Several pointings were used for the observations on the Mars Express
spacecraft. The pointings refered in this description file are used
in data product labels as values of the keyword
    SPACECRAFT_POINTING_MODE.
The values that can be used are indicated in CAPITALS in this file.
More detailed information on pointing modes, its definitions and
corresponding parameters can be found in [1]. Users are advised to
use the ESOC attitude information or the derived SPICE C-kernels [2]
to compute geometry related information.


The INERT pointing is used to point the instrument platform towards a fixed
direction in right ascension and declination. Keywords RIGHT_ASCENSION
and DECLINATION are used to described these. The not applicable value
for these two keywords are -1e32.

The NADIR pointing mode is used for science observations nominally around
the pericentre. In this pointing mode the Z-axis of the spacecraft points
towards the centre of Mars and the X-axis perpendicular to the ground track.

The ACROSSTRACK pointing mode is a derivative of the NADIR pointing mode
using a constant offset angle with respect to the ground track on the
surface of Mars perpendicular to the flight-direction of the spacecraft.
The angle for this observation is given with the keyword
OFFSET_ANGLE, measured in degrees and defines the separation angle between
the tracked point and subsatellite point from Mars center.
The angle is considered positive, if the resulting Z-axis of the space-
craft is closer to the orbital poles (cross product between s/c position
and velocity) then the Z axis for the nadir observation.
The non-applicable value is -1e32.

The ALONGTRACK pointing mode is a derivative of the NADIR pointing mode
using a constant offset angle with respect to the ground track on the
surface of Mars in the flight direction of a spacecraft.
The angle for this observation is given with the keyword OFFSET_ANGLE,
measured in degrees and defines the separation angle between the
tracked point and subsatellite point from Mars center. The angle is
considered positive if the projection of the line-of-sight of the space-
craft is ahead of the subsatellite point on the ground track.
The non-applicable value is -1e32.

The SPOT pointing mode is aimed at pointing the optical instruments to a
Mars surface feature and tracking it.
In the operational planning, the point on the surface is specified with its
planetocentric latitude/longitude (Mars is assumed an ellipsoid) and its
altitude (counted in the normal direction above the specified point on the
ellipsoid).
In addition the angle between the east direction and the projection on the
surface of the slit of the optical instruments is kept constant. By doing so
the instrument observes the same line on the surface throughout the complete
observation.

The SLEW pointing mode is the mode that is used to perform a smooth transition
between two different pointing modes.  During the operational planning, the
flight dynamics team implements the slew geometry following different criteria
(e.g. avoiding illumination of side panels of the S/C).

The SPECULAR reflection pointing mode is aimed at pointing the High-Gain
Antenna to a locus of surface points that result in specular reflection of the
radio wave if Mars were smooth. That part of the signal power which is reflected
toward Earth is received at the ground station. To compute the point of specular
reflection a sphere model of the Mars surface is taken.

The EARTH pointing mode is used for science data and housekeeping
telemetry download, spacecraft tracking and commanding and for
Radio Science experiments. The EARTH pointing mode is defined as
- the High Gain Antenna boresight axis pointing to the Earth
- the Y axis of the spacecraft frame, closest to the ecliptic plane
  normal.
 Remark:
 The HGA axis is 5 degrees offset from the spacecraft frame X axis
 towards the Z axis.

The OTRK (Object Tracking) pointing mode is used during observations of Phobos
and Deimos. Due to deviations of the s/c position along its orbit an Inertial
pointing would lead to a mis-pointing. With the OTRK mode the planned s/c
orientation at the planned time (relative to periapsis) is adapted by Flight
Dynamics to ensure the target is at the planned position in the field-of-view.
The X axis of the spacecraft frame is aligned perpendicular to the object
velocity vector.

The DERIVED pointing mode is a modification or combination derived from
other Mars Express pointing modes.

REFERENCES:
[1] Payload Operations Service to Flight Dynamics Interface
    Control Document, MEX-ESC-ID-5501, ESA/ESOC/Flight_Dynamics
    Issue 1.4, January 2006
[2] Mars Express SPICE Data Set, ESA/ESTEC/SCI-SO, MEX-M-SPICE-6-V1.0