A satellite is a complex technical system made up of several parts: the platform, the instrument, and antennae. The platform manages the scientific mission entrusted to the satellite, provides it with energy – thanks especially to its solar panels – and carries the measuring instrument. It also guarantees the necessary stability and alignment for the instrument to take measurements, and acts as a communications interface with the ground segment. A satellite has to survive on its own, since there is no after sales service in space. Autonomy and efficiency are absolutely essential.
To increase the likelihood that it will carry out its mission in accordance with what it is expected to do, in all possible situations and during its entire lifetime (this is what is known as its reliability), the satellite must meet a very high level of requirements, that of being space-qualified. This is true during design, manufacturing and integration. The problem is that a satellite leads a pretty hectic life. When it is launched, and then when it is put into orbit, it is bumped and jolted around a great deal. It is then exposed to an extremely hostile environment, bombarded by solar radiation and heavy ions*, and surrounded by several tens of thousands of pieces of space debris. It is constantly subjected to extreme changes of temperature (from + 100 °C in full sunlight to -100 °C in the shade). And on top of that, it is surrounded by the vacuum of space, in a state of near weightlessness. Before it is launched, therefore, it undergoes a long series of tests that reproduce such extreme conditions.
To increase the likelihood that it will carry out its mission in accordance with what it is expected to do, in all possible situations and during its entire lifetime (this is what is known as its reliability), the satellite must meet a very high level of requirements, that of being space-qualified. This is true during design, manufacturing and integration. The problem is that a satellite leads a pretty hectic life. When it is launched, and then when it is put into orbit, it is bumped and jolted around a great deal. It is then exposed to an extremely hostile environment, bombarded by solar radiation and heavy ions*, and surrounded by several tens of thousands of pieces of space debris. It is constantly subjected to extreme changes of temperature (from + 100 °C in full sunlight to -100 °C in the shade). And on top of that, it is surrounded by the vacuum of space, in a state of near weightlessness. Before it is launched, therefore, it undergoes a long series of tests that reproduce such extreme conditions.
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Continuity and quality of data
Finding ways of combining scientific goals and technology is at the heart of the collaboration between researchers, agencies and industry when defi ning space missions for climate observation. The state of one component of each sphere of the Earth system, for instance the humidity of a soil, the composition of an area of vegetation, or the state of the surface of a lake, can be characterised by a wavelength that is specifi c to a measuring instrument, which may be either active or passive. And in the electromagnetic spectrum, there is a particularly wide range of wavelengths to choose from! Passive instruments react to 'natural' wavelengths (sunlight, infrared, etc) in the same way as a camera. They make up the family of radiometers, which is divided into subfamilies according to the range of wavelengths processed, the wavelengths to be measured (called channels), and other characteristics requested by scientists. Some examples: optical cameras, spectrometers*, multispectral* or hyperspectral* instruments, microwave radiometers, and sounders. Active instruments generate electromagnetic waves which are then refl ected off the surfaces that are targeted (these are known as lidars, SAR, altimeter radars, etc.) The signal that returns to the instrument contains information about the object observed. How does this end up as a visible 'image'? The instrument and/or the ground segment process this signal with powerful computers in order to extract the 'useful' part which contains information about the target, from the surrounding 'noise' of the measurement.
Scientists frequently carry out field campaigns to update correspondence tables between measurements and physical and chemical properties of the target. [It is essential to make sure that measurement remains stable for the whole length of the mission. This calibration is carried out, for instance, by aiming the instrument at a known terrestrial target and making sure that there is no measurement drift. Measurements from space are based on a precise reference, that of the geoid. Although the Earth appears to be a near-perfect sphere, it is not in fact a uniform object with regard to mass, and therefore not with regard to gravimetry*, which has an effect on measurement references. This is why space missions are regularly launched with the aim of modelling the gravity field, and hence the measurement reference, as accurately as possible.
Tomorrow's satellites and instruments are being prepared today with the space agencies. Among the areas for research and development (R&D): instruments that can carry out measurements for a longer time, more accurately, and that can 'see' better; and satellites that are lighter, more manoeuvrable and able to transmit more information that is vital for the monitoring of the climate.
Dominique Murat
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