DESCRIPTION OF THE CONTENTS OF 'nominal_roll_attitude.dat'

The 'nominal_roll_attitude.dat' file contains the value for the
nominal roll angle of SOHO at any given time, in tabulated form.

The nominal roll angle always used to be zero. However, with the high
gain antenna parked in a fixed position since the summer of 2003, SOHO
has to turn upside down during half of its halo orbit in order to send
data to Earth through the high gain antenna. In other words, the
nominal roll switches between 0 and 180 degrees every 3 months.

If the roll angle of SOHO is zero, the spacecraft +Z axis is aligned
with the North Pole of the Sun ("right side up"). If the roll angle is
180, spacecraft -Z axis is aligned with the North Pole of the Sun
("upside down").

The value of the nominal roll attitude in the file will be either 0 or
180. There will be no actual roll determination to refine this figure,
because file reflects the value of the nominal spacecraft roll, not
the actual spacecraft roll. For this reason, the value should only be
used for periods when the spacecraft is in Normal Mode (i.e. not
during manoeuvres or other special operations).

The time stamp for each roll change reflects the moment when the new
nominal roll becomes effective:

  - The time stamp typically corresponds to a 180 roll manoeuvre, and
    the actual roll of the spacecraft would transition from 0 to 180 
    or from 180 to 0 at about the indicated time.  The determination 
    of this time will be made by the FOT. Please note that roll 
    manoeuvres are not instantaneous, they last for about 26 minutes.
    At the end of a roll manoeuvre, the spacecraft is not yet in Normal
    Mode, i.e. there is no guarantee that the actual spacecraft roll 
    is very close to the nominal roll. 

  - When the spacecraft is not in Normal Mode for an extended time
    period, the time in the 'nominal_roll_attitude.dat' file will 
    reflect the moment at which it has been decided to change the 
    nominal roll of the spacecraft. Whenever SOHO goes into normal
    mode again, its actual roll attitude will be as close as possible 
    to this nominal value.

  - For example, on 8 July 2003 SOHO was in ESR when such a
    decision was taken. The nominal roll angle of SOHO was up until
    then 0 (zero) degrees. But the actual roll angle of the spacecraft
    was about 60 degrees when it was decided to go to a 180 degree
    nominal roll attitude at about 13:00 UT. When the spacecraft went
    into normal mode again, the actual roll attitude had been set
    already to 180 degrees, (it did not go first to 0). In this case,
    the time recorded in the 'nominal_roll_attitude.info' for the
    nominal roll change is 13 UT.  This does not mean that at 13 UT the
    roll attitude of SOHO was changed to 180 degrees, but that the next
    time SOHO went into normal mode after 13 UT, its roll attitude was
    as close as possible to 180 degrees.

There is a SolarSoft routine (called "get_soho_roll") available to
read this file and give the correct value for any given date.

For finding out the mode of the spacecraft, see the ancillary
spacecraft mode files at 'data/ancillary/scmode' in our web pages.

Luis Sanchez, 2003-07-09
Stein Haugan, 2004-03-16