Nominal Observation Mode
On Proba-V, when nominal observation mode is active, the operational scenario is fully automatic and performed on-board, based on a reference scenario which can be tuned by a number of configurable parameters.
The nominal scenario orbit has the following phases:
Starting from north to south (descending part of the orbit), the satellite is in a geodetic attitude with yaw steering correction. It images areas with land coverage, within specific latitude restrictions and illuminated with a minimum sun angle. During this phase, the integration time is continuously updated to optimize the instrument settings w.r.t. the light conditions of the imaged area.
When -56 degrees latitude is reached (latitude of Cape Horn), the satellite stops imaging and autonomously enters in a sun-bathing mode, changing the attitude to maximize the power input from the sun. During this phase, the instrument is set into idle mode.
When entering eclipse on the ascending part of the orbit, the satellite returns to a geodetic pointing mode.
On the exit of eclipse, the instrument is re-activated to reach thermal stability by the time the first land areas are in visibility.
The reaction wheel momentum off-loading is autonomously performed when the instrument is not imaging. The momentum offloading management is de-activated 10s before the next imaging sequence to give some time for the platform to stabilize its attitude.
This cycle repeats itself autonomously without the need of commands from ground.
The above approach allows to activate continuous acquisitions of all scientifically interesting parts of the earth (land defined in the land-sea mask), with a single telecommand, which is the command to enter nominal observation mode. It activates the required units, commands the correct attitude, enables the autonomous instrument operations and runs the reference scenario continuously.
Example of PROBA-V one day coverage of the 3 cameras (green=right camera, red= center camera, blue = left camera)
The mission Principal Investigator (PI) has the possibility to request specific calibration activities. These are crucial for maintaining the quality of the scientific data and for correcting for instrument degradation. On average 3 specific calibrations are executed every day, they include acquisition over specific ocean sites for Rayleigh, Deep Convective Clouds and Sun glint calibration, while desert site calibration are part of nominal acquisition. To limit the load on the Mission Operations Centre for these regular activities, also these calibrations activities are automated.
In calibration mode, first the required off-pointing is commanded (if any), followed by the reconfiguration of the instrument. For calibration activities, typically, different instrument settings are required compared to nominal observations. This is not limited to instrument settings only, but can also include changes in the compression ratios of the science data (some calibrations require uncompressed data for example).
A special calibration request is the moon calibration. As this is not a zone on earth and the calibration is executed once every 28 days based on a specific moon phase angle, the handling of this request is slightly different from the other calibration requests. The moon requests have a separate onboard queue and the required maneuver is calculated on the ground due to the sporadic nature of the request.