15 Sep 2010 17:16

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.

DDOR
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.

OCM
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:

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

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

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

PFS
The instrument was not operated during the reporting period.

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

VeRa
No radio science observations were made during the reporting period.

VIRTIS
VIRTIS-M was operated in the visual channels only.

VMC
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

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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.