SPACECRAFT POINTING MODE AND DESCRIPTIONS Version 2.4 Written by Maud Barthelemy 2/10/2006 Modified by Maud Barthelemy 30/01/2007 Modified by Maud Barthelemy after ASPERA review 13/02/2009 17/03/2009 This document describes the values for the SPACECRAFT_POINTING_MODE keyword. First, the definition of the SPACECRAFT_POINTING_MODE keyword is: "The spacecraft_pointing_mode element provides the pointing mode of the spacecraft. The definition of the modes and the standard values are given via the SPACECRAFT_POINTING_MODE_DESC element, which shall always accompany this keyword." Reference: VEX-ESF-IF-5005, Issue 2.10, 2008-10-28 Acronyms and glossary FD Flight Dynamics HGA High Gain Antenna LoS Line Of Sight SA Solar Array s/c Spacecraft SOC Science Operation Center Yaw steering: Rotation around Z(yaw) axis 1) NADIR This pointing mode is used for Venus observation . This pointing mode is applicable to the science case #1. It is defined as follows: - The Z axis of the s/c points towards the Venus centre. - The X axis of the s/c is perpendicular to the ground track. There are two possible selections of the X-axis. The solution normally preferred maintains the cold face of the s/c (-X face) in shadow. The solutions are: o close to the orbital north pole (the orbital north pole vector is defined as the cross product of the position vector of the s/c and the velocity vector of the s/c o close to the orbital south pole. In practice, for half Venus year (when the sun is on the north side of the orbit) the first solution is selected. 2) ACROSSTRACK The ACROSSTRACK pointing mode is computed from the basic nadir pointing by applying a rotation around the s/c Y axis. The rotation angle shall be computed such that the projection of the s/c Z axis, which is the LoS, onto the Venus surface is separated by a constant predefined angle from the sub-satellite point as seen from the Venus centre. The across track rotation angle is considered positive if the resulting Z-axis of the S/C is closer to the orbital pole (cross product between S/C position and velocity) than the Z-axis for the nadir observation. The angle is measured in degrees and shall be specified in the data product label. For simplicity a spherical model of Venus surface shall be taken with radius equal the mean Venus equatorial radius. 3) ALONGTRACK The ALONGTRACK pointing mode is computed from the basic nadir pointing by applying a rotation around the s/c X axis. The rotation angle shall be computed such that the projection of the s/c Z axis, which is the LoS, onto the Venus surface is separated by a constant predefined angle from the sub-satellite point as seen from the Venus centre. The along track rotation angle is considered positive if the projection of the LoS of the S/C is ahead of the sub-satellite point on the ground track. The angle is measured in degrees and shall be specified in the data product label. For simplicity a spherical model of Venus surface shall be taken with radius equal the mean Venus equatorial radius. 4) EARTH The Earth pointing mode is used for science data and housekeeping telemetry downlink, s/c tracking and s/c commanding. In addition Earth pointing may be used for Radio Science observations (part of science case #8, science case #9 and science case #10). It is defined as follows: - HGA boresight axis pointing to the Earth (either HGA 1 or 2), - Y-axis of the S/C (SA axis) is either closest to the ecliptic plane normal or perpendicular to the plane defined by the earth and sun directions. - In addition, in case of the Y axis ecliptic option, the direction of the Y axis (north/south) can be selected. 5) INERT Inertial attitude is used to point a payload towards a fix direction. The rotational degree of freedom around this direction can be used to e.g. optimise power/thermal conditions. The direction is given in right ascension and declination, angles that are given in the data product label. This pointing is used for science cases #5, #6, #7 and may also be used for science case #10. 6) NADIR_POW Nadir pointing mode is used for Venus observation for longer durations at high distances. This pointing is applicable to science cases #1 and #2. It is defined as follows: - LoS axis of the s/c pointing towards the Venus centre, - Y-axis of the s/c perpendicular to the s/c-Sun line. Please note that the LoS (Line of Sight) corresponds either to the s/c +Z axis (for optical instruments) or to the HGA axis (for bi-static radio sounding). There are two possible selections of the X-axis. The one is selected such that no sun illumination of the -X face of the S/C occurs. This pointing results in high rates when the Sun is close to the orbital plane. Maximum s/c angular rates and acceleration must be observed when selecting this profile (i.e. the profile shall not be selected if the resulting rates/accelerations exceed a threshold). 7) SPECULAR The specular pointing is intended to be used for radio science. For this a pointing axis (usually the HGA1 or HGA2 boresight) points towards the specular point on the Venus surface with respect to Earth and the spacecraft. The SC Y-axis is oriented such that the power on the solar arrays is optimised. Normally (i.e. for pointing axes close to the SC x-z-plane and if the sun direction is not too closely aligned with the pointing axis) there are two possible attitudes having the Y-axis perpendicular to the Sun direction. The selection is made taking s/c thermal considerations into account (normally the solution with no sun on the -X face is chosen). For pointing axes that are not in the s/c x-z-plane, the Y-axis cannot be oriented perpendicular to the Sun direction, if the Sun is too closely aligned with the pointing direction. In this case the attitude is defined such that the s/c Y-axis is as close as possible to the plane perpendicular to the Sun direction. This profile is applicable to science case #4 . 8) CUSTOM CUSTOM pointing is the pointing profile aimed at the implementation of user specific pointing requirements that cannot be accommodated via a specific interface. This profile is applicable to science case #8 . 9) MOSAIC The aim of mosaic pointing is to cover the full Venus disc from apocentre by performing a raster motion. This pointing is applicable to science case #3. The mosaic pointing consists of a 3x3 square raster. The angular step between raster points is assumed to be 0.062 rad. The dwell times at raster points is proposed to be 640 seconds and the slew time 1 minute. For each of the raster points the attitude is computed based on the nadir pointing with yaw steering for power optimisation (see NADIR_POW) and applying one rotation along the S/C Y axis and one rotation along the S/C X axis (in this order). A mosaic is hence fully defined by specifying for each raster point, the rotation angles, the dwell time, and the slew times between raster points. 10) SPOT This spot pointing allows to direct a pointing axis (specified in the s/c-frame) towards a point on (or over) the Venus surface. The spot is defined by longitude, latitude and height. The roll around the pointing axis is defined such that an instrument slit is aligned with a surface direction (aligned here means that the projection of the vectors in the plane perpendicular to the pointing axis are aligned). The direction on the surface is defined by the angle from the east direction at the selected spot (measured positively towards north). The slit direction must be different from the specified pointing axis. 11) SPOT_POW SPOT_POW is the power optimised spot pointing mode. This spot pointing allows to direct a pointing axis (specified in the s/c-frame) towards a point on (or over) the Venus surface. The spot is defined by longitude, latitude and height. The s/c Y-axis is oriented such that the power on the solar arrays is optimised. Normally (i.e. for pointing axes close to the s/c x-z-plane and if the sun direction is not too closely aligned with the pointing axis) there are two possible attitudes having the Y-axis perpendicular to the Sun direction. The selection is made taking s/c thermal considerations into account (normally the solution with no sun on the -X face is chosen). For pointing axes that are not in the s/c x-z-plane the Y-axis cannot be put perpendicular to the Sun direction, if the Sun is too closely aligned with the pointing direction. In this case the attitude is defined such that the s/c Y-axis is as close as possible to the plane perpendicular to the Sun direction. 12) LIMB_SUN LIMB_SUN is a limb pointing with point in limb defined relative to the Sun. A pointing axis (specified in the s/c-frame) is directed towards a point on the limb. The point on the limb is defined by an azimuth angle on the limb. As seen from the spacecraft this angle increases in counter clockwise direction. The reference of this azimuth angle is defined such that bright limb (i.e. the Sun direction projected in the plane perpendicular to the Venus-s/c direction) is at +90deg. The roll around the pointing axis is defined such that a second axis (specified in the s/c-frame, that must be aligned with the pointing axis) is aligned with the radial direction from Venus centre to the selected point on the limb (aligned here means that the projection of the vectors in the plane perpendicular to the pointing axis are aligned). 13) LIMB_POLE LIMB_POLE is a limb pointing with point on limb defined relative to Venus rotational pole. A pointing axis (specified in the SC-frame) is directed towards a point on the limb. The point on the limb is defined by an azimuth angle on the limb. As seen from the spacecraft this angle increases in counter clockwise direction. The reference of this azimuth angle is defined such the Venus rotational north pole (projected in the plane perpendicular to the Venus-SC-direction) is at +90deg. The roll around the pointing axis is defined such that a second axis (specified in the SC-frame, that must be aligned with the pointing axis) is aligned with the radial direction from Venus centre to the selected point on the limb (aligned here means that the projection of the vectors in the plane perpendicular to the pointing axis are aligned). 14) SLEW SLEW pointing mode is the mode that is used to perform a smooth transition between two different pointing modes. 15) PLANET This pointing allows to track a planet, the Sun or the Earth-Moon with a pointing axis (specified in the SC-frame). The SC Y-axis is oriented such that the power on the solar arrays is optimised. Normally (i.e. for pointing axes close to the SC x-z-plane and if the sun direction is not too closely aligned with the pointing axis) there are two possible attitudes having the Y-axis perpendicular to the Sun direction. The selection is made taking s/c thermal considerations into account (normally the solution with no sun on the -X face is chosen). For pointing axes that are not in the s/c x-z-plane the Y-axis cannot be put perpendicular to the Sun direction, if the Sun is too closely aligned with the pointing direction. In this case the attitude is defined such that the s/c Y-axis is as close as possible to the plane perpendicular to the Sun direction. Note that for the special case in which the pointing axis is directed to the Sun, the Y-axis is oriented towards the s/c orbital north or south pole with respect to the Sun. This profile is applicable to science case #11. The following values are user defined. They are not described in the FD document [VEX-ESF-IF-5005]. 16) TRACKING This pointing mode is used by VMC for Spot pointing modes. In 99% of the cases, they use the spot pointing power optimised. However, in very rare occasions they use the non power optimised. When Venus, the spacecraft and the sun are in a straight line, this would require an instantaneous 180 degrees flip to keep the spacecraft in a power optimised mode. As this cannot be operated, the power optimised spot pointing cannot be used. See defintion of the Spot pointing modes above. 17) LIMB The LIMB value is part of the standard PDS value. It is used by VMC only and with a special usage. VMC indicates "LIMB" pointing whenever the Limb is in the LoS and whatever the actual pointing mode is (INERT, LIMB_POLE, LIMB_SUN). As the OBSERVATION_TYPE is yet used to give detailed information on the science done during the observation, the team uses the SPACECRAFT_POINTING_MODE keyword to give scientific information on the type of observation.