PRoVisG

Precise 3D Measurements of Objects at Apollo 14 Landing Site from LROC NAC Stereo Images

Saturday, May 22, 2010

Can we measure the size and shape of equipment and objects of the Apollo missions using today’s orbital images from LROC cameras? As an example to demonstrate the precise 3D measurement capability of LROC NAC stereo imagery, three objects (including the Lunar Module, ALSEP equipment, and Turtle Rock) and the astronauts’ traverse at the Apollo 14 landing site are measured and their 3D models are reconstructed. Such 3D measurements and models can be used for mission planning and space human research analysis for future landed missions [NASA/GSFC/Arizona State University/The Ohio State University].

Using two high-resolution (.5 m/pixel) LROC NAC images (M114064206 and M111708164) taken from two separate orbits (1943 & 1596, respectively), we can form a stereo image pair for 3D measurements at the Apollo 14 landing site. Thanks to the high image resolution of LROC NAC cameras,  among the visible objects are the lunar module (Antares), ALSEP (Apollo Lunar Surface Experiments Package, west of Antares), a rock nicknamed Turtle Rock (north of Antares), and multiple astronaut traverse lines indicated by disturbed soils. Photogrammetric data processing methods can be used to identify the objects and measure their sizes and shapes. Such information can then be employed to reconstruct 3D models of the objects, which are displayed on the digital terrain model (DTM) of the site that is automatically generated from the same data set.  Additionally, Cone Crater is at the northeast end of the astronaut traverse.

In the zoomed image below, the lunar module can be identified by its top (red points) and the shadow (green lines). These points are identified and measured in the two stereo images and their corresponding 3D ground coordinates are computed. Note that the shadow analysis uses different times and sun angles of the two images. In addition, the nearby terrain is measured at the selected points on the ground (green points) as a reference. From these measurements, we can compute the height and diameter of the lunar module. As the result, the height of the lunar module (descent stage) is estimated as 3.0 m, compared to the design specification of 3.2 m. On the other hand, the shadow analysis resulted in a height of the lunar module of 3.2 m. Furthermore, using a least squares fitting to a circle the diameter of the lunar module is computed as 4.4 m, compared to the design data of 4.2 m.

ALSEP and Turtle Rock are relatively small in the vertical direction. The LROC NAC image resolution and the imaging geometry would not be able to resolve for the height that is less than 1 m. However, we can measure their horizontal dimensions: 2 m x 1 m for ALSEP (yellow points), and 1.5 m x 1 m for Turtle Rock (green points).

Based on the above 3D photogrammetric measurements and the additional information from the design specifications, we created 3D models of the selected objects of the landing site in the AutoCAD system. We then imported these 3D object models into ArcGIS to combine the 3D models with the DTM of the landing site. Generally, a CAD package for 3-D modeling (e.g., AutoCAD or Google Sketchup) and a GIS package (e.g., Autodesk 3D Map or ArchGIS) can be employed for the visualization purpose in this case.

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