used to eliminate the error accumulation of Mars rover positioning using only ground sensors and images.The resolution of Mars satellite images is getting higher and higher. For example, the resolution of HiRISE image is 30cm, which provides favorable conditions for this purpose.The orthophoto image generated from the rover image can be superimposed and compared with the high-resolution image;Mars rovers are observed in multiple HiRISE images, which can directly locate and eliminate accumulated errors;For the feature of the Spirit landing area with many stones, the positioning of the rover on the satellite image can be achieved by matching the stones extracted from the rover image and HiRISE image respectively.In the process of business operation, these methods are basically implemented manually.The planetary remote sensing mapping and navigation positioning research group of the Institute of Remote Sensing of the Chinese Academy of Sciences has recently made positive progress in the positioning of the rover integrated with ground and satellite images, realizing the automatic registration of ground images and high-resolution satellite images and the positioning of the rover, and the positioning accuracy is better than one pixel of satellite images (HiRISE image 30cm).
world record
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The highest linear speed of the Spirit and Opportunity rovers is 3.7 cm/s (Guinness World Record of Land Speed on MarsRecord).[3]
