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.
Safety mode was, however, triggered on four separate occasions on days, 12, 19, 20 and 30 by the Y-gyro health monitor status change during the program design implementation and testing period. A few seconds after the safety mode trigger on day 28, a 'thruster-on' condition occurred (in autonomous mode) resulting in the following sequence of events:
Estimates suggest that approximately 10% of the remaining propane was consumed during this one event ('thruster on' condition in safety mode).
On day 30, safety mode triggering occurred during LOS and at AOS the spacecraft was in autonomous mode with AOCS control switching rapidly between RCE1 and RCE2. It is believed that the sequence of events was as follows:
It is estimated that between 400 and 700 grains of propane were consumed by this event - note that a 'normal' safety mode trigger uses between 20 and 80 grams of propane.
A further safety mode trigger on day 21 was caused by an erroneous conclusion by the automatic command program that one slew had completed (use of bad quality data despite a quality check), and initiation of a second slew whilst the first was still in progress with subsequent incorrect attitude of the spacecraft. Additional quality checking of the telemetry data has been incorporated.
On day 40, the gyro box temperature increased from 67° to 74° over a period of approximately 5 hours. Subsequently, oscillatory behaviour of both temperature and X-gyro motor current was observed, culminating in a maximum temperature of 76.6°C and 'full scale' current (255 mA = 255 counts). The X-gyro was switched off on day 42 and since then the temperature has varied between 62°C and 64°C. Note that the number of Y-gyro health monitor status changes was one in the period 12/2 to 28/2.
There have been no changes in the status of the payload hardware CMA2 has been operated continuously for each observation since 15/2 but shows no evidence of nominal behaviour. Additional calibration data from the CRAB observation (day 36) has led to a refined adjustment procedure of the ME detector preamplifier gains to compensate for gas leakage.
2. Performance and Operations
Tables 1 and 2 on p. 7/8 give the current performance parameters of the EXOSAT instruments.
During the period 1.1.85 - 28.2.85 loss of observation time has resulted from the X-gyro malfunction (14 hrs, day 1), solar activity (4 hrs, day 4), and various safety mode triggers (days 13,19,21,28,31; 9,8,9,20,8 hrs)
Note that the Skew(S) gyro is now used instead of the X-gyro for spacecraft pointing control.
On day 15 a 'natural' occultation of the X-ray source 4U1530-44 was carried out, mainly to verify the required ground system software and operational procedures. All aspects of the system performed satisfactorily and a strategy for use of the hydrazine orbit modification facility for mission lifetime extension or occultation observations can be developed (ref. under 'Future Plans').
In order to avoid spurious safety mode triggering and the inherent danger of excessive propane loss (eg. event on day 28) when the Y-gyro health status switches from 'good' to 'fail' (X-gyro health status is always 'fail'), safety monitoring tasks have been implemented in software. The hardware safety-monitoring circuitry is disabled and an OBC program monitors continuously the status of various sun presence indicators and the rate of decrease of pressure in the plenum chambers of the RCE. If a genuine safety condition is indicated (sun in forbidden zone thruster stuck open) the program re-enables the hardware function, which executes autonomous mode in the normal manner. The spurious 'two gyro fail' situation is therefore ignored.
A risk with this operational procedure is an OBC software halt/corruption during LOS followed by a genuine safety mode condition which would not then be recognized. Measures have been taken to reduce the likelihood of spurious OBC malfunction, and the possibility of re-enabling the hardware function, should a genuine OBC halt occur (CPU overload error), is being assessed. In any case the probability of a genuine safety mode condition during LOS is low (none since the launch); safety conditions tend to occur when operations on the satellite are in progress. Together with the low probability of a CPU halt, this risk of non-recognition of a safety mode condition during LOS is considered justifiably small, given the current hardware status.
Attention is drawn to the article (p.40) which discusses the dependence of CMA1 temperature on -angle and integrated on-time of the Al 28V power line and the implications for the planning of long-duration observations.
3. Observation Output
Both FOT production and automatic analysis of the data are completed at the Observatory within about 20 days of the observation, with the exception of a small backlog in the analysis of ME experiment data. FOT's are typically available at ESOC some 10 days after the observation. Automatic analysis is carried out within 20 days of the observation for LE1 and GS experiments and currently, for the ME, with an additional delay of approximately 80 days, although it is expected that this will soon be eliminated. Note that all auto output is checked for scientific integrity which, together with sorting, labelling etc. is a further overhead on the time between observation and despatch of the output to the P.I.
A 'problem' has occurred with the use of the ME background monitoring program MHTR3. Data generated in this mode between day 352 (1984) and day 50 (1985) is affected whenever MHTR3 was used with a sample rate of the ME QE counter of 32 Hz (all other sample rates generate the correct data). Samples 1,3 etc. of the packet (ID = 1072) contain the correct data giving the count rate integrated over 1/32s, but samples 2,4 etc. should be ignored (usually zero, occasionally a random value). MHTR3 has been modified to eliminate this 'error' and all samples in packet 1072 can be regarded as correct after day 50 (1985). Prior to day 352 (1984), sampling at 32Hz was not available as an option.
Standard LE1 CMA background images for the four filters LEXAN 3000 Angstroms, LEXAN 4000 Angstroms, Aluminium/Parylene. and Boron have been generated and are available from the Observatory on request. For the LEXAN 3000 Angstrom and Al/P filters, data from single long observations has been accumulated whilst for the Boron and LEXAN 4000 Angstrom filter a number of observations have been combined. Full FOV images, central 1 degree-square images and maximum resolution central images for all filters are available. CRAB calibration data (day 36) is also available on request.
4. Future Plans
EXOSAT's natural lifetime will terminate during April 1986 if the active on-board orbit control system (hydrazine) is not used to increase the perigee height prior to entry into the dense atmosphere. A 'delta-V' capability of 170 m s-1 can provide a maximum mission extension of slightly in excess of 12 months. However the trade-off between remaining propane, use of extra propane during the orbit modification and/or anomalous situa- tions, and the uncertain hardware situation as the mission progresses (particularly the gyro state-of-health) is difficult to assess. Attention is drawn to the article on p.48 which discusses the current resources and concludes that on balance, extension of the mission should not be undertaken immediately but delayed until early 1986.
Procedures for fuel saving have been implemented - most observations are now carried out with satellite roll information deter- mined from the fine sun sensor and 1 star, and using the OBC program Sun Motion Correction. Manoeuvre are planned to be executed at 42°/hr with the exception of some long Y-slews Naturally slew durations are longer but this is matched by increased observation times for AO-3 as a rule and observational efficiency should not decrease.
Work is in progress on the definition of operational procedures and additional software requirements for the situation of two gyros failed, the so-called 'back-up' mode. Two options are available for manoeuvres, namely 'star hopping' whereby a star is 'moved' successively from one edge of the star tracker FOV to the other (less than or equal to 4 degrees), and 'direct thruster demand' whereby specific impulses are given to the appropriate thruster without the normal closed-loop control of the manoeuvre by the gyros.
A significant increase has been observed recently in the Y-gyro spin motor current instability and hence the frequency of status changes from 'GOOD' to 'FAIL' of the health monitor, for example more than 250 occurrences within a period of 2 days, starting on day 083. Initial investigations by the contractor (Ferranti) suggest that the spin motor current variations are probably caused by occasional minor ball bearing cage instability and that this should not give cause for any additional concern about expected gyro lifetime. There has been no impact on operations, although naturally an increased probability of a spurious safety mode condition (2 gyros registering 'FAIL') occurring. Note that safety monitoring is now carried out by OBC software (ref. Express No.9, p.4) and that, in the event of an OBC malfunction, operational procedures guard against spurious triggering of safety mode should the Y-gyro health status change.
There have been no changes in the status of the payload hardware.
2. Performance and Operations
Tables 1 and 2 on p.4/5 give the current performance parameters of the EXOSAT instruments.
During the period 1.3.85 to 30.4.85, loss of normal observation time occurred on days 114 to 116 because of solar activity Throughout the entire period it proved possible to replace the observing program with certain 'LE only' observations.
A second propane gauging operation (ref. Express No.9, p.51) was carried out in orbit 181 during the long observation of 4U1700-37 (constant angle). Analysis of the data by Marconi Space Systems (MSS) indicates a continuing discrepancy with respect to the propane logging results both in total remaining propane (3.5 kg/5.1 kg as on 7.4.85) and usage during the period 9.2.85 to 7.4.85 (900 gm/420 gm). Further work is in progress to resolve the problem as a matter of urgency, given that the worst case figures would imply a mission lifetime termination at the end of 1985.
EXOSAT's fourth eclipse season, covering a period of 12 orbits from 16.3.85 to 23.4.85 with eclipses varying in duration from a few minutes to approximately 1 hour, has just ended.
A number of significant outstanding actions from the EXOSAT data analysis workshop (ref. Express No.8, p.23) have recently been completed and are referenced here:
3. Observation Output
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.
4. Future Plans
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.
There have been no changes in the status of the spacecraft hardware.
Reference was made in Express No.7 p.2 to an error in the star tracker calibration reference points (Local Lord Points) and the implication for possible (small) systematic errors in determined pointing positions. SODERN have provided new reference data which ESOC and the Observatory Team are currently using to check previous pointings and the positions of known X-ray sources. Details will be given in the next issue of the Express.
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.
2. Performance and Operation
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.
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.
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:
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.
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:
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.
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.
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.
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.
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.
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
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.
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).
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
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:
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.
3. Status of the "current" data
Please note that "final" refers to the final results of the GROUND calibration analysis, except if otherwise stated.
4. Differences between LE1 and LE2
The energy and position grids, the mass absorption coefficients (data types Al-A4) and the optics efficiency (B1-B2) are the same for both experiments. Filter thicknesses (B3-BC) and the misalignments (BD) differ.
CMA data types (with the exception of the efficiency - data type Cl) are identified for both experiments: see explanation in section 5.
PSD data types refer to each experiment.
The only grating information available (see section 3 above) is for LE1.
5. Future Updates
The final values for data types El and E4 (LE2 only) have been put in the calibration history and will be available on the FOT.
The final values for data type Cl (LE2 only) are available and should be put in the calibration history in the near future.
6. Source of the data
The data in the LE CCF has been derived from the analysis of the ground calibration data obtained during four calibration tests at the Long Beam Test Facility of the Max Planck Institut fur Extraterrestrische Physik, Garching. The analysis has been a collaborative effort between the LE hardware institutes - Space Research Laboratory (Utrecht), Cosmic Ray Working Group (Leiden), and Mullard Space Science Laboratory - and the EXOSAT Observatory (LC for the CMA, JD for the PSD).
For the CMA data it should be noted that the detector on LE1 was replaced after the ground calibration. The data in the CCF is therefore a copy of the corresponding LE2 CCF, with the exception of data type Cl. The latter contains the efficiency of the detector previously mounted on LE1, considered to be a "standard" CMA. The efficiency of LE2 as a function of energy is, however, abnormal.
The use of LE2 data for LE1 should have no effect on the PSF/LSF parameters (data types C6-CD), since these are dominated by the optics.
A difference in the sum signal-related data (C2-C5, CE. CF) because of a possible difference in gain between the two detectors, should also be small.
All the CMA calibration data is based on empirical fits to the raw data; no specific modelling is implied.
For the PSD the LE1 and LE2 CCP are specific to each experiment The LE2 PSD has not been operational. The LE1 CCF is valid for PSD observations from launch to day 179. The PSD was operated on the nominal AGC setting of 0.66.
The LE1 PSD calibration data is derived from the ground calibration measurements with the addition of model data to data types D3-DA, where insufficient experimental results were available. The model data takes account of the PSD response/X-ray optics and is validated by comparison with a number of experimental control points. For tiie grating the analysis of the ground calibration data is still in progress at Utrecht.
7. A note on interpolation
Observatory software uses a cubic spline interpolation for all parameter-parameter functions and a simple two-dimensional interpolation for positional dependency. Information on this will be included in the specific documents referred to in Section 1.
8. Linearisation Coefficients
These coefficients are not part of the CCF. The actual numbers and the procedure used for the linearisation are not particularly relevant for the observer. For general information, however, a brief description is provided of the linearisation process and also an indication of the origin and status of the coefficients themselves.
The linearisation corrects for:
The linearisation does NOT correct for (ref. FOTH: Issue 2):
The linearisation coefficients used by the FOT production software were generated using data taken on the ground (Long Beam Tests) with the following exceptions:
All the sets of coefficients (with the exception of the central channel correction for LE2 PSD), are considered final at the time of generation. For LE1 PSD (which is now operated at a reduced gain) an evolution can be expected.
HEASARC Home | Observatories | Archive | Calibration | Software | Tools | Students/Teachers/Public
Last modified: Thursday, 26-Jun-2003 13:48:32 EDT