Detectors - Vegetation System
In good design logic, an instrument is more often than not built around its sensor, and advances are made in remote sensing instruments at the rate at which these components progress. It thus seemed sensible to us to present the Vegetation sensors in this first article on the payload.
There are two types of instrument on-board SPOT4: besides VEGETATION, two high spatial resolution instruments provide a detailed view of 60 x 60 km2 zones with resolutions of 10 and 20 metres.
The observation principle is the same for both types of instrument. The image is gained owing to the presence of a line of sensors in the focal plane of the collecting optics. A line is scanned by electronically reading the elementary sensors and line-by-line progress across the track is provided by the movement of the satellite. From this brief explanation of how the device works, we can see the advantages of designs with no moving parts where the mechanical assembly is simple. The sensors, on the other hand, are complex.
Sensors of Visible Channels B0, B2, B3
The sensor chosen to equip the three cameras of the "visible" channels is the Thomson TH7811, which is also used in the two HRVIR instruments.
This sensor contains 1728 photosensitive elements, each 13 microns square, associated with two analog shift registers. A third register combines the information coming from the first two (figure 1). The video signal is delivered through an output amplifier. The information collected (photons) is converted into charge packets that accumulate before being transferred along the registers. The operating principle is the following: on a p-type silicon substrate, a dielectric layer of silica is created, on top of which is deposited an electrode, the whole (electrode - dielectric - substrate) forming a MOS (Metal-Oxide-Semiconductor) capacitance for storing the charges (figure 2). Applying voltages to these electrodes generates potential wells in the substrate. If two near potential wells have different depths, the charges accumulate in the deeper one. Changing the potential of one of the elements thus allows the charges to be transferred, their progression along the registers being produced by switching the voltages applied to the electrodes at a rate fixed by the control clock (figure 3).
The Thomson 7811 sensor is the result of a series of improvements made to a product that already equips the SPOT2 and SPOT3 satellites, in particular the addition of an anti-blooming system (figure 4) which minimizes the effects of specular reflections.
Sensor of Mid-Infrared Channel MIR
The sensor used for the mid-infrared channel of VEGETATION is an improved version of the Thomson TH31900 mounted on the two HRVIR instruments.
This sensor is made up of 3000 GaInAs/InP photodiodes with a pitch of 26 µm, read and multiplexed by conventional charge coupled devices. The line is composed of 10 modules of 300 diodes each, mounted, glued and perfectly aligned on a ceramic support. Assembly is accurate to + 1 µm with a single non-operating diode between two modules. The tracks needed by the clock signals, power supplies and video outputs are implanted on the ceramic support. More than 3000 connections are made in aluminium wire welded ultrasonically. The ceramic support with its 10 wired modules is enclosed in a titanium cover having an optical window. The support-cover seal is made by compressing a joint of indium. A copper insert buried in the ceramic base of the unit ensures a uniform thermal profile. Two thermistors are mounted at each end of the copper insert.
Taking advantage of the experience gained in the production of the flight models for the HRVIR, we have made several important changes for VEGETATION:
- Increased dimensions for the InP/GaInAs/InP wafers to improve the quality and homogeneity of the 300 diodes composing each module. This improvement has resulted in more uniform radiometric performances and a better efficiency.
- Changes to the ceramic cases, which are now made using a technology called co-sired : i.e. in stacked "sheets" of ceramic. This technology enables the electrical interconnections to be made by screen printing. This method reduces the risks of electrical faults; it is the same as the one used for making multi-layer printed circuits.
- The sensors mounted on the HRVIR instruments have 3000 elementary 30 x 30 µm² sensors in staggered configuration of five with a pitch of 26 µm. This geometry is very penalizing for VEGETATION image quality (high geometrical distortion at the edge of the field). Technological progress, notably the reduction of noise and the dark signal, has allowed the fabrication process to be modified so as to obtain a perfect line with elementary sensors 20 x 30 µm² at a pitch of 26 µm. For the needs of VEGETATION, only 1728 of the 3000 available on the line are used.
The electric, geometrical and radiometric performance levels required for the visible or MIR sensors are often at the limit of what is feasible. To respect these requirements it is necessary to have specific, very sophisticated measuring equipment and strict selection is indispensable in parts fabrication. All this leads to high costs and the setting up of very elaborate technical procedures; these are the price to be paid for success with a very high performance imaging instrument.