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No. 238 - Atmospheric Drag Experiment campaign #6

No. 238 - Atmospheric Drag Experiment campaign #6

Report for the period 28 August to 24 September 2011 (week 35 - week 38)The reporting period covers four weeks of Venus Express mission operations. During this period routine planned operations were conducted at very low telemetry downlink data rates. The sixth Atmospheric Drag Experiment campaign began on 13 September and continued through to 24 September.

Cebreros ground station

Bad weather at Cebreros (CEB) on 31 August 2011 resulted in a loss of downlinked data as localized heavy rain reduced the signal strength. Standard practice is for data to be retained on the spacecraft until it has been verified to be stored at the ground station. On this occasion the data were successfully copied to another part of the on-board memory for secure storage, and were downlinked later. All the data were successfully retrieved.

During the CEB communications passes on 31 August and 2 September the on-board lock was lost on the telecommand uplink due to a malfunction on the X-Band Low Power Amplifier (X-LPA). As soon as the on-board lock was lost, the data downlinks were stopped. A re-sweep was done with the X-Band High Power Amplifier (X-HPA), after which the science data downlink was resumed. No data were lost.

The X-LPA experienced temporary problems again on 4 and 5 September, and on 9 and 10 September, during which times the station had to use the X-HPA as the prime amplifier.

During the science dumps on 8 September a glitch on the downlink caused the loss of some VIRTIS (imaging spectrometer) and MAG (magnetometer) data. The copy of the data on the spacecraft was retained for a later downlink attempt. A retrieval was again performed at the end of the Cebreros downlink pass, but the gaps were not filled, indicating that the data must have been lost in the space link. The reason is still unknown. The data were copied into another part of the on-board memory and dumped successfully in the following days. No data were lost.

The X-Band High Power Amplifier (X-HPA), used for uplink communications to the spacecraft, turned itself off on 11 September. The cause was determined to be because of a too-tight limit adjustment. The control software monitors various parameters in the unit, and compares them with pre-determined safety limits. If a parameter exceeds the limits, the unit can automatically shut down to prevent damage until the cause is determined. In this case, the pre-determined limits of a unit parameter were set too tight, and the value reached was still within safe limits. The parameter limits were analyzed and changed to a wider, but still safe, range by the operations team at the station.

Aerobraking review board

The Aerobraking Review Board met at ESA's European Space Astronomy Centre (ESAC) in Spain, on 5 September. Aerobraking would involve lowering the spacecraft deep enough into the upper atmosphere that atmospheric drag would significantly alter the spacecraft orbit, while also providing information about atmospheric density. The Board looked at the unique science that could be achieved, as well as the operational knowledge that would be gained by carrying out an aerobraking experiment. Aerobraking techniques can be used to change a spacecraft orbit without the use of significant amounts of fuel; for example, to change a spacecraft's initial planetary orbit and achieve the final orbit required for the scientific mission. However, the spacecraft must be designed for the physical and thermal stresses that would be experienced as a result of the atmospheric drag; Venus Express was not designed for this. After reviewing the science that could be achieved and the effect on the current science plans, and considering the possible risk to the spacecraft, the Board decided to delay the execution of aerobraking until the end of the mission. The science and operations experience can still be achieved, but at a later date.

Spacecraft clock synchronised

The clock on the Venus Express spacecraft is a simple count-up timer, typical of those used on spacecraft. These clocks save weight and cost but they tend to drift and at regular intervals have to be synchronised – or correlated - with atomic clocks on Earth. One of these regular time correlations was performed before the start of the Cebreros communications pass on 6 September.

Atmospheric Drag Experiment campaign #6

The sixth Atmospheric Drag Experiment (ADE) campaign was carried out from 12 to 24 September. These campaigns provide measurements that are used to improve models of Venus' upper atmosphere, especially over the northern pole of the planet. This is the latest in a series that have been performed on a regular basis since 2008.

Venus Express is in a highly eccentric orbit that takes it out to about 66,000 kilometres from the planet when at apocentre, but much closer to the planet's surface when the spacecraft is at pericentre. For the ADE campaign, the entire spacecraft body and the attitude-sensing gyroscopes act as an experiment to detect the drag of the atmosphere – and hence to determine its density - by its integrated effect on the spacecraft's attitude and orbit position. During the campaign the spacecraft pericentre is allowed to go much lower than normal, in this case it decreased to 175 kilometres as the spacecraft flew over the northern pole of Venus. During close approach, the spacecraft attitude is maintained by reaction wheels, so that any drag from the extreme upper atmosphere will be detected by the on-board attitude gyroscopes. The amount of drag experienced by the spacecraft during the ADE is significantly less than that which would be experienced in an aerobraking orbit change, and is maintained at a drag level which is known to cause no problems on the spacecraft.

During this period, the ESA ground station at New Norcia (NNO), Australia, and the NASA Deep Space Network (DSN) station in Canberra, Australia, provided tracking at pericentre.

The regular wheel off-loadings (WOL) that are used to release the accumulated spacecraft momentum every day were performed in the middle of the daily Cebreros communications passes. That provided at least one hour of tracking after the WOLs, allowing even the minor orbit perturbations from the WOLs to be determined and removed from the orbit data, resulting in improved accuracy of the ADE results.

Summary of main activities

During the reporting period, routine mission operations were conducted using the ESA Cebreros (CEB) ground station. Four ADE tracking experiments were carried out using ESA's New Norcia (NNO) ground station and one with the NASA Deep Space Network (DSS 43) antenna in Canberra.

The table below shows a chronology of the main spacecraft bus activities in the reporting period:

Main activities during reporting period

MET
(Day)

Date
(DD-MON-YYYY)

DOY

Main Activity

MET = Mission elapsed time; DOY = Day of year

2119 28-Aug-2011 240 CEB communications pass.
2120 29-Aug-2011 241 CEB communications pass.
Pre-aerodrag with solar array tilting
2121 30-Aug-2011 242 CEB communications pass.
2122 31-Aug-2011 243 CEB communications pass.
Batteries EOC set to 24.4 V
2123 01-Sep-2011 244 CEB communications pass.
2124 02-Sep-2011 245 CEB communications pass.
2125 03-Sep-2011 246 CEB communications pass.
2126 04-Sep-2011 247 CEB communications pass.
2127 05-Sep-2011 248 CEB communications pass. Time correlation done after pass.
2128 06-Sep-2011 249 Shortened CEB communications pass due to CEB maintenance.
2129 07-Sep-2011 250 CEB communications pass. Downlink telemetry rate increased to 76 kbps.
2130 08-Sep-2011 251 CEB communications pass.CEB communications pass. Uplink telecommand rate increased to 2 kbps.
2131 09-Sep-2011 252 CEB communications pass.
2132 10-Sep-2011 253 CEB communications pass.
2133 11-Sep-2011 254 CEB communications pass.
2134 12-Sep-2011 255 CEB communications pass.
2135 13-Sep-2011 256 CEB communications pass. ADE #6 started. NNO tracking at pericentre.
2136 14-Sep-2011 257 CEB communications pass. ADE #6 on-going. NNO tracking at pericentre.
2137 15-Sep-2011 258 CEB communications pass. ADE #6 on-going. NNO tracking at pericentre
2138 16-Sep-2011 259 CEB communications pass. ADE #6 on-going. NNO tracking at pericentre.
2139 17-Sep-2011 260 CEB communications pass. ADE #6 on-going.
DSS 43 tracking at pericentre.
2140 18-Sep-2011 261 ADE #6 pass over NNO, CEB Communication pass, WOL during pass.
2141 19-Sep-2011 262 ADE #6 pass over NNO, CEB Communication pass, WOL during pass.
2142 20-Sep-2011 263 ADE #6 pass over NNO, CEB Communication pass, WOL during pass.
2143 21-Sep-2011 264 ADE #6 pass over NNO, CEB Communication pass, WOL during pass.
2144 22-Sep-2011 265 ADE #6 pass over NNO, CEB Communication pass, WOL during pass.
2145 23-Sep-2011 266 ADE #6 pass over NNO, CEB Communication pass, WOL during pass.
2146 24-Sep-2011 267 ADE #6 pass over NNO, End of ADE #6, CEB Communication pass.

At the end of the reporting period on 24 September 2011, Venus Express was at 252 million kilometers from Earth. The one-way signal travel time was 839 seconds.

At the end of the reporting period, the final oxidizer mass was estimated to be 32.108 kilograms, and the final fuel mass estimate was 19.995 kilograms.

Payload Activities

The instruments were operated nominally according to the plans of each instrument team.

ASPERA The instrument was regularly operated as part of the routine plan. On 16 September 2011, a few data packets were lost due to a minor data hang-up which occasionally happens.
MAG The instrument was regularly operated as part of the routine plan. A glitch during the science downlink on 8 September caused the delayed downlink of some MAG (magnetometer) data.
PFS The instrument was not operated.
SPICAV The instrument was regularly operated as part of the routine plan.
VMC The instrument was regularly operated as part of the routine plan.
VeRa The instrument was regularly operated as part of the routine plan.
VIRTIS The instrument was regularly operated as part of the routine plan. A glitch during the science downlink on 18 September caused the delayed downlink of some VIRTIS (imaging spectrometer) data.

Future Milestones

  • Start of the nineteenth eclipse season. As the spacecraft passes into and then out of the shadow of Venus, the SPICAV and VIRTIS instruments can measure the changes in solar radiation as Sunlight passes through different altitudes of the atmosphere.
  • Start of the twelfth Earth occultation season, when Venus is between the Earth and the spacecraft for part of each orbit. The spacecraft can point towards Earth and transmit a high-accuracy signal through the Venusian atmosphere at entry and exit from the occultation. The changes in that signal are recorded on Earth, and these changes can be used to calculate atmospheric parameters.


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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #301 through MOR #304. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.

Last Update: 1 September 2019
9-Mar-2021 11:16 UT

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