The test and measurement vessel (BEM) Monge is a ship of the French Navy that doesn’t go unnoticed when it is docked at the Brest naval base. With its imposing white silhouette measuring over 220 meters in length, 26 meters in width, and its three large radars named Armor 1, Armor 2, and Normandie, it has literally become an iconic element of the Brest harbor.
Especially when it emits a strange green beam that doesn’t fail to attract attention: the lidar (laser imagine detection and ranging) mesospheric. “There was already one on the predecessor of the Monge, the Poincaré, but since then, we have refined our knowledge to have this instrument, one of the most performant in the world,” attests civil engineer Laurent, head of a technical department of the Directorate General of Armaments (DGA), aboard the Monge for the past twenty years.
The role of this green laser is very precise: to study the atmosphere up to one hundred kilometers, the limit of what we can call space, in order to guide the missile tests of the DGA. “It really helps us to characterize, in terms of density, humidity, pressure, and temperature, the layer of the atmosphere,” specifies the commander of the Monge, Captain Alban Doulcet. “The lidar is a unique object in Europe, which feeds our scientific models, including those of the DGA, and allows us to better exploit the data we will gather on the day of the launch.”
Its operating principle is quite simple: vertically emit a powerful laser beam. “In the eye, we see a continuous line but in reality, it is a series of light pulses composed of photons, at 100 hertz,” specifies the civil engineer. “And it will always be green in color, which corresponds to the adapted wavelength, namely 332 nanometers.”
Regarding the processing of the data, it gets a bit more complicated. “Some of the photons, which compose the light, are absorbed by the atmospheric particles which release absorbed energy by emitting another photon. A proportion of these retro-diffused photons return downwards and are collected thanks to the reception telescopes,” specifies the DGA civil engineer. “The number of received photons will then be proportional to a certain altitude.”
“In Brest, we only carry out simple missions to verify that everything is working properly. The lidar is a fantastic instrument but even the slightest cloud blocks the measurement.” Nevertheless, in Brest, it is primarily verification exercises that are carried out at night when weather conditions are favorable: clear skies, without clouds.
(Context: The Monge is a unique vessel used for missile testing and data collection) (Fact Check: The Monge is equipped with specific instruments dedicated to data collection and missile tracking)
Furthermore, the Monge becomes a naval platform that moves, and we can observe things where others cannot. And especially, sometimes very distant things, since the Monge can also be used to track satellites, the International Space Station (ISS), becoming a true link between the sea and space.
(Context: The Monge is equipped with radar and tracking systems for missile testing) (Fact Check: The Monge’s radars are unique in Europe and are used for tracking missiles and other objects)
