PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2017-05-12 NOTE = "Description of the DOCUMENT/NAVASSMT directory contents for the MLA nav data assessments documents." END_OBJECT = TEXT END The NAVASSMT (Navigation Assessment) directory contains documentation and supporting images and data that describe the refinements to the navigation trajectory computed by the MLA team from MLA data. Overview: In the last year of the MESSENGER mission, the spacecraft was operated at altitudes much lower than originally anticipated. The use of the MESSENGER-derived gravity field enabled the design of an ambitious plan to acquire scientific data from very low altitudes. Over several periods of a few weeks each in late summer 2014 and in spring 2015 in particular, the periapsis altitudes were below 25 km, and down to 5 km. The resulting complex mission operations relied on accurate and robust spacecraft navigation by KinetX Aerospace, Inc. Because the processes established during cruise and higher-periapsis orbit phases could not be matured in time for these low-altitude operations, the MLA team provided support to the MESSENGER project and the KinetX navigation team to assess in near-realtime the performance of orbit predictions. The rapid, quantitative data proved crucial to confidently execute the mission as planned. An additional thrusting maneuver was performed to avoid possible early impact after prediction solutions were observed to drift from the actual trajectory assessed through the MLA altimetry data. The plots and quantitative data were produced daily during mission operations. They were key to the Navigation Team as they executed complex navigation and maneuver planning. As such, we expect them to be of interest to the wider planetary community. These predictions show showing that onboard instruments have been, and can be, used during complex mission operations to substantially reduce risk. For example, such a scenario could apply to the Europa Multiple Flyby Mission, and we expect that this will be a valuable archive. This archive contains the following files: NAVINFO.TXT -- This file. MLATRACKADJUSTMENTS_SUMMARY.PNG -- A png image showing a summary of every adjustment made to the spacecraft position using MLA data for nav orbit predictions. The data shown here are contained in MLATRACKADJUSTMENTS.TAB as well as in the RDR_PLOTS and SPK_PLOTS directories. MLATRACKADJUSTMENTS_SUMMARY.LBL -- PDS label for the track adjustments plot. MLATRACKADJUSTMENTS.TAB -- A summary file of the adjustments made to the spacecraft position using nav orbits and MLA data. The adjustments are described below. This file contains 2031 lines, equal to the number of files in RDR_PLOTS directory. MLATRACKADJUSTMENTS.LBL -- PDS label describing the format and values in the corresponding table file. RDR_PLOTS/ - A directory containing PNG images of MLA RDRs (based on nav orbits, an existing MLA DEM, and calculated after correction) and RMS values before and after correction. Each short MLA track was used to calculate a single ACR (Along-track, aCross-track, and Radial, in km) spacecraft location correction for a particular nav predictive solution. Note that each RDR may be evaluated using more than one SPK trajectory kernel (*.bsp). Each image file is accompanied by a PDS label. The files in this directory are further organized into year/month sudirectories. This directory contains 2031 files, corresponding to every point shown in MLATRACKADJUSTMENTS_SUMMARY.PNG. Each line in MLATRACKADJUSTMENTS.TAB corresponds to the results showin in one of the files in this directory. The filenames are of the format MLA_ODP.PNG where MLA - instrument name yymmddhhmm - identifies the time at which the MLA CDR/RDR was collected nnn - the nav team orbit solution number v - the nav team kernel version number PNG - Portable Network Graphics, a raster graphics file format that supports lossless data compression A sample file name is MLA1504271613_OD428P0.PNG SPK_PLOTS/ - A directory containing PNG images of the ACR corrections made to the spacecraft position. The files are in the format EVALSPK_381P0_ACRKM.PNG where the values between the '_' correspond to the particular navigation prediction solution. Three graphs are shown in each file, corresponding to corrections in each direction (Along, aCross and Radial) for all the MLA RDRs processed using this SPK. Each point represents the solution for a single RDR. The 0 value corresponds to a major spacecraft maneuver, such as a burn. The lighter coloured lines correspond to smaller maneuvers, for which new solutions were not calculated by the nav team. Each image file is accompanied by a PDS label. There are 52 SPKs used. As there is overlap in the time frame between SPK segments used from various solutions, a single RDR may be present as a value in up to 5 different files. The filenames are of the format EVALSPK_P_ACRKM.PNG EVALSPK - stands for "Evaluation SPK" nnn - the nav team orbit solution number v - the nav team kernel version number ACRKM - stands for Along-track, aCross-track, and Radial, in kilometers PNG - Portable Network Graphics, a raster graphics file format that supports lossless data compression A sample file name is EVALSPK_431P2_ACRKM.PNG SPK/ - A directory contains fragments of the navigation team's predicted orbit solutions that were relevant to the time periods over which MLA data were used to evaluate spacecraft position. The orbit solutions were not modified by the MLA team. They are provided for ease of use of the dataset. Each SPK kernel file is accompanied by a PDS label. IMPORTANT NOTE: These SPK (*.bsp) fragments are those of predicted orbits and were further refined by the navigation team prior to publication at NAIF. The files provided here do not have continuous time coverage and are not intended to be used in any context outside this MLA study. The definitive orbits published by the navigation team were not available at the time these studies were done. The final SPKs, which are the ones that should be used for all scientific analysis, may be found at the NAIF PDS node in the MESS-E/V/H-SPICE-6-V1.0 data set. The filenames are of the format NAVSPK_ODP.BSP NAVSPK - stands for "Navigation SPK" OD - stands for "Orbit Determination" number nnn - the nav team orbit solution number v - the nav team kernel version number BSP - SPICE convention for SPK kernel file extensions A sample file name is NAVSPK_OD431P2.BSP