Calibration

Parasol in-flight calibration and performances

During the Parasol Calibration phase, ended on July 12^th 2005, a large set of vicarious methods, mainly historically developed for Polder [1] and furthermore improved, has been used to in-flight calibrate the instrument and to estimate radiometric and geometric performances. In addition, the multi-temporal behavior has been assessed based on the analysis of the 18-months level-1 data archive.

Radiométric Performances

The Polder/Parasol instrument has no internal calibration source for in-flight on board calibration. However calibration is ensured by an approach based on both pre-flight modeling and observation of specific references of Earth targets. These natural targets present well-known reflectance signatures: sunglint, desert areas, Rayleigh scattering, bright clouds...

1 - Absolute calibration

The Parasol absolute calibration principle is to achieve absolute calibration of shorter wavelengths through a method based on Rayleigh scattering acquisitions over oceanic sites, and to transferred this calibration to longer wavelengths through interband method using specular reflection (sunglint) or bright diffuse clouds. After this first step, the procedure refines the estimation by a more global adjustment (iteration) between calibration over Rayleigh scattering, sunglint, and bright clouds.
A very good agreement is observed between results of these 3 methods: accordance is found to be within 1 to 2% for all spectral bands (see Figure 1). The only exception is the 443nm spectral band, because of important and un-corrigible stray-light contamination.
In addition, a cross-calibration was performed using desert sites (20 selected sites) and using Polder-2 as a reference sensor. This result shows that Parasol and Polder-2 are excellently cross-calibrated.

Figure 1. Consistency between in-flight absolute calibration derived from 4 methods:
Rayleigh scattering, sunglint, bright clouds, desert sites.
Values are referring to the present level-1 calibration.

Using these different calibration methods, a multi-temporal survey of the radiometric sensitivity was performed based on the 18-months archive of Parasol level-1 data. A slight temporal drift was evidenced and corrected : variations are very small for red and near infra-red spectral bands (from 670 to 1020nm), and are moderate with nearly -1,5% and -3,0% for 565nm and 490nm spectral bands respectively as shown on Figure 2.

Figure 2. Multi-temporal decrease of the Parasol instrumental sensitivity with time.

Important remark: The use of 443-nm spectral band is strongly not recommended.

2 - Other radiometric parameters

Other radiometric parameters are necessary to optimally process the level-1 data. In particular, multi-angular and in-polarization characterization, which are the main interests of POLDER instruments, must be in-flight checked and possibly adjusted. The following parameters, have been checked using various methods based on observation of natural targets, the accuracy of results being of course dependent on the method:

• low frequency multi-angular response: no evolution detected using absolute and inter-band methods- checked within 1-2%
• high-frequency multi-angular response: in-flight verification using statistical acquisitions over bright targets - accuracy better than 0.2%
• polarization sensitivity of the wide field-of-view optic: no evolution detected over un-polarized targets - checked for 490P, 670P and 865P within 0.5%
• non-linearity of the instrumental response: no-evolution detected over bright targets - checked within 0.4%

Geomatric Performances

1 - Geometric calibration

The geometric calibration consists in verifying and possibly adjusting the rotation matrix between imaging frames defined by the optics and the CCD matrix, used to geometrically resample images on the POLDER level-1 grid.
The geometric calibration is performed through a spatio-triangulation algorithm based on correlation between two images from the same orbit and viewing the same unique point at the Earth surface.
The process is iterative (initial parameters are pre-flight estimated) and improvement of each iterative step was validated through the multi-angular registration performance. Only two iterations were necessary to assure the convergence of the multi-angular accuracy.
Finally, no variation was found with geographic position (i.e. latitude), and no temporal variation with time was evidenced on available level-1 data (18 months).

	Bias in micro-radian		Version 2
	roll	-317.2
	pitch	-955.1
	yaw	-521.7

Table 1. In-flight estimated biases through geometric calibration.

2 - Geometric performances

The geometric performance is described through 5 indicators:

• the absolute localization accuracy: it represents the distance between the estimated position of a given point on the level-1 POLDER grid and its reel position. The localization performance was estimated after geometric re-calibration of the instrument, using control points from Végétation/SPOT5 images. Localization performance was estimated at 2.27km, while the specification requires 4km max (with a 2km objective).
   
• the multi-polarization registration accuracy: it represents the radius of the circle containing the 3 measurements corresponding to acquisitions through the 3 orientations of the polarizer for a given spectral band (among 490, 670, and 865). The specification requires a radius of 0.05 pixel Max. The multi-polarization mean performance was estimated through a correlation algorithm at about 0.05 pixel at 670, and within 0.05 pixel for 490 and 865.
   
• the multi-spectral registration accuracy: it represents the radius of the circle containing the 9 measurements corresponding to acquisitions through the 9 spectral bands (one turn of the filter rotating wheel). The specification requires a radius of 0.10 pixel Max. The multi-spectral performance was estimated through a correlation algorithm at less than 0.09 pixel Max and the specification is met.
   
• the multi-angular registration accuracy: it represents the radius of the circle containing the (up to) 16 measurements corresponding to acquisitions of the 16 viewing directions (i.e. 16 turns of the rotating wheels). The specification requires a radius of 0.10 pixel RMS. The multi-spectral performance, was estimated through a correlation algorithm and after geometric re-calibration of the instrument, at better than 0.10 pixel for 7 successive viewing directions, but up to 0.13 pixel RMS for all viewing directions. Nevertheless, the specification is considered as almost met.
   
• the multi-temporal registration accuracy: it represents the radius of the circle containing all measurements acquired during one month of acquisitions. The specification requires a radius of 0.125 pixel RMS. Due to its definition, this total multi-temporal performance includes a multi-angular part and a purely multi-temporal part. Only the multi-temporal part was estimated after geometric re-calibration of the instrument at 0.103 pixel RMS. Considering this result, the total multi-temporal performance, not strictly estimated, is therefore considered met.

Conclusions

Parasol system is now in-flight calibrated and radiometric/geometric performances are fully compliant with mission specifications.

References

[1] Hagolle O., P. Goloub, P.-Y. Deschamps, H. Cosnefroy, X. Briottet, T. Bailleul, J.M.Nicolas, F. Parol, B. Lafrance, and M. Herman, "Results of POLDER In-Flight Calibration," IEEE Transactions on Geoscience and Remote Sensing, vol. 37, pp. 1550-1566, 1999.
[2] Fougnie, B., G. Bracco, B. Lafrance, C. Ruffel, O. Hagolle, C. Tinel, In-flight Performances of PARASOL Inside the Aqua-Train Atmospheric Observatory, A33C-915, AGU Fall Meeting, 5-9th December, 2005.