1.

INTRODUCTION

Today Earth observation from space is oriented towards security, whether civilian or military. It provides armies with the strategic data necessary for setting up modern defense to guarantee reliability, regularity and discretion of the information supplied. It is also used for the prevention of civilian risks (pollution, natural catastrophes, climatic change) and the management of terrestrial resources.

Coming after the SPOT program, it was decided to go ahead with the PLEIADES HR program. PLEIADES HR is the optical high resolution component of a larger optical and radar multi-sensors system : ORFEO, which is developed in cooperation between France and Italy for dual Civilian and Defense use. ALCATEL SPACE has been entrusted by CNES with the development of the high resolution camera of the Earth observation satellites PLEIADES HR.

The PLEIADES HR camera concept has emerged from a design-to-cost approach. An appropriate and rigorous trade-off was made between performance and associated technology, so as to guarantee low cost. The selected technologies allow high performance associated with development risk control.

This paper introduces the main constraints of conception taken into account, then presents the design and highlights the key features and performances of the camera.

2.

CONSTRAINTS FOR CONCEPTION

The constraints for the camera conception are from different kinds:

3.

CAMERA DESIGN OVERVIEW

3.2

Camera overall configuration

An overview of the camera is presented below (Fig. 1). Starting from the light beam entrance, it is constituted of:

Fig. 1.

Camera exploded view

• - The optical telescope based on the KORSCH combination. The imager design is an all-reflective 4 mirrors telescope with light-weighted Zerodur optics and a carbon structure.

• - The shutter mechanism located behind the M1 mirror. It protects the detection unit from direct solar entrance.

• - A thermal refocusing device included in the mirror M2 assembly. It allows in orbit focusing of the telescope.

• - The detection unit mounted on the telescope and composed of two main assemblies : the focal plane assembly (FPA) ensuring the spectral separation in the field of view and transforming the incoming optical signal in electrons, and the video electronics interfacing with the FPA and the satellite on board image chain.

• - The service electronics located in the bus and composed of two unit : a dedicated unit for video power supply and the instrument in/out unit which interfaces with the satellite and ensures the instrument command and control.

4.

CAMERA OPTICS

The optical design of PLEIADES HR (Fig. 2) is based on a compact KORSCH combination, assembling an on axis part (M1 + M2 collector mirrors) and an off axis part (M3 + MR mirrors) feeding the different focal planes. The design features a 650 mm aperture and a 12.9 m focal length.

Fig. 2.

Optical configuration

This optical lay-out provides great interest in term of accommodation. Its low speed (~F/20) allows an easy folding beam behind the M1 mirror. The volume is then restricted to less than 1.9 m length and 1.2 m in maximum diameter. More over, the low speed leads to a positioning sensitivity in the class of 10 μm focus (the more constrained axis).

The four light-weighted Zerodur (class 0) mirrors equipped with monolithic Invar mirror fixation devices (MFD) provide high performance optics (Table 2).

Table 2:

Mirrors design and performance

In order to not distort the optical surfaces, the MFD are designed to provide decoupling between the mirror and the interface plane against flatness errors, thermal expansion differences and also limit to a reasonable extent, the risks of dynamic coupling between the mirror and the support structure.

The production of the MFD makes use of wire electroerosion, proven to give good results for such types of application.

A key technology for manufacturing of high performance optics is the polishing technique, which is used to do at least the final correctives steps of surface generation. Ultrafine mechanical computer controlled polishing machine (CCPM) has been selected because it is well operational for large optics with low F number, it allows to have low roughness surface (<3A) and surface flatness compatible with Marechal’s condition (RMS<λ/28), it is demonstrated for off-axis aspherical polishing. Furthermore the selected CCPM for the production of the M2 mirror is based on a magnetorheological principle leading drastic improvements of the final quality, compared to conventional CCPM’s. But it is limited to typically 300 mm in diameter.

The radiometric requirement is met with protected silver coating on the mirrors surface.

Within the PLEIADES HR study special effort was devoted to the camera alignment procedure. This starts at design. The secondary and the tertiary mirrors are left respectively with five and two degrees of freedom which are carefully designed for precise alignment. Alignment of the elements requires computer aid and highly computer aided interferometric measuring technology. The telescope is then aligned to the required root mean square wave front error (accuracy ≤ 12 nm RMS) and output pupil accuracy positioning (≤1.7mm) in relation with output pupil diaphragm position.

On one hand the two diaphragms located in the intermediate focal plane and at the output pupil, and on the other hand the mask in the shade of the M2 mirror on the M1 mirror prevent from stray light on the detector.

5.

OPTO-MECHANICAL DESIGN AND MATERIAL

Under a stringent thermal environment, the sensitivity of the compact optical lay-out combined to a relatively high-resolution / pupil diameter-ratio, lead to a very high stability requirement on the structure to fulfill the performances; especially for the M1-M2 link, as the defocus (along optical axis) is a major contributor to telescope WFE degradation.

Therefore carbon / carbon (C/C) composite has been selected by ALCATEL SPACE as a key material for camera structure for the following reasons (Table 3) :

Table 3.

C/C properties

However the choice of materials has been done also to reach the stability needs within the lowest cost.

Thus, different structural architectures and materials have been investigated to finally select, as the best appropriate candidate (Fig. 3) :

Fig. 3.

External camera view

The primary structure equipped with thermal control design provides an interesting mass (<125 Kg) for such a telescope which has to withstand high loads and presents a very high stability performance. This architecture allows a versatile design which can be easily adapted for different telescope geometries and offers a large compatibility with the existing launchers.

A robust thermal control is implemented and consists of multi-layer insulation (MLI) for efficient thermal protection of the overall camera and passive cooling plus heaters monitored by high precision platinum probes (PI regulation). A radiated thermal control in the M1 mirror light-weighting cells drives the camera WFE stability, whereas the detection unit is isolated and makes use of heat pipes and dedicated radiator to drain the power dissipation.

A thermal refocalisation device included in the M2 structure offers possibility of re-align optics during flight mission. This solution is lighter (mass) and cheaper than a mechanical one, in preserving performances. The requirement is an ability of 33,5μm PTV displacement for the M2 mirror with stability (focus : ± 0.8μm -tilts : ± 5μrad) and no mirror distortion induced. The design is based on the temperature controlling of an aluminum ring located between mirror and structure (Fig. 4). The functioning temperature range [17°C ; 34.5°C] is compatible with a 23°C thermal controlled camera.

Fig. 4.

Thermal refocalisation device

The main purpose of the shutter mechanism is to protect the detection unit from direct solar entrance. It shall be closed in less than 2 second in case of satellite loss of attitude and is located in a limited volume behind the M1 mirror. An hybrid stepper motor with redundant windings moves a flap from the one to the other steady state position : open and close.

6.

FOCAL PLANE DESIGN

The focal plane (Fig. 5) is the heart of the highly integration detection subassembly (Fig. 7). The size of the observed image is close to 400 mm and is analyzed in 30,000 samples in Pan and 7,500 in Xs.

Fig. 5.

Focal plane assembly

The Pan band is constituted of 5 TDI mode CCD arrays, which the pixels are of PhotoMOS type with lateral anti-blooming structure. The image section has 6000 columns of active pixels each 13 μm square and is clocked continuously to give a time-delay-and-integrate (TDI) function. The transfer of charge along the CCD is made synchronous with the velocity of the scan image. The integration time is m times longer than a single-detector integration time, where m is the number of stages in a row. Since the noise is proportional to m^1/2, the SNR improvement over a single detector is also m^1/2. Within the image section are 4 separately connected electrode groups to enable the TDI length to be varied. The electrodes of the image section are of the four-phase type. The full-well capacity (> 130 K electrons) is largely set by the channel doping and the anti-blooming “barrier height”, i.e. the well depth at which charge spills to drain. Below the image section is a register that is split into 10 sections, each with a separate output circuit.

The colored bands (XS) are constituted of 5 CCD four linear arrays. The spacing between the centers of each line and the next is 936 μm allowing the necessary area for the readout registers and associated bus structures for each line. The photo-sensing element of the pixel is a photodiode. Each line of pixels has 1,500 photoelements on a 52 μm pitch and the size of each photoelement is 52 μm square. Signal charge generated in the photodiode is initially collected under an adjacent CCD electrode. Transfer of charge from the storage gate to the read-out register is under control of a transfer gate. Within the storage gate area is an antiblooming structure. The full-well capacity meets the 130 K electrons minimum saturation requirement. The structure of the readout registers is composed with 2 register elements for each of the photo-detection pixel sites, using two phase technology. Thus the readout register is operated at twice the required pixel frequency and the output signal is making up after summing in the output circuit.

The standard detector package consists of a co-fired Aluminum Nitride (AlN) ceramic body with dual in line pins on the underside. The window is in BaK50 with an antireflection coating on both sides.

The spectral selection is made by optical filters placed very close in front of the detectors. Pan filters and XS strip lines filters (Fig. 6) are space-qualified multi-layer coating deposited on glass substrates. Each filter is composed of high-pass filter and low-pass filter. An absorbing material deposited between the XS filters isolates each band from the others to avoid inter-band stray-light.

Fig. 6.

XS filter design

Fig. 7.

Detection unit

One of the key features of the FPA is the extensive use of mechanical parts made of SiC. This material offers a very good thermal conductance leading to a simplified thermal concept, a low CTE for acceptable dimensional stability thanks to this class of camera featuring a low optics speed, and a very high mechanical rigidity for a low mass.

A Sic main structure ensures accurate positioning and thermal dissipation.

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 XS/XS in the field registration) and folding with long roof Sic mirror separates Pan and XS image (Pan/XS 1.58 mrad maximum in the field separation).

For its integration in the focal plane, each detector is equipped with a mechanical frame of SiC glued on the detector package in AlN. This frame fixes the detector on the focal plane structure and supports the front-end electronics with its shielding. This innovative architecture provides a versatile building block for a variety of imaging requirements.

The high integration level requires to use specific connecting parts : the link is made with flexible circuits optimized to operate at frequency close to 7 MHz and with the constraint of distance of typically 10 cm between the focal plane and the video electronics.

7.

CAMERA ELECTRONICS

The video electronics are mounted on the telescope (behind the focal plane), all video functions associated to one or two CCD are integrated on a single board, and the 70 video chains interface the compression and mass memory unit with 15 redundant very high speed digital links (Agilent HDMP-1032).

The design of the video electronics is driven by the high data rate required by the high resolution, the video signal profile and the stringent specified radiometric accuracy. The pixel frequency required is 6.5 MHz for Pan band and 3.7 MHz for XS bands. For video processing, CCD phase driving and digitally programmable delay no satisfactory standard component has been found. For this reason, CNES and ALCATEL SPACE have developed 3 analog/mixed ASICs corresponding to these 3 functions. They have been designed for 1 to 10 megapixel/s high accuracy readout of CCD detectors and have been developed in RHBIC3 BiCMOS 1.2 μm Rad Tolerant technology.

The CLBNG ASIC is a versatile wide-band analog CCD front-end circuit for video acquisition and processing. Its architecture is fully differential and allows 2 multiplexed inputs. It includes correlated double sampling, 12 bit DAC for offset correction and a digitally programmable internal voltage reference. It provides simple and efficient interfaces with most A/D converters.

The FAST ASIC is a versatile phase generator circuit for driving CCD. It includes 2 independent channels programmed by a common serial interface (setup of rise/fall time); high and low levels are externally tuned. It provides compatibility with most CCD available from the market.

The TRIM ASIC is a versatile digitally programmable delay circuit. It provides delay on rising edge and falling edge independent and includes 4 channels with delay range from 0 to 100 ns with 1 ns step.

On the basis of these ASICs, two types of hybrid have been developed :

Due to their low power and size budgets, these basic functions provide a low mass, low volume and low power consumption for the highly integrated detection unit.

The dedicated unit for video power supply is located in the bus and consists in 4 independent board set (3 boards combining with 1 or 2 Pan CCD and 1 board combining with all XS CCDs). Its design is based on ALCATEL standard converter modules (CVST40) which use high output flyback converter with very high accuracy in a wide output power range.

Fig. 8.

Instrument in/out functional diagram

The instrument in/out unit consists in two redundant board sets located in the bus. It serves as interface between the bus and the instrument by means of a 1553 bus. Its main tasks are the control and management of the different components of the instrument, which are, in addition to itself :

The core of the unit is implemented into two FPGAs : ACTEL 54SX72 (which are the biggest ITAR free device available); the capacity provided by these devices is enough for the PLEIADES HR instrument requirement.

8.

PERFORMANCE ESTIMATES

The PLEIADES HR instrument performances are listed in Table 4.

Table 4.

Instrument performances summary

9.

CONCLUSION

ALCATEL SPACE offers a family of small-satellite space borne imaging cameras enable to satisfy the space remote sensing market (defense and security customers, scientists and civilian institutional customers, commercial customers), based on its experience gained over the previous decades and now with the PLEIADES HR camera program.

The cameras fully meet panchromatic and multispectral systems sensing requirements. These camera designs deliver superior performance using a low power, low mass and scalable architecture. The overall design philosophy is to use proven technology and in-house products for a low-risk and low cost-system.

This complete range of innovative architecture provides a versatile approach for a variety of imaging requirements and covers a wide variety of areas for possible missions, such as wide-area surveillance and multi-spectral remote sensing : military information, range finding, mapping, ground, ocean and resources supervision.