1-

INTRODUCTION

Since years, the Earth Observation needs even higher performance detection systems. For Infra Red domain, current quantic detectors require to cool down the detectors and optics of the focal plane to cryogenic temperature (90 K to 100K), in order to limit as lowest as possible parasitic light and temperature effects that would saturate detectors, making them non-operational. The challenge consists in managing simultaneously thermal cooling and stability as well as mechanical discoupling and accurate focal plane components location.

In the frame of the IASI instrument (Infrared Atmospheric Sounding Interferometer), issued from a collaboration between French space agency (CNES) and EUMETSAT, Alcatel SPace Industries (ASPI) develops a very efficient passive cryocooler, under CNES contract. This cryocooler will be tested on ground and delivered until end of 2001. It is designed to cool down the IASI Infra Red cryogenic detectors and optics below 100 K. The selected concept guarantees high thermal stability and very accurate location of the focal plane components, one of the permanent design drivers being the thermoelastic constraints induced by cryogenic temperature. So, the CBS design is supported by detailed thermal and mechanical modelings, as well as technological tests, and will be validated through global CBS performance tests.

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MAIN REQUIREMENTS

The CBS is part of the IASI instrument, as presented in figure-1. It takes on board the IASI cryogenic Infra Red detectors (3 to 15μm band) and their dedicated cryogenic optics. It appears that the IASI detection chain performance highly depends on the CBS detectors efficiency. This is closely linked to the ratio between the optical usefull flux reaching the detectors microlenses and the thermal noise on the detectors. The CBS shall then maximise the usefull flux via high accuracy optical pointing and stability. This is achieved by mechanical stiffness and discoupling, precise machining and finishing, limitation and anticipation of the thermoelastic distortions due to cooling down. Furthermore, a very efficient thermal control is necessary to optimise the radiometric performance, by decreasing the detectors proper temperature below 100K and reducing the detectors neighbouring environment temperature in order to limit as lowest as possible the parasitic fluxes. The CBS general layout is exposed in figure-2.

Figure-1 :

CBS is part of the IASI instrument

Figure-2 :

discoupling & stability in a compact layout

Therefore the CBS architecture is a compromise between thermal discoupling and stability, as well as mechanical stiffness, discoupling and stability. These thermal and mechanical often conflicting requirements are fully overlapped within the following strong constraints :

The CBS design has then to fullfil the required biginning of life performances synthesized in table-1 and the stability presented in table-2 hereafter.

Table-1 :

CBS requirements in BOL (Beginning of Life) constitute a real challenge

Table-2 :

stability requirements are a basis of CBS design

An important request concerns the level of contamination on the optics and detectors. Indeed, molecular and particular contamination could reduce the usefull optical flux on the detectors, decreasing the detection performances. The CBS manufacturing and AIT activities shall then be compatible of the severe contamination levels requested at IASI delivery (molecular < 500ppm & particular < 10^-7g/cm²). Besides this, an in orbit CBS decontamination heating system shall be implemented, as well as the possibility of heating during early orbit phases.

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CBS OPTICAL ARCHITECTURE

The CBS aims to collect the optical signal coming from the interferometer, to split it into three spectral bands and to convert it into electrical signal delivered to IASI instrument treatment electronic (see figure-3).

Figure-3 :

The CBS is the eyes of IASI

The transformation of the final optical signal into the electrical one is the task of the CAU (detectors). The COU (lenses & dichroics) collect, split and focus the optical flux. CAU and COU are mounted into the CBS cryogenic focale plane (3^rd stage optical box), as presented in figure-4.

Figure-4 :

the CBS optical box houses optics & detectors

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CBS THERMAL ARCHITECTURE

The CBS being a purely passive cryogenic radiator, its thermal efficiency depends mainly on the three following parameters :

Figure-5 :

the CBS 3 stages assembly equipped with detectors, cold links & optics

Figure-6 :

discoupling &high rejection capability are the drivers of the thermal concept

Figure-7 :

a sunshield to protect the 3 CBS stages

The thermal regulation of the focal plane (detectors and optics) is completed by :

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CBS MECHANICAL ARCHITECTURE

The CBS mechanical design, in terms of materials, manufacturing and inter-stages filtering fixation

solution, is driven by the following main requirements :

Note : the use of Titanium for the stages avoid any sliding effects at fibreglass fixation interfaces.

Figure-8 :

1^st * 2^nd lightweighted but highly stable frames

Figure-9 :

a stiffened radiative plate

Figure-10 :

an optical box precisely machined

Figure-11 :

the 3^rd stage assembly constitutes the heart of the CBS

Figure-12 :

discoupling blades play a major role in the thermo-mechanical behaviour

The sunshield design is a compromise between mass saving and thermoelastic stability. It is composed of :

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CBS MAIN PERFORMANCES

The detailed analyses and associated technological tests results are compliant with the requested performance.

Optics alignment is better than 40μm (for the most sever ones), all contributors included (manufacturing, integration, cooling down, gradients, launch residual, gravity and ageing).

One of the main challenge is related to the 3^rd stage operational temperature, which is on the way of being successfully realised. The typical computed BOL temperature of the cryogenic detectors and optics is 92 K, in maximum hot environmental and orbital case. This temperature level leads to the flux balance exposed in table-3 and the following temperature spreading (see figure-13).

Figure-13 :

92 K on focal plane represents 200 K of decrease on 200 mm

Table-3 :

the hot case CBS power budget illustrates thermal control efficiency

The thermal stability is better than specified. In particular, the results of a refined coupled transient analysis between detectors (internal model) and CBS, show that detectors fluctuation during observation of a typical flight scene cycle (duration around 10s) is less than 0.01 K.

The mechanical performance is also achieved. The 1^st frequency (nominal value) is 130 Hz, partially correlated via sine vibration tests on a test mokup (see section 7) and the nominal mass is 21.8 kg.

The CBS performance is secured by refined analyses,using detailed thermal mathematical models (see figure-14) as well as detailed mechanical ones (see figure-15).

Figure-14 :

highly detailed thermal models for accurate thermal predictions

Figure-15 :

highly detailed mechanical models ensure mechanical/thermoelastic behaviour

CBS design and analyses are based on a technological test program, successfully completed : b fibreglass mechanical characterisation ;

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CBS DEVELOPMENT & VERIFICATION LOGIC

The CBS performances will be verified through a global test campaign (see figure-17) on the protoflight (PFM).

Before going to the PFM test sequence, a preliminary dynamic test has been performed with flight representative blades, in order to verify strength and stiffness (see figure-16).

Figure-16 :

a successful dynamic elementary test

Figure-17 :

a manufacturing & test sequence to confirm CBS performance

The first major key event will be the thermal cryo-optic test sequence to verify the temperature level and the optical performance in operational conditions. For this purpose, a specific 20 K cryogenic panel (with appropriate pyramidal active shape for high emittance need at low temperature) is developed, as well as specific optical and radiometric devices.

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CONCLUSION

After a design phase supported by thermal and mechanical detailed analyses, associated to technological tests, the CBS PFM is now manufactured and ready for assembly phase.

This first important step enables to be confident in the CBS capability to fulfil the required performance, which will be validated in 2001 through the test sequence previously presented.

The CBS solid concept developed by ALCATEL, enhances the interest of passive cryocooler systems, thanks to the following great advantages :