3.1

- Detection electronics

3.1.1

– Electrical architecture :

The SEDHI concept requires to implement on a single board all electronic functions associated to 1 or 2 detectors, from the video signal pre-amplification to the digitised video frame, and including various functions such as video signal processing, detector low noise supply and polarisation generation, detector and video processing chain sequencer, detector clock drivers, synchronisation interface, TM/TC interfaces, …

The functional organisation of this board, named MVP (Module Video de Proximité), is shown on the next figure.

A full detection electronic is constituted of several MVPs, depending on number of detector in the focal plane, and a MSP board (Module de Servitude de Proximité), in charge of MVP synchronisation and MVPs’ interfaces with power supply and TM/TC equipments.

Stacked MVPs and MSP constitute de detection electronic box. This box is mounted very close to the focal plane. Detectors are connected to detection electronics by the mean of short flexible printed circuits.

One MVP is able to process up to 10 CCD outputs, at a maximum pixel rate of 10 megapixel per second. Furthermore, due to high pixel frequency, detector and video processing chain clocks must be phased with an accuracy of a few nanoseconds. For this purpose, MVPs include several programmable phase trimmers in CCD phase driver blocks in association with video processing chains.

Each CCD output is processed by a video processing chain whose main characteristics are: pixel frequency from 1 to 10 MHz, correlated double sampling, 12 bit A/D conversion, digitally programmable phase of sampling clocks on a 100 ns range with 1 ns resolution, black level compensation.

Figure 4:

Organisation of a MVP

Each CCD clock is driven by a CCD phase driver whose main characteristics are : capability to drive up to 1.5 nF input, with 5 ns rise or fall time, digitally programmable rise and fall time, from 5 to 110 ns, independently digitally programmable delay on a 100 ns range with 1 ns step, adjustable low and high output level

The high output data rate of a MVP (up to 1.2 Gigabit/sec) is transmitted downstream through few LNTHD (Liaison Numérique Très Haut Débit), which are serial or serial/parallel integrated digital interfaces.

It must be noticed that there is a high level of modularity and standardisation at the MVP level because one MVP is associated to one type of detector. With SEDHI architecture, the same MVP can be used each time the corresponding type of detector is used.

3.1.2 –

Technological implementation :

Future High Resolution instruments require rather large quantities of few basic functions such as video processing chains, CCD phase drivers or LNTHD. For example, quantities are respectively estimated to 70, 180 and 15, in the case of one Pleiades camera.

Consequently, feasibility of these future HR instruments will depend strongly on the availability of those basic functions with good power and size budgets.

Unfortunately, standard component from manufacturers have scarcely the right characteristics, in term of functionalities, performances or capability to withstand space environment.

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 chosen to develop 3 analogue/mixed ASICs corresponding to these 3 functions. The main characteristics of these 3 functions are listed hereafter.

ASIC ARC

Digitally programmable delay, delay on rising edge and falling edge independent, 4 channels, delay range from 0 to 100 ns with 1 ns step, CMOS technology, prototyped by CNES.

Video processing ASIC (CLBNG)

Simple or double correlated sampling, 1 to10 megapixel/s capability, two channels multiplexing, fully differential, internal reference, complementary BiCMOS technology, Alcatel Space development.

CCD phase driving ASIC (FAST)

Up to 1.5 nF driving capability with 5 ns rising or falling edges, digitally programmable rise and fall time from 5 to 110 ns, independently adjustable low and high output level, 2 channels, complementary BiCMOS technology, Alcatel Space development.

On the basis of these ASICs, two types of hybrids are developed :

• • Video processing hybrid, which includes two complete video processing chains. Each processing chain is based on a CLBNG ASIC, an ARC ASIC, a 12 bit 10 MSPS A/D converter from the commercial market, a digital ASIC for offset correction processing and few passive components. Compared to video processing chain of SPOT5/HELIOS2 generation, these new chains exhibit a gain of two for the maximum pixel frequency and a gain of five for power and size

• • CCD phase driver hybrid (HTDN), which is able to drive 4 CCD phases, and based on 2 FASTs, an ARC and few passive components

Up to that step, these technological basic functions, ASIC or hybrids, are compatible with most detection electronic architectures, as described above. In that prospect, they can be considered as standard basic functions.

Due to their low power and size budgets, these basic functions fulfil the requirements of limiting the mass, volume and power consumption of the complete subsystem which is one of the main goals of the SEDHI concept.

Technological implementation of next generation detection electronics is summarised on the next figure.

Figure 5:

technological implementation

The preliminary drawing of a MVP based on the described electrical and technological definition is shown hereafter :

Figure 6:

view of a MVP

3.2

- Mechanical concept

The mechanical and thermal architecture is split in three distinguish parts corresponding to separate hardware modules: the focal plane, the electronics modules, the focal plane radiator.

Each element is supported independently by the telescope’s structure with the help of mechanical frames and supporting blades.

Several configurations of focal plane may be considered depending on the application, but the most standard will use linear detectors with a beam splitter in order to compensate the distance between staggered modules.

A focal plane structure holds the detectors and beam splitter mirrors and insures the thermal conductivity between the detectors and the external thermal drain. The thermal flux dissipated by the detectors are dissipated in space through the focal plane space radiator.

Each electronics PCB is mounted in an individual mechanical frame to constitute a MVP. These modules are stacked together and screwed on a specific mechanical frame.

This architecture authorizes a very flexible mechanical architecture, which can be easily adapted to a wide range of focal plane requirements (number of detectors, length of the focal plane..), with a minimization of engineering efforts.

The following figure shows a focal plane constituted of 2 lines of 5 linear detector, with the associated MVP and focal plane radiator.

3.3 -

Thermal concept

The thermal concept must handle the heat generated by the detectors in the focal plane and by the MVPs.

The focal plane radiator size depends directly on the detectors power dissipation which is directly related to their operating frequency. The radiator sketched on next figure must handle a detector power dissipation close to 20 W. The focal plane structure is thermally coupled to the radiator by cooper wires and/or heat pipes. The radiator is fixed on the telescope structure by insulation GFRP blades.

The MVP are thermally regulated by the radiate surface opposite to the MVP mechanical interface. The available surface is sufficient to limit the temperature of each module to a temperature compatible with the radiometric requirements. No thermal drain is required as the heat is conducted through the mechanical structure of each MVP.

Figure 7:

example of a SEDHI mechanical arrangement

3.4 -

Adaptability to a large scale of missions

The concept of SEDHI can be advantageously used for instrument with lower number of detectors. This may occur in applications using long linear arrays with very small pixel pitches. In this case the number of detectors butted in the focal plane is smaller.

The following figure shows a configuration with two detectors (a panchromatic and a multispectral detector for instance, which is a configuration commonly identified in the current missions), showing the good adaptability and versatility of the SEDHI concept.

Figure 8:

view of a SEDHI with two detectors mounted on a TMA telescope

SEDHI may also be envisaged on instruments with a larger number of detector lines or with very large focal plane sizes (larger than 50 centimeters), or with more than 2 detector lines.

Increasing the number of detector lines may be justified if some redundancy is required at detector level. The high integration of electronic functions in the SEDHI concept increase the reliability performances of each detection chain.

The modularity of the concept facilitates the redundancy at detector level or at electronic functions level. It is not foreseen to implement redundancy inside each MVP, but at MSP level where the power supply interface functions, command and control function may be duplicated if required by a specific application.

The distance between detector lines in the focal plane may also be a critical point with regard to the ground processing which will be required to register the images acquired from each line of detectors. Some applications will need to reduce the distance between these lines. This can be performed with the help of beam splitters or by integrating detector dies on a focal plane mechanical interface. In each case, the focal plane will be very integrated and the concept must be compatible with the small size and volumes available for the electronics. Integrating the complete proximity electronics on one PCB (the MVP) allow to reduce the width of electronic modules and thus will ease the reduction of the distance between detector dies.

The following figures shows a SEDHI with 3 lines of detectors (2 redunded panchromatic lines and one multispectral line), with a physical distance between detector lines smaller than 30 mm, and the complete subsystem mounted on a Cassegrain telescope.

Figure 9:

SEDHI with 3 lines of detectors mounted on a Cassegrain telescope