Scheduling of a number of approved AO-3 observations started immediately
after the COPS approval of the programme. Approximately 70 AO-2 pointings
(15% of the programme) are outstanding and will be interleaved with the
AO-3 observations.
Several anomalous 'events' have occurred during the period 1.1.85 -
28.2.85 following the X-gyro malfunction on day 366 (84).
During the normal recovery procedure from the safety mode event of day
366 associated with the X-gyro switch off (Express
No. 8 p.2), a second safety mode was triggered on day 1 when the AOCS
outer loop was closed on two axes only with the Skew(S) gyro in operation.
Later analysis showed that outer loop closure must be carried out simultaneously
on all three axes when the S-gyro is active in the control loop.
Excessive fuel consumption (propane) has occurred between days 3 and
10 because of double-sided limit cycling on the X-axis caused by multiple
firing of the thruster on both sides of the cycle. Analysis of the software
flow charts suggested that this behaviour could arise if certain control
parameters had an incorrect value (eg. a corruption of memory); this was
verified by subsequent readout of the RAM locations and the problem 'solved'
by re-writing the locations to their correct values. On two further occasions,
RAM Data has been found to be corrupt and was re-written from ground.
A review of the X-gyro malfunction event was held at ESOC on 10/1 and
recommended that, since the gyro appeared to function nominally in terms
of spin rate etc. and that the anomaly might be related to power supply
problems, the gyro should remain on continuously but not be used for control.
Although the input current continued to exhibit its high anomalous state
with periodic reversion to nominal, no evidence existed to suggest that
the X-gyro could not be used again in the event of a further gyro failure.
Status changes of the Y-gyro health monitor have occurred in the recent
past. Since the X-gyro switch-on above and associated higher temperature
of the gyro box the frequency has increased giving rise to an increased
probability of safety mode triggering (2 gyros 'failed' - X-gyro health
monitor is always 'fail' after the event on day 366). Because of this,
the hardware safety monitoring circuitry has been disabled and the relevant
tasks implemented in an OBC program which essentially re-enables the hardware
function should a genuine 'safety mode' condition occur.
A few minor modifications to the Automatic Analysis software have been
implemented:
Attention is drawn to the note on page 57 concerning organization of
the Interactive Analysis system.
A number of minor modifications have been introduced into the specification
of the EXOSAT Observation Log (Express No. 9, p.44). This log should be
complete by 1.7.85 and copies will be available on.request from the Observatory
in the form of computer listings (or magnetic tape) comprising one line
for each stable pointing. Details of present and all further specification
changes will be published in the next issues of the Express. Note that
the Log will identify data available in the archive and should facilitate
its exploitation.
Work is continuing on the definition and implementation of the 'back
up' mode of operation of the AOCS for the situation of two gyros failed
(ref. Express No.9, p.6). Only the 'direct
thruster demand' operation will be defined since the alternative -
'star hopping - would require an unacceptable level of software development.
During investigation of the GSPC dead time effects (ref. p.67), a malfunction
of the electronic single channel analyser for energy discrimination has
been discovered. The upper level threshold is commandable in 4 steps (equivalent
to ADC channels 60, 120, 180, 240 i.e.~10, 20, 30 & 40 keV at gain =
1) with the nominal value being channel 240. This threshold has shown a
gradual deterioration in sharpness of cut-off, manifested initially as
a few counts remaining between channels 240 and 255 in spectra observed
during 1984 and presently as a rather smeared-out cut-off with significant
counts still at channel 255 (ref. p.68). Note that this precludes any correlation
between OEP counts and spectral counts, and users should not, therefore,
use the QEP counts for flux or dead time estimates.
Tables 1 and 2 on p.5/6 give the current performance parameters of the
EXOSAT instruments.
A power failure at ESOC occurred on 22.6.85 and resulted in the loss
of approximately two hours of observing time with considerable restructuring
of observation start and end times. Recent estimates of the remaining attitude/manoeuvre
gas(propane) using the two methods of logging and gauging (ref. Express
No.9 p.51) indicate approximately 4 kgs as of June 9th 1985. With a current
gas consumption of ~240 gm/month (based on the logging data) and predicted
use for stabilization during an orbit manoeuvre to extend the mission beyond
its natural end in April 1986, careful use and timing of the orbit maneouvres
should permit a full observation programme until late 1986 (provided operations
remain nominal and albeit with considerable uncertainty!). Regular gauging
exercises and continual logging will be carried out to maintain realistic
estimates of mission lifetime in order to optimise the programme where
possible. With respect to gas conservation measures (ref. Express No.9 p.6), use of 1 star and the sun
for attitude determination can increase the probability of tracking an
incorrect (single) star, hence confirmation of pointing will always be
done with two stars. This will add a slight overhead (few minutes) to
total manoeuvre times from source to source.
3. On-board Software
Attention is drawn to the article on p.71 concerning potential (rare)
errors in packet reference times of certain time-critical OBC programs
when executed in specific 'program slots'.
Following the important discovery of quasi-periodic oscillations from
X-ray binaries, a new OBC mode (MHER7) has been specified and is being
developed/tested to be ready for appropriate observations of galactic centre
sources in August/September. This mode will provide high time resolution
intensity samples (submillisecond) and/or limited spectral information
(maximum 4 energy bands). A detailed specification and operational procedures
will be given in the August issue of the Express, but in the meantime PI's
with observations for which this mode is relevant should contact A. Parmar
at the Observatory.
4. Observation Output
With reference to the original specification of the observation log
(Express No.9 p.44), note that a minor change has occurred: the accumulation
time for ME energy histograms will be shown to a resolution of 0.1s and
not 1s.
Note that data archive listings derived from the log have a layout which
broadly follows the specification (p.72). In the Observation log listings,
however, the PI name will be replaced by payload and OBC configuration
details, as described in Express no. 9.
Details of the GSPC calibration history and status are given on pp.51-66.
An update to the GSPC CCF will be implemented shortly (description in the
next issue of the Express). A second data analysis workshop held at ESOC
on 22/23 May 1985, was attended by 7 scientists active in EXOSAT data analysis.
Presentations were given by Observatory Team members and discussions highlighted
a number of areas for further action (future articles in the Express).
One area of immediate interest concerned the off-axis point spread function
of the LE CMA and the definition of its functional form. At present, the
best description of the off-axis PSF can be given as a set of 43 images
of Cyg X-2 (raster scan calibration, Lexan 3000 A filter). No change to
the CCF is currently planned. These images, together with the standard
set of background images per filter (ref. Express No.9 p.5) are available
on request. Demonstrations of the interactive Analysis System were given
to illustrate general and specific aspects of the analysis of EXOSAT data.
5. Future Plans
AO-4 will be issued in August 1985 with a response required by 1st January
1986. Selection of the AO-4 programme, with a duration from March to the
end of the mission will be undertaken by the Committee on Observation Proposal
Selection (COPS) in February next year.
Volume 12: OBSERVATORY STATUS AS OF 31.8.85
The AO-3 programme of observations is 38% complete with many of the
important galactic centre observations carried out in the last two months.
Because of the planned modifications to EXOSAT's orbit in order to extend
the mission lifetime, advance time lines for both the A0-3 programme post-January
14th 1986 and the approved AO-4 observations will be more liable to real
time changes. In principle, an orbit modification per orbit may be executed
and PI's are advised that observation times may change with little prior
notice, although the Observatory Team will strive to minimise the subsidiary
effects of any unavoidable changes, particularly in respect of co-ordinated
observations.
1. Hardware
There have been no changes in the status of the spacecraft hardware.
On day 232 (August 20th 1985) at 01.02Z, the monitored HT's of ME detector
C suddenly decreased from 2195V to 2048V for the Argon supply and from
2003V to 1926V for the Xenon supply. Both HT's were immediately switched
off. Subsequent tests indicate that the Detector C monitored HT's are stable
at 77V and 147V below the nominal values for Xenon and Argon respectively.
Background energy spectra in the Argon and Xenon ranges suggest normal
proportional counter operation at a gas gain about a factor of S lower
than nominal, not consistent with the above reduced voltages on the detector
anodes. Further investigation and analysis of data is in progress, however
Detector C is currently not operated. The other seven detectors of the
ME experiment continue to function satisfactorily.
2. Performance and Operations
Tables 1 and 2 on p.5/6 give the current performance parameters of the
EXOSAT instruments.
New star tracker calibration reference data (Local Lord Points) has
been implemented on 30.7.85 and subsequent measurements indeed show an
improvement in star separation errors. A sample of X-ray objects with accurately
known optical/radio positions has been analysed to include the improved
calibration data together with a number of other effects such as the 5"
offset of the Y-axis limit-cycle (ref. Express
No. 4 p.34) - refer to the article on p.53. A clear improvement in
the correlation between EXOSAT position and known RA, DEC has been
demonstrated and EXOSAT source positions can now generally be quoted
to an accuracy of about 5 or 6 arcseconds.
Some observers have reported anomalies in the power spectra of data
obtained with the GSPC experiment and tic OBC in DIRECT mode, specifically
spikes in the spectra at the software cycle frequency and at associated
harmonics. This is believed to result from use of an SOS set-up which samples
the GSPC E channel at 2K s-1 and misses two sample slots/SWC (ref. Express
No. 5 p.38) separated by 0.5 x SWC. In spectral analysis (arguably the
GSPC's prime function), this minor loss of samples can be ignored whereas
for timing analysis when data is summed over 100.000's of SWC's, corrections
must be incorporated arbitrarily into the transform. All SDS configurations
have been modified to avoid this problem and PI's who wish to use the GSPC
DIRECT mode for timing studies should contact the Observatory.
Minor loss of observing time has occurred on a number of occasions for
a variety of reasons, viz: power failure at VILSPA (day 193,~1 hr), solar
activity (day 198, LE1 only operated for ~18 hrs), antenna problems (day
213/214, ~2 hrs) and real time graphics computer hardware faults (day 223/224,
LE1 not operated because no real time monitor of the detector gain was
possible).
3. On-board Software
Attention is drawn to the note on p.76 giving details of the use of
MHER7, which processes the ME experiment data and provides high time resolution
(sub-millisecond) intensity samples with an option of limited energy resolution.
Specification, development and initial testing of MHTR4, a further high
time resolution ME application program, has been completed. Operational
use of MHTR4 will start as soon as realistic tests have been carried out
on a bright X-ray source. A detailed specification and operational procedures
will be given in the next issue of the Express, but in the meantime PI's
with observations for which this mode may be suitable (eg. bright sources,
for which extremely high time resolution is required or when the use of
MHER7 is CPU-limited) should contact A. Parmar at the Observatory. Basically,
MHTR4 processes ME energy data and sets a single bit to '1' or 'O' per
sample corresponding to the presence or absence of a photon within a defined
selectable energy range. In its expected normal mode of operation, it will
provide 0.25 msec time resolution data and use 58% of the available telemetry.
4. Future Plans
Given the current estimates of the remaining attitude control gas and
the natural decay of the EXOSAT orbit in April 1986, the strategy for raising
the orbit perigee height to extend the lifetime beyond its natural termination
will consist of the following:
- review of fuel situation at beginning of 1986.
- Delta V manoeuvre of 10 ms-1 (maximum possible) on 14.1.86.
- Delta V manoeuvre of 10 ms-1 on 14.2.86.
- review of fuel situation in March 1986 and scheduling of further Delta
V's.
These two 'Delta V's' in January and February will extend the mission
lifetime by about 6 weeks to the end of May 1986 and will provide realistic
figures of attitude gas usage for stabilisation during the manoeuvre, an
important input to the planning of any further orbit changes.
Now that regular gauging exercises are carried out to estimate attitude
gas usage, it should prove possible by recognising changes in thermal
inertial of the propane at the liquid/gas transition to identify with some
confidence a point in time when approximately 1 kg of gas remains, sufficient
for about 4 months of operation. At this stage a programme of observations
with the LE1 grating will be assessed and attempts made to position the
grating correctly in the FOV of the X-ray beam in order to devote the final
few orbits of the mission to important grating observations.
One final major update of the FOT Handbook is planned and should be
complete by the end of the year. It will consist mainly of detailed specifications
of new OBC modes (e.g. MHER7, MHTR4 etc.) and descriptions of the recent
calibrations and CCF modifications -most of this information has already
been published in various issues of the Express. Users who received update
no. 2 or collec- ted it at the Observatory will automatically receive the
relevant pages of update no. 3.
Volume 13: OBSERVATORY STATUS AS OF 31.10.85
AO-3 is approximately 50% complete.
1. Hardware
There have been no changes in the status of the spacecraft or payload
hardware.
Following the X-gyro malfunction on day 366 (1984) and subsequent attempts
at re-activation (ref. Express No.9 p.2),
the Contractor (Ferranti) has
analysed all relevant data from the failure itself and further tests, specifically
with regard to re-use of the X- gyro should one of the Y, Z or skew gyros
fail. The main conclusion of this work is that it would be necessary to
switch off the X-gyro some 40 minutes after power-on because the temperature
limit of 85°C for safe operation would be reached. Effectively, the X-gyro
cannot therefore be used and must be considered as failed. Work has progressed
on the definition of on-board software and operating procedures for a 'two
gyro failure' mode (ref. p.45) and will be complete shortly.
Analysis of the ME Detector C malfunction on day 232 (ref Express No.12 p.2) suggests a component
failure in the HT convertor low voltage supply
circuitry common to both Argon and Xenon detectors such that the monitored
HT has an arbitrary value and the actual HT delivered to the detector anodes
is considerably lower, consistent with the observed factor of 5 reduction
in gas gain. It is not anticipated that Detector C will be operated during
the remainder of the mission. Although this malfunction bears some resemblance
to the behaviour exhibited by CMA2 (ref. Express No.2 p.4) in terms of
a severely reduced gain operation and rather arbitrary HT monitor values,
it is believed unlikely that the two problems are related particularly
as CMA2 was operated satisfactorily for a period of about 30 hours some
months after the initial 'failure'
2. Performance and Operations
A general increase in the > Emin, > Emax, Guard count rates of
all ME detectors and in the GSPC count rates (PM, SCA(E) and QEP) has been
observed over the previous 12 months. This is, believed to reflect an increase
in solar activity throughout the approach to solar maximum. It has not
led to any measurable degradation in instrument performance.
A number of users have reported the presence of 'spikes' in the normal
(background) energy spectra of individual Xenon detectors These 'spikes'
occur in channel 137 (at nominal gain), differ in intensity in each detector
eg. ~1 ct/min for detector E, less than or equal to 0.01 ct/min for detector
F and show a strong intensity and weak amplitude dependence with electronic
gain. Background-subtracted spectra are, of course, clean. Electronic tests
show no evidence of counts in this channel and, since internal detector
discharge would be unlikely to give a constant amplitude signal, the explanation
is probably in terms of pick-up on the signal lines from a source of system
level interference.
A loss of approximately 10 hours of observation time occurred on day
295 (H2215-086) when a klystron was replaced in the uplink power amplifier
of the antenna at VILSPA.
3. On-board Software
Attention is drawn to the article on p.34
describing the new OBC modes MHTR4 and MHTR5, and the layout of the
data on the FOT (including MHER7).
Printed on p.37 is a copy of an article already mailed to all P.I's, summarising
the optimum OBC configurations recommended by the Observatory for observations
of specific categories of X-ray sources.
Users are reminded of the discussion (Express No.11, p.71) of potential
errors in packet reference times (PRT) when high time resolution programs
are executed in high-number slots (5 and 6) Such programs are now executed
in the low--number slots to avoid this problem. Three further 'timing anomalies'
exist and are summarized below:
- A second error can occur in the PRT's for HER6 and any other mode which
calculates a time resolution of the PRT greater than 1 SWC, such as MDIR2,
MPULS, MPULS2. This error, which occurs randomly and rarely, was first
detected during the design of MHER7 and was corrected for MHER6 packets
on day 281 (1985). Prior to this, the least significant bit of the most
significant PRT word is very occasionally not updated when the least significant
word re-cycles. The error is detectable and the correct timing word can
be determined fairly obviously, since the interval between packet reference
times should be constant. For the MDIR2, MPULS and MPULS2 programs, the
error potentially still exists and modifications to the code will be incorporated
shortly (details in the next Express).
- Initial data packets from MHER7 and MHER6, that is the first and second
packets produced after start-up, can contain incorrect data. The error
is recognised by non-statistical excess counts in particular bytes which
are clearly not indicative of source variability. Although the effect is
rare, users are recommended to ignore the first two packets produced by
MHER6 or MHER7 if inclusion in the analysis would give odd results. The
problem, which is under investigation, is believed to be caused by an error
in synchronization between clearing of output buffer areas and program
initiation.
- Recently, a telemetry load restriction of less than or equal to 85%
has been applied to specific combinations of OBC modes to avoid continuous
telemetry overload alarms. These are thought to result from 'beating' between
the output frequencies of programs producing at specific instants in time
large packets of data eg. MHER4 is conjunction with many other programs.
Again, the problem is under investigation and seems to have appeared only
within the last few months because of increased use of higher time resolutions
for all programs. Note that the normal telemetry usage figure has been
about 99% and considerable effort will be made to 'retrieve' the missing
margin.
4. Observation Output
Following the failure of ME detector C, a software modification to the
ME automatic analysis has been implemented to account for 3 detectors only
in experiment half 1.
Some delay has therefore occurred in production and despatch of the
ME automatic analysis output for the period 232 (1985) to 254 (1985).
5. Future Plans
Now that EXOSAT is probably within its last year of operations, serious
thought is being given to a post-operations exploitation phase in terms
of continuing provision of the Interactive Analysis System for guest investigators
and maintaining a science team for support of the IA system and to act
as a focus for EXOSAT data analysis and research. Further details will
be given in a future issue of the Express.
Volume 14: OBSERVATORY STATUS AS OF 31.12.85
Now that the strategy for extending the mission lifetime has been fixed,
the March/April schedule will be determined on the assumption that 10 ms-1
thrusts take place as planned on 1/3 and 8/3. Should this strategy alter
for any reason, some re-arrangement of the time line at short notice can
be expected.
1. Hardware
There have been no changes in the status of the spacecraft or payload
hardware.
Tests on all malfunctioning elements of the payload with the exception
of the grating mechanism of telescope no.1 have been carried out recently.
CMA2 and detector C of the ME experiment continue to show normal counter
operation but at a much reduced gain. Both PSD's still exhibit the 'failure
modes' as described previously, namely rapid increase in 'LEP' counts on
the PSD1 grid and a very high count rate on the PSD2 guard.
2. Performance and Operations
All slewing is now carried out at the lowest possible slew rate of 43°/hr.
Double limit cycling about the X-axis and therefore excessive fuel usage
has occurred frequently during 85°/hr and rarely during 43°/hr slews. Such
cycling uses an additional amount of propane equivalent to about 20 hours
of operation in order to save 2 to 3 hours of manoeuvre time and, although
the duration of certain manoeuvres will be of the order of 5 to 6 hours,
the saving at this stage of the mission is considered justified.
Readers are reminded of the note in Express
No.9 p.6 concerning measures
implemented to conserve fuel including the use of the Fine Sun Sensor (FSS)
and 1 star for satellite attitude control and reconstitution from January
1985. Since June 1985, when procedural and software modifications were introduced
to allow reconstitution from 2 stars while controlling via 1 star and the
FSS, it has become apparent that uncertainties in the FSS calibration can
lead to a reduction of pointing accuracy, in the worst cases of the order
of several arc seconds for long observations. Further calibration of
the FSS is planned and corrected attitude data will be published as appropriate,
however in the meantime PI's primarily interested in position determination
should contact the Observatory for the most up-to-date correction.
Minor loss of observation time because of solar activity has occurred
in the period 1/11/85 - 31/12/85.
3. On-board Software
Reference is made to the description in Express No.13 (p.3) of the packet
timing or data anomalies in certain OBC modes or combinations of modes.
Further investigation has shown that the first and second packet data
content problem (point (2) of the referenced article) was caused by an
error in the ground software protocol, present since launch and corrected
on day 324 (85). Mote that the first and second data packets produced by
any program executed prior to this date could in principle contain invalid
data and users are therefore recommended to exclude these packets from
the analysis.
Use of the standard OBC mode configuration defined in Express No.13
p.38 has partially eliminated the occurrence of telemetry overload
alarms at 85% load, (Express No.13 p.4)
although the problem still exists and is under investigation.
A potential system level error in the treatment of overspill counts
for certain OBC modes, namely GHEBL2, GHEBL4 and MHER2 has recently been
noted. Users are reminded that 'overspill' counting should be extremely
rare, given the 8 bit counting (less than or equal to 255 counts), spectral
shape, line strengths and time resolutions involved and to our knowledge
has occurred only during an observation of Sco X-1 in the GHEBL4 packets.
Nevertheless, a design specification error for certain situations in the
definition of which channel has counted over 255 counts, as described in
the OBC software documentation, resulted in the FOT production software
combining the MSB of channel n with the LSB of channel n-1. This error
has been corrected on day 350 (1985). Prior to this date, 'overspill' counting
can be recognised by counts in the dedicated 'overspill' channels and data
in any such packets should not be used in the analysis.
4. Future Plans
During the EXOSAT hardware development phase and the initial design
for the Observatory system, a principle decision was made on cost grounds
to re-use the Hewlett Packard 1000 series computer systems, already procured
for instrument calibration and check-out facilities, at ESOC for EXOSAT
data analysis and support to the operational programme. These systems,
adequately upgraded during the operation phase, have supported throughout
the first years of the mission many aspects of mission planning, production
of the automatic analysis, calibration and instrument performance assessment,
development and provision of the interactive data analysis system and the
research activities of the team. It has been obvious for some time that
tt0e execution of these tasks approaches the CPU limitation of the HP-1000
systems, which are suited more to instrument control and data analysis
than to scientific data analysis in its full sense. Also, their 16-bit
word length and consequent 32K byte address space impose a severe constraint
on software development and prevent the transport of software from more
modern machines with 32-bit word length.
The discovery of quasi-periodic oscillations from a number of binary
X-ray sources and the general importance of X-ray variability study and
timing analysis have increased substantially the demand for CPU power.
The Observatory is therefore procuring a Dec Microvax Q7 system, comprising
71 MB disc, 2 MB memory, 1600 bpi tape deck and 8-port I/O interface together
with an additional 71 MB disc, two terminals and laser printer. Timing
analysis and spectral fitting software will be implemented on the Microvax
and will be available to users of the IA system by the early summer. Note
that the Microvax will be connected directly to the existing HP systems
in order to facilitate file transfer.
The AO-4 observation programme is expected to include a number of long
observations which in certain cases and normally in agreement with the
PI may be terminated before the approved time has been used. To maximise
the scientific return from EXOSAT in its final operational phase by making
use of any observing time made available in this way, the Observatory proposes
to identify from the approved AO-4 programme a number of observations which,with
the prior approval of the PI, can be scheduled at very short notice. Clearly
the OBC and P/L configuration must be determined in advance, but this constraint
is felt justified to optimise the programme. Once the AO-4 programme has
been selected, Pi's can contact the the mission planning group to discuss
the suitability of scheduling their observation in this way.
A paper has been prepared for the Feb. 1986 Science Programme Committee
(SPC) meeting outlining the Agency's plans for support to the EXOSAT user
community for the next several years. The plans will be discussed with
the scientific advisory bodies, the Astrophysics Working Group (AWG) and
the Space Science Advisory Committee (SSAC) in January 1986.
Volume 15: OBSERVATORY STATUS AS OF 28.2.86
In the light of the spacecraft hardware anomaly described below the
strategy for orbit perigee raising to extend the mission lifetime is under
review particularly with respect to the capability of the RCE1 system to
maintain stability during the thrust and possible use of the redundant
RCE2 system, despite its known 'thruster-on' failures (ref. Express No.4, p.34 and Express No.9 p.3). Note that complete failure
of the RCE1 plenum valve would require immediate action to ensure the
safety of the satellite and procedures have been defined for this
event. Furthermore, additional software and operational procedures are
under definition and development to support mission operations with
RCE2, although doubts remain about the feasibility.
Because of the above uncertainty over mission duration, and capability
of extension, and the importance of carrying out priority AO-4 observations,
the observation programme for March and April 1986 has been completely
revised in order to maximise the science output during the remaining phase
of the mission. Outstanding AO-3, AO-2 and AO-1 observations (ref. pp.
7/8 ) together with the remainder of the A0-4 programme will be scheduled
as and when the mission profile permits. Inevitably, and regrettably, a
number of approved observations may not be carried out.
1. Hardware
Readers are reminded (Express No.9 p.4)
that following the X-gyro malfunction
in January 1985 an OBC program was implemented to monitor the status of
a number of sun presence indicators and the rate of pressure decrease in
the plenum chamber of the RCE in order to avoid spurious triggering of
safety mode by the unstable Y-gyro health status monitor. Note that the
RCE consists of two redundant parts, RCE1 and RCE2, served by a common
propane tank (for mass distribution reasons in fact two tanks permanently
coupled) with separate plenum chambers and valves. On day 44 (1986), the
plenum pressure in RCE1 failed for the first time to reach the demand value
of 0.72 Bar and subsequent tests indicated a severely reduced gas flow
rate through the valve and a delay between commanding the valve open and
onset of gas flow. From the available data it has not so far proved possible
to distinguish between a continuing or a 'step-function' degradation in
performance of the valve. Operations can, however, still be carried out
normally and measures have been implemented to reduce the frequency of
valve commanding and hence to minimise further degradation.
2. Performance and Operation
Improved star tracker calibration data have been in use since 30.7.85
(ref. Express No.12 p.2). All previous
pointings from launch to 30.7.85
have been re-analysed Wi ti the new calibration data to give a set of corrected
pointings for each observation. These will be included in a future re-analysis
of all EXOSAT data, but in the meantime observers primarily interested
in positional accuracy should contact the Observatory. Note that in the
majority of cases, the difference in pointing is not significant.
Uncertainties in the fine sun sensor (FSS) calibration - see Express No.14 p.2 - are under investigation
and will be corrected systematically
in the re-analysis exercise. Where positional accuracy is the prime interest
for an observation with the FSS used for control, P.I's should again contact
the Observatory for the most accurate pointing data.
There has been only a minor impact on operations from the malfunction
of the RCE1 plenum valve (Section 1) and no impact on performance during
stable pointing and manoeuvres, although the options for orbit modification
are severely affected.
Solar activity has caused minor loss of observation time on several
occasions, however, a huge solar 'proton' event on 6/2 (0918 Z) prevented
scientific operations between 6/2 and 9/2 (days 37-40). P.I's with observations
immediately following this period are advised that a reduction in average
background count rates of about 20-25% was observed.
3. Observation Output
The Observation Log (ref. Express No.9 p.44) has recently been updated
to include all observations performed to the end of 1985 and printed copies
of the log with chronological or RA-ordered listings of the observations
can be obtained on request from the Observatory. Software to access the
log is under development with the intention of providing a remote-access
'browsing' facility. Note that an earlier copy of the log, complete up
to January 1985, is available for distribution (quoting reference V/49/)
from the Centre de Donnecs Stellaires at Strasbourg.
Now that the EXOSAT Bibliography has become rather extensive future
issues of the Express will list only additions during the previous period
and the complete bibliography will be established on a computer file with
access provided as for the observation log.
Attention is drawn to the archive release list on p.46 Since the establishment
of the procedure for requesting archive data some 200 tapes have been authorised
for release.
4. Future Plans
For the remainder of the operational phase, the EXOSAT Express will
continue to be published bi-monthly. In the post-operational phase (ref.
p. 37), the frequency of publication will
reduce gradually from 4 issues/year to 2 issues/year.
As the end of the mission approaches consideration is being given to
the re-analysis, after an operational 'wind-down' period, of all EXOSAT
data as a pre-requisite to the establishment of the results data base and
final catalogues. Details will be given in a future issue of the Express.
OBSERVATORY STATUS AS OF 31.5.86
1. Spacecraft Status
The valve leading to the plenum chamber of the RCE1 system was found
on 1983 February 13th to be degrading such that the gas flow was reduced
to 7% of the nominal (see Express No.15
p.2). The operation of this valve
continued to deteriorate until by March 18th the flow was reduced to 2%
of nominal. Later that day the valve failed to operate sufficiently to
pressurize the plenum chamber to the level required to maintain stable
pointing. The spacecraft was commanded into autonomous safety mode with
the Y-axis pointed at the sun. The use of the redundant RCE2 system as
the only other means for maneouvre, attitude control and orbit changes
involved the risk of a thruster on condition which had in the past been
experienced five times. This fault always occurred with the +Y thruster
resulting in a rapid spin up of the spacecraft.
The RCE2 thruster on anomaly was believed to be related to temperature
changes causing small variations in either the leakage current, forward
bias or gain of the preamplifier transistor of the RCE drive. Since the
main source of heat is the electronics itself, a possible solution was
suggested that involved an RCE2 on-off cycle of approx. 1 to 80, thus maintaining
a lower average temperature. An on-board computer (OBC) program was developed
to perform the RCE2 on/off duty cycle and the necessary patches to the
AOCS made to avoid problems when the RCE was off. In this mode the spacecraft
pointing would drift by a few arc minutes during the RCE off phase.
The first attempt to initiate this procedure was made on March 20th.
However, as soon as the inner loop was closed (such that the gyros maintain
a stable pointing configuration) a safety mode was triggered and, at the
same time, the Y-thruster-on problem recurred. The observed spin rate increased
to 1500 deg/hour with tumbling around all axes. Telemetry was lost probably
because the transmitter (powered from the sun bus) went off. After more
than two hours during which telecommands to re-acquire the sun were sent
blind, telemetry was re-acquired. The spacecraft was again left in autonomous
safety mode.
The OBC program was modified to monitor the RCE for a thruster-on condition
and to turn off the RCE if such a condition occurred it was thought that
the safety mode trigger and thruster-on may have been the result of the
non-zero spin rate when the inner loop was closed. On March 27th the spin
of the spacecraft was slowed to effectively zero by changing the orientation
of the solar panel (solar sailing) and inner loop closure was reattempted.
This time stable pointing was achieved.
The various patches made to the AOCS meant that normal slewing was no
longer possible. Slews were achieved by changing the bias on the gyros
so that the pointing gradually drifted across the sky at a rate of 10°/hour.
The observation program was restarted on 27th March. The first observations
indicated that the pointing could be maintained to within 5 arc minutes.
It was no longer possible to close the outer loop between the star tracker
and the AOCS. However the star tracker readout could still be used by the
spacecraft controller to correct for any drift in the gyros. The propane
usage was increased during this interval to 15 gm/day (against 8 gm/day
previously) although this was eventually reduced to 11 gm/day by making
further adjustments to the AOCS.
Operations proceeded (relatively) normally until on April 9th at 22.40
UT, shortly after a perigee, an AOCS anomaly occurred where the gyro offset
became so large that the counter measuring the offset overflowed. While
RCE on-off commands were being issued from the ground to try to counter
this problem, unknown to the spacecraft controller a safety mode had been
triggered by the AOCS. This resulted in the spacecraft attempting to acquire
the sun. However, an RCE off command from the ground interrupted this process
and resulted in the spacecraft rotating past the sun. On the next revolution
the RCE was commanded on again, but the +Y thruster-on problem occurred
causing the spacecraft to tumble out of control. Telemetry was lost at
22.58 UT and not recovered again.
Repeated commanding to reacquire the sun over the following hours and
days did not have any effect. The batteries only had sufficient charge
for two and a quarter hours. After they were exhausted it would have required
a chance alignment of the solar panels for there to be any hope of a successful
command. Also it was very probable that the propane was exhausted during
the final spin up. EXOSAT re-entered the atmosphere at 00.48 UT on May
6th above the Pacific Ocean south of New Zealand at longitude 171.4°E latitude
56.0° S. The figure on p.5 shows the ground-track for the two final revolutions.
It is worth thanking the personnel of the operations division for their
support at all hours of the day and night. In particular Dietmar Heger
and Peter Prior played a crucial role.
During the final two weeks when the pointing was maintained to only
a few arc minutes precision it is still possible to reconstruct the attitude
using the star tracker (see FOT Handbook Sect.3.4 p.4/17). During the intervals
when the tracker was off, it may be possible to reconstruct the attitude
using the gyro readouts; this is currently under investigation.
2. The Future
It is planned that the ground segment of the Observatory will be relocated
within the Astrophysics Division of the Space Science Department at ESTEC
towards the end of this year, beginning of next year. The plans for the
move have not yet been finalised but it is hoped that the interactive support
and archive tape production will continue uninterrupted.
It is worth emphasising that while the operational phase is now over,
the task of analysing the data acquired has only just begun in earnest.
Almost two-thirds of the total data archive is now in the public domain.
Astronomers from all over the world are free to exploit this resource over
the years to come.
- Nick White
EXOSAT EXPRESS VOLUME 17
FOREWORD
This is the first issue of the Express in EXOSAT's post-operational
phase and the last to come from ESOC before the Observatory moves to the
Space Science Department at ESTEC in two stages around the end of the year.
Details are given overleaf.
No serious interruptions are foreseen to the facilities offered at the
Observatory. In this issue your attention is drawn to the new procedure
for requesting data from the EXOSAT archive and to work concerning the
relative alignment of the LE1 and star tracker coordinate systems, that
is particularly important for the determination of accurate source positions.
Henceforth the EXPRESS, which will come out less often than hitherto,
will be mainly concerned with outstanding calibration items and data analysis
topics. If there is anything you would like to see discussed in these pages
please contact the Editor. Also, please keep us supplied with preprints
and reprints of all EXOSAT related work so our complete records can be
maintained.
EXOSAT EXPRESS
Editor: Andy Pollock
Published by: EXOSAT Observatory Tel: 06151-886-716
ESOC Telex: 419453/419441
Robert Bosch Str.5 Telefax: 886/662/611
61 Darmstadt
W. Germany.
(UNTIL THE END OF THE YEAR).
FOREWORD
It is now one year since the previous Express appeared and in the intervening
time the observatory team has been relocated at ESTEC. This relocation
was achieved with minimal disruption such that the dispatch of data tapes
and the use of the interactive analysis facility has continued without
problems. The major activity within the observatory team is the establishment
of an EXOSAT database that can be accessed remotely via networks. The design
and implementation of this database is described in detail here and we
welcome any comments from the community. As part of this program the observatory
micro-vax has now been put on the Space Physics Analysis Network, SPAN.
A description of the SPAN network is given, as well as instructions on
how access to the observatory micro-vax.
The number of requests for data from the EXOSAT archive continue to
be made at the rate of ~5 per month, with each one typically asking for
10 observations. It is worth noting that the one year proprietary rights
of the principal investigators has now passed for the entire mission and
essentially all EXOSAT observations are now available in the archive. The
procedure for making archive requests is repeated in this issue.
This issue contains several articles on final updates to the ME and
LEIT calibrations. A new CCF will be issued to all the major EXOSAT analysis
centers in early October. In general these revisions to the calibrations
represent minor changes and should not affect results that have already
been published.
A description of the EXOSAT experience in participating in multiwavelength
campaigns is given, plus some recommendations on how such campaigns might
be better organized in the future.
The EXOSAT Express will now be issued approximately once per year. It
is expected that the next issue will appear towards the end of 1988 and
will contain further details of how to access the database, as well as
addressing any other calibration problems that may have come to light in
the intervening period.
THE EXOSAT EXPRESS
Editor: N.E. White
Published by: EXOSAT Observatory/SAE
Astrophysics Division of the Space Science Department of ESA
ESTEC
2200 AG Noordwijk
The Netherlands
Tel: (31)1719 84446
Telex: 30098
Fax no: (31)1719 17400
SPAN: REQUEST::EXOSAT
Bitnet: MAILEX@HNOES1O
A GUIDE TO THE LE CCF
1. Introduction
Some information is presented here to aid EXOSAT observers in the use
of the Low Energy Telescope Current Calibration File to process scientific
data. For the layout of the CCF readers are referred to the EXOSAT Observers
Guide Part III (the Final Observation Tape Handbook, referred to below
as "the FOTH"), section 3.7.1, Rev.1.
This document gives an overview of the contents and omissions of the
current CCF, what is meant by "current data", whether the data
is preliminary or final and includes a few suggestions on how to use the
data.
It is intended to print update notes (when changes to the CCF content
are made) and specific documents concerning:
- the CMA effective area (in preparation) the CMA point spread function
- the CMA sum signal and its use
- the PSD effective area
- the PSD point spread function
- the PSD energy resolution
- the grating
2. The "current" calibration data
This document refers to the CCF contained on the latest FOT's produced
at the time of writing.
The overall SHF key is the time from which the data is valid and refers
to the beginning of the mission (early experiment switch on).
It should be stressed that ALL the data in the CCF is based on GROUND
CALIBRATION measurements. The times indicated for the single data types
refer either to the beginning of the mission, or to the time when the data
was put into the file. They are not relevant and can be ignored.
All the data in the CCF is VALID FROM THE BEGINNING OF THE MISSION.
However not all of it is yet in a final form (future calibration updates
- see FOTH Sect. 5 - will still refer to the beginning of the mission,
i.e. replace the current values).
Changes in the CCF are described below to enable observers to relate
the CCF contents of a FOT to a particular stage of the calibration analysis.
Not all the changes have been recorded in the Calibration History, (FOTH
Section 5) since in the early period of the mission the Status of some
ground calibration data was still highly preliminary.
The starting point for the Calibration History is dated day 277 1983.
This date appears as the "last update of file" as well as the
"last update of cal. history". The overall SHF key is set to
day 1 1980. Some early AO-1 FOTs may contain this CCF (indicated below
as CCF O).
The change-between CCF 0 and CCF 1 concerns data type B1 (optics effective
area). In CCF 0 this contained two errors, i.e. one point in the positional
grids giving the effective area as a function of position was incorrectly
set to -1 (both CMA and PSD grids). This was point no. 39 of the grids
(respectively bytes 332-3 for the CMA and 460-1 for the PSD. The correct
values are 806 (CMA) and 695 (PSD). This applies to both LE1 and LE2.
A further change has been made to the calibration history and transmitted
to the FOT Production Team. CCF 2 is being written onto the more recent
FOTs.
CCF 2 applies to LE2 only (although few FOTs will be actually produced
since the experiment is presently inoperative): the "last update"
time will be day 340 (the overall SHF key is unchanged) and the data types
involved are El and E4, which have been replaced by the final data.
Any CCF with a "last update" time earlier than day 277 1983
should be regarded as highly preliminary. This will mainly apply to Performance
Verification FOTs. The following stages in CCF updating have occurred.
CCF-2 is the pre-launch CCF and should NOT be used at all. All the content
is either preliminary or not defined. Most data types are also NOT consistent
with the present FOTH specifications. This CCF should be disregarded.
CCF-1 is the first post-launch CCF supplied to FOT Production. The
following data types are different from CCF O: A4, B1 (the "missing
point" described above), B2 (no information on the Aluminium/Parylene
filter support grid), B4 to B6 and B8 to BA (different thicknesses for
the filters), BD (preliminary misalignments), D1.
-angle and integrated on-time of the Al
28V power line and the implications for the planning of long-duration
observations.