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ROSAT Guest Observer Facility

Important ROSAT Dates


Below are listed the more important dates associated with ROSAT operations:

1999 February 12
ROSAT turned off!

1998 December 8
PSPC-B put back into the focal plane of the XRT to use remaining gas for observations.

1998 September 20
Spacecraft lost attitude control and the HRI was severely damaged during an accidental slew near the sun.

1998 August 21
Nominal timeline was resumed with WFC star tracker in the attitude control system.

1998 June 1
Eight-year anniversary of ROSAT's launch.

1998 April 28
Last XRD star tracker failed. Attempts are made to add the WFC star tracker to the attitude control system.Timeline suspended

1997 June 1
Seven-year anniversary of ROSAT's launch.

1997 January 17
Routine reset of the spacecraft clock was initiated.

1996 December 19
ROSAT satellite entered a safe mode caused by an erroneous signal from the star tracker. Normal timeline operation was resumed about 11 hours later.

1996 June 1
Six-year anniversary of ROSAT's launch.

1996 May 15
AO-7 proposals due.

1996 April 29
ROSAT long-term science mission plan session at San Diego HEAD meeting.

1995 December 2
First AMCS safe mode since 29 October 1994, Eight-hour duration.

1995 October 1
HRI hot spot reported.

1995 September 25-29
Wuerzburg Meeting: "Roentgenstrahlung from the Universe"

1995 July 3
New AMCS software tested for operating with fewer gyros.

1995 June 1
Five-year anniversary of ROSAT's launch.

1995 May 15
AO-6 proposals due.

1994 November 3
Safe mode triggered by solar eclipse. Timeline restarted the following day.

1994 October 31
Release of the WGAcat ROSAT source catalog.

1994 October 26
Release of the MPE ROSAT source catalog.

1994 September 5-11
Final normal timeline operation of the PSPC.

1994 June 1
Four-year anniversary of ROSAT's launch.

1994 April 25-28
Pointing difficulties.

1994 March 31
AO-5 proposals due.

1994 February 27
New angular-momentum-stabilized operating mode for ROSAT to compensate for lack of startracker and sunsensor information.

1994 Feb 2
A reduced pointing observing program began, due to AMCS problems.

Dec 1993-Jan 1994
In December and 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 resulted in the safe mode being triggered.

It soon became clear that the problem was related to the new attitude control strategy, which became active after the Z-gyro failure that occurred in November 1993. For more information see rosat status report #83. In December 1993 and January 1994 ROSAT, was only partially operational. Observations actually carried out are listed in dec93-feb94.obs.

1993 December 18
ROSAT entered safemode again. During the days that followed, different attempts to recover from the safemode were only partially successful and never for more than one orbit. On December 22, the wobble was disabled, which seemed to help avoid safe mode.

1993 November 26
Timeline operations were resumed with a modified observing program, constructed under the boundary conditions that slews occur only on the day side and no observations be carried out with a sun cone angle between 85 and 95 degrees.

1993 November 18
Z-axis gyro failed permanently, sending ROSAT into safe mode.

1993 September
A roll angle error was found. The effect of this roll error was not included in the current SASS processing. (It will be included in the "Rev1" reprocessing of the entire dataset planned for the near future.)

The error was small, being 0.185 degrees for the PSPC. This error corresponded to a position error of 6 arcseconds at the radius of the support ring.

1993 September 22
Survey Data made available for background estimations.

1993 August 10
The Off-Axis Point Spread Function algorithm was released.

1993 August 1
MPE and GSFC (in consultation with SAO) concluded, through independent analysis of calibration and other data, that there was some temporal variation of the PSPC gain that was not included in the gain correction process currently applied to data in SASS.

This effect showed up primarily as inconsistencies between calibration spectra of N132D taken at different epochs across the ROSAT mission. Another artifact of the problem was the highly significant residuals in number of spectra compared to the expected model. These appeared as large negative residuals below about 0.20 keV and large positive residuals between about 0.20 to 0.4 keV.

1993 June
The HRI was moved into the focus on June 10 for the last time in AO-3. The PSPC gas-flow rate was reduced the same day. The PSPC had time until June 15th (the beginning of AO-4) to settle down at the new equilibrium. Immediately after the switch-on on June 15th, an extended radioactive source calibration was started, which was followed by one orbit of filter-wheel-closed background measurements. This procedure was repeated the next day. For the first week after the switch-on, the radioactive calibrations were repeated once per day. Background measurements are taken once per week for one orbit.

The PSPC performance at reduced flow rate looked good. The gain dropped slightly, from channel 103 before to 100 after the switchover.

Attitude error: two cases were found where the AMCS after a slew locked on to a wrong star pattern-due to the reduced accuracy of the onboard gyro system. In both cases, only two stars were available in the startracker field of view and the commanded star pattern (essentially star separation and the magnitudes) were fulfilled also by a nearby star pattern, about two degrees off from the commanded position. Both the on-board and the ground system attitude software system therefore assumed a correct pointing and could not recognize the error.

1993 May
The malfunction of STC 1 was traced to a failure in the conventional RAM area. This failure was similar to the cause of the complete loss of STC 2 in autumn 1990, when a much more vital memory cell was lost, rendering STC 2 unusable. The STC 1 anomaly has stabilized at a rather lower failure rate than observed in January/February, corresponding on average to a 5-10 percent loss of observing time.

1993 Jan 10
Problems with startracker 1. In about 30 percent of the slews to a new target position, the mapping period of the startracker at the end of the slew takes more time (from 1 up to 20 minutes) for the tracker to enter the pointing mode. This lead to a minor shortening of some of the observations, while a few targets were lost.

German and UK data entered the US archive later the same month.

1993 Feb 24
1000 days in orbit celebration

1993 Jan 12
A new PSPC response matrix was released

1992 Nov 1
The US ROSAT Public Data Archive (USRPDA), located at the Goddard Space Flight Center in Greenbelt, MD, officially opened.

1992 March
The MPE/GSFC/SAO ROSAT calibration team found that the HRI point-spread function had an unexpected component, which redistributes roughly 10 percent of point-source photons beyond a central core (radius ~ 10 arcsec) and into a region that extends out to about 5 arcmin, an effect that is relatively independent of energy.

1992 Feb 06-24
A large transient in the ROSAT Z-gyro current occurred late on Feb 06, reminiscent of that which immediately preceded the loss of the Y-gyro in 1991 May 12. The s/c was put into safe mode at that time and observations were suspended.

The Z-gyro, although continuously with too high a current, was still giving meaningful readings of angular velocities. However, its output signal seemed to be scaled wrongly, indicating speeds slower by a factor of three or four than the spacecraft actually was performing.

Evaluation of the Z-gyro malfunction proved that the measurement outputs of the Z-gyro were still valid, however, the scale factor had changed by a constant factor of 2.8. The most convincing explanation of the available data implied that one of three phases of the gyro motor failed, and consequently the motor current had increased.

To compensate for the decreased Z-gyro output, a new change of the onboard software was developed and tested on the GSOC simulator over Feb 18-19. This software patch contained the change of the Z-gyro scale factor by 280 percent.

This software update meant that the scale factors of the other gyros cannot be commanded anymore, which fortunately has little impact on mission operations.

The software patch was successfully up-linked on Feb 20 and passed a first test in the form of a slew to the north ecliptic pole on the same day. After the slew, the attitude was correct and reference stars were identified.

ROSAT went back on the Mission Timeline on 1992 Feb 24 and the first three slews were successfully performed. This success indicated that the new gyro scale factor was correctly determined and appeared to be quite stable.

1991 Nov 04
The new AMCS onboard software allowed a successful resumption of normal pointing operations, marking the end of the reduced pointing phase (which followed the loss of the Y-gyro on May 12) and the start of AO-2. More than two hundred pointings were carried out in the reduced pointing phase, most of them lasted one day (and were selected from AO-1 targets with approved times approximately great 10^4~s). The scheduling constraints varied quite frequently and required rapid adjustments by both MPE and GSOC mission planning staff.

1991 Oct 14
During October, the PSPC-B developed a slightly higher background rate extending over the whole sensitive area of the detector, but confined to the lowest pulse height channels. A hot spot near the edge of the field of view also was observed. Investigations revealed that the hot-spot count rate had increased from about 0.001 to about 0.01 cts s^{-1} over the preceeding approximately six-month period. Purging of the detector gas and switching the gas supply from tank B to tank A (to exclude the possibility of different gas composition) reduced the hot-spot count rate and the count rate in the lowest pulse height channels by a factor of approximately two. Although no immediate danger for the detector operation could be identified, the high voltage of the detector was lowered from 3060 to 3000 volts on October 14 . As a consequence, the PSPC-B gas gain dropped by approximately 30 percent, and thus the lower energy range limit of the detector was raised from about 70 to about 100 eV. Calibrations at the reduced voltage have shown that there is no change in the spectral resolution of the detector

1991 July 05-08
After a long slew on Jul 05, ROSAT became lost due to Earth-block and lack of guide stars. Consequently, the s/c went into safe mode due to operating too long without reference stars. Successful recovery procedures were carried out over the weekend of Jul 06-07.

1991 May 18 - Nov 04
Reduced-Pointing mode, whereby slews and target acquisition were tightly constrained due to the use of the X-gyro in the control loop. Slews were reduced to one direction and constrained to be short, guide stars needed to be available when the satellite was over Weilheim (i.e. target declinations were constrained to be > +20 degrees), a reduced sun cone angle was enforced (90 +/-5 degrees), only one target was observed per day, and only PSPC observations were allowed.

Also during this period, an increased level of solar activity led to high levels of radiation, causing the detector to safe itself several times. These conditions resulted in a yield of about 30,000 seconds of ROSAT data for each approximately one-day observation. New AMCS on-board software enabled a successful resumption of normal pointing operations on 1991 Nov 04.

1991 May 12-18
It became clear that WFC count rates were systematically less than expected based on the WFC survey. A calibration observation of RE1629+781 confirmed the gain loss, and showed that a new operating voltage was required. The WFC count rates were determined to be sim 20 percent of the expected values and the backup detector degradation was even more severe than that suffered by the primary.

The Y-axis gyro on ROSAT failed, sending the satellite into safe mode. The Z- and S-gyros functioned normally, but the X-gyro is problematic (and was removed from the control loop during the All-Sky Survey after its drift rate exceeded specifications). Attitude control was successfully regained on May 18. There appears to be no hope of retrieving use of the Y-gyro.

1991 Apr 26-30
Difficulties were encountered with the uplinking of new software patches developed to address the problems of maintaining known attitude, resulting in the satellite switching to safe mode. By Apr 30 the problems were resolved.

1991 Apr 06-10 and Apr 21
ROSAT suffered a loss of attitude on Apr 06, and subsequently ended up in safe mode as a result of the extra safety procedures implemented after the loss of PSPC-C. Observations scheduled for 1991 April 06-10 were lost. A further attitude loss occurred on Apr 21 resulting in the loss of an additional day of data before normal operations were restored.

1991 Mar 25-27
Solar flare activity in 1991 much enlarged the radiation belts and increased the PSPC-B background, causing a loss of observations due to the PSPC-B count rate exceeded the danger threshold. Adjustments to the tolerances led to resumed operations, with 80 percent of the pre-flare efficiency.

1991 Feb 08
AO-1 pointed observations were begun using (the reserve) PSPC-B. Some survey observations were made to make up for some losses of July 1990.

1991 Jan 25
With all but the last week of the six-month all-sky survey completed, an onboard computer glitch caused the ROSAT spacecraft to tumble out of control for approximately 15 hours. Housekeeping data taken during the tumbling showed that the satellite scanned across the sun, burning a hole (and hence destroying) in the window of PSPC-C and damaging the S2a filter in use on the WFC. During the subsequent contact cycle, the status of the spacecraft was recognized and the proper safemode reached. The (final) one-week (approximately) strip of the sky that went unobserved during this time was surveyed later between 1991 August 03-13, thus completing the All-Sky Survey.

1991 Jan 15-18
At 07:10 UT, the survey rate was increased to 1.2678 degrees per day.

1990 Dec 22-26
Gyro malfunction and subsequent loss of telescope attitude.

1990 September 08
The ROSAT AMCS star tracker STC-2 stopped working continuously during the scan mode and has since failed completely. STC-1 remains fully operational, and there is only a minor decrease in expected attitude accuracy.

1990 August 17-19
Valve which regulates the flow of gas into the PSPC-C failed on Aug 17. The gas pressure dropped to a point where the high voltage was automatically switched off. No PSPC data were taken between 1990 Aug 17 14:00 UT and Aug 19 00:00 UT, but a backup value was switched on. Since no area of the sky went totally unobserved, the decision was made not to alter the offset angle and to proceed with the survey timeline as planned. WFC survey observations continued as planned during this period.

1990 July 30
ROSAT all-sky survey begins (using PSPC-C in the focal plane)

1990 July 16-29
PVC phase pointed observations using the PSPC (Jul 16-23) and HRI (Jul 24-29).

1990 July 11-15
As part of the PVC phase, the ROSAT `mini-survey' was performed.

1990 July 01
One of the two WFC background monitoring instruments (GM tubes) malfunctioned, causing WFC operational difficulties.

1990 July 01
XRT/HRI First Light Observation of Cyg X-2.

1990 June 17
WFC First Light Observation

1990 June 16
Start of scientific observations with the First Light Observation of the XRT/PSPC-C pointing at the LMC.

1990 June 01
Launch of ROSAT spacecraft on a Delta II from Cape Canaveral, USA, and in-orbit deployment. Initial switch-on and on-board engineering tests were performed without any indications of hardware or software problems.

ROSAT GOF


Curator: Michael Arida (SP Sys); arida@milkyway.gsfc.nasa.gov
HEASARC Guest Observer Facility
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This file was last modified on Tuesday, 09-Aug-2005 11:47:43 EDT

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