Map registration is a technique that matches (“registers”) a location in an image to the identical location on a map. Astrobotic’s landing technology registers high-quality, real-time camera images taken during the lander’s descent with terrain maps derived from orbital imagery to achieve safe landing at a precise destination. This capability is key for Astrobotic’s 2015 mission to the Lacus Mortis pit: it enables Astrobotic’s Griffin lander to fly directly over the pit, capturing close-up views of the floor and walls that cannot be seen from orbit or the ground, and then to land within 100m of the pit.
Traditionally, lunar landers have descended blind, using radio to determine their approximate trajectory during descent. They followed a pre-planned trajectory to a final descent altitude near the ground. At the final descent altitude, the computer or a human pilot used RADAR to refine the altitude and velocity estimates and gently touch down on whatever happens to lie beneath them. Because of uncertainty in radio localization and variation in lunar gravity, landing using radio and RADAR alone is accurate to approximately 10km. While this accuracy is good enough for landing in flat, safe regions of the Moon, much greater precision is necessary to safely land near a lunar pit or on the rim of a crater.
Map registration acts in place of radio orbit determination to provide better than 100m landing accuracy. As the lander descends, on-board cameras image the surface of the Moon. These images are registered to maps from the Lunar Reconnaissance Orbiter so that position and attitude can be triangulated. Velocity is then measured as the rate of change in the triangulated location.
Over the next four months, Astrobotic will conduct a series of tests to evaluate the performance of its prototype landing sensors and software. One of these tests is to evaluate the map registration accuracy of the proposed sensor package.
The following graphic shows the standalone sensor package that will be used for this testing. The package contains a camera and an IMU (inertial measurement unit) for gathering the data necessary for map registration. It also contains a GPS and compass – which only function on Earth – for comparison to measure the accuracy of the map registration algorithms.
The dome-shaped element on top of the sensor box is the GPS. The cut away shows the power board, IMU, compass, and camera. The camera is a machine vision board-level sensor with an attached lens. The camera is relatively low resolution (1.5 MP) with fast frame rate and low distortion, and is optimal for capturing images during descent from 20km to 100m. This sensor package could be used for high-resolution mapping on Earth or the Moon. The software can operate entirely without GPS, which would be a boon for terrestrial-based UAVs.
Astrobotic will collect data with this sensor in a series of helicopter and fixed wing flights to evaluate the accuracy of the system at a range of altitudes. A culminating test will integrate the map registration sensor with LIDAR and visual odometry cameras to evaluate accuracy for all phases of landing.