Venus Express Science Operations
The role of VSOC
The primary tasks of the VSOC are: to support the Principal Investigator teams in the preparation of the operational telecommand files which are sent to VMOC (Venus Express Mission Operations Centre) at ESOC for uplink to the spacecraft; and to ensure that all data collected during the mission are archived correctly so that the scientific community can access them during and after the mission.
In particular VSOC is responsible for:
- Together with the instrument teams, defining the scientific operations for all mission phases;
- All aspects of (science) mission planning;
- Support to the PI teams for issues related to payload operations;
- Validation of spacecraft command sequences sent to VMOC;
- Providing a ‘quick-look’ science data facility which provides an opportunity for rapid feedback into operational planning;
- Data archiving, including preparation of the archive and validation of content.
The broad spectrum of mission goals, the complexity of the payload, and the existence of severe operational limitations due to re-use of the Mars Express spacecraft design justify the need for careful planning of observations in order to effectively use the spacecraft and payload capabilities and to maximise overall science return. Science planning is a complex process that takes into account requests from all experiments and balances these with the operational constraints. The entire planning process involves close collaboration between the PI teams, VMOC and VSOC. Trade-offs have to be made between different observations and spacecraft pointing in order to meet the mission scientific objectives while still respecting the spacecraft constraints. Following discussions with all parties VSOC consolidates the various inputs into a schedule of observations.
During routine operations three planning cycles are used: Long-term planning began about 2 years prior to launch. During this period VSOC and the Venus Express Science Teams produced the Science Activity Plan (SAP) which outlines science activities for the entire mission. As the observation period approaches, VSOC initiates the medium-term planning which covers periods of a month and involves adding further details to the SAP. During this phase pointing profiles are fixed and instrument operations are defined at mode level (eg science, calibration or standby). Certain resources, such as the data volume distribution across the instruments and the power consumption are firmly fixed. This exercise results in the Medium Term Plan (MTP). The final cycle, short-term planning, covers periods of a week. The MTP is expanded and the final operational details, including the payload telecommand sequences and instantiated parameter values, are produced. Following verification by VMOC these data are uplinked to the spacecraft.
Planning Tools
A number of customised software planning tools have been developed by VSOC. The Venus Express Project Test Bed (PTB) allows real-time simulation of the spacecraft behaviour and its environment. It is used by VSOC and the PI teams to simulate the mission and to generate top-level timelines (used in the long- and medium-term planning).
MAPPS, the Mapping and Payload Planning Software, is a reduced version of the PTB which allows the user to display a 2-D map of Venus in order to visualise the spacecraft orbit over the planet. It shows key aspects such as spacecraft position over the target, Sun and Earth positions, and day/night boundaries. MAPPS facilitates the computation and display of items such as the projected field-of-view of an imaging instrument on the surface of Venus.
The Experiment Planning System (EPS) is embedded within MAPPS. It models the experiments so that the proposed timelines, including details such as payload mode, pointing mode, data volume requirements, power usage etc., can be validated. EPS, used for short-term planning, allows the VSOC to verify that the timeline is feasible and compliant with all system constraints.
Pointing modes
A key aspect of the planning is to define the pointing mode to be used during the different observations. In the original mission scenario four modes were identified:
- NADIR: this mode is used for looking at the planet.
- INERTIAL: used to stare at a fixed direction in space. This mode is employed for stellar and solar occultations and for inertial limb crossings.In the latter, the bore-sight of the spacecraft will slowly sink through the limb. A modification to this mode, the LIMB mode, was later introduced to allow tracking of a fixed altitude in the limb.
- MOSAIC: this mode facilitates the creation of a large image from a series of smaller ones
- CUSTOM: this is applied mostly for the radio science instrument when using atmospheric refraction of radio waves to determine atmospheric temperature at different heights.
A number of additional pointing modes have subsequently been introduced. These are primarily used for observations with SPICAV, VIRTIS and VMC:
- PENDULUM: this permits NADIR observations in the ascending branch of the orbit in hot seasons when the thermal constraints on the spacecraft normally would prevent any science operations. This has resulted in a factor of 2-3 increase in the number of NADIR-type observations. In this mode the spacecraft alternates between a NADIR orientation which exposes sensitive faces of the spacecraft to full Sun, followed by a slew to a cooling attitude in which excessive heat can be radiated away. These two orientations are repeated over a period of, typically, 9 hours.
- SPOT TRACK: used to continuously track a point on the surface of Venus. The latitude, longitude and height above surface of the target point are pre-programmed into the observation schedule. This was introduced to avoid the inherent drifting that occurs with observations made in NADIR mode.
- PLANET TRACK: this mode facilitates the continuous tracking of other celestial bodies by actively steering the spacecraft. Celestial coordinates are pre-programmed into the schedule. It is used to make long distance observations of Earth, Mercury and Mars.
Science Data and Archiving
Telemetry from the spacecraft is received at VMOC from where it is distributed to the PI teams and to VSOC. Upon receipt of the telemetry VSOC perform a quick-look analysis to identify possible performance issues and to provide quick feedback into the planning process. They also transform the spacecraft attitude and trajectory data into ‘SPICE’ kernels, a data format widely used by the planetary community for geometrical and other calculations. Data from the flight dynamics team are converted into MAPPS compatible format and provided to the PI teams so that they have the latest attitude and orbit files available for science planning.
Each PI team has a proprietary period of 6 months during which period they calibrate and process the raw data, analyse the scientific products and prepare the data (raw and calibrated) to be delivered to the ESA Planetary Science Archive. VSOC also validate the data and perform quality checks prior to the public release of the data.
More detailed information on the science planning activities for Venus Express can be found the following documents (see Related Links in the right-hand menu):
Titov, D.V. et al., "Venus Express science planning", Planetary and Space Science, Volume 54, Issue 13-14, p. 1279-1297, November 2006, DOI:10.1016/j.pss.2006.04.017
Koschny, D. et al., "Venus Express Science Planning and Commanding", ESA SP-1295, p. 1013, November 2007