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High-Precision Heading Determination Based on the Sun for Mars Rover 时间:2018-10-03 22:50   来源:未知   作者:admin   点击: 次 Abstract:Since the American Mars Exploration Rover Opportunity landed on Mars in 2004, it has travelled more than 40 km, and heading-determination technology based on its sun sensor has played an important role in safe driving of the rover. A high-precision heading-determination method will always play a significant role in the rover’s autonomous navigation system, and the precision of the measured heading strongly affects the navigation results. In order to improve the heading precision to the 1-arcminute level, this paper puts forward a novel calibration algorithm for solving the comparable distortion of large-field sun sensor by introducing an antisymmetric matrix. The sun sensor and inclinometer alignment model are then described in detail to maintain a high-precision horizon datum, and a strict sun image centroid-extraction algorithm combining subpixel edge detection with circle or ellipse fitting is presented. A prototype comprising a sun sensor, electronic inclinometer, and chip-scale atomic clock is developed for testing the algorithms, models, and methods presented in this paper. Three field tests were conducted in different months during 2017. The results show that the precision of the heading determination reaches 0.28–0.97′ (1σ) and the centroid error of the sun image and the sun elevation are major factors that affect the heading precision.
1. Introduction
  Spirit and Opportunity, the twin rovers of NASA’s Mars Exploration Mission, landed on Mars in 2004. Some discoveries of the rovers, such as the evidence of liquid water on the surface of Mars, have been a source of excitement. Opportunity has been kept in service for 13 years and has travelled over 42.195 km, becoming a veritable marathon champion. High-precision attitude-determination technology of Mars rovers, especially the heading-determination technology, has played an important role in safe driving and the realization of scientific goals. Because there is no satellite navigation system like GPS on Mars, the rovers utilize Pancam cameras as sun sensors for heading, which can restrict the error growth of the IMU and improve the dead-reckoning accuracy [1]. When the rover remains static, the sun sensor makes a 10-minute tracking observation of the sun, and the quaternion estimator (QUEST) method is used to calculate the attitude and heading, the precision of which reaches 1.5°. Because the field view of sun sensor on Spirit and Opportunity is only 16°, a rotation platform with a dial is needed. The rover first rotates the sun sensor to the predicted elevation of the sun and then horizontally scans the sky to find the sun [2]. Long-term searching and observation of the sun do not fulfill the real-time navigation requirement for Mars rovers, and the low-precision heading greatly affects the dead reckoning.
Several Jet Propulsion Laboratory (JPL) studies have also used the sun sensor for heading determination for Rocky 7 and FIDO field rover, but the method is similar to that of Spirit and Opportunity, and the precision in the field test is also to within a few degrees [3, 4]. Deans et al. bundled a fish-eye camera and an inclinometer together, and the precision of the heading determination is superior to 1° [5]. Most published heading-determination methods require long-duration observations only when inclinometer data is unavailable. Enright et al. and Furgale et al. use a digital sun sensor with 140° field of view and an inclinometer called HMR-3000 for heading, and practical tests on Earth indicate that the precision reaches 1° [6, 7]. Yang et al. use a large-field-of-view sun sensor for heading determination, and the precision reaches 0.1123° when observing the sun for 30 minutes, which is the best reported precision so far [8]. Illyas et al. present a novel algorithm for micro-planetary rover-heading determination using a low-cost sun sensor, and a large number of experiments show that the heading precision reaches 0.09° (1σ), which plays an important role in reducing the accumulated heading error of MEMS sensors [9]. In a GPS-denied environment, visual navigation can provide accurate localization [10], but the error grows sharply with the distance travelled. Lambert et al. develop a novel approach incorporating the sun sensor and inclinometer measurements directly into the visual odometry pipeline to reduce the error growth of path estimation, and the resulting localization error is only 1.1% of a 10-km distance travelled [11].
   The Mars rover-heading determination method via the inertial navigation system and star sensor has also been thoroughly researched. A novel method based on star sensors and the strap-down inertial navigation system (SINS) is put forward to accurately determine the rover’s position and attitude, and the initial position and attitude determination for planetary rovers by INS/Star Sensor integration is researched [12]. A high-precision SINS/star sensor deeply integrated navigation scheme is effected by He et al. [13]. In recent years, the inertial navigation system, star sensor, and visual navigation system have been integrated to improve the navigation accuracy and reliability [14]. However, Mars has some atmosphere, which makes stars invisible in the daytime. Furthermore, the rovers always move during the day and rest at night, which makes the star sensor difficult to apply.