Fast and precise localization of Tianwen-1 Mars roverlanding site

Abstract

<p indent=0mm>At 7:18 on May 15, 2021, Beijing time, more than <sc>9 months</sc> after the launch of China’s first Mars exploration mission Tianwen-1, the lander successfully landed in the pre-selected landing area in the southern Utopia Plain of Mars. The overall terrain in this area is relatively flat, making it easier for the Tianwen-1 lander to land safely, but there are still scattered obstacles such as craters, sand dunes, and ridges. Therefore, the Tianwen-1 lander still needs to be accurately located, in order to plan the path of the rover, preventing the rover from overturning due to obstructive terrain. Besides, incorporating the follow-up data of the rover into the unified Martian geographic coordinate system also needs the accurate landing site. The location of the landing site can be estimated according to the lander’s landing orbit data, however, due to the parachute landing method, the error of the estimated location is large, so the image obtained after the landing of the Tianwen-1 lander should be used to accurately locate the landing site. Mars is far away from the Earth, and data transmission channel is limited. After the Tianwen-1 landed, the relevant image data received until May 22, 2021 only had the following three types of data: Remote sensing images obtained by the orbiter, a descent image during hovering obstacle avoidance and slow descent phase, and the ring-shot stereo image pairs of the Mars rover’s navigation camera on the landing platform after landing. In order to locate the landing site of the Mars rover as soon as possible, this article proposes a precise positioning method based on the above limited data. At first, using the digital orthophoto map (DOM) generated by the orbiter remote sensing image as the base map, extract the craters, sand dunes, craters and ridges in the pre-selected landing area, and calculate the topological relationship between them. In addition, the topological relationship between the crater, sand dune, crater and ridge around the landing site is also calculated based on the ring-shot stereo image pair of the navigation camera. The second step is to perform fuzzy matching between the topological relationship of the feature geomorphology calculated by the navigation camera’s ring-shooting stereo pair and the topological relationship of the feature geomorphology extracted from the DOM of the orbiter remote sensing image to obtain several suspected landing sites, and then to determine one of them as the initial position of the landing site based on the descent image during the hovering obstacle avoidance and slow descent phase obtained near the landing site. Finally, through the multi-image space resection of the navigation camera images, the precise position of the landing site is calculated as (109.925°E, 25.066°N). This method makes full use of the limited data at the initial stage of the landing, and realizes the first-time positioning of the Tianwen-1 lander’s landing site. It has strong timeliness and ensures the safe movement of the rover. On June 6, 2021, U.S. time, the HiRISE camera on the Mars reconnaissance orbiter (MRO) captured high-resolution remote sensing images of the Tianwen-1 lander and the Mars rover. The position of the Tianwen-1 lander on the image is the same as that in this article, which further verifies the accuracy of the positioning method.