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Rosat Status report 83

		ROSAT Status Report # 83 
			Feb 2nd 1994

=                                                                       =
=              ROSAT NEWS No. 25   ---      1-Feb-1994                  =
=                                                                       =
=                 ROSAT Scientific Data Center at the                   =
=       Max-Planck Institut fuer Extraterrestrische Physik (MPE)        =
=                 Postfach 1603, 85740 Garching, FRG                    =
=  e-mail addresses (Uli Zimmermann):                                   =
=   rosat_svc@mpe-garching.mpg.de (Internet) or                         =
=   MPE::ROSAT_SVC (SPAN)                                               =
=  ROSAT Service Area read-access via anonymous ftp:                    =
=   ftp rosat_svc.mpe-garching.mpg.de       user: anonymous             =
=   WFC Archive access via telnet/ftp ait.physik.uni-tuebingen.de       =
=                                user: xuv  (password: xuv_archive)     =

This issue reports on:
   SATELLITE STATUS: understanding the problems
   MISSION PLANNING: what was observed in the Dec 93/Jan 94 period 
   ROSAT DATA ARCHIVE (RDA):  now data can be retrieved ONLINE 
   WFC EVENT FILES, gaps in - Problems with Exposure Determination
   EXSAS/MIDAS: OSF/1 implementation available

SATELLITE STATUS: understanding the problems
In the following we give a detailed summary of the present problems with 
the ROSAT attitude system, including some history of the trials to 
overcome them (collected by Martin K"urster, MPE). Although repeating 
some of the info we sent out earlier, it may help to better understand 
the context.

1. What happened

After Xmas and in early January there were a number of occasions during 
which problems in ROSAT's attitude measurement and control system (AMCS) 
caused the satellite to drift away from its nominal pointing direction. 
This ended up with the safe mode being triggered. Currently, while the 
problem is being investigated, ROSAT is remaining in safe-mode. 

It soon became clear that the problem is related to the new attitude 
control strategy which became active after the Z-gyro failure that 
occured in November 1993. To remind you of the geometry: ROSAT 
originally had four gyros to measure rotations around three spatial 
axes; the X-axis is pointing to the sun, the Z-axis is the telescope 
pointing direction, and the Y-axis is perpendicular to X and Z; the 
axis of the fourth (redundant) gyro is inclined with respect to all 
of the other three axes. The Y-gyro was lost earlier in the mission. 
With the failure of the Z-gyro the attitude solution must now be 
derived from the following information.

a) If the star tracker has identified a star pattern, this information 
   is    exclusively used for the attitude solution (stable pointing); 
   this is unchanged.
b) If no star tracker information is available (during slew, earth 
   block, or when the star pattern cannot be identified) the 3-D 
   attitude solution can no longer be derived from the readings of 
   three gyros (as was the case previously), but must be deduced from 
   the combined readings of the X-gyro and S-gyro, the sun sensor(s), 
   and the magnetometers.

All critical situations occured when at the same time ROSAT was in the 
earth's shadow (i.e. no sun sensor readings could be used) and also no 
star tracker information was available (e.g. during an earth block of 
the sky region the telescope was pointing to). In these cases the 
attitude solution must be derived from the two gyros and the 
magnetometers. Since the gyro measurements have been routinely used 
during the whole mission, the error must be related to the 
magnetometer readings.

In the first instances drifts occuring during earth blocks on the 
night side were typically of the order 25 deg. When ROSAT left the 
earth's shadow the fine sun sensor provided the information that the 
maximum sun angle (20 deg) was exceeded and triggered the safe mode. 
At this time it was believed that only slow drifts occured during the 
night side, and that the sun angle could not be exceeded by much. As 
a quick measure to prevent frequent safe modes (which can lead up to 
almost a day of idle time) the attitude control software was modified 
in such a way that it allowed for 2.5 min (later 5 min) during which 
the fine sun sensor could try to move the satellite back to nominal 
sun angle before triggering safe mode. 

This measure did not help when a drift of 110 deg occured, so that 
after sunrise also the coarse sun sensor (with a field of view of 
45 deg radius) did not `see' the sun and triggered safe mode. After 
it was clear that such large drifts can occur it was decided that 
ROSAT was not to be recovered from safe mode before a detailed 
understanding of the problem was gained. Otherwise it could not be 
excluded that the telescope would accidentally point to the sun with 
focal plane instruments (FI) unsecured.

2. What causes the problem

A detailed analysis of the drift events revealed the following. For 
operational reasons magnetometer readings can be used only in 
intervals of two minutes. In the critical shadow-and-earth-block 
situation two spatial coordinates are constantly given by the gyros, 
but the third component is available only in two-minute intervals. 
If some kind of external torque is present (see below) the satellite 
begins to drift around some spatial axis, and for two minutes, only 
two components of this drift (parallel to the X- and S-gyro) are 
measured and compensated for. The third drift component 
(perpendicular to the X-S-plane) is only recognized two minutes later. 
In some unfortunate situations, the satellite is oriented such that 
the earth's magnetic field vector is lying near the plane defined by 
the axes of the X- and S-gyros. Then the third component is not 
available from the magnetometers, the drift is not recognized and 
can continue. 

While it was clear from the beginning that these critical 
geometrical situations can occur, it was not realized that torques 
strong enough to cause significant drifts in a few minutes are being 
exerted on the satellite. The source of these torques has now been 
identified to be the magnetic torquers which are used to prevent the 
reaction wheels from accumulating too high rotation rates and 
saturating. The torquers produce a magnetic field that tends to 
align (by rotation of the satellite) with the terrestrial field 
thereby making it possible to slow the rotation rate of the reaction 
wheels (counter-rotation). To make this discharge of the reaction 
wheels as smooth as possible it is performed almost permanently in 
small portions. Thus during the 2 min. without magnetic field 
measurements a number of discharge events can take place. As 
simulations performed by DASA (the company responsible for the AMCS 
system) show the strength of these torques is of the right order to 
produce the observed drifts.

3. Measures taken to solve the problem

The identification of the source of the disturbances leads to a 
quite straightforward cure of the problem, i.e. the suppression of 
magnetic reaction wheel discharge during night time. The capacity 
of the reaction wheels is sufficient to restrict their discharge to 
the times of sun presence. However, substantial modifications of 
the onboard software are required which need to be tested in a 
simulator on ground. Also two days of in-orbit tests with secured 
FI are foreseen.

4. A temporary reduced pointing phase

To make use of the time before the new software will become 
available for use it is planned to circumvent the critical 
situations (nightly earth blocks) by means of an appropriately 
devised timeline. Three cases can be distinguished:

a) During full-sun orbits (a few days once a month) the sun sensor 
   can be used together with the gyros so that a regular automatic 
   timeline can be applied.
b) During times where there is a continuous viewing zone in the sky 
   (also a few days per month) targets located in this zone can be 
   observed, because    attitude reference is given without 
   interruption by the star tracker.
c) At all other times those regions in the sky are identified that do 
   not go into earth block during the shadow phase. Targets located 
   in these regions will then be identified. Since these regions move 
   on the sky, target changes are foreseen once a day. They will be 
   made during sun presence. As a security against the rare situation 
   that after a slew the star tracker does not manage to lock-on to a 
   star pattern during day time a change of the FI software is 
   currently being made which automatically protects the FI when the 
   night begins.
This temporary observing program will begin on Wednesday, Feb. 2, 1994, 
i.e. at the beginning of the next full-sun orbit phase.

5. Planned return to nominal action

Nominal actions will be resumed when the new AMCS software is available 
and tested. This is estimated to happen at mid-February.

MISSION PLANNING: what was observed in the Dec 93/Jan 94 period ?
In December 1993 and January 1994 ROSAT was only partially operational.
Users that would like to know whether a specific observation in the
Dec/Jan timeline was performed, can look to the file dec-jan94.obs
in directory timeline in the ROSAT Service Area. For HRI observations 
an entry means that the observation looks normal from the aspect
protocol, but the useful data time has not yet been evaluated. For 
PSPC data the accepted time, as derived from the SASS processing is
noted in the last column.

*								    *
*US note: the file can also be found on the legacy ftp area under   *
*rosat/timelines/dec-jan94.obs					    *
*								    *

>From February 10 on all publicly available data sets in the RDA will be 
accessible online from the ROSAT Service Area via anonymous ftp 
(access see header of ROSAT NEWS). The data are stored in compressed 
form on magnetic discs and therefore, after transfer, have to be 
decompressed at the users site. Thus the network load is minimized.

At this time data retrieval from UNIX systems is straightforward and
easy to do. For retrievals from VMS systems things are more heavy and not 
yet tested. We will try to find also for VMS users a nice solution.

Layout of the data in the archive is as follows:
Data from a specific observation, identified by its sequence number
and an additional identifier (p for PSPC, f for PSPC with filter and h 
for HRI data), are contained in a proper subdirectory of directory 
'archive/data'. PSPC data with sequence number 123456, for example, are 
stored in directory archive/data/123456p. 

The archive data have been compressed with the gzip utility that is 
used by most data/software distribution centers. It is publicly 
available (no cost) and more effective than e.g. the UNIX compress 
utility. There are versions for most operating systems including VMS. 

If the gzip utility is not already installed on your system, ask your
system manager to do so. He can get the software either directly from 
one of the FTP servers or likewise from the ROSAT Service Area (read 
file gzip.info in directory archive/tools).

You may, of course, also in future send your archive requests via e-mail
to us and receive the data on cassette. This is especially recommended 
if datasets are very long or the network connections not sufficiently
stable. A request form is available in directory archive.

This info plus an example session for UNIX users can be found in the
file getdata.info in directory archive (a file getdata-vms.info will be
placed later there to inform on vms specifics). Please send us your 
comments on experience with the online data retrieval.

WFC EVENT FILES, gaps in - Problems with Exposure Determination
In the course of statistical investigations of a large sample of WFC 
sources we discovered discrepancies up to 10%-20% in the count rates of 
different observations of bright non-variable sources. This was found to 
be due to data gaps in the event files, not recorded in the TIM_SEL and 
TIM_INT descriptors. Further analysis showed that the start and end 
times of each slot (OBI) as recorded in the TIM_SEL descriptor correctly 
refer to the times when the WFC was on target and recording events. 
Events were however only copied into the event files delivered to us 
from the Rutherford-Appleton-Lab for time periods where a valid attitude 
from the WFC startracker was available, thus resulting in an 
inconsistency of the data and the descriptors of these files. We are in
contact with the UK Science and Data Centres about this problem. The 
problem results in an over-estimation of the exposure time and 
consequently an underestimation of the source count rates in the 
affected observations. A detailed investigation of the event files of 
421 observations carried out at the GXUVDC showed that 50 % of the 
observations are not affected at all and that for about 90 % of the 
observations the error in the exposure is less than 10 % (comparable to 
the uncertainty in the calibration of the effective areas of the WFC). 
However, about 1 % of the observations have errors larger than 30 %. 

The standard analysis software run in Tuebingen has been revised 
(V4.00 2/94), to correctly treat the problem for all data distributed 
after January 15.

A preliminary correction has been applied to the exposure maps available 
online in the public ROSAT/WFC archive. Note that values in the 
TIM_INT and TIM_SEL descriptors were not changed and that a residual 
error of the exposure values remains (98 % of the corrected exposure 
maps have errors less than 10 % and about 90 % have errors less than 
5 %). A complete reprocessing of the data in the  public online archive 
will occure at a later time.

There are several different ways for owners of WFC data to check the 
status of their old data and to work around the problem:

1. - The exposure time (descriptor TIM_INT) will be changed to the 
     correct value by running the EXSAS correction package in the 
     attitude mode. Analysis making use of the EXSAS correction package 
     in attitude mode at all times has resulted in the correct count 
2. - PIs may order revised data tapes and standard analysis print outs 
     produced with the new software version from the GXUVDC.
3. - For public observations a corrected exposure value may be obtained 
     by down-loading the corrected exposure map from the ROSAT/WFC 
     archive account.
     Note the above limitations of the corrected exposure maps at the 
     present time.
4. - Observers interested in timing analysis may check for gaps in the 
     data by binning the event file in short (5 sec) intervals and 
     removing the empty bins. We do suggest, however, to order a 
     revised data set from the GXUVDC in such cases.

Please don't hesitate to contact the GXUVDC at the address given in the 
header. A detailed description of how to log into the ROSAT/WFC online 
archive was given in ROSAT News #19.  

EXSAS/MIDAS: OSF/1 implementation available
All institutes that returned the EXSAS Questionnaire (you may also get it 
from the Service Area, directory exsas) to us should have received the 
new EXSAS software in the meantime. 

A version of MIDAS/EXSAS for the OSF/1 operating system on DEC Alpha
computers is now in use at the data center and may be requested from us.

====================== end of ROSAT NEWS ================================