Why did an Earth observation satellite turn to the moon?

A few months ago, between June 19 and 29, the Pléiades Neo 3 satellite acquired a whole series of pictures of the Moon. Recently put on orbit by a Vega launcher, on April 29, 2021, this satellite is in the acceptance phase. That is to say that before it is put into service, scheduled for the end of the year, the ground teams ensure that the satellite and its payload are operating nominally. As Gil Denis, engineer at Airbus Defense and Space explains, blogger on space andearth observation, « observing the moon is part of this process that spans several months “. Technically, it is ” absolute radiometric calibration, carried out by Cnes “. Explanations.

This series of observations of the Moon may come as a surprise from a satellite designed to observe the Earth, but it is necessary. There is indeed a ” real technical interest in observing the Moon “. Surprisingly, these ” images are necessary to verify the correct operation of l’instrument d’observation ».

To understand this little-known aspect of the Pleiades satellite recipe, you should know that Airbus Defense and Space, which markets the satellite data, needs to verify ” that what the satellite sees at a distance, that is to say the reflectance, an intrinsic characteristic of the observed ground, corresponds well to the reality on the ground “. In other words and in a simpler way, what interests customers is that the data digital « show what is really on the ground, that the colors views in images translate parameters physical observed terrain “. Calibration is required.

Observe the Moon to better observe the Earth

And to do it from orbit, it is necessary to calibrate the satellite, in particular from to the moon. Ideally, one would think that it is better to observe terrestrial regions but ” very few regions, the polar zones for example, lend themselves to these observations in particular because one does not know precisely the models of brightness and the nature of the soils, andatmosphere terrestrial disrupts the measurements ”. Conversely, the Moon is perfect for this calibration because “ it has no atmosphere and the necessary information is known with precision ».

To carry out this calibration from the Moon, he “ s‘acts of‘estimate an absolute radiometric calibration coefficient “, Which can be done using a” detailed model of lunar reflectance as a function of the relative positions of the Sun, Moon and satellite “. This operation is done at the time of Full moon when the face is lit from the front, closest to the Earth. This type of calibration is performed at regular intervals, throughout the life of the satellite, ” because the characteristics of the instruments change over time and that this must be taken into account to ensure at all times that what the digital data shows perfectly reproduces what is on the ground ».

To carry out these acquisitions, a special so-called “inertial” guidance was put in place: ” at the time scale of the shot of a few seconds, the Moon does not “scroll” under the satellite, but remains in a fixed direction ».

If the observations are technical, the images produced are all the same very photogenic, even bluffing because of the sharpness of the images which “ illustrates the performance of the satellite “. The instrument which produces images at 30 cm from resolution on the surface of the Earth obviously does not offer us the same resolution. At this ” distance, nearly 360,000 kilometers from Earth, the resolution on the lunar soil is not 30 cm: each pixel covers a square with a side of about 175 meters “. We cannot therefore see the machines of the missions Apollo, but the relief and shadow of the craters, as well as the terminator observed by Pleiades Neo 3 might suggest that the satellite is much closer to the Moon than it actually is.