Next: Known Problems and Bugs
Up: Mission History
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
- 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
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
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 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 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) `` 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 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 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 ( 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 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 over the
preceeding 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 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
This software update meant that the scale factors of the other gyros
cannot be commanded anymore, which fortunately has little impact on
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
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 ) 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
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
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\
(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
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.
|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 |
Table A.4: List of NASA/GSFC ROSAT GOF Status Reports
35 || Oct 27 || Meeting agenda |
|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 |
Table A.4: List of NASA/GSFC ROSAT GOF Status Reports (cont. 1)
70 || Oct 28 || ROSAT Science Symposium schedule |
|71 || Nov 13 || ROSAT Z-axis gyro fails |
72 || || |
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 |
90 || || |
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) |
Table A.4: List of NASA/GSFC ROSAT GOF Status Reports (cont. 2)
100 || Aug 01 || MPE Report # 30 |
Next: Known Problems and Bugs
Up: Mission History
If you have problems/suggestions please send mail to