No. 241 – Nominal operations; solar eclipse; Earth occultation season #12
03 January 2012 19:22
Report for the period 20 November to 17 December 2011
This reporting period covers four weeks of Venus Express mission operations, from 20 November to 17 December 2011. It includes routine planned operations at moderate but increasing telemetry downlink data rates, the end of solar eclipse season #19 and continuing Earth occultation season #12.
Cebreros ground station
Scheduled maintenance of the Cebreros ground station (CEB), a routine task, was performed on 13 and 14 December.
The orbit of Venus Express was designed to be 24 hours long, so that communication with Earth would be scheduled during normal working hours at Cebreros, minimising the mission’s operating costs. Regular maintenance is necessary for any complex antenna and its associated ground system. In the case of CEB, when carried out during normal working hours, regular maintenance interrupts the nominal downlink times from the spacecraft. However, it generally only reduces the downlink time by one to two hours, and thus has only a small effect on the reduction in the amount of science data that can be downlinked on such days. If the maintenance takes place during periods when the instruments produce large amounts of data, the data that cannot be downlinked immediately are retained on the spacecraft and downlinked over the next one or two orbits.
End of solar eclipse season #19
The 19th eclipse season ended on 4 December. The spacecraft no longer passes through the shadow of Venus at any point during its orbit.
Change to battery charge state
Due to the end of eclipse season, the end-of-charge state of the batteries was lowered from 100% to 80% on 6 December. When not in eclipse season, the lower maximum charge lowers the 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.
Battery deep discharge test
Spacecraft batteries degrade over time, and in predictable ways. These predictions form the battery modelling of anticipated battery performance. One parameter modelled in this way is the battery internal resistance, and this can be calculated from the rate at which the batteries discharge when in use. A deep discharge test was performed to assess the health of the batteries and to calibrate the discharge modelling. During the test, the spacecraft's solar panels are turned away from the Sun and the battery is used to power the spacecraft systems. The battery is discharged longer during these tests than is done during eclipses. This allows for a better calculation of the battery’s internal resistance – a key parameter of battery health – as well as the efficiency of the battery control electronics. The discharge is also compared to predictive models of power output over time. While it is typically referred to as 'the' battery, the system uses three individual batteries. All continue to be in good health, as predicted for this phase of the mission.
Solar array test
The battery deep discharge test allows to test the efficiency and performance of the solar panels as well. The test confirmed that the galium arsenide (GaAs) cells continue to operate with no defective cells or strings, and with no signs of degradation in efficiency.
Twelfth occultation season
The twelfth Earth occultation season continued during this period. During the occultation seasons, the orbit places Venus between the spacecraft and Earth for part of each orbit. This alignment allows the use of the spacecraft’s Earth communications signal to probe the planet’s atmosphere.
The ESA ground station at New Norcia (NNO) station in Australia was used for occultation measurements, as the station faced Venus when the spacecraft was at pericentre.
Shortly before the Venus Express spacecraft reaches pericentre, it is pointed to Earth and broadcasts its default signal. The signal used for occultation measurements must be far more accurate than required during a communications pass, hence it is generated using a special oscillator. As the spacecraft moves behind the planet, this highly accurate signal passes through the Venusian atmosphere before being blocked by the planet. The refracted signal can be measured and processed to yield details about the atmosphere that cannot be obtained in other ways. These occultation measurements are performed when Venus Express is at its pericentre, as and when the Earth-Venus-spacecraft geometry allows.
High accuracy spacecraft ranging
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.)
Summary of main activities
At the end of the reporting period on 17 December 2011, Venus Express was at 204.7 million kilometers from Earth. The one-way signal travel time was 682 seconds. The final oxidizer mass was 30.089 kg and the final fuel mass was 18.699 kg.
The instruments were operated nominally according to the plans of each instrument team.