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No. 225 - End of quadrature operations

No. 225 - End of quadrature operations

Report for Period 15 August to 11 September 2010 (week 33 - week 36)The reporting period, covering four weeks of Venus Express mission operations, was devoted to routine science activities and included the last quadrature operations. The swap from high gain antenna 1 (HGA1) to HGA2 and the associated 180° flip of the spacecraft around its Z-axis were successfully executed on 15 August. On 21 August, the regular DDOR measurements were made. The quadrature phase ended on 26 August. A routine orbit control manoeuvre was performed on 28 August. The current eclipse season, which started already on 30 July, will end at the start of the next reporting period.

End of inbound quadrature
On 26 August, Venus Express exited the quadrature phase which had started on 21 July (see also status report no. 224, linked from the right-hand menu). As part of the quadrature operations a swap was made from HGA1 to HGA2 for use during Earth communication passes. The swap took place on 15 August, just before the Cebreros communication pass.

The reason for the high gain antenna swap is that the spacecraft attitude for continuing the use of HGA1 during Earth pointings would lead to illumination of spacecraft faces that are not designed to cope with such exposure to the Sun. HGA1 can only be used for Earth communications during the periods when the Sun-spacecraft-Earth angle is smaller than 90°. During the coming months, when this angle is greater than 90°, HGA2 will be used instead. In January 2011, in the middle of the mission's next (outbound) quadrature phase that marks the transition again to Sun-spacecraft-Earth angles of less than 90°, a swap back to HGA1 will be made.

With the conclusion of the quadrature operations the 10° tilt on the spacecraft attitude during Earth communication periods has been removed. The fake ephemerides that were needed to maintain this tilted attitude were removed as well.

On 21 August (DOY 233) delta differential one-way ranging (DDOR) was performed using two ground stations simultaneously to track the spacecraft. These measurements are performed regularly to support the accurate determination of the ephemeris for the planet Venus that is maintained by NASA's Solar System Dynamics Group.

The DDOR measurement principle uses a technique to highly accurately determine the position of a transmitter in space and relies on observing the time delay between reception of the signal from the transmitter by two ground stations at different positions on Earth. In this case the Venus Express spacecraft was the transmitter and the two ground stations were New Norcia in west Australia and Cebreros in Avila, Spain.

An orbit control manoeuvre (OCM) was executed on 28 August. It was a prograde manoeuvre at apocentre, with the intention of raising the pericentre altitude by 24.4 km and increasing the orbital period by 41.4 seconds. The manoeuvre had a nominal magnitude of 1.512 m/s.

The calibration within the orbit determination showed a slight over-performance of 1.45% (+21.9 mm/s) with a 1-sigma uncertainty of 0.07% (1.1 mm/s). The pericentre altitude was increased by 0.335 km more than planned and the orbital period was increased by 0.6 seconds more than planned.

Summary of main activities
During the reporting period mission operations have been conducted with the support of the ESA Cebreros ground station. The communication passes over Cebreros were performed daily. During the first two weeks, however, three Cebreros passes were skipped (DOY 229, 231 and 236) as part of the quadrature operations. In addition, Cebreros passes were regularly shortened to allow more time for science observations.

The table below shows a chronology of the main activities:

Date DOY Main Activity
1741 15/08/10 227 Swap from HGA1 to HGA2.
Telemetry bit rate lowered from 228 kbps to 38 kbps
1742 16/08/10 228 Shortened Cebreros communications pass
1743 17/08/10 229 Cebreros pass skipped
1744 18/08/10 230 Shortened Cebreros communications pass
1745 19/08/10 231 Cebreros pass skipped
1747 21/08/10 233 DDOR with Cebreros and New Norcia.
Telemetry bit rate increased to 45 kbps
1748 22/08/10 234 Shortened Cebreros communications pass
1749 23/08/10 235 Shortened Cebreros communications pass
1750 24/08/10 236 Cebreros pass skipped
1751 25/08/10 237 Shortened Cebreros communications pass
1752 26/08/10 238 Shortened Cebreros communications pass.
End of inbound quadrature.
Fake ephemerides removed
1753 27/08/10 239 Shortened Cebreros communications pass
1754 28/08/10 240 Shortened Cebreros communications pass.
Orbit Control Manoeuvre
1755 29/08/10 241 Shortened Cebreros communications pass
1756 30/08/10 242 Shortened Cebreros communications pass.
Telemetry bit rate increased to 57 Kbps
1757 31/08/10 243 Shortened Cebreros communications pass
1758 01/09/10 244 Shortened Cebreros communications pass
1764 07/09/10 250 Cebreros pass shortened for Mars Express
1767 10/09/10 253 Telemetry bit rate increased to 76 Kbps.
Cebreros pass shortened for Mars Express

MET = Mission elapsed time; DOY = Day of year

At the end of the last Cebreros pass in the reporting period (DOY 254) Venus Express was orbiting Venus at 76 million km from the Earth. The one-way signal travel time was 253 seconds.

Payload Activities

The instrument was regularly operated as part of the routine plan.

The instrument was regularly operated as part of the routine plan.

The instrument was not operated during the reporting period.

The instrument was regularly operated as part of the routine plan.

No radio science observations were made during the reporting period.

VIRTIS-M was operated in the visual channels only.

The instrument was regularly operated as part of the routine plan.

Future Milestones

  • 13 September 2010: End of eclipse season
  • October 2010: Atmospheric drag experiment campaign #4
  • 29 October 2010: Venus at inferior conjunction
  • 31 December 2010: Start of outbound quadrature phase

Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #249 through #252. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.

Last Update: 1 September 2019
23-Feb-2020 19:13 UT

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