Method for acquiring images of a terrestrial zone using a spacecraft comprising a laser transmission module

A method for acquiring images by using a spacecraft, said spacecraft being in a moving orbit around the Earth, the spacecraft comprising:

• an observation instrument associated with a field of view that is fixed in the spacecraft frame of reference, said field of view being defined, around an optical line of sight, by a sensor array located in a focal plane of the optics of the observation instrument,
• a laser transmission module associated with a laser line of sight that is fixed in the spacecraft frame of reference, said laser transmission module being located in the focal plane or in a secondary focal plane of the optics or in an intermediate focal plane of part of the optics, the laser transmission module being adapted to transmit data comprising images acquired by the sensor array of the observation instrument, to a laser reception module,
• said method comprising one or more phases of image acquisition by the sensor array of the observation instrument, and a phase of data transmission by the laser transmission module, said transmitted data comprising one or more images acquired by the sensor array of the observation instrument,
• wherein, during each acquisition phase and each transmission phase, the attitude of the spacecraft is controlled according to a setpoint on the Earth's surface that was predetermined for the acquisition or transmission phase in progress, said attitude control comprising:
• a pointing modification step during which the attitude of the spacecraft is modified so as to orient the optical line of sight or the laser line of sight towards the setpoint,
• a pointing immobilization step during a time interval referred to as the immobilization period, during which the attitude of the spacecraft is controlled so that the optical line of sight or the laser line of sight is kept oriented towards said setpoint during the immobilization period,
• wherein, the spacecraft having a ground speed Vsol and the observation instrument being associated with a spatial resolution Rs along a direction of travel, the immobilization period is greater than Rs/Vsol for each acquisition phase,
• wherein, a ground footprint of the field of view being of length L along the direction of travel, said method comprises at least two successive phases of acquiring images of two parts P1 and P2 of a terrestrial area Z, wherein the pointing modification step of the image acquisition phase for part P2 is executed during a time interval of duration β such that the value Vsol·(α+β)·1/L is less than a value R, R being equal to 3, a being the immobilization period of the acquisition phase for part P1.