1.

PRESENTATION OF THE HIGH RESOLUTION MAPPING FOCAL PLANE

EADS-SODERN started in 1980s the development of civil space-borne Earth surface mapping focal pane assemblies with the SPOT1 push-broom satellite of CNES. Afterwards EADS-SODERN has developed a series of focal panes, which have been progressively adapted to match the technical evolution of satellites in the programmes SPOT and then Pleiades. This development policy has resulted in equipment with remarkable operating reliability and with technical design which continues to use the best space-rated technology.

Table 1 shows the most striking features and the trend of resolution improvement (the decrease of ground sampling distance GSD) from 10 m to better than 3 m for the SPOT 5 satellite and better than 0.7 m for the Pleiades HR satellite Panchromatic (PAN) spectral band.

Table. 1.

Typical features (GSD and swath width)

Of course, this list of features is not compete and doe not show all the technological improvements made in such various fields as optics with spectral separators, mechanics and materials and monolithicline detectors.

2.

COMPARAISON SPOT5 / PLEIADES HR

The Pleiades FP (Focal Plane) new generation of instrument represents a breakthrough in comparison with the previous SPOT5 instruments as a significant step in GSD is made. And contrary to SPOT5 dioptric telescope which is optimised with a significant glass thickness in the focal pane allowing spectral separation and optical image division by prisms, the Pleiades FP takes into account the catoptric telescope design that excludes optical corrector. The comparison of the spectral separator architecture is summarized in table 2.

Table. 2.

Spectral function fulfilling (Comparison SPOT 5 HRG / Pleiades HR)

Table. 2.

The SPOT5 spectral separator consists of elementary prisms. The angles of these prisms and the dichroic coating deposited on the surfaces are calculated in such a way that each interface between prisms acts as a long-pass filter i.e. transmits longer wavelengths and reflects shorter wavelengths. The beam, which is of polychromatic light at the input, is progressively filtered in channels B1, B2 and so on. Beams derived by total reflection are filtered at the specified bandwidth thanks to low-pass filters paced at the outputs of the beam splitter.

The Pleiades instrument concept authorises only very limited glass thickness in the optical path, therefore optical glass thickness is less than few mllimetres. This imposes to discard the beam splitter concept in favour of a new approach with a small patterned filter, cast in one piece, mounted on the detector. As depicted in table 2 the beam, which is polychromatic at the input, is filtered by coatings with multi-layer dielectric structures on the front and rear sides of the filter.

Besides the spectral separator aspect, the XS and PAN lines of sight are approximately few mrad apart, i.e. few cm in the focal pane of the telescope. Indeed in push-broom mode, the separation of the PAN from the XS CCD-lines, i.e. the in-field angular separation PAN/XS, causes a time delay of the colour images. Studies have been performed to minimize this time delay taking into account the design of the line features and the detector package size. However with the increasing of the focal length the PAN-sharpened images (merging of high resolution panchromatic and lower resolution multispectral images to a high resolution colour image) of Pleiades FP are more accurate even if the FP widths are similar (table 3).

Table. 3.

Size of image pane Comparison SPOT 5 HRG / Pleiades HR

Moreover the size of the focal pane is also linked to the size of the pixel. The architecture of Pleiades HR is based on a TDI (Time Delay Integration) mode CCD array for the PAN band to improve the radiometric performances of the instrument. The available TDI technology allows a pixel size of about 13 μm square. Thus for the PAN channel, this aspect and the required Swath / GSD ratio lead to replace the SPOT5 monolithic detector with 2 staggered linear arrays of 12 000 photodiodes by 5 TDI detectors with 6 000 columns. The resulting lengths of the focal panes are compared in table 3.

3.

PLEIADES HR CONSIDERATIONS

The man driving parameter on the optica architecture of Pleiades FP is the realization of 400 mm length continuous retinas. First of all the angular separation between PAN and XS lines is based on the use of a SiC central mirror of approximately 400 mm length. As depicted in [1] and figure 1 the continuous line is obtained by so-called optica butting technique on each spectral channel PAN and XS. Two optical spitting pane mirrors (~ 80 mm length) per retina reflect the light rays towards two detectors when three detectors are illuminated in direct path for each channel.

Fig. 1.

Pleiades HR optical architecture

Arrangement of CCD lines in the focal plane is depicted in figure 2. The TDI PAN detector are slightly tilted in the focal pane in order to minimize the mismatch between the charges transfer rows and the image motion direction across the array due to telescope distortion.

Fig. 2.

Pleiades HR pixel layout

A specific feature which has a great impact on the FP AIT approach is related to the telescope optical interface that offers significant incident angles of the light paths. As illustrated figure 3, the relative positions of each spitting mirror and the dedicated detectors have to be accurately controlled with reference to the exit pupil centre O. This aspect is still more complex due to the arrangement of CCD lines as explained above and the performances of the telescope including the pupil change locations corresponding with the different image positions in the FOV, the accuracy of the paraxial approximation and so on.

Fig. 3.

Pleiades light paths (paraxial approximation)

Moreover for the assessment of the total geometric image pane quality on each axis, the summary budget combines the three-dimensional tolerancing contributions of each component, structure, mirror and detection module.

The depth of focus δ of an optical system is expressed as the axial displacement that the image may experience before the resultant image blur becomes excessive. According to the classical theory of diffraction:

Decreasing the numerical aperture (NA) on Pleiades FP is thus particularly attractive. But due to significant incident angles and the implementation of TDI detectors, lateral shifts become unacceptable more rapidly than blurring. Therefore, depth of focus tolerancing, i.e. retinas flatness, reman critical in Pleiades FP.

In addition to those previous considerations, the reduction of the numerical aperture and the implementation of opaque diaphragms for straylight purpose inside the focal pane reduce the CCD dies visual accessibility during AIT sequences. Sensitivity to particulate contamination is also increased by the reduction of the numerical aperture.

4.

AIT SEQUENCE

The assembly chronology is presented in the flow chart figure 4. Three stages are critical:

Fig. 4.

Principe of FP flow chart

Manufacturing of detection module

The detection module (figure 5) constitutes the whole detection. They consist of:

Fig. 5.

Detection modules

• - one CCD (panchromatic or multispectral),

• - the mechanical frame glued o the opposite rear of the detector package (This mechanical frame screwed on the structure, ensure an isostatic mounting on the structure and evacuate the thermal heating.),

• - the associated front-end analogue electronics,

• - the associated filter paced in front of and dose to the CCD window (only for Pleiades FP).

One of the key features of detection module is the choice of the material that insures stiffness and thermal stability with a reasonable mass, i.e. SiC on Pleiades FP. As depicted in the previous chapter, we also implement patterned filter on Pleiades detection module. Both aspects lead to a very critical AIT sequence.

Manufacturing of equipped structure

The manufacturing of equipped structure is a specific phase on Pleiades FP programme because of the implementation of the two different kinds of mirrors (figure 6).

Fig. 6.

Pleiades equipped structure

During this sequence the mirrors but also the mechanical interface of the focal pane are set with a few ten micrometers tolerancing. The adjustment goals are defined at any stage of the process by a realistic optical modelling.

5.

GROUND SUPPORT EQUIPMENT

The facilities described here are extremely rigid structures with granite reference panes. They consist of two basic functiona parts:

For Pleiades HR AIT sequence, three specific integration and measuring benches, corresponding with the different assembly and reaization stages, are used:

The BMV equipment

This equipment is specifically designed for applications such that high accuracy integration process. It also aiows precise inspections in particular taking into account the glass thickness in case of die CCD and filters.

The system performance quaity over the entire range of the BMV is summarized in table 4.

Table 4.

BMV platform features

Axis	Translation capabilities	Resolution	Accuracy
X, Y and Z	200 mm	≤ 0.1 μm	≤ 2 μm

Fig. 7.

BMV facility

The 3-dimensionnal measuring machine

A classical co-ordinate measuring machine (CMM) enables the geometric constitution of the equipped structure which is obtaned on the basis of adjustment and metrology of mirrors locaization with a rate accuracy of few microns in adjustment and in knowledge. During the assembly process the probing device scans successively the reference frame located on the structure and the mirrors.

The figure 8 shows the Pleiades FP and adjustment tools during the engineering model assembly

Fig. 8.

Assembly of mirrors on the structure on Pleiades EM model

The ZEMAX optica software is used as support for this integration to determine the fine adjustment corrections of the optical components.

Fig. 9.

Pleiades FP rays tracing

The BCR12 equipment

The BCR12 equipment, figure 10, is specifically designed and built for focal panes integration and control. The man dement is a high precision platform, with three axes air bearing stages. This platform aiows also immunity from vibration transmitted by the floor.

Fig. 10.

BCR12 facility

The system performance quality over the entire range of the BCR12 is summarized in Table 5.

Table 5.

BCR12 platform features

Axis	Translation capabilities	Resolution	Accuracy
X	440 mm	≤ 0.1 μm	≤ 2 μm
Y (axial sighting)	9 mm	≤ 0.1 μm	≤ 2 μm
Z	140 mm	≤ 0.1 μm	≤ 2 μm

An optical bench is implemented on a long travel linear stage moving aong the horizontal (X) axis. It is basically made with one integrating sphere light source, lenses, filter wheel, CCD camera and reference reticle. The measurement process with the motorized platform consists in:

• - coarse superposition and adjustment of the reticle at the position of the pixel under inspection,

• - analysis of the pattern on the camera,

• - fine superposition and foclisation of both pixel and reticle.

Reticle and lenses are specificity designed for Pleiades FP assumng glass thickness and back focal length.

To guarantee the conformity of the test equipment, we designed a metrological standard, figure 11, with the same Pleiades FP mechanical interface and a glass window with multiple scae graticules etched on it.

Fig. 11.

Pleiades metrological standard

The assembly process of detection module on the structure is obtained on the basis of repeated phases of adjustment and metrology of pixel localization. This process is particularly achieved with a rate accuracy of few microns in adjustment for the TDI row orientations.

6.

CONCLUSION

This paper ams at demonstrating how thanks to its extensive experience accumulated on the occasion of SPOT programme and on military programmes during thirty years, EADS-SODERN is able to propose solutions to the chaienges linked with high performances focal panes integration.

In the frame of the French Pleiades programme, EADS-SODERN has developed dedicated integration and test facilities to fully manufacture and characterize the geometrical performances of focal pane. The first results obtained on breadboards and engineering model have demonstrated the high level of geometrical performances of the facilities and make us confident for the achievement of the programme.