No. 240 – Nominal operations; start of solar eclipse season #19; Earth occultation season #12
Cebreros ground station
The software of the Intermediate Frequency and Modem System (IFMS) was upgraded on 26 October, to prepare the station for Phobos-Soil (Phobos-Grunt) support. The new functionalities and the upgrade do not affect Venus Express in any way.
The Intermediate Frequency and Modem System (IFMS) 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 synchronization among different channels, several sampling rates, quantization levels, and modes of operation.
Due to bad weather at Cebreros on 2 November 2011, some MAG (magnetometer) and VIRTIS (imaging spectrometer) data packets were not received on the ground for eight minutes. These packets were recovered on-board the spacecraft, and downlinked to the ground on 6 November; no data were lost.
On 3 November, the start of the Cebreros pass had to be delayed by one hour, as the observed signal-to-noise ratio from the spacecraft was extremely marginal and unstable. Such degradation of the signal is usually due to temporary ionization of the upper atmosphere.
High accuracy spacecraft ranging
A Delta Differential One-way Ranging (DDOR) measurement was performed on 12 November.
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.)
Change to battery charge state
The spacecraft has a regular and predicted period of time when it is in the dark for a portion of each orbit. This is known as an eclipse season. As the nineteenth eclipse season had just started, the end-of-charge of the batteries was set to 100% on 23 October. This ensures that there is sufficient power in the battery to support emergency operations if the spacecraft goes into a ‘safe mode’ immediately after one of the regular daily eclipses. When not in eclipse season, the maximum charge is set at 80%, which is less stressful on the battery, and prolongs 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.
Occultation season #12
The twelfth Earth occultation season started on 15 November 2011. During this season, the alignment of Earth and Venus as seen by the spacecraft in orbit allows the use of the communications signal sent from the spacecraft to Earth to probe Venus’s atmosphere.
Shortly before Venus Express 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.
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 must be synchronized – or correlated - with atomic clocks on Earth at regular intervals. One of these regular time correlations was performed on 3 November.
Summary of main activities
The table below shows a chronology of the main spacecraft bus activities in the reporting period:
|Main activities during reporting period|
|2175||23-Oct-2011||296||CEB communications pass. Battery end-of-charge level set to 100%.|
|2176||24-Oct-2011||297||CEB communications pass.|
|2177||25-Oct-2011||298||CEB communications pass.|
|2178||26-Oct-2011||299||CEB communications pass.|
|2179||27-Oct-2011||300||CEB communications pass.|
|2180||28-Oct-2011||301||CEB communications pass.|
|2181||29-Oct-2011||302||CEB communications pass.|
|2182||30-Oct-2011||303||CEB communications pass.|
|2183||31-Oct-2011||304||CEB communications pass.|
|2184||01-Nov-2011||305||CEB communications pass.|
|2185||02-Nov-2011||306||CEB communications pass.|
|2186||03-Nov-2011||307||CEB communications pass.|
|2187||04-Nov-2011||308||CEB communications pass.|
|2188||05-Nov-2011||309||CEB communications pass|
|2189||06-Nov-2011||310||CEB communications pass.|
|2190||07-Nov-2011||311||CEB communications pass.|
|2191||08-Nov-2011||312||CEB communications pass.|
|2192||09-Nov-2011||313||CEB communications pass. A Delta-DOR measurement was taken using CEB and NNO as part of on-going update of the ephemeris of Venus.|
|2193||10-Nov-2011||314||CEB communications pass.|
|2194||11-Nov-2011||315||CEB communications pass.|
|2195||12-Nov-2011||316||CEB communications pass.|
|2196||13-Nov-2011||317||CEB communications pass.|
|2197||14-Nov-2011||318||Short CEB communications pass. NNO ground station used for occultation measurements.|
|2198||15-Nov-2011||319||Short CEB communications pass. NNO occultation pass.|
|2199||16-Nov-2011||320||CEB communications pass.|
|2200||17-Nov-2011||321||CEB communications pass. NNO occultation pass.|
|2201||18-Nov-2011||322||CEB communications pass.|
|2202||19-Nov-2011||323||CEB communications pass.|
At the end of the reporting period on 19 November 2011, Venus Express was 224 million kilometres from Earth. The one-way signal travel time was 748 seconds. The final oxidizer mass was estimated to be 30.316 kilograms, and the final fuel mass estimate was 18.845 kilograms.
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.|
|MAG||The instrument was regularly operated as part of the routine plan.|
|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 operated for the occultation passes, at orbit pericentre.|
|VIRTIS||The instrument was regularly operated as part of the routine plan.|
A previously seen temperature alarm was activated on 6 November when the temperature of the radiator of VIRTIS-M (moderate resolution channel) briefly exceeded its limit. The time spent above the -80°C limit was about 16 minutes – relatively brief. As observed in the past, this is caused by the geometry of the eclipse season. During the season, the VIRTIS instrument’s heat radiator is exposed to light reflected from Venusian clouds in certain orientations. This albedo reflection is enough to heat the radiator, raising its temperature to slightly higher than normal. This has been investigated thoroughly in the past, and does not damage the instrument.
- Start of the seventh Atmospheric Drag Experiment (ADE) season in January 2012. The natural decrease in pericentre height is necessary to probe the extreme upper atmosphere during ADE, hence the nominal orbit raising manoeuvres can be delayed.
- Orbital correction manoeuvres will be carried out to raise the pericentre after the ADE.
- Start of the twentieth eclipse season on 8 February 2012. As the spacecraft passes into and then out of the shadow of Venus, the SPICAV and VIRTIS spectrometers can measure the changes in solar radiation as it passes through the different altitudes of the atmosphere.
- Quadrature operations: as the Earth-Venus-Sun angle exceeds 90 degrees, changes in antenna usage and restrictive thermal constraints become necessary. Quadrature season starts in March 2012.