Description of the EROS MSI IMAGES WITH GEOMETRY BACKPLANES bundle V1.0 =========================================================== Bundle Generation Date: 2017-06-16 Peer Review: 2017 Asteroid Review, Fri Feb 10 00:00:00 MST 2017 Discipline node: Small Bodies Node Content description for the EROS MSI IMAGES WITH GEOMETRY BACKPLANES bundle ==================================================================== The Near Earth Asteroid Rendezvous (NEAR) was the first mission flown under NASA's Discovery program and the first mission ever to orbit an asteroid. NEAR was launched on February 17, 1996 and entered orbit around asteroid 433 Eros on February 14, 2000. It measured the bulk properties, composition, mineralogy, morphology, internal mass distribution, and magnetic field of Eros. During its year of orbital operations, NEAR's multispectral imager (MSI) returned over 160,000 images of the surface including a series taken during the spacecraft's historic controlled descent to the surface on February 12, 2001. MSI consists of an optical system and a Si CCD focal plane unit. The optical system is made up of a refractive telescope with an eight-position filter wheel covering the wavelength range from visible to near-infrared (450-1100 nm). The seven color filters are designed to discriminate iron-containing silicate minerals. A broadband filter was primarily used for low-light imaging and optical navigation. The CCD is a frame transfer unit with electronic shuttering, with a field of view of 51.5 x 39.4 mrad divided into an array of 244 x 537 pixels. The MSI backplane files contain geometric information for 93,397 images of the surface of asteroid 433 Eros. Each backplane file includes for each pixel of an MSI image its location on the surface of the asteroid (both as x,y,z, and lat,lon coordinates), the solar incidence angle, emission (viewing) angle, and solar phase angle at the time of acquisition, and average values for the surface slope relative to gravity, the elevation relative to gravity, the average gravitational acceleration, and the average gravitational potential, assuming an asteroid of uniform density. Data ==== The location, illumination, and geometric parameters for each MSI pixel are calculated using spacecraft position and observational geometry derived from the Gaskell (2008) shape model that has been archived with the PDS. This shape model was produced using stereophotoclinometry (SPC, see Gaskell et al. (2008) for a description of the SPC technique). The model has 3,145,728 triangular plates, for an average plate area of 362 m^2, and an average plate edge length of 29 m. A byproduct of the SPC technique is updated pointing information for the NEAR spacecraft relative to the asteroid. SUMFILES are generated that solve for the information that typically exists in NAIF SPK and CK kernels, namely the spacecraft trajectory and attitude. Each of the MSI backplane files is a 16-band image cube. The bands of the cube contain the information detailed below calculated for the center of each pixel: 1: Pixel values (I/F) of the original FITS images delivered to the PDS by the NEAR team [Murchie et al., 2001], linearly resampled to rectify the image by squaring the pixels. The original image size is 537 x 244 pixels and the rectified size is 537 x 412 pixels. 2: x coordinate (kilometers) of the intercept with the surface of the asteroid in body-fixed reference frame 3: y coordinate (kilometers) of the intercept with the surface of the asteroid in body-fixed reference frame 4: z coordinate (kilometers) of the intercept with the surface of the asteroid in body-fixed reference frame 5: latitude (planetocentric, degrees) 6: longitude (degrees east) 7: radial distance (kilometers) from the asteroid center of figure 8: solar incidence angle (degrees) 9: emission angle (degrees) 10: solar phase angle (degrees) 11: horizontal pixel scale (kilometers) 12: vertical pixel scale (kilometers) 13: average surface slope relative to gravity (degrees) 14: average elevation relative to gravity (meters) 15: average gravitational acceleration (meters/[s^2]) 16: average gravitational potential (J/kg) The last four values were calculated using the method described by Cheng et al. (2002), using a constant density for Eros of 2.67 g/cm^3 and a rotation rate of 0.000331165762 rad/s. With this density and rotation rate, the gravitational potential on the surface can be computed via integration (see Cheng et al. (2002) for details). A reference gravitational potential equivalent to a 'geoid' on Earth can be obtained from the area-averaged root-mean-square of this gravitational potential. This reference potential provides the elevation on the surface of the asteroid once divided by the local magnitude of the gravitational acceleration. The density and rotation also provides via integration the magnitude and direction of the gravitational acceleration at the surface. The slope given is obtained by taking the inverse cosine of the dot product of the normal to each plate model facet with the vector describing the direction of the gravitational acceleration at the center of each plate model facet. All of the MSI backplanes are 32-bit (floating point) binary images, in the Flexible Image Transport System (FITS) format. They may be read into the U.S. Geological Survey's Integrated Software for Imagers and Spectrometers (ISIS, version 3) using the command 'fits2isis' on the FITS file: fits2isis from=MSI_BACKPLANES.fit to=output.cub This will generate an ISIS cube file named 'output.cub'. The ISIS package called 'qview' allows the image and backplanes to be displayed. For locations that do not intercept the surface of Eros, the backplane value is set to -1e32. To open one of the backplane FITS files in an image analysis package such as ENVI, set the header offset to 2880 bytes. Coordinate System ================= A planetocentric coordinate system is employed, which is body-centered, using the center of figure as the origin. The latitude is defined by the angle between the equatorial plane and a vector extending from the origin of the coordinate system to the relevant point on the surface. Latitude is measured from -90 degrees at the south pole to +90 degrees at the north pole. Longitude extends from 0 to 360 degrees, with values increasing eastward (i.e., it is a right-handed coordinate system) from the prime meridian. References ========== Cheng, A.F., O. Barnouin-Jha, L. Prockter, M. T. Zuber, G. Neumann, D. E. Smith, J. Garvin, M. Robinson, J. Veverka, and P. Thomas, Small-scale topography of 433 Eros from laser altimetry and imaging, Icarus 155, 51-74, 2002. Gaskell, R.W., O. Barnouin-Jha, D.J. Scheeres, A.S. Konopliv, T. Mukai, S. Abe, J. Saito, M. Ishiguro, T. Kubota, T. Hashimoto, J. Kawaguchi, M. Yoshikawa, K. Shirakawa, T. Kominato, N. Hirata, and H. Demura, Characterizing and navigating small bodies with imaging data, Meteoritics and Planetary Science 43, 1049-1061, 2008. Gaskell R. W., Gaskell Eros Shape Model V1.0. NEAR-A-MSI-5-EROSSHAPE-V1.0. NASA Planetary Data System, 2008. Li, H., M.S. Robinson, and S. Murchie, Preliminary remediation of scattered light in NEAR MSI images, Icarus 155, 244-252, 2002. Murchie S., Taylor H., NEAR_A_MSI_3_EDR_EROS_ORBIT_V1_0, NASA Planetary Data System, 2001. Known issues or problems with the data ====================================== Pixel Location Uncertainty =========================== Our best assessments indicate that the position of the MSI backplane data possesses less than 0.5 MSI pixel error relative to the Gaskell (2008) shape model that is archived in the PDS. Frequently, the images are located relative to this model to within 0.1 MSI pixel, which is substantially better than prior results obtained using the original Navigation NAIF SPICE ephemeris solutions (SPKs) and the pointing data (CKs). Limitations =========== The MSI backplanes were constructed for 54,073 deblurred images, and 39,324 cleaned and deblurred images - a total of 93,397 files. Refer to Li et al. (2002) for information on the cleaning and deblurring process. The NEAR-Shoemaker mission's MSI team delivered a total of 121,147 Eros images that were cleaned and deblurred. Backplanes were not constructed for images that were not registered to the shape model due to poor quality, had too little of the asteroid in the field of view, or had no asteroid in the field of view. References: Gaskell R. W., Gaskell Eros Shape Model V1.0. NEAR-A-MSI-5-EROSSHAPE-V1.0. NASA Planetary Data System, 2008. Li, H., M. S. Robinson, and S. Murchie, Preliminary remediation of scattered light in NEAR MSI images, Icarus 155, 244-252, 2002.