FR2730808A1 - Calibration system for radiometer mounted on board satellite - Google Patents

Abstract

The system is intended for calibration by measuring the solar radiation during the flight of a terrestrial satellite. The satellite carries a radiometer (10) which is subject to the solar radiation via calibrating device (26) consisting of a diffuser (34). The device operates for a given angle of incidence of radiation w.r.t. the radiometer. The measured value of the diffused radiation is compared with a value which is held as a reference created during calibration of the radiometer on the ground. The measurements are repeated several times during the year, when the satellite is in predetermined positions defining the required angles for measurement.

Description

 The present invention relates to a method of solar calibration in flight of a radiometer on board a terrestrial satellite as well as a radiometer intended to be on board of a satellite and comprising a device for solar calibration in flight of at least one radiometric channel of the radiometer.

 A radiometer is an instrument for measuring the energy flow carried by radiation.

 Radiometers are notably used to carry out climatic measurements.

 To this end, earth observation satellites are equipped with a radiometer which includes a rotary scanning head making it possible to reconstruct the radiation field coming from all the points of the scene seen from the satellite.

 These radiometers have a relatively small number of channels, generally between 2 and 5, with a fairly wide spectral band, in the visible or infrared wavelength ranges.

 An example of a radiometer on board a scientific satellite is described and represented in the article entitled "The ScaRaB earth radiation budget scanning radiometer published in the journal METROLOGIA (1991, ne28, pages 261 to 264).

 Each of the radiometric channels generally uses a very sensitive pyroelectric type sensor, the response of which requires regular calibration so that the corresponding radiometric channel provides a quantitative measurement with sufficient precision.

 It has already been proposed to use the radiometer to calibrate in flight the different radiometric channels.

 The on-board calibration sources vary depending on the nature of the radiometric channels.

 Some channels require a black body or a black body simulator. These calibration reference sources do not present any problem of stability over time and it is therefore possible to perform, reliably over time, regular calibration of these radiometric channels.

 On the other hand, for other channels such as the "visible" and "solar" channels, the reference sources for the calibration are constituted by lamps, or groups of lamps, with filament which themselves exhibit a significant drift in over time.

 This calibration means does not make it possible to characterize the variations in the response of the radiometric channel for short wavelengths (visible plus ultra-violet).

 To correct these variations, a channel calibration must be carried out with a source characterized by its energy and spatial stability and whose spectrum must cover the entire spectral band.

 The sun is the source of radiance that best meets these two conditions.

 In addition, it is desirable that the spectrum of the ideal source of calibration is the most similar to that of the albedo observed.

 In this respect, it will be noted that the solar spectrum is close to the spectrum of the scenes observed by the radiometer.

 It thus appeared that a solar calibration device in flight on board the radiometer would bring a performance gain on the final precision of the measurements made by the radiometer.

 The object of the invention is to propose a new solar calibration device in flight which makes it possible to perform, at regular and relatively frequent intervals, an absolute or relative solar calibration of the radiometric channels and which makes it possible to improve the final performance of the measured radiance.

To this end, the invention proposes a device for solar calibration in flight of a radiometer on board a terrestrial satellite whose orbit is heliosynchronous, characterized in that it consists
- subject the on-board radiometer to solar radiation, with the interposition of a solar calibration device in flight comprising a diffuser, for at least a determined angle of incidence of solar radiation relative to the radiometer, and to measure the value in flight radiation scattered by means of at least one radiometric channel of the radiometer; then
- compare the measured value of the scattered radiation with a reference value established during a preliminary calibration step on the ground of the radiometer during which the radiometer was subjected to solar radiation with the same angle of incidence, and during from which the value of the radiation scattered by the solar calibration device in flight was measured.

According to other characteristics of the process
- the step of in-flight measurement of solar radiation is repeated several times during a year at determined instants of the orbit of the earth around the sun and of the orbit of the satellite around the earth, instants at which said angle of incidence has a known determined value and to which the in-flight solar calibration device is subjected to solar radiation, the prior ground calibration step having made it possible to establish a series of reference values corresponding to several known determined values the angle of incidence
the first in-flight solar calibration is carried out at the first instant following the putting into orbit of the satellite and at which the angle of incidence of the solar radiation corresponds to a determined value known of the angle of incidence, and it is optionally carried out a correction of the reference values established on the ground as a function of the difference between the first value measured in flight and the associated reference value established on the ground
- Said determined angle of incidence of solar radiation relative to the radiometer corresponds to a determined angle of the earth-sun direction relative to the plane of the heliosynchronous orbit.

 The invention also provides a radiometer intended in particular for implementing the method according to the invention.

 To this end, the invention provides a radiometer on board a sun synchronous satellite, characterized in that it comprises a device for solar calibration in flight of at least one radiometric channel of the radiometer, characterized in that the calibration device solar in flight comprises means for transforming the incident solar radiation into uniform radiation over the field of the radiometric channel, with a given radiance.

According to other characteristics of the radiometer
the transformation means comprise a prism for deflecting the incident solar beam
- the prism is a prism with total reflection
the transformation means include means for attenuating and homogenizing the flow of the incident solar beam
- the attenuation and homogenization means include at least one diffuser per transmission
- the attenuation and homogenization means include a stack of diffusers by transmission
- the mitigation and standardization means include an afocal objective arranged upstream of the diffuser by transmission
- the attenuation and homogenization means are located between the deflection prism and the entry of the radiometric channel;
- The in-flight solar calibration device includes an input baffle adapted to the field of the optical system of the in-flight solar calibration device and which protects the device against radiation and / or stray light;
the solar in-flight calibration device is fixed relative to the radiometer frame which rotates a scanning head comprising at least one radiometric channel
the solar calibration device in flight is movable relative to the radiometer frame which rotates a scanning head comprising at least one radiometric channel so as to allow in particular an orientation of a prism of the device with respect to the incident beam of the solar radiation.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows for the understanding of which reference will be made to the accompanying drawings in which
- Figure 1 is a schematic perspective view of a radiometer equipped with a solar calibration device in flight produced in accordance with the teachings of the invention
- Figure 2 is a diagram for illustrating the two main areas of visibility of the sun from the radiometer as a function of the position of the satellite in its orbit
FIGS. 3A and 3B are two diagrams which illustrate the variation of the orientation of the sun during the year and which show the zones of the orbit of the satellite usable for carrying out a solar calibration
- Figure 4 is a diagram which illustrates the relative movement of the direction of the sun with respect to an instrument mark of the radiometer
FIG. 5 is a graph making it possible to illustrate the determination of the instants at which a solar calibration in flight can be carried out in accordance with the teachings of the invention;
- Figure 6 is a perspective diagram which illustrates the arrangement of the main components of an embodiment of the in-flight solar calibration device with which the radiometer of Figure 1 is equipped
- Figure 7 is a schematic detail view which illustrates the arrangement of the optical components of the means for transforming the incident solar beam with respect to the radiometric channel
- Figure 8 is a view similar to that of Figure 7 which illustrates a second embodiment of the means for attenuation and homogenization of the incident solar beam.

 There is shown in Figure 1 a radiometer 10 whose general design is known and for the detailed knowledge of which we can for example refer to the aforementioned article in the journal "METROLOGIA".

 The radiometer 10 comprises a frame 12 intended to be fixed to a platform 14 of a meteorological satellite 16 (see FIG. 2).

 The frame 12 rotates, around a geometric axis Ys, a scanning optical head 18 comprising for example four radiometric channels 20.

 The radiometer 10 also includes an electronic unit 22 and an antenna 24.

 In accordance with the teachings of the invention, the radiometer is equipped with a device 26 for in-flight solar calibration which is on board the radiometer and which is illustrated diagrammatically in FIG. 1.

 The design of the onboard flight solar calibration device will be described in more detail below.

 It will however be noted, as of now, that the device 26 comprises a sighting cone 28 which determines an AVS axis for aiming the sun.

 In accordance with the teachings of the invention, the installation of the in-flight solar calibration device 26 on the radiometer 10 is a function of the determination of the areas of visibility of the sun by the aiming cone 28 as a function of the position of the satellite 16 on its orbit, knowledge of the sun's trajectory in the instrument reference frame of the radiometer (during the satellite mission), and determination of the instants of calibration in flight.

 To determine the visibility of the sun by the solar calibration device in flight 26, it is necessary to take into account the masking or occultation of solar radiation by the earth, by the satellite 16, and in particular by its platform 14 as well as by d other instruments carried by the platform 14.

 Satellite 14 is a sun-synchronous meteorological satellite so that it flies over a region of the terrestrial globe T at an hour which depends only on latitude.

 Satellite 14 is therefore on a mission in a sun synchronous orbit whose celestial longitude of the ascending node derives from 360 per year in the direct direction. The plane of the orbit thus retains a fixed position in space relative to the direction of the sun S.

 The orbit is thus defined by the inclination of the plane of the orbit, that is to say the angle made by the plane of the orbit with the plane of the equator which, in the case of a Sun-synchronous orbit, is close to 100, and by the period of revolution, that is to say the time necessary for the satellite to achieve exactly one revolution in its orbit.

 The orbit is also characterized by the right ascent of the ascending node of the orbit, the ascending node being the point of this orbit where the satellite flies over the equator while passing from the southern hemisphere to the northern hemisphere. Its right ascent is the angle made, from the center of the earth, by its direction with that of the vernal point.

 As an example, the sun synchronous orbit of satellite 14 is calibrated in such a way that the time at the descending node is 10:00 a.m.

 As can be seen in FIG. 2, there is therefore a first shadow zone Z1 which is linked to the geometrical characteristics of the orbit and which is located on the side of the ascending node of the orbit.

 When the satellite is in the zone illuminated by the sun S, there is a zone Z2 in which the radiometer 10 is in the shade of the platform 14. A solar calibration is therefore not possible in these two zones from orbit.

 The arrangement and configuration of the various instruments, such as the radiometer 10, are constant on the platform 14 during the orbit and it is therefore possible to verify, when designing the satellite 16, that no instrument obstructs the viewing direction of the sun.

 The diagram in FIG. 2 shows that there is a first “sunny” zone of the radiometer 10 at the passage of the north pole corresponding to a journey of approximately 28 minutes and a second zone situated at the passage of the south pole corresponding to a journey of about 15 minutes.

 The sun S actually makes an average angle of about 30 relative to the plane of Figure 2.

 The orientation of the sun therefore varies during the year due to the inclination of the earth's axis T and its elliptical orbit around the sun during the year. This results in a variation in the size of the usable "sunshine" zones of the radiometer 10 during the year.

 This phenomenon is illustrated in FIGS. 3A and 3B, the first of which represents the size of the areas which can be used in winter and the second of which illustrates the size of the areas which can be used in summer.

 Figures 2, 3A and 3B show that it is possible to perform solar calibration only in the two areas near the north and south poles.

 However, the area near the north pole is preferable because the field of view of the radiometer is then clear towards the sun.

 On a given date, the direction of illumination by the sun S describes a cone C whose half-angle at the top is equal to the angle between the direction of the sun and the normal to the plane of the orbit, and whose axis is this normal.

 The design of the satellite is such that it ensures a constant orientation of the axis of the radiometer which is always tangent to the orbit.

 For this purpose, the axis of rotation of the satellite is perpendicular to the plane of the orbit.

 There is shown in Figure 4, by arrow F, the direction of movement of the sun.

 The half-angle of the cone described by the direction of the sun varies according to the date.

 To identify the direction of the sun relative to the instrument, two angles can be defined.

 The first teta angle is the declination of the sun, that is to say the angle between the earth-sun direction and the equatorial plane. This angle varies depending on the season between +23 27 and - 23 27.

 The second angle is the angle psi between the plane of the orbit and the earth-sun direction. This direction should theoretically remain constant since the orbit is heliosynchronous and it must be equal to 30 ".

 However, the psi angle varies during the year between the values 23 2 and 34 4.

 The graph in FIG. 5 makes it possible to represent the position of the sun S during the year as a function of the angles psi on the abscissa and teta on the ordinate.

 Several traces of TOi orbits have also been shown in FIG. 5.

 The intersection of the traces TOi with the curve of the sun CS gives the position of the sun for a given orbit.

 The right vertical axis of the graph in FIG. 5 is graduated in time to determine the instant of solar calibration, the time "t" being equal to O at the descending node of the orbit.

 As we have seen with reference to FIG. 4, the sun describes a cone C with an axis parallel to the axis Xs of the satellite frame of reference and of half-angle at the top which is the angle complementary to the angle psi and which will therefore vary during the year.

 Note also that the sun is located in the satellite frame by the azimuth angle and by an elevation angle in the axes of the instrument frame.

 A calculation of the apparent angular speed of the sun shows that it is not necessary to follow the sun in elevation during the calibration phase.

 In FIG. 5, three traces of orbits TO1, TO2 and TO3 have been represented.

 Each of these traces intercepts the position curve of the sun CS in two or four positions of the sun S.

 These positions, for each of the orbits considered, are characterized by the same angle psi of incidence of solar radiation.

 It has thus been shown that it is possible to determine orbits and, as will be explained later, instants of the path of these orbits for which the value of the angle of incidence of solar radiation is precisely known. compared to the satellite and therefore compared to the radiometer and its solar calibration device in flight 26.

 In the example illustrated in FIG. 5, the choice of only three angles of incidence makes it possible to carry out, over the course of a year, eight solar calibrations in flight which are all almost equally distributed over time.

 The instants of calibration in flight vary according to the orbit TOi chosen and they are determined directly on the graph in FIG. 5.

 For example, for the angle of incidence corresponding to the trace of the orbit TO1, it will be possible to use a solar calibration in flight for the day ne125 at approximately 70 minutes after the passage of the descending node and for the day n 185 at approximately 69 minutes after passing the descending node.

 In addition, the values chosen for the incidence of solar radiation make it possible to envisage the arrangement of a solar calibration device in flight 26 fixed relative to the frame 12 of the radiometer 10.

 In accordance with the teachings of the invention, the process for determining the instants of calibration in flight has just been exposed, as well as the directions of observation of the sun at these instants of calibration, these parameters being perfectly determined.

 The solar flight calibration method according to the invention will now be described.

 It is proposed to use solar calibration in flight only as a relative calibration to characterize the variations in the normalized spectral response during the satellite mission and to allow regular calibration of the reference sources of the on-board radiometric channels. on the radiometer.

 Without departing from the scope of the invention, it is possible to use the calibration method according to the invention as an absolute calibration method.

 The calibration process begins with a step of calibration on the ground by the solar calibration device in flight which is carried out on the ground by observing the sun through the device 26.

 Contrary to the known technique, this observation is made through the solar calibration device in flight 26 which, as will be seen below, comprises an on-board diffuser, while the previously used solution called for a standard diffuser used at ground for solar calibration.

 If the solar in-flight calibration device is fixed, it is necessary to place the entire radiometer on an adjustable mechanism making it possible to aim the sun by the solar in-flight calibration device 26, and this for all angles of incidence determined to calibrate.

A simple turntable with indexed positions may suffice.

 Calibration measurements made on the ground to determine reference values, must simulate the measurement that will be made in flight.

 We must in particular modify the equatorial movement to simulate the orbital rotation of 360 in 100 minutes.

 For each angle of incidence to be calibrated, it is desirable to make several measurements at slightly different angles so as to perform a flight calibration on several consecutive orbits.

 In the case where the calibration device 26 is mobile, it suffices to provide, for the calibration on the ground, a position of the device for which the direction of sight is in the plane of the scanning of the radiometer. It is then possible to position the radiometer on the equatorial mount used for testing the satellite, so as to aim at the sun in this position. It is then easy to rotate the head 18 between the aiming position and the calibration position, then to compare the two.

 Once the ground calibration operations of the solar flight calibration device have been carried out, it will be possible, as soon as the satellite is in orbit, to carry out regular flight calibration operations at the instants determined above, then to carry out a recalibration of the reference sources as a function of the comparison between the calibration reference values established on the ground and the solar calibration values measured in flight.

 In flight, the first solar calibration whose angular incidence is known will serve as a relative reference. Depending on the possible difference between the results of this first solar calibration and the results of the ground calibration for the same angle of incidence, it is possible to readjust the calibration curve in flight.

 Then, the in-flight calibrations are carried out at regular intervals, as explained above, corrected for variations in incidence and they will make it possible to follow the degradation of the normalized spectral response.

 It has been assumed that the normalized spectral response is stable between the time when the ground calibration is carried out and the time when the first solar flight calibration is carried out.

 The procedure for triggering a solar calibration in flight is as follows.

 Before the calibration instant, a shutter mechanism (not shown) is opened and the corresponding radiometric channel is placed in the angular position for solar calibration.

 The sun will pass in front of the solar calibration device in flight 26 with an apparent angular speed.

 Depending on the opening field of the solar calibration device in flight, the scrolling time during which it receives solar radiation is approximately 100 seconds, during which time data can be acquired.

 It is possible to repeat this operation on several successive orbits and to carry out a treatment on the posterior ground to determine the filtered solar radiance, taking into account the fact that the variation of the angle of incidence of the solar radiation during these successive orbits acceptable for the diffuser incorporated in the solar in-flight calibration device.

 The diagram in FIG. 6 shows that the in-flight solar calibration device 26 is essentially constituted by a baffle 30, by a prism with total reflection 32 and by an optical diffuser-attenuator system 34.

 The cabinet 30 which is located in the extension of the aiming cone has an AVS solar aiming axis and it is designed in particular to protect the device from stray light and radiation.

 The solar beam which has entered the device through the cone 28 and the baffle 30 then reaches the prism with total reflection 32 which makes it possible to ensure a deflection, or bending, of the incident solar beam in order to orient it along the ACE axis. of the solar calibration channel.

 This bending between the AVS and ACE axes is made necessary by the arrangement of the instrument on the satellite and by the orientation of the scanning head 18.

 The user of a total reflection prism made for example of silica guarantees very great stability over time, this material being almost unalterable.

Total reflection guarantees a reflection coefficient of 100% for a small range of incidences, whatever the wavelength in the spectral band
the considered.

 The angle of the prism is calculated according to the index of the material at the various wavelengths, taking into account variations as a function of temperature, so as to have a margin of safety with respect to the angle of total reflection.

 Ground calibration is considerably simplified because only purely geometric parameters are to be taken into account.

 The prism may have a reflecting face which makes an angle of about 50 with the entry face.

The angle being different from 45, this difference must be taken into account for the determination of the calibration instants.

 The entry face of the prism is not frosted because, otherwise, part of the scattered rays would not be reflected and the exit flow would be asymmetrical. In addition, the rays retroreflected by the exit face will be evacuated outside the prism.

 In accordance with the teachings of the invention, the attenuation device 26 also comprises means for attenuation and for homogenization of the incident solar beam constituted by an optical diffuser-attenuator system.

 In the embodiment shown diagrammatically in FIGS. 6 and 7, this optical system consists of a stack 34 of transmission diffusers 36 which are made of silica.

 The stack of diffusers 36 is interposed between the outlet of the prism 32 and the inlet of the radiometric channel 20 carried by the head 18.

 The realization of the optical diffuser-attenuator system by stacking diffusers by transmission 36 is particularly simple to implement.

 The attenuation of the solar flux depends on the number of diffusers 36 stacked and the homogenization due to the successive diffusions makes the device practically insensitive to variations in the angle of incidence.

 Attenuation is in all cases made necessary because the incident solar flux is on average ten times greater than the dynamics of the radiometric channels.

 In the variant embodiment illustrated in FIG. 8, the optical diffuser-attenuator system is constituted by a transmission diffuser 36 and by an afocal objective 38 interposed between the output of the prism 32 and the channel 20.

 The angular magnification of the afocal objective is chosen to allow the beam to be attenuated and so as to reduce the sensitivity to a variation in the angle of incidence at the entrance to the objective.

 The diffuser 36 then serves to standardize and homogenize the beam in order to obtain uniform illumination of the radiometric channel 20.

 However, this solution has an entrance pupil of reduced dimensions and it imposes additional constraints as for the positioning of the cabinet 30.

 The design of the in-flight solar calibration device 26 is therefore such that it makes it possible to transform the incident solar radiation into uniform radiation over the field of the radiometric channel with a given radiance thanks to the bending of the beam, to the uniformization over the pupil, attenuation of the beam and taking into account possible variations in the angle of incidence of solar radiation by at least one diffuser per transmission.

 It should be noted that the choice of silica to produce the main optical components of the solar calibration device in flight, a material which is recognized for its stability in a space environment, makes this device almost insensitive to temperature variations as well as to phenomena. contamination and aging of optical components.

 In order to further improve the reliability of the solar calibration device in flight, it is possible to provide a shutter mechanism which will protect all the components of the space environment during periods of non-use of the solar calibration device 26.

 The error due to the aging of the components is therefore practically zero, as is the error due to the effects of contamination.

 The accuracy of the solar flight calibration device according to the invention also depends on the accuracy during the ground calibration operation.

 A first source of error would result from a misalignment of the solar calibration device with respect to a reference optical cube of the radiometer, the second source of error being the misalignment between the optical cube of the instrument and the satellite reference optical cube.

 These errors are not significant and will remain constant during the mission of the satellite.

 As a relative calibration is carried out in flight, these errors will not enter the error report. It is only necessary to ensure that the alignment accuracy on the ground is good enough for the sun to remain in the field of the cabinet.

 A source of error also results from the identification of the direction of solar aiming in flight.

 The attitude errors of the satellite as well as possible distortions of its structure during the mission must be taken into account.

 Finally, account must be taken of the alignment accuracy of the radiometric channel with the calibration system.

 Tests carried out on devices according to the state of the art have demonstrated that the angular depointing of the channel could be of the order of 0.2. Due to a stacking solution of diffusers by transmission, this deflection is almost negligible.

Claims (15)

 1. Solar calibration device in flight of a radiometer (10) on board a terrestrial satellite (16) whose orbit is heliosynchronous, characterized in that it consists  - subjecting the on-board radiometer (10) to solar radiation, with the interposition of a solar calibration device in flight (26) comprising a diffuser (34), for at least a determined angle of incidence of solar radiation relative to the radiometer, and to measure in flight the value of the radiation scattered by means of at least one radiometric channel (20) of the radiometer (10);  then  - comparing the value measured in flight of the scattered radiation with a reference value established during a preliminary calibration step on the ground of the radiometer (10) during which the radiometer (10) was subjected to solar radiation with the same angle of incidence and during which the value of the radiation scattered by the solar flight calibration device (26) has been measured.  2. Calibration method according to claim 1, characterized in that it consists in repeating the step of in-flight measurement of the solar radiation several times during a year at determined instants of the orbit of the earth around of the sun and the orbit of the satellite (16) around the earth (T), instants at which said angle of incidence has a known determined value and at which the solar calibration device in flight is subjected to solar radiation, the preliminary step of ground calibration having made it possible to establish a series of reference values corresponding to several determined values known of the angle of incidence.  3. Calibration method according to claim 2, characterized in that the first solar calibration in flight is carried out at the first instant following the putting into orbit of the satellite and at which the angle of incidence of the solar radiation corresponds to a determined value. known of the angle of incidence, and in that it is possibly a correction of the reference values established on the ground as a function of the difference between the first value measured in flight and the associated reference value established on the ground .  4. Calibration method according to any one of claims 1 to 3, characterized in that said determined angle of incidence of solar radiation with respect to the radiometer (10) corresponds to a determined angle of the direction earth-sun with respect to to the plan of the sun-synchronous orbit.  5. Radiometer (10) on board a sun-synchronous satellite (16), characterized in that it comprises a device (26) for solar calibration in flight of at least one radiometric channel (20) of the radiometer (10), and in that the in-flight solar calibration device (26) comprises means (30, 32, 34) for transforming the incident solar radiation into uniform radiation over the field of the radiometric channel (20), with a given radiance.  6. Radiometer according to claim 5, characterized in that the transformation means comprise a prism (32) for deflecting the incident solar beam.  7. Radiometer according to claim 6, characterized in that the prism is a prism with total reflection.  8. Radiometer according to claim 5, characterized in that the transformation means comprise means (34) for attenuation and homogenization of the flux of the incident solar beam.  9. Radiometer according to claim 8, characterized in that the attenuation and homogenization means comprise at least one diffuser per transmission (36).  10. Radiometer according to claim 9, characterized in that the attenuation and homogenization means comprise a stack of diffusers by transmission (36).  11. Radiometer according to claim 9, characterized in that the attenuation and homogenization means comprise an afocal objective (38) arranged upstream of the diffuser by transmission (36).  12. Radiometer according to any one of claims 8 to 11 taken in combination with one of claims 6 or 7, characterized in that the attenuation and homogenization means (34) are located between the deflection prism ( 32) and the entrance to the radiometric channel (20).  13. Radiometer according to claim 5, characterized in that the in-flight solar calibration device (26) comprises an input baffle (30) adapted to the field of the optical system (32, 34) of the solar calibration device in theft (26) and which protects the device against radiation and / or stray light.  14. Radiometer according to any one of claims 5 to 13, characterized in that the in-flight solar calibration device (26) is fixed relative to the frame (12) of the radiometer (10) which rotates a head of scanning (18) comprising at least one radiometric channel (20).  15. Radiometer according to any one of claims 5 to 13, characterized in that the in-flight solar calibration device (26) is movable relative to the frame (12) of the radiometer (10) which rotates a head of scanning (18) comprising at least one radiometric channel (20), so as to allow in particular an orientation of a prism of the device with respect to the incident beam of solar radiation.