December04, 2001
Scientific observations with RXTE have resumed with a series of successful uninterrupted pointings, and the recent attitude anomaly is believed to be resolved. For the time being there continue to be some restrictions on maneuvers, with most slews being scheduled during Mission Operations Center prime shifts. These restrictions will probably be lifted within the next few days, allowing RXTE to return to nominal operations.
It is believed that the initial attitude anomaly was caused by a "bad star" (possibly a variable or transient) in the catalog used by the star trackers. A similar problem was encountered last year. The bad star is being replaced, and the entire star catalog reviewed to attempt to prevent similar problems in the future.
November 27, 2001
On 2001 Nov 25th, RXTE slews were commanded off and the PCA and ASM
safed, following attitude control system (ACS) anomalies in which both
Fixed Head Star Tracker (FHST) Kalman filters failed to update
correctly. Slews were reenabled and the instruments turned on again
for a brief diagnostic interval on Nov 26th, after which science
observations were again suspended and the satellite put back into
inertial hold pending further study of the problem.
Thus, science data were not obtained from day 330 03:45-19:16 UT, and
from day 330 ~22:00 UT to the present time. Spacecraft power and PCA,
HEXTE, and ASM instrument health remain otherwise nominal. The cause of
the ACS anomaly is being investigated by spacecraft engineers.
Please refer back to this page for future updates.
November 26, 2001
PCA Team member Craig Markwardt, widely recognized as a man
with a great attitude, has successfully completed his effort to
recover the true attitude information of the RXTE satellite from
2000 September 6-12. During that time, RXTE gradually lost contact
with some guide stars, and drifted up to 55 arcmin from the nominal
pointing direction. Because the satellite was unaware of the pointing
error, attitude data in the ACS files for the affected observations
do not reflect reality. Observations falling in this time period were
renamed with the integers F, T and U following the original OBSID, in
order to flag the user that something was amiss with these files.
About 125 observations were affected. Since the attitude drift
is slow and small, most observations contain the source within the collimator,
but at an offset compared to the nominal pointing direction. This can cause
faint sources to fall below the noise level. For all other sources, true
knowledge of the pointing and off-axis response allow users to recover the
true source flux.
Since the RXTE star trackers scan their FOV for the brightest stars,
it was possible in most cases to derive a solution of the spacecraft
attitude from this data. This reconstructed attitude solution was performed
on the ground by a collaboration of GSFC flight dynamics facility personnel
and workers at CSC. The ground-derived attitude solution was used to
apply a correct to every affected file during the period of bad attitude.
These corrected data are now available in the RXTE archive, with the same
obsid names. The new files have added FITS HISTORY keywords describing the
attidude correction. The housekeeping files containing the *original*
attitude data are still in the archive, with orig_ preceding thier file
names, (for example, orig_FH0e_c97a482-c97a634).
The following keyword also appears in each of the corrected files:
Detailed information about the ground solution, the correction,
and which observations are affected can be found on Craig Markwardt's
Attitude Anomaly page at:
http://asd.gsfc.nasa.gov/users/craigm/xteatterr/
September 27, 2000
Between 2000 Sept 6 16:00 - Sept 13 15:40 (UT), the RXTE satellite
suffered errors in attitude determination. Corrective action was
taken, and the problem is believed solved. However, RXTE users should
be aware that data obtained during this interval may be subject to
attitude errors of up to 1 degree which will not be reflected in the
attitude file. Quantities such as the pointing position, Earth limb
angle, offset angle etc, are very likely to be incorrect, and cannot
be reconstructed. Effects on your analysis may include an apparent
reduction in the flux measured by the PCA and HEXTE instruments of up
to a factor of 5, and poor spectral fitting results. Please be
EXTREMELY cautious before drawing any scientific conclusions from the
affected observations.
PIs of affected observations have been informed directly. Note that
the data from public TOOs performed during this period have been
released to the community, despite the uncertainty in attitude.
Ultimately, in the RXTE Archive, all affected obsIDs will be tagged to
indicate the unreliability of the attitude data.
RXTE in Inertial Hold: ACS Anomaly
Old Data Gets New Attitude
ACSCORR = T / ACS errors are corrected
RXTE Attitude Errors: 2000 Sept 6-13