ESA Science & Technology - No. 243 – Atmospheric Drag Experiment campaign #7 and pericentre raising

Cebreros ground station
Scheduled maintenance was carried out at ESA's Cebreros ground station (CEB) from 09:30 to 14:30 CET on 25 and 26 January 2012. Due to this, the time available to downlink Venus Express science data was reduced on those days.

CEB uses three copies of a software system, known as the Intermediate Frequency and Modem System (IFMS). This was designed to meet ESA's Earth Station communication and ranging/ doppler signal processing requirements for deep space missions. It also operates with the high Doppler rates encountered with near-Earth spacecraft. A complex system, it includes digital signal processing and data processing subsystems that have the capacity for multiple channel reception with accurate synchronisation among different channels, several sampling rates, quantisation levels, and modes of operation.

The software of all three of the CEB IFMS systems was updated without any impact on operations. The software was also validated by the Mars Express team using their spacecraft. However, the same could not be done with Venus Express, as the required conditions were not met.

On 31 January, a power outage at the Mission Operations Centre caused the prime Mission Control Server (MCS) to go down; this occurred before the CEB pass had started. The backup MCS (located in a separate building which was not affected by the power outage) was configured for use in place of the primary just before the beginning of the pass. The CEB pass was handled using this backup computer without problems.

High accuracy spacecraft ranging
ESA's CEB and New Norcia (NNO) ground stations were used for a Delta Differential One-way Ranging (delta-DOR, or DDOR) measurement on 20 January. However, a problem with the new IFMS software caused the loss of all the data.

These measurements are carried out with the Venus Express spacecraft on a regular basis to support the accurate determination of the ephemeris for the planet Venus that is maintained by NASA's Solar System Dynamics Group. The DDOR measurements pinpoint the location of the Venus Express spacecraft; the location of the spacecraft with respect to the planet centre is known to high accuracy. Combining the two allows the orbit of Venus to be determined with very high accuracy. Modern-day DDOR campaigns are carrying on a tradition started by ancient civilizations - the orbit of Venus has been measured throughout the ages, with increasing accuracy; it is increasingly important in the space age.

The DDOR technique uses two widely separated antennas to simultaneously track the location of a transmitter in space in order to measure the time delay between signals arriving at the two stations. Theoretically, the delay depends only on the positions of the two antennas and the spacecraft. In reality, it is affected by several sources of error: for example, the radio waves travelling through the troposphere, ionosphere and solar plasma, and clock instabilities at the ground station. DDOR corrects these errors by 'tracking' a quasar in a direction close to the spacecraft for calibration. The quasar's direction is already known to very high accuracy by astronomical measurements, typically to better than 50 billionths of a degree (a nanoradian). The quasar is usually within 10 degrees of the spacecraft so that their signal paths through Earth's atmosphere are similar. In principle, the delay time of the quasar is subtracted from that of the spacecraft's to provide the DDOR measurement.

(For more information about DDOR, see the ESA Bulletin article “Delta-DOR – A New Technique for ESA's Deep Space Navigation” - link in right-hand column.)

Preparations for next eclipse season
The next reporting period will include the start of a new eclipse season. To prepare for this, the end-of-charge state of the batteries was raised from 80% to 100% on 3 February.

When not in eclipse season, the lower maximum charge reduces stress on the battery, prolonging its life. The lower end-of-charge also leaves sufficient reserves of power for the spacecraft to go into safe mode if necessary, and still have enough time to last for the longest possible recovery sequence to a Sun-pointing attitude. Once the battery is needed to power the spacecraft during eclipses, its end-of-charge state is raised back to 100%.

Atmospheric Drag Experiment campaign #7
The seventh ADE campaign started on 8 January 2012, as the pericentre altitude approached 175 kilometres above the Venusian surface. DSS-43, NASA's 70-metre Deep Space Network antenna located near Canberra, Australia (CAN), was used to support the ADE daily, from 15 through 19 January. The campaign concluded on 19 January.

This campaign is the latest in a series that have been carried out regularly since 2008. Such campaigns provide measurements that are used to improve models of Venus' upper atmosphere, especially over the planet's north pole.

Venus Express's parking orbit is highly eccentric; it takes the spacecraft out to roughly 66,000 kilometres from the planet at its apocentre, and 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 instrument that detects the drag exerted by the atmosphere. By measuring its integrated effect on the spacecraft's attitude and position in orbit, the atmospheric density can be determined. During the campaign, the spacecraft's pericentre is allowed to decrease beyond its nominal altitude, down to 175 kilometres in this case (see figure) as the spacecraft flew over the northern pole of Venus. During closest approach, the spacecraft's solar panels are tilted to induce a very tiny roll in the spacecraft. This roll is countered by the reaction wheels, and the change in reaction wheel speeds can be used to determine the density of the atmosphere at this extreme upper altitude. The amount of drag experienced by the spacecraft is significantly lower than that which would be experienced during an aerobraking orbit change. The drag is maintained at this level, which is known to cause no problems for the spacecraft.

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. This provided at least one hour of tracking after the WOLs, allowing even minor orbit perturbations from the WOLs to be determined and removed from the orbit data, thus improving the accuracy of the ADE results.

Change in Venus Express's pericentre altitude with time, starting from 13 November 2011 (orbit number 2032) and ending on 4 February 2012 (orbit 2115). Credit: ESA

Orbit control manoeuvres
Two orbit control maneuvers (OCM) were performed to raise the pericentre altitude after the seventh Atmospheric Drag Experiment (ADE) campaign. These were performed successfully on 22 January, to adjust the apocentre height, and on 24 January, to raise the pericentre height. The change in pericentre height is shown in the plot showing change in pericentre with time, for the seventh ADE.

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

At the end of the reporting period on 4 February 2012, Venus Express was at a distance of 161.8 million kilometres from Earth. The one-way signal travel time was 539 seconds. The final oxidizer mass was 28.466 kg and the final fuel mass was 17.657 kg.

Payload activities

Each instrument is assigned a fixed amount of storage in the spacecraft memory, where its data can be stored until it is transmitted to the ground. The VMC (monitoring camera) reached its memory limit on 26 January, and about 12 Mbytes of data were overwritten. Data that are overwritten cannot be recovered.

A common ASPERA (neutral and charged particle analyzer) anomaly recurred on 20 January. Due to a glitch during the initialisation of its movable scan platform, the science data generated for about 100 minutes were lost. ASPERA worked correctly after the next 'power on', without needing any manual intervention.

On 19 January, the VMC (monitoring camera) suffered a recurrence of a previous anomaly. The high-speed link between the solid-state memory modules (SSMM) on the spacecraft bus and the VMC camera disconnected unexpectedly. As a consequence, the camera could not transfer and store its data in the SSMM. About 40 minutes of data and instrument telemetry were lost and could not be recovered.

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

 
Future milestones

• Start of eclipse season #20 on 7 February 2012.
• Inbound quadrature operations start on 2 March 2012: as the Earth-Venus-Sun angle exceeds 90 degrees, changes in antenna usage and restrictive thermal constraints become necessary.

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Legal disclaimer
This report is based on three ESOC mission operations reports, MOR #321 through MOR #323. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.