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A.2 Important Dates


Below we list the more important dates associated with ROSAT operations. As is true for all missions, ROSAT has also experienced a number of additional (minor) operational difficulties since launch. A summary of all the periods lost due to satellite problems (the so-called black-list) can be found in the ROSAT Newsletter.

1990 June 1
Launch of ROSAT spacecraft on a Delta II from Cape Canaveral, USA, and in-orbit deployment. Initial switch-on and on-board engineering tests performed without any indications of hardware or software problems.   

1990 June 16
Start of scientific observations with the first light observation of the XRT/PSPC-C pointing at the LMC.    

1990 June 17
WFC First Light Observation of the meaty source RE1629+780, a serendipitous discovery during the PSPC calibration observation of Abell 2256 on 17 June 1990.

1990 July 1
XRT/HRI first light observation of Cyg X-2.

1990 July 1 TBR
One of the two WFC background monitoring instruments (GM tubes) malfunctions, giving WFC operational difficulties.

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

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

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

1990 August 17-19
Valve which regulates the flow of gas into the PSPC-C failed on August 17. The gas pressure dropped to point where the high voltage was automatically switched off. No PSPC data were taken between 1990 August 17 14:00 UT and August 19 00:00 UT when 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 September 8
The ROSAT AMCS star tracker STC-2 (Sect. 1.2.5) 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 December 22-26
Gyro malfunction and subsequent loss of telescope attitude.

1991 January 5-6
Loss of attitude.   

1991 January 15-18
At 07:10 UT, the survey rate was increased to tex2html_wrap_inline18099 per day.

1991 January 25
With all but the last week of the six month all-sky survey completed, an almost fatal OBC glitch on 1991 January 25 at 20:18 UT caused the S/C to start spinning in an uncontrolled manner for tex2html_wrap_inline18101 hours, with all instruments still operational.   The failure is not fully understood, however, its origin may lie in a build-up of radiation-induced events, inhibiting the switchover to the redundant OBC CPU, as should automatically occur after a halt in AMCS telemetry. Housekeeping data taken during the tumbling showed that the satellite scanned across the sun, burning a hole in the window of PSPC-C (and hence destroying the detector) and damaging the S2a filter in use on the WFC.       This event was probably the origin of the degradation in WFC efficiency (although not recognized until 1991 May). Three hours after the loss of attitude, the S/C power dropped and non-essential loads (including the scientific payload) were automatically turned off, in which state it remained until January 26 when GSOC successfully managed to revive it during a ground contact period.     The (final) `` tex2html_wrap_inline18103 1-week'' strip of the sky which went unobserved during this time was surveyed later between 1991 August 3-13, thus completing the all-sky survey.   

1991 February 8
AO-1 pointed observations begun using (the reserve) PSPC-B.

1991 February 16-18
Survey observations to make up lost exposure from July 1990.  

1991 March 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% of the pre-flare efficiency

1991 April 6-10 & April 21
ROSAT suffered a loss of attitude on April 6, 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 April 6-10 were lost.  A further attitude loss occurred on April 21 resulting in the loss of an additional day of data before normal operations were restored.  

1991 April 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 April 30, the problems were resolved.  

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 tex2html_wrap_inline18105 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 (Sect. 1.2.3) functioned normally, but the X-gyro was 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 May 18 - November 4
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 ( tex2html_wrap_inline18109 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 tex2html_wrap_inline18111 30,000 seconds of ROSAT data for each day of observation. New AMCS on-board software enabled a successful resumption of normal pointing operations on 1991 November 4.

1991 July 5-8
After a long slew on July 5, 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 July 6-7.

1991 August 3-13
Survey completion observations of the stripe of the sky unsurveyed during the scheduled all-sky survey phase due to the solar slew on 1991 January 25.

1991 October 11 (S/C clock 42910000)
During the preceeding months, PSPC-B had 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 was also observed. Investigations revealed that the hot spot count rate had increased from about 0.001 to about 0.01 tex2html_wrap_inline18113 over the preceeding tex2html_wrap_inline18115 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 approximately a factor of two. Although no immediate danger for the detector operation could be identified, the high voltage of the detector was lowered from 3060 V to 3000 V on October 11. As a consequence, the PSPC-B gas gain dropped by approximately 30% and thus the lower PI channel  limit of the detector was raised from about 8 to 11. Calibrations at the reduced voltage have shown that there is no change in the spectral resolution of the detector   

1991 November 4
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 1991 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 tex2html_wrap_inline18117  s). The scheduling constraints varied quite frequently and required rapid adjustments by both MPE and GSOC mission planning staff.

1992 February 6-24
A large transient in the ROSAT Z-gyro current occurred on February 6, 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 operating continuously with too high a current, was still giving meaningful readings of angular velocities. However, its output signal seemed to be scaled wrongly, indicating a factor of 3-4 slower speeds than the spacecraft was actually 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 aconstant factor of 2.8. The most convincing explanation of the available data implied that 1 of 3 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 February 18-19. This software patch contained the change of the Z-gyro scale factor by 280%. 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 February 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 February 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.

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 tex2html_wrap_inline18119 ) and into a region that extends out to about 5', an effect which is relatively independent of energy.

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

1993 January 10
Problems with STC-1. In about 30% of the slews to a new target position, the mapping period of the star tracker at the end of the slew takes more time (from 1 up to 20 minutes) until the tracker enters the pointing mode. This lead to a minor shortening of some of the observations, while a few targets were lost.

German and U.K. data entered the U.S. archive later the same month.

1993 January 12
New PSPC response matrix released.

1993 February 24
1000 days in orbit celebration, production of T-shirts, Tausend und Eine Nacht mit ROSAT.   

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, where a much more vital memory cell was lost, which rendered 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 % loss of observing time.  

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 15 (the beginning of AO-4) to settle down at the new equilibrium. Immediately after the switch-on on June 15, an extended radioactive source calibration was started which was followed by 1 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.

Two cases were found where, due to the reduced accuracy of the onboard gyro system, the AMCS after a slew locked on to an incorrect star pattern. In both cases, only two stars were available in the star tracker field of view and the commanded star pattern (essentially star separation and the magnitudes) were fulfilled also by a nearby star pattern, about 2 degrees off from the commanded position. Both the onboard and the ground system attitude software system therefore assumed a correct pointing and could not recognize the error.  

1993 August 1
MPE and GSFC (in consultation with SAO) have concluded, through independent analysis of calibration and other data, that there is some temporal variation of the PSPC gain which is 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 is highly significant residuals in number of spectra compared to the expected model, these appear as large negative residuals below about 0.20 keV and large positive residuals between about 0.20 to 0.4 keV.

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

1993 September 22
Survey data made available for background estimations.

1993 September
A roll angle error was found. The effect of this roll error is not included in the current SASS\ processing.  (It will be included in the REV-1 reprocessing of the entire dataset planned for the near future.)

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

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 December TBD
End of six-month block of PSPC observations in AO-4.

1993 December 18
ROSAT entered safemode again.

After Christmas 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. ROSAT remained in safe-mode while the problem was investigated. 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 1993 November.  

1993 December 22
The wobble was disabled to reduce attitude control problems.    

1994 February 2
A temporary observing program began to test some solutions to the pointing problem.

1994 February 26
The observing program restarted at midnight of February 26/27 (UT), beginning with five days of PSPC observations.

1994 March 4
HRI observations began.    

1994 June 30
The PSPC was moved into focus for a last seven-day period lasting from 1994 June 30 to July 6. This ended the regular ROSAT PSPC guest observer program.   


Number Date Major Content
1 1991 Mar 21 Overview of project software
2 Apr 03 Satellite & processing status
3 Apr 25 AO-2 notes and blacklist
4 May 15 Gyro problems
5 Jun 10 Reduced Pointing Timeline
6 Jul 05 AO-2 IUC meeting report
7 Jul 17 Satellite safed
8 Aug 05 SASS status
9 Aug 19 Timeline
10 Sep 06 SASS status
11 Sep 12 Timeline update
12 Oct 04 Attitude software update
13 Oct 21 SASS & satellite status
14 Nov 14 AO-3 announcement
15 Nov 20 Guidelines for AO-3
16 13 Dec Databases
17 1992 Jan 7 Mission Planning Status
18 Feb 11 Z-gyro problem
19 Feb 25 general status
20 Feb 28 February 5-28 lost AO-2 sequences
21 Mar 24 April - June AO-2 timeline
22 Mar 31 HRI Spatial Response
23 Apr 8 Proc. problems, WFC bright source catalogue
24 May 11 AO-3 results
25 May 22 Science bulletin: Geminga
26 Jun 11 AO-1 & AO-2 holdovers
27 Jun 11 AO-3 timeline
28 Jul 9 rosserv ftp account announced
29 Aug 7 MPE Report # 10, calibration
30 Aug 18 IDL routines
31 Sep 8 U.S. workshop announcement
32 Sep 24 Workshop reminder
33 Oct 1 MPE Report #11
34 Oct 6 Workshop update
35 Oct 27 Meeting agenda
Table A.4: List of NASA/GSFC ROSAT GOF Status Reports



Number Date Major Content
36 Nov 16 Public Archive Opens
37 Nov 17 AO-4 announcement
38 Nov 25 AO-3 timeline
39 Dec 1 New matrix
40 Dec 9 Master Observation List available
41 Dec 14 U.K. bulletin, matrix bug
42 Dec 17 Archive access
43 Dec 23 HRI performance update
44 Dec 24 PROS hints & pointers
45 1993 Jan 13 New matrix
46 Jan 15 U.K. & German data released to archive
47 Never Existed
48 Jan 26 Archive note
49 Jan 29 RPS warning
50 Feb 3 MPE bulletin # 15
51 Feb 3 Archive note
52 Mar 1 MPE Report # 16
53 Mar 26 Hints & Pointers
54 Mar 30 Report from calibration workshop
55 Apr 19 ftp moves to legacy
56 May 6 U.K. & German data release
57 May 6 MPE Report # 17
58 May 10 AO-4 targets
59 May 12 More U.K. & German data in archive
60 May 20 1993 workshop
61 Jun 2 AO-4 timeline
62 Jul 13 MPE Report # 19: Reduced PSPC gas flow
63 Jul 14 Hints & Pointers # 9
64 Aug 1 PSPC gain problem
65 Aug 10 Off-Axis PSPC PSF
66 Sep 22 Survey Data available for bgd estimation
67 Oct 13 Error in roll angle
68 Oct 25 The OGIP ROSAT PSPC Calibration Guide
69 Oct 26 U.S. ROSAT Public Data Archive questionnaire
70 Oct 28 ROSAT Science Symposium schedule
Table A.4: List of NASA/GSFC ROSAT GOF Status Reports (cont. 1)



Number Date Major Content
71 Nov 13 ROSAT Z-axis gyro fails
73 Nov 30 ROSAT operations
74 Dec 9 new Ftools release
75 Dec 13 Caution to check coordinates
76 Dec 15 ROSAT sequence-naming conventions
77 Dec 16 New time lines on FTP
78 Dec 22 PSPC PI channels and bands
79 Dec 23 MPE Report # 24
80 Dec 29 Hints & pointers for PROS users
81 1994 Jan 1 AO-5 announcement
82 Jan 28 ROSAT in safe-hold mode
83 Feb 2 MPE Report # 25
84 Feb 7 RPS5 Note
85 Feb 16 Off-axis point spread function for the ROSAT HRI
86 Mar 1 ROSAT satellite status, naming conventions, PROSCON
87 MPE Report # 26
88 Mar 14 Short-term timelines
89 Apr 1 MPE Report # 27
91 Apr 22 ROSAT archive enhancements
92 Apr 25 Correction to the PSPC off-axis PSF memo
93 May 04 MPE Report # 28
94 May 19 Data set naming conventions
95 Jun 21 New information available
96 Jun 27 Hints and pointers for PROS users #11
97 Jul 05 MPE Report # 29
98 Jul 15 Archive Note
99 Jul 27 Hints & pointers for PROS #12 (PROS 2.3.1 patch)
100 Aug 01 MPE Report # 30
Table A.4: List of NASA/GSFC ROSAT GOF Status Reports (cont. 2)


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