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Remote Sensing from Space

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Results from the ATLAS missions

During the first ATLAS mission in March 1992, at the middle of the descending part of Solar cycle 22, the Sun was quite active and a cluster of sunspots passed over the Solar disk from one limb to the other. The data obtained on the two successive Solar observation periods of days 85 and 89 shows a difference of more than 1.5Wm², which is more than 0.1%.
The SOLCON left and right channel measurements of the differential absolute radiometer, characterized independently, show a difference of 0.06% consistent with the level of uncertainly on the evaluated absolute accuracy of SOLCON. On days 91 and 92, the irradiance values increased as the sunspots were reaching the Solar limb. The situation encountered during this mission is very favorable to the detection of time stamp errors in databases. This is indeed illustrated with the ACRIM II daily mean observations, in which a two day shift is identified  (Data can be acquired from the jpl.simdac anonymous ftp server, April 14th, 1995, file). During the second ATLAS mission in April, 1993, the Sun was very quiet and the Solar irradiance very stable. Very little to no sunspots were observed. Over the whole ATLAS 2 mission the measured mean Solar constant of each of the five Solar observation periods did not change more than 0.3 Wm². As for the first mission, the dispersion during each orbit was of the order of 0.5 Wm². This is mainly due to Solar noise as the SOLCON instrument noise is about an order of magnitude smaller. Of particular importance to this mission is the fact that simultaneously to ACR and SOLCON, the UARS and ERBS spacecrafts as well as the European Retrievable Carrier (EURECA) were carrying six other radiometers to measure the Solar irradiance. It was the first time that such a space radiometric comparisons was performed. Compared to usual ground radiometric comparison conditions at high altitude sites like Table Mountain Observatory (TMO) or WRCD/Davos (CH), the Solar source target was obviously much more stable in the absence of circumsolar sky radiation and variable absorption of the atmosphere. A mean Solar constant was calculated of 1366.22 Wm² and for each radiometric channel an "adjustment factor" (see table beneath) was determined realizing de facto the definition of the Space Absolute Radiometric Reference (SARR).

 
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Table: "Adjustment factors" for the radiometers participating de facto in the space radiometric comparison realized during the ATLAS 2 mission. Note that for the eight radiometers, the dispersion is less than ± 0.1 %


The ADJUSTMENT FACTOR (or multiplication factor) given for each instrument enables all the data to be reduced to the SPACE ABSOLUTE RADIOMETRIC REFERENCE. The resulting Solar constant value provides the state of the art accuracy.
During the third ATLAS mission, in October 1994, the Sun was again very quiet and the Solar irradiance very stable. Over the whole mission the measured mean Solar constant of each of the four Solar periods did not change more than 0.4 Wm².
Compared to the previous mission, the Solar constant was about 0.3 Wm² lower as one would expect in the descending phase of activity of Solar cycle 22.
With the results of SOLCON during ATLAS 3 we could validate the actual ERBS accuracy by using its Space Absolute Radiometric Reference (SARR) 1.000453 adjustment factor.
Indeed we find that some ERBS values agree very well within 0.2 Wm² but that some others are scattered by ± 0.6 Wm². This is due probably either to the effect of Solar noise undersampled or to some intrinsic ERBS noise.
The same verification will be done later on the ACRIM II and the ACR instruments flown on ATLAS when the data are made available.
During the ATLAS 3 mission, an unscheduled experiment with SOLCON pointing successively into the shuttle velocity vector and free space, provided a measurement of the density of air.
With the assumption that the conversion factor of the kinetic energy of particles to heat is 0.4, we found a density of 5 10-12 Kg m³ at 295 Km altitude on 11/11/1994 at 18H GMT day time of the ATLAS 3 orbit in agreement with the current air density models.
This result shows that radiometers measuring the Solar constant and aiming in the velocity vector will be affected by an error function of the air density. It demonstrates also that a SOLCON radiometer like instrument could be designed specially for the purpose of air density measurements. This would be useful for monitoring the flight conditions of the space station.

 
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Compared results obtained by the SOLCON experiment during ATLAS-I

All the data shown are reduced to the SARR scale by application of the ad hoc adjustment coefficient to the data of ACRIM II, ERB, ERBS and the SOLCON Left and Right radiometric channels. A shift of two days is found in the ACRIM II database.

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Compared results obtained by the SOLCON experiment during ATLAS-II

All the data shown are reduced to the SARR scale by application of the ad hoc adjustment coefficient to the data of ACRIM II, ERB, ERBS and the SOLCON Left and Right radiometric channels. A shift of two days is found in the ACRIM II database.

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Compared results obtained by the SOLCON experiment during ATLAS-III

All the data shown are reduced to the SARR scale by application of the ad hoc adjustment coefficient to the data of ACRIM II, ERB, ERBS and the SOLCON Left and Right radiometric channels. A shift of two days is found in the ACRIM II database.

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