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No. 221 - Start of Earth occultation season #9

No. 221 - Start of Earth occultation season #9

Report for Period 25 April to 22 May 2010 (week 17 - week 20)The Venus Express spacecraft and instruments performed nominally during the reporting period. However, two occurrences of a minor anomaly with ASPERA led to the loss of part of the science data taken by the instrument on 27 April and on 7 May. On 30 April the mission's ninth Earth occultation season started, which will last until 11 July 2010. The current eclipse season, which started already on 7 April, is ongoing and will last until end May 2010. On 14 May the regular DDOR measurements were performed.

During the eclipse season the Venus Express spacecraft regularly passes through the shadow of Venus, with the Sun being eclipsed by the planet's disk as seen from the spacecraft's perspective.

VeRa Earth occultation season #9
The mission's ninth season of Earth occultations started on 30 April 2010 (DOY 120). During this season, around the pericentre passages the Earth is occulted by the disk of Venus, as seen from the spacecraft. This condition allows for unique measurements to investigate the fine structure of the neutral atmosphere and ionosphere of Venus.

The occultation measurements are part of the Venus Express Radio Science Investigations (RSI) performed under the Venus Radio Science experiment (VeRa). For these investigations the VeRa team makes use of the radio links of the spacecraft's communications system. The spacecraft's High Gain Antenna (HGA) is pointed toward the Earth before the approach to occultation. The on-board ultra-stable oscillator (USO) is used to generate a very stable signal which is sent to Earth over the HGA in simultaneously the S-band and X-band. Just before and after the occultation the transmitted radio signal passes through the ionosphere and atmosphere of Venus. The effect that this has on the received signal gives valuable information on the characteristics of the atmosphere (such as temperature, density and pressure) as a function of altitude.

During the reporting period the 35m ESA deep space antenna at New Norcia, Australia, was used for receiving the spacecraft's radio signals on ground.

Note: no occultation measurements were performed during the previous occultation season (#8), which ran from early December 2009 to early February 2010.

Mosaic images with star tracker
On 13 May (DOY 133) a test was performed using one of the star trackers to take several images with a limited field of view to be combined to make a larger mosaic. See also status report no. 179 in right-hand menu for earlier imaging tests with the star tracker.

DDOR
On 14 May (DOY 134) 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.

Summary of main activities
During the reporting period mission operations have been conducted with the support of the ESA Cebreros (CEB) ground station. Communication passes over Cebreros were performed daily. Additional passes for the RSI occultation campaign were performed around pericentre, with the New Norcia (NNO) ground station.

The table below shows a chronology of the main activities:

MET
(Day)
Date DOY Main Activity
1634 30/04/10 120 RSI Occultation over NNO around pericentre
1635 01/05/10 121 RSI Occultation over NNO around pericentre
1637 03/05/10 123 RSI Occultation over NNO around pericentre
1638 04/05/10 124 RSI Occultation over NNO around pericentre.
Patch uploaded for ASPERA IMA
1640 06/05/10 126 RSI Occultation over NNO around pericentre
1641 07/05/10 127 RSI Occultation over NNO around pericentre
1643 09/05/10 129 Bad weather at the Cebreros ground station, impacting data return. Science data stored on-board for later retrieval
1645 11/05/10 131 RSI Occultation over NNO around pericentre.
Shortened Cebreros pass due to planned ground station maintenance activities
1646 12/05/10 132 Shortened Cebreros pass due to planned ground station maintenance activities
1647 13/05/10 133 RSI Occultation over NNO around pericentre.
Test of mosaic imaging with star tracker #2
1648 14/05/10 134 DDOR with Cebreros and NNO
1649 15/05/10 135 RSI Occultation over NNO around pericentre
1652 18/05/10 138 RSI Occultation over NNO around pericentre
1653 19/05/10 139 RSI Occultation over NNO around pericentre.
All science data stored on-board on DOY 129 was successfully retrieved on-ground.
The telemetry bit rate was increased from 91 kbps to 114 kbps
1656 22/05/10 142 RSI Occultation over NNO around pericentre

MET = Mission elapsed time; DOY = Day of year

At the end of the last Cebreros pass in the reporting period (DOY 142) Venus Express was orbiting Venus at 200.9 million km from the Earth. The one-way signal travel time was 670 seconds.

Payload Activities

ASPERA
The instrument was regularly operated as part of the routine plan. On 4 May (DOY 124) the new patch for the ASPERA Ion Mass Analyser (IMA) was uplinked successfully.

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
On twelve orbits in the reporting period, RSI experiments took place around pericentre as part of the Earth occultation campaign #9, with twelve dedicated passes over the NNO ground station.

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

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

Future Milestones

  • 30 May 2010: End of eclipse season
  • 11 July 2010: End of Earth occultation season #9
  • 21 July 2010: Start of next quadrature phase (inbound)


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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #233 through #236. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.

Last Update: 1 September 2019
15-Jul-2020 13:07 UT

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