2

INTRODUCTION

Following the SPOT satellites family, within the future Franco Italian ORFEO system dedicated to optical and radar Earth observation, CNES, who is in charge of the optical component, imagined a system whose performances are significantly enhanced compared to the SPOT series and achieve a range of operational missions widened to defense and civil safety domains. It is composed of two satellites equipped with optical high resolution cameras. The agility of the satellites allows a great capacity of daily image acquisitions. The so called PLEIADES-HR satellites, will deliver panchromatic images with broad field and 70cm ground sample distance at nadir. The panchromatic acquisition (Pan) is supplemented by four multispectral channels (MS) with narrow spectral bands and 2.8m ground sample distance at nadir.

Thales Alenia Space, in charge of designing, building and testing the optical instrument, has recently successfully completed the development of the Proto-Flight Model with outstanding image quality performances.

3

REQUIREMENTS

The PLEIADES-HR camera achieves all the image quality requirements (field of view, ground sample distance (GSD), modulation transfer function (MTF), signal to noise ratio (SNR)) while respecting the drastic restrictions of mass and volume imposed by satellite accommodation and performance. The main requirements that have led to the camera design choices are summarized hereafter:

(*)

The MTF performance on final images is 0.2 after image restoration.

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TELESCOPE OPTICAL DESIGN

The optical architecture of the telescope results from the optimization of radiometric performances compatible with volume restrictions, respect to the broad field of view and spectral range required. The four mirrors Korsch telescope, including a folding mirror, appears to be the best choice. The following figures recall the optical principle of the telescope (Fig. 1 and Fig. 2):

Fig. 1

: CodeV 3D view of PLEIADES-HR telescope

Fig. 2

: CodeV 2D view of PLEIADES-HR

The front cavity is a light collector constituted by two centered mirrors. The primary mirror (M1) is a conic surface not far from a parabola. The secondary mirror (M2) is aspherical. The entrance pupil is located on the primary mirror. In the back cavity, behind M1, is located an intermediate image plane where the beams converge then diverge and are folded up by a long rectangular folding plane mirror (MR) towards the tertiary aspherical mirror (M3) which gives a corrected image at the focal plane (PF).

The beams are folded up along the field of view so as to locate the focal plane on a cold side of the satellite, due to thermal control constraint.

As for the beams coming from M3 towards the focal plane not being blocked by MR, the field of view is slightly shifted perpendicular to the image line. Thus, MR is shifted from the M1-M2 optical axis and M3 is off-axis as shown on Fig. 1.

The real exit pupil of the telescope is located between M3 and the focal plane in the same plane than MR.

Both M1 and M2 are centered and circular mirrors. On the contrary, MR and M3 are very long “field” mirrors. M3 is off-axis with a low aperture number. Its use by sub-pupils along the field, makes it more difficult to manufacture as the wave front flatness must be guaranteed on every elementary zone while strictly restricting tilt and focus variations between them so as to control distortion effects at the focal plane. Therefore, this mirror is the main polishing difficulty, achieving the accuracy of traditional polishing technique. MR is subject to the same type of constraints, but as it is plane, its realization is less critical.

The telescope field of view allows the simultaneous imaging of a panchromatic line and four multispectral lines with a slight field separation compliant to mission requirements.

The main optical characteristics and sensitivities are given below:

Because of the high axial magnification of M2, the front cavity needs a very accurate control of the M1M2 distance within a few microns obtained thanks to a carbon-carbon composite cylinder. A thermal refocusing device, linked to M2, is also implemented in order to compensate offsets and long term drifts.

The back cavity is not particularly sensitive to misalignments. Its design results from a compromise between compactness and distortion that is all the more high as M3 gets closer to the image plane.

The mechanical architecture is shown on the following CAD-slice view.

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TELESCOPE ASSEMBLY

The most critical integration phase consists in a careful pre-assembly of the mirrors in the mechanical structure. All the mirrors, equipped with their mechanical fixation devices (MFD), were designed, manufactured and coated by the SESO Company in Aix-en-Provence. The mirror fixation devices (MFD) are designed to minimize the deformation of the mirrors by filtering small displacements and local flatness irregularity at interface.

As to respect the drastic cleanliness constraints required, all the integration and test phases are carried on in the class100 clean room.

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DETECTION UNIT

The focal plane (Fig. 14) is the heart of the highly integrated detection subassembly (Fig. 15). The size of the observed image is close to 400mm and is analyzed in 30,000 samples in Pan and 7,500 samples in MS.

Fig. 3

: CAD view of the mechanical architecture of PLEIADES-HR camera

Fig. 4

M1 on its transport interface with transfer tool

Fig. 5

M1 with its protection cover being integrated on the base-plate structure

Fig. 6

integration of the M2 sub-system: mirror, baffle, refocusing device, radiator and spider

Fig. 7

(left) M3 on its panel, (right) MR on its transport interface and cover

Fig. 8

back cavity mechanical structure ready for mirrors integration

Fig. 9

(left) MR in the back cavity and exit pupil reticule (right) trace of the beam on the reticule

Fig. 10

M2 maintained on the alignment tool prealigned in front of the telescope

Fig. 11

interferometric configuration for M2 alignment – vacuum chamber opened

Fig. 12

interferometric configuration for M2 alignment - vacuum chamber closed

Fig. 13

aligned telescope ready for detection unit integration

Fig. 14

Focal plane concept

Fig. 15

Detection Unit

Fig. 16

PFM undergoing optical measurement (MTF)

Fig. 17

PFM camera ready for mechanical tests

Fig. 18

PFM undergoing radiometric measurement

Fig. 19

Pan MTF measurements based on contrast evaluation method

Fig. 20

Predicted Pan EOL MTF based on ground measurements

Fig. 21

Pan SNR measured with 13 active TDI lines

Fig. 22

Predicted MS EOL MTF based on ground measurements

Fig. 23

On ground MS SNR measurements synthesis

The Pan band is constituted of 5 back-thinned TDI mode CCD arrays, whose pixels are PhotoMOS type with lateral anti-blooming structure. The image section has 6000 columns of active square pixels, each 13×13µm².

The colored bands (MS) are constituted of 5 CCD four linear arrays. The spacing between the centres of two consecutive line is 936 µm. Each line of pixels has 1,500 photo-elements on a 52 µm pitch and the size of each photo-element is 52×52 µm².

The spectral selection is made by optical filters placed very close in front of the detectors. Pan filters and MS stripe-line filters are space qualified multi-layer coatings deposited on glass substrates. An absorbing material deposited between the MS filters isolates each band from the others to avoid inter-band straylight.

A SiC main structure ensures accurate positioning and thermal dissipation of the equipped detectors.

For an easy high level image products ground processing, optical butting with splitting zerodur© mirrors provides the continuity of the detection lines (Pan/Pan and MS/MS in the field registration) and folding with long roof SiC mirror separates Pan and MS image.

The link between the Focal Plane and the Video Electronics is made with flexible circuits optimized to operate at frequency close to 7 MHz.

Radiometric performances are optimized thanks to video processing entirely integrated on the focal plane.

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CAMERA INTEGRATION AND TESTS

After electrical integration and radiometric performance verification, the PFM Detection Unit is mounted on the telescope for final qualification test campaign.

The first step consists in MTF measurements along the Pan line. This is done via a contrast evaluation method, using a collimator that lights the camera over the full entrance pupil.

The MTF performance is deduced from the acquisition of images of a square wave pattern whose spatial frequency is close to the Nyquist frequency of the Pan pixel, located at the image plane of the collimator.

The MTF performance measured at that step confirmed the high level image quality expected at telescope step and authorized the instrument to go through the qualification mechanical tests.

Due to the mechanical configuration of the instrument inside the platform, the quasi-static and sine levels tests cover the loads induced by acoustic, shock and thermal environments.

All simulation levels have been passed in record time with no performance degradation.

The comparison between measurements (MTF and geometrical tests) before and after mechanical tests prove the perfect stability of the instrument.

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FINAL PERFORMANCE TEST RESULTS

The main image quality parameter is the SNR. MTF product which determines the useful support of the MTF and hence image resolution. The performances apply for each spectral band, throughout the life of the satellite, excluding the compression effect and outside inter-array zones.

The PFM optical instrument shows outstanding image quality performance compared to the required level.

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CONCLUSION

The optical architecture of the PLEIADES-HR camera, based on a light and compact Korsch telescope and a highly integrated detection unit, achieves the image quality requirements with margin. Instrument low mass and high compactness are major and mandatory contributors to the satellite agility performance.

Achieving these performances needs the use of advanced technologies and benefits from a highly efficient thermal and mechanical architecture that ensures very high in-flight stability of the camera.

Through the development of the PLEIADES-HR camera, Thales Alenia Space has confirmed its successful experience in designing, building and testing high resolution optical instruments, with a special notice to the high performance optical facilities and skills available in Cannes for optical measurements and alignment.

PFM development has been successfully achieved. Outstanding MTF and SNR performances have been reached. The FM2 camera integration is in progress towards a delivery planned Q2-2009.