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Sun and Moon Observation

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2.4.1 Sun and Moon Observation

All measurement activities are planned relative to solar and lunar constellations. SCIAMACHY operations are either sun or moon fixed, but not Earth fixed. Since solar and lunar constellations, as viewed from ENVISAT’s orbit, are determined by the orbital motion of the platform, the orientation of the orbital plane, the lunar motion around Earth and the Earth’s movement around the sun, the observing conditions can be completely predicted by orbit analysis. These predicted conditions are then translated into configurable instrument states and timelines.


Sun Occultation

The mean local time of 10 a.m. during descending node crossing leaves the sun always to the left of the flight direction, i.e. at azimuth angles > 180°. The elevation of the sun varies between approx. -70° and +70°. Sunrise occurs after ascending node crossing when ENVISAT moves towards the North Pole on the eclipse side of the orbit. The sun becomes visible at the Earth’s limb left of the flight direction at middle to high geographic latitudes. The exact latitude is dependent on the actual position of the true sun relativeto the Earth (true sun reflects the actual annual orbital motion of the Earth contrary to the mean sun which is characterized by assuming a constant Earth orbital velocity). In summer, sun occultation measurements start when the spacecraft has reached geographic latitudes of about 27° north while in winter the sub-satellite point moves up to about 75° North. At sunrise the solar elevation is identical to the elevation of the Earth’s limb, i.e. approx. 27.2°. The azimuth angle at sunrise has a mean value of 330°, corresponding to the mean local time at descending node crossing of 10 a.m. and changes over a year due to the apparent motion of the true sun. Caused by the orbital motion of ENVISAT, the sun rises almost vertically through SCIAMACHY’s limb TCFoV. In an occultation measurement, the ASM has to acquire the sun at an angle of about 330° and to follow the slightly changing azimuth as the sun moves higher. In the elevation direction the sun must be tracked by the ESM up to the maximum elevation angle of 19.5°, limited by the TCFoV. From the Earth’s limb up to an elevation angle of 25.2°, corresponding to an altitude of 100 km, the sunlight is absorbed by the Earth’s atmosphere. Thus the sun serves both as a target for probing the atmospheric trace gas constituents (altitude < 100 km) and for calibration and monitoring measurements (altitude > 100 km). Therefore the total time of the sunrise in the limb TCFoV is referred to as Sun Occultation & Calibration (SO&C) window.

Observation of sunrise from a spacecraft is affected by the refractive properties of the Earth’s atmosphere. The refraction angle depends on the Earth’s radius, the scale height of the exponentially decreasing refractivity profile, the refractivity and the height of the tangent point of the incident rays. For visible light the refraction angle amounts to approx. 70 arcmin at the horizon (h = 0 km), i.e. SCIAMACHY can observe the first solar photons when the sun/moon is still well below the geometric horizon. As the unrefracted sun rises, the refracted image of the disk is distorted by differential refraction. At an altitude of h » 17 km refraction has become so small that refracted image and solar disk overlap. Below this height the angular rate of the rising sun/moon as defined by the moving spacecraft is larger than the variable rate of their refracted images. At low altitudes, measuring the sun can become difficult due to obscuration by or reflected radiation from clouds. On-board control of the scan mirrors during occultation uses the Sun Follower with its relatively wide field of view of 0.72° × 2.2°.

Sub-solar Observations

The sub-solar port above the ESM provides additional access to the sun above the atmosphere. Because sun viewing in this configuration does not involve the ASM, measurements of this kind can be used to monitor the behaviour of the ASM mirror. The sun is visible through the sub-solar port when the sun has reached its highest elevation. This occurs at an azimuth angle of 270°. The sub-solar elevation angle changes continuously with season. Therefore the sun moves up and down over a year along the elongated dimension of the sub-solar TCFoV when passing through the window. The duration of a sub-solar measurement is defined by the time it takes the sun to pass through the azimuth dimension of the sub-solar port, reduced by the small aperture stop to only 0.72°. This interval amounts to 21 sec, with the sun being fully visible for a short period of only 3.5 sec.


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