No. 238 - Atmospheric Drag Experiment campaign #6
24 Sep 2011 09:38
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:
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.
The instruments were operated nominally according to the plans of each instrument team.