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
Version 2.5 for Earth pointing during Quadrature
19/01/2010
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
a/ Default pointing
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.
b/ Pointing during Quadrature
During Quadrature periods the pointing is a biased Earth pointing. To avoid
the illumintation of the VMC Field of View, the s/c is tilted by 10 degrees
on the +Y axis.
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.