The first four months of SOHO's life in orbit were devoted to the verification and commissioning of the spacecraft and its experiments, and to the manoeuvres needed to inject it into its final halo orbit around the L1 Lagrangian point. This article reviews the work that took place during this period to ensure that SOHO would be on station and fully operational as quickly as possible.
The scientific mission of the SOHO spacecraft was described in detail in ESA Bulletin 84 (November 1995), while the first scientific results were presented in the August 1996 issue (Bulletin No. 87).
Figure 1. The SOHO Spacecraft
Figure 2. Launch of SOHO on 2 December 1995 from Cape Canaveral aboard an Atlas II-AS vehicle
SOHO was launched from Cape Canaveral in Florida on 2 December 1995 at 03:08 Eastern Standard Time by a NASA-provided Atlas IIAS launcher (the launch window on this particular day was 51 min, closing at 03:25 EST). The spacecraft was separated from the launcher within 0.021 s of the predicted time and the attitude rates and errors were all within specification.
The Centaur upper stage initially achieved a 185 km x 175 km parking orbit. Following an 80 min coast period, the Centaur's second burn injected SOHO into a transfer trajectory towards the L1 Lagrangian point.
SOHO's first mid-course manoeuvre was performed during 3/4 December and its high accuracy allowed the second, which was actually an orbit-shaping manoeuvre, to be postponed until 4 January 1996. It was followed on 14 February by the halo-orbit injection manoeuvre, with a further small trim manoeuvre on 20 March.
SOHO's final operational orbit is a so-called Class II orbit, with an anticlockwise rotation (when looking at the Sun), at a distance of about 1.5 million kilometres from Earth. The period of this halo orbit around the L1 Lagrangian point is approximately 178 days.
Figure 3. The SOHO transfer trajectory and final halo orbit around the first Lagrangian point (L1)
The commissioning activities that took place during SOHO's journey to the halo orbit were part of an exhaustive, predefined scenario laid down in the Launch and Early Orbit, and Integrated Orbit Checkout Plans drawn up by ESA and NASA.
Immediately after launch, the performance of the spacecraft's subsystems was evaluated. Spacecraft commissioning then began and continued throughout the transfer phase to, and after SOHO's injection into, the final halo orbit. A total of 42 different commissioning tests were performed, some of which were repeated several times, to verify the overall health and performances of the various spacecraft subsystems. The commissioning testing also had to be carefully interleaved with the experiment switch-on and instrument-commissioning activities being conducted by the scientific investigators.
The SOHO observatory was then formally transferred to the scientific community on 16 April 1996.
SOHO Commissioning and Early Operations 1. Launch and Early Orbit Phase: 2/21 December 1995 - Service Module: switch-on and deployment activities; commissioning; power budget subsystems evaluation - First mid-course correction manoeuvre - Experiments: GOLF door closure verification, MDI start, CCD bakeout 2. Early Experiment Operations: 21 December 1995/2 January 1996 - Service Module: subsystem evaluation; fuel budget - Experiments: GOLF 2, VIRGO 1, MDI 1 &2(partial), SUMER 1, CDS 1, LASCO-EIT 1, UVCS 1 &2(partial), SWAN 1 &2, CELIAS 1 (partial), CEPAC 1-5 3. Second Mid-course Correction to Maximum Distance: 3/16 January 1996 - Orbital shaping manoeuvres (2nd mid-course correction manoeuvre) - No experiment activity - Service Module: commissioning; subsystem evaluation; RF budget 4. Maximum Distance to Halo-Orbit Injection: 11/14 February 1996 - Halo-orbit injection - Service Module: subsystems and thermoelastic evaluation, pointing and microvibration budget - Experiments: GOLF 3 & 4, VIRGO 2 & 3, MDI 1 &2,SUMER 2, EIT 2, UVCS 2, SWAN 3-5, CELIAS 1 & 2, CDS & LASCO - Experiments: MDI, SUMER, CDS, LASCO, UVCS, joint calibrations, joint activities with Service Module 5. Halo Orbit: 13/29 March 1996 - Service Module: subsystems evaluation and final budgets - Experiments: remaining commissioning activities; First Month of SOHO Science SOHO declared fully commissioned on 16 April 1996.
SOHO's solar arrays were deployed flawlessly a few minutes after the spacecraft's separation from its Atlas launcher. The discharge from the batteries at the time of solar-array deployment was not more than 7 Ah per battery (each of 24 Ah capacity) and with a stable temperature of 23°C.
A full power-budget analyis was completed just after launch and repeated, including a trend analysis, at the end of the commissioning phase in March 1996. SOHO's power system proved to be working nominally and the estimated power margin after five years in orbit is predicted to be 7%.
Radio-frequency (RF) subsystem
The first RF signal from SOHO was acquired by the Madrid ground station after the spacecraft's separation from the launcher, 2 h 1 min 41 s after lift-off. The communications uplink was established immediately after this first signal acquisition for the main and redundant transponders.
The RF subsystem was then fully verified the day after launch, again at the beginning of January when SOHO was at almost maximum distance from the Earth (i.e. 1.5 million km), and once more at the end of March during the final commissioning phase. The design margins of 3 dB on the telemetry and telecommand links have been confirmed as being met.
The antenna pointing mechanism and the high-gain antenna performed nominally during a scheduled 360° roll of the spacecraft. The redundant transponder and high-power amplifier have also been exercised and behaved nominally, and the redundant receiver has been fully tested.
The RF system is therefore confirmed to be working nominally.
SOHO's data- handling subsystem has also been shown to be working correctly. The on-board time used to correlate all SOHO data is well within the design specification of ±5 ms of drift over 2.5 days.
The solid-state recorder, with its full capacity of 2 Gbit, is being used every day to retrieve the data generated out of ground station visibility. Its automatic onboard error detection and correction functions have been exercised, with Single-Event Upsets (SEUs) being detected at an average rate of one per minute. All errors have been corrected, which results in a zero bit error rate. The onboard tape recorder has also been successfully commissioned.
All of the operational data-handling modes have been exercised and shown to be working correctly.
Attitude and orbit control subsystem
Figure 4. The electrical architecture of the SOHO spacecraft
All the Attitude and Orbit Control subsystem modes have been tested, including the emergency Sun reacquisition mode, which was triggered on 3 December 1995. Both the nominal and redundant chains have been tested.
Reaction wheels 1,2 and 3 are used nominally, and number 4 has also been commissioned. The reaction-wheel capacity in combination with the spacecraft perturbation torques is such that wheel momentum management is necessary only once every eight weeks on average.
The sensitivity of the Star Sensor Unit (SSU) to Single Event Upsets (SEUs) has led to the attitude-control unit software triggering a change in AOCS operating mode from the normal star- tracker-based mode to a mode relying on gyroscopes on a number of occasions. A software patch to minimise this occurrence has therefore been defined and uploaded (after extensive validation using the ESTEC independent software validation facility).
Microvibrations and pointing
Figure 5. Microvibration performance of the SOHO spacecraft's reaction wheels (RW1 and RW2) in pitch and yaw, as detected by the MDI experiment on 20 March 1996. It can be seen here that the Y-axis disturbance due to RW1 was 0.01 arcsec at 40.72 Hz
A series of spacecraft commissioning tests and experiments were conducted in February/ March 1996 to evaluate the spacecraft's microvibration and pointing-stability performance, covering:
The microvibrations generated by the UVCS (Ultraviolet Coronagraph Spectrometer) roll at 20 Hz and 10 Hz, the SUMER focussing mechanism, wheels 1,2,3 and 4 at various speeds and other instrument mechanisms were evaluated. The comparison between the test results and the analysis showed no major discrepancies, with most performance figures better than predicted.
The objective of this commissioning test was to verify SOHO's absolute pointing using data obtained from the CDS, EIT, LASCO, MDI, SUMER, UVCS and VIRGO scientific instruments. The results are summarised in Table 1. The in-orbit data are representative of the actual line-of- sight pointing of the instruments when the spacecraft is attitude-controlled using the fine-pointing Sun sensor, and therefore the end-to-end absolute pointing performance can be assessed. The ground-test data, by contrast, reflect only the pointing performances at the experiment interfaces (reference optical cubes).
Table 1. SOHO's absolute pointing performance Expt. CDS EIT LASCO MDI SUMER UVCS Axis In Orbit Y 3'57" 3'20" 2'49" 2'34" 5'15" 3'20" Z -24" -12" 31" +1'25" 45" 37 Before Y 44" 37" -1'51" 1'12" -58" 3'46" Launch Z -1'34" -38" -1'27" 45" 1'32" 2"
The results of these tests showed the spacecraft's absolute pointing performance to be within specification. After the commissioning phase, SOHO spacecraft pointing has been readjusted to compensate for the small observed bias of the LASCO and EIT instruments towards the south pole of the Sun (about 3.5 arcmin). A number of other experiments, such as UCVS, SUMER, CDS and MDI, could realign their lines of sight individually.
SOHO's propulsion subsystem behaved nominally during the early-orbit, manoeuvring and wheel- management phases.
Two full assessments of the spacecraft's thermal performance were conducted, during the early-orbit phase (8 December 1995) and late in the commissioning phase (27 March 1996) when the experiments were operational. Mathematical thermal models had been run using solar constant values provided by the VIRGO experiments in order to evaluate the ageing of the Sun shield.
All the spacecraft temperatures were found to be well within their design limits, and are expected to remain so until the end of the nominal mission lifetime.
The SOHO software has performed flawlessly since launch, with more than 90% of the nominal processing having been exercised during the commissioning phase. However, several software patches have had to be uploaded into the Data-Handling Subsystem memory, including one to overcome the emergency Sun reacquisition problem mentioned above, and another to overcome a problem with the VIRGO experiment's cover, described below. Both patches were successfully loaded and have worked adequately. A patch was also loaded into the Attitude and Orbit Control Subsystem memory to improve the SEU filtering. Another software patch to reduce the number of mode switches caused by high-energy particle impacts on the SSU's Charge Coupled Devices (CCDs) has since been prepared and uploaded.
The VIRGO experiment's sensors were commissioned during the first weeks in orbit. One of them, the Luminosity Oscillation Imager (LOI) sensor did not perform correctly. Subsequent investigations conducted by the Principal Investigator, with support from ESA, indicated that the problem was due to the LOI cover not staying open. A solution based on a software patch residing in the data-handling chain was devised based on a combination of mathematical simulation of the mechanism's behaviour and physical testing on a spare model of the VIRGO experiment in the laboratory.
The patch was uploaded on 26 March 1996 and the LOI cover was opened successfully on the second attempt.
On 29 March 1996 the spacecraft was judged as fulfilling all of the specified system requirements and SOHO was handed over to the solar-physics community to begin its scientific mission. The first scientific results were presented to the World at large at a Press Conference in Paris on 2 May 1996 and described in some detail in the previous issue of the ESA Bulletin (No. 87, pp. 7-24).
Many teams and individuals, too numerous to list here, contributed to make the SOHO launch operations and commissioning phase a resounding success, including key staff from Matra Marconi Space (France and UK), Saab (Sweden), Galileo (Italy), and NASA and its contractors, as well as the ESA Team. Their contributions are gratefully acknowledged.