Taking advantage of what we can’t see

Although the term occultation could lend itself to very different interpretations, according to the imagination of each one, we are going to talk about a very specific one, the “Radio Ocultation”, RO, as it is known in the field of the satellite exploration and its applications to weather forecasting.
To explain what it is, we must explain the phenomenon of refraction. We are very familiar with the refraction of light as it passes through a body of water; We know that light rays alter their direction, they “refract” and in this way, for example, for those who see the rowing of a boat from under the water, it turns out that the image that is seen does not correspond with the expected reality, rowing It is not in the apparent straight line due to refraction.
The handle of a garbage collector, when immersed in water, seems to break.
When going from one medium to another with different density (from air to water as in the example), the direction of the ray of light changes, the ray is refracted. The angle formed between the original ray line and the direction of the new ray is called the “refractive index”.
This is “seen” with visible light, but also with other non-visible rays, such as the familiar GPS signals.
And a very important detail, the index of refraction, depends on the density of the medium, if this is the atmospheric air, will be affected by humidity and temperature.
The GPS signals go from the satellite (Global Positioning System) located in geostationary orbit to other satellite in low orbits (LEO) and pass through the atmosphere, which, depending on the concentration of water vapor and the temperature, refract them.
An important part for climate analysts is the establishment of the altitude of the “tropopause”, which is the part of the atmosphere that lies between the troposphere and the stratosphere.
The radio concealment technique was originally developed at Stanford University and JPL (Jet Propulsion Laboratory) who gave it this name, RO. In 1995 the studies and the precise measurements of these deviations began, to try the correlation with the atmospheric characteristics, the meteorological predictions and the study of the climatic change.
If the signal is picked up by several satellites at a time, the measured deviation will correspond to different atmospheric layers, rather, to different conditions of the same atmosphere at different altitudes.
A decade later there is already an operational system for climate prediction and climate research, the GRAS system, which is aboard the EUMETSAT series of EPS satellites.
The figure shows the satellite that emits the GPS signal and three satellites in LEO orbit, which capture the same signal with different refractions depending on the atmospheric layers it crosses.

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