ESA Science & Technology - No. 245 - Start of the mission's longest Earth occultation season; temporary star tracker blinding

Cebreros ground station
On 20 March at 09:19 UTC, when signal from the spacecraft (acquisition of signal, AOS) was due to be received, an electronic unit in the ground station experienced a minor problem. The AOS was late, but no science data was lost, as a redundant unit was switched into use.

No serious problems were experienced with the ground station during this reporting period.

High accuracy spacecraft ranging
ESA's Cebreros (CEB) and New Norcia (NNO) ground stations were used for a Delta Differential One-way Ranging (delta-DOR, or DDOR) measurement on 17 March.

For more information about DDOR, see the "Delta-DOR measurements" link in the right-hand column.

Solid State Mass Memory warm reboot
During the first file transfer between the ground and the spacecraft on 29 March, the Solid State Mass Memory (SSMM), the main memory bank of the spacecraft instruments and computer, experienced a problem when it tried to create a new file. Following this, it did not respond to some commands that were sent to troubleshoot the problem. This type of problem occurs irregularly, but is not unusual. The SSMM was restarted without being powered off, in a process known as a warm reboot. It responded as expected, and normal operations were restored.

Quadrature operations
The spacecraft began operating in the inbound quadrature configuration on 2 March, two days prior to the start of this planning period. (See Status Report 244 for further details.)

During quadrature operations the Venus Express High Gain Antenna (HGA) is swapped and HGA2 takes over from HGA1. The commands to swap and switch to HGA2 as the primary were uplinked to the spacecraft on 23 March. These included on-board commands to be followed in case of problems caused by the swap and potential loss of contact with Earth. In case of loss of contact after the swap, the spacecraft would have switched back to HGA1 and entered safe mode on 27 March, one day after the swap.

The actual swap on board the spacecraft took place on 26 March. This operation involves the configuration of on-board software, plus attitude re-orientation and a reconfiguration of the telemetry, tracking and control (TT&C) software. Such an operation normally takes less than one hour, and is executed out of pass, shortly before the acquisition of signal (AOS) with the Cebreros ground station. The bit rate was changed to 38 kbps, compatible with the HGA2 link budget, and the 'safing' commands (to switch to safe mode in case of any problems) were deleted on-board.

Star tracker blinding and temporary loss of science

As part of routine operations, the command sequences for science operations (which include both instrument commands and spacecraft attitude changes) are uploaded to the spacecraft daily. The spacecraft then stops communicating with Earth while science operations are carried out, typically for 12 to 14 hours. After this, the spacecraft is programmed to point towards Earth again, where it detects the uplink signal from the Cebreros ground station near Madrid, Spain. The flight control team studies the telemetry to assess spacecraft health, and instructs the spacecraft to begin transmitting stored science and spacecraft telemetry data to the ground station for recording and transmission to the European Space Operations Centre (ESOC) in Darmstadt, Germany.

On 6 March, when the spacecraft turned to Earth and the flight control team assessed the spacecraft health, they found that Venus Express's primary star tracker (STR) was blinded and was not tracking stars – it had been in that state for about 8 hours. The STR lost lock on 6 March at 01:41 UTC. It was suspected that high levels of solar activity had caused the problem; a pair of massive solar flares that occurred early in the morning of 7 March had generate a Coronal Mass Ejection that resulted in a proton storm.

The team immediately switched the redundant STR into use, before the spacecraft was out of view of the Cebreros station in Spain. However, the secondary STR was in the same condition as the primary. Unable to restore star tracking right away, the flight control team stopped the execution of stored commands, and allowed the spacecraft to remain pointing at Earth. Communication was then lost as the Earth rotated and Venus dipped under the horizon with respect to the Cebreros ground station.

In addition to using the star trackers to determine the spacecraft attitude, the gyroscopes can also be used. The gyroscopes sense the spacecraft's movement and can be used to track its motion. While they are accurate enough for several applications, their accuracy drifts over time unless they are calibrated using a more accurate source of attitude information: the star trackers. Without the star trackers to calibrate the gyroscopes, their accuracy would drift over the next few days. Without the spacecraft pointing information from the gyroscopes, the science data would be of limited use.

The team also noticed that solar activity, rather than decreasing, was still increasing. Therefore it was considered safer to leave the spacecraft in Earth-pointing mode while the star tracker anomaly was analysed. In parallel, extra ground station coverage was requested from the rest of the ESA Tracking Network, and was granted on the station at New Norcia, Australia.

On 7 March and 8 March 2012, the star trackers showed no change and continued to be blind. The light detectors were overexcited by the solar radiation, and the extra energy in the detectors was not dissipating. In addition, another solar flare had been observed that was also likely to hit Venus Express. Therefore, the detectors were left turned off on 7 and 8 March, while the radiation environment was monitored.

In the meantime, the spacecraft control team was busy performing a few necessary checks and activities:

• deleting already uploaded commands;
• reconfiguring parameters used in emergency safe modes to ensure that the star trackers are ignored;
• performing various tasks to calibrate the gyroscopes;
• activating back-up transponders as a precaution;
• negotiating emergency ground station coverage from various ESA ground stations;
• performing occasional tests of the star trackers to monitor their condition and detect changes;
• generating and uploading modified spacecraft commands.

On 9 March, the radiation levels finally dropped, and the star tracker detectors began to recover. Plans were made to resume routine operations and begin science observations again. Procedures were drafted and reviewed to ensure that all systems were checked, and found to be acceptable, before switching back to nominal star tracker use. These checks continued through 10 and 11 March.

On 12 March, all systems checked out as normal. The SSMM was checked, the star trackers functioned nominally in autonomous tracking mode, the accumulated momentum in the spacecraft was dumped with a small burn of the spacecraft thrusters, and all the electronics were functioning normally.

No science activities were performed from 16:28 UTC on 7 March, until 13:26 UTC on 12 March, when the spacecraft was returned to executing the pre-planned science command sequences. Subsequent checks show that no other spacecraft systems were affected by the solar radiation event.

Summary of main activities
The table below shows a chronology of the main spacecraft bus activities in this reporting period.

At the end of the reporting period on 31 March, Venus Express was at 100.9 million kilometres from Earth. The one-way signal travel time was 336.3 seconds.  The final oxidizer mass was 28.316 kg and the final fuel mass was 17.563 kg.

Scientific focus
This reporting period, which falls under MTP 77 (the 77th month of science operations, or the 77th Medium Term Plan), covers the period from 4 March through 31 March 2012.

Standard nadir pointing during this observing period did not illuminate areas of the spacecraft that have thermal restrictions, making this MTP 'cold'. This geometry makes the science observations much easier to perform.

During this MTP, local time at ascending node (LTAN) starts in the first orbit at about 00:00 hours local time. This is midnight, with the ascending branch of the Venus Express orbit directly between the two terminators on the night side. The LTAN at the end of the planning period was about 03:00 hours local time.

Change in Venus Express's local time at ascending node. Credit: ESA

The entire reporting period fell during the twentieth eclipse season. During an eclipse, the viewing conditions are dark enough for the Venus Monitoring Camera (VMC) to take low-resolution images of the planet's surface through windows in the atmosphere that allow certain infrared radiation frequencies to be detected.

A long Earth occultation season started towards the end of this MTP, on 27 March, and will last for almost 7 more months, until 06 October 2012 (in MTP 84). This is much longer than a normal occultation season because the occultation occurs during the apparent retrograde motion of Venus from Earth.

Venus Express coverage of Venus by orbital tracks during the 77th medium term plan, 4 March to 31 March 2012. Credit: ESA

During this reporting period, observations on the descending arc were ideal for studying the day-side of the planet. The spacecraft orbit ascended from South to North on the night side, allowing most observations to be carried out in the attitude most favourable for thermal constraints, and keeping all the faces of the spacecraft that have thermal restrictions in the dark. As the spacecraft had not been heated by earlier observations, it was possible to carry out hot observations after it passed pericentre near the North Pole. For some orbits, the start of the Cebreros Earth communications pass was delayed until about two hours after pericentre to allow this viewing of the day-side.

The surface targets included Ishtar Terra in the Northern Hemisphere, as well as Bell Regio and Eastern Eistla Regio, which have been identified as 'hotspots', i.e. a surface manifestation of a mantle upwelling.

The Venus Express Infrared and Thermal Imaging Spectrometer (VIRTIS) visible channel carried out observations in limb tracking mode every second orbit during this reporting period. These observations will be used to map the airglow of atomic oxygen.

Payload activities

The instruments were operated nominally according to the plans of each instrument team, except during the shutdown period caused by the star tracker anomaly.

Future Milestones

• Continuation of the twentieth solar eclipse season
• Continuation of the thirteenth Earth occultation season
• Exit from quadrature season attitude
• Beginning of the eighth Atmospheric Drag Experiment campaign

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Legal disclaimer
This report is based on three ESOC mission operations reports, MOR #328 through MOR #331, as well as the MTP077 Master Science Report. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.