ELS Blockage Produced by the ASPERA-4 Scanner and the Venus Express -Y Solar Array NASA ROSES Solar System Workings (SSW) Grant 80NSSC21K0151 Author: R. A. Frahm Southwest Research Institute 6220 Culebra Road San Antonio, TX 78238 PI: Shaousi Xu University of California at Berkeley Berkeley, CA 94710 Date: 10 January 2023 Version 1.0 Table of Contents 1.0 Contents 1.1 -Y Solar Array Angle 1.2 ASPERA-4 Scanner Angle 2.0 Table Contents 2.1 Table Header 2.2 Table Values 3.0 ASPERA-4 ELS File Use 4.0 Use for Non-ASPERA-4 ELS 1.0 Contents The file ASPERA4Scanner_NYSA_Blockage_Tables.txt contains information specifically determined for the Electron Spectrometer (ELS) as part of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) experiment on the Venus Express spacecraft. This document is a description of the contents of the tables in the file ASPERA4Scanner_NYSA_Blockage_Tables.txt. ASPERA4Scanner_NYSA_Blockage_Tables.txt contains 52 header lines describing the content followed by 361 tables. Each table begins with three header lines which includes the -Y solar array angle. There is one table for each -Y solar array angle. The table is a two-dimensional array which is ASPERA-4 scanner angle by the Electron Spectrometer sector number. Each table is generated for a particular position of the -Y solar array on the Venus Express spacecraft. There are 181 data lines in the two-dimensional array, one l;ine for each scanner angle. The values in the table represent the percentage of blockage for each particular ELS sector that is caused by the spacecraft of its appendages. Blocked sectors are viewing electrons which are emitted or reflected by the spacecraft and do not originate from the Venus environment. These electrons are modified by the surfaces of the spacecraft and are not considered pristine. 1.1 -Y Solar Array Angle The -Y solar array angle is determined as the angle between the SPICE defined frame VEX_SA-Y_ZERO and the SPICE defined frame VEX_SA-Y. The angle ranges from 0 deg to 360 deg and is defined as a right hand rotation. The 0 deg -Y solar array angle table is identical to the 360 deg -Y solar array angle. The zero degree position occurs when the VEX_SA-Y_ZERO and VEX_SA-Y frames align. The definition of these frames are included in the file VEX_V11.TF, where the 11 is the most recent version number. The SPICE file for the Venus Express Spacecraft may be found archived on the PDS Navigation Node web site: https://naif.jpl.nasa.gov/pub/naif/VEX/kernels/fk/ 1.2 ASPERA-4 Scanner Angle The ASPERA-4 scanner angle is determined as the angle between the SPICE defined frame VEX_ASPERA4_URF and the SPICE defined frame VEX_ASPERA4_SAF. The angle ranges from 0 deg to 180 deg and is always positive. The zero degree position occurs when the VEX_ASPERA4_URF and VEX_ASPERA4_SAF frames align. The definition of these frames are included in the file VEX_V11.TF, where the 11 is the most recent version number. The SPICE file for the Venus Express Spacecraft may be found archived on the PDS Navigation Node web site: https://naif.jpl.nasa.gov/pub/naif/VEX/kernels/fk/ 2.0 Table Contents Each table contains three header lines beginning with the "#" character followed by 181 lines describing the percentage of blockage for a sector with respect to the ASPERA-4 scanner rotation angle. Angles are given at the resolution of 1 degree (integer values). Blockage values are given at a resolution of 1% (integer values). 2.1 Table Header The first three lines of a table are marked with a "#" in column 1. The first line contains the -Y solar array angle in deg after the leading "#". An example of the first header line is: # 37 deg is the solar array rotation angle where the -Y solar array rotation angle is 37 deg in the above example. The second and third header lines describe the contents of each column in the table. An example of the second and third header lines are: #Scanner Electron Anode Number #Ang 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 The first column is the scanner angle in degrees and the next 16 columns represent the percentage blockage of the sector marked in that column. 2.2 Table Values Since the first column is the scanner angle and the next 16 columns represent the percentage blockage of the sector marked in that column, let's take the example table where the -Y solar array rotation angle is 37 deg and examine one of the rows. The 21st row of the table is as follows: 20 96 0 0 0 0 0 0 0 0 0 0 0 65 100 100 100 Here, the ASPERA-4 scanner angle of 20 deg is seen as the first entry in the row. The last value in the table represents sector 15 and has a value of 100. This means that there is 100% blockage of sector 15 because it views the entire spacecraft and/or its appendages. For the same scanner angle, sector 12 is a value of 65. This means that 65% of sector 12 view field is blocked by the spacecraft or its appendages and 35% of sector 12 views into open space. 3.0 ASPERA-4 ELS File Use This information is used to select the sensors which are to be used in a high- level process. The contained information can be used to determine which sectors of the Electron Spectrometer on Venus Express are blocked by the spacecraft or its appendages. The user will have to decide on what level of blockage is appropriate for their application. For example, the pitch angle distributions (PADs) archived as VExELSPADRG files with in the Planetary Data System are made from sectors where the blockage is 0%, thus completely view open space. Given a particular time, the Venus Express SPICE kernels archived at the PDS Navigation node for the Venus Express spacecraft can be interpreted to determine the values of the orientation of the -Y solar array zero position (frame VEX_SA-Y_ZERO) and the current solar array position (frame VEX_SA-Y) to determine the -Y solar array rotation angle. At the same time, the Venus Express SPICE kernels archived at the PDS Navigation node for the Venus Express spacecraft can be interpreted to determine the values of the position of the ASPERA-4 scanner (frame VEX_ASPERA4_URF) and the position of the rotated instruments on the ASPERA-4 scanner (frame VEX_ASPERA4_SAF) to determine the ASPERA-4 scanner angle. Since the tables are generated at integer values of the solar array offset, Tables can be found which bracket the solar array angle. Linear interpolation can be applied to determine a table at the spacecraft solar array position. Using this interpolated table, the scanner angle rows which bracket the scanner can be found. Linear interpolation can then determine the blockage values for each of the sectors of the Electron Spectrometer on the Venus Express Spacecraft. 4.0 Use for Non-ASPERA-4 ELS These tables are developed specifically for the Electron Spectrometer as part of the ASPERA-4 experiment on the Venus Express spacecraft. These tables were generated for the particular positions of the -Y solar array and ASPERA-4 scanner on the Venus Express spacecraft and are reflect the view of the ELS sectors. They should not be used for any other spacecraft, instrument, or component of an instrument.