Optical Assembly

Mounted on the instrument structure via quasi-isostatic mounts, 2 cameras, oriented at +20°/-20° with respect to Nadir, ensure the optical imaging and spectral filtering, the signal detection and video generation.

Each camera is composed of a detection unit coupled to a refractive telescope that provides the image quality and the focusing on the CCD detector thanks to a 11 lenses optical combination. Two parallel blades on the telescopes front side realize the spectral filtering and the radiation shielding (SUPRASIL blade). The telescope focus, key optical quality parameter, is thermally compensated and controlled. The telescope characteristics are:

Based on a Thomson TH 7834 CCD (12000 pixels of 6,5 x 6,5μm²) and its proximity electronics, the detection unit is directly mounted on the rear side of the telescopes to ensure the focus stability performance. In this configuration, the CCD, controlled at 24 °C, is thermally and mechanically coupled to the telescopes. The proximity electronics dissipated power is evacuated thanks to specific radiators mounted on the top of the detection units. The detection unit sequencing and the acquisition of the video signal is performed by the MVS (Module Video Stereo).

Fig.2:

HRS telescopes during integration and alignment on the main structure

Structures

Ultra light and highly stable, the main structure, made of Al honeycomb / CFRP sandwich, featuring new cyanate material (low thermal and moisture expansion figures), ensures a stable accommodation of the instrument units and decoupled interfaces with the satellite. In addition, secondary structures are implemented to support the active thermal control (MVS and Optical Assembly thermal hoods) and to avoid sun entrance in the cameras (sun cap).

Thermal control

The Optical Assembly accurate thermal control (+ 0,5°C temperature and thermal gradient stabilities), avoiding the use of any refocusing mechanism, is achieved thanks to a thermal enclosure concept that radiatively controls the high thermal inertia assembly via a combination of active and passive thermal control. Instrument control electronics

Accommodated inside a single unit, the instrument control electronics (MVS) provides management, housekeeping, detection sequencing and video electronics.

The management and housekeeping electronics realize the implementation of instrument modes and monitoring, the active heat control of the instrument and the standard SPOT5 interfaces with the satellite (DC/DC conversion of the satellite power bus and OBDH data handling bus). A CCD sequencing electronics, a Video electronics at 4,1Mpixels.s^-1, capable of up to 10 Mpixels.s^-1 and a 12 bits analog to digital conversion complete the detection chain and provide HRS images digital information to the satellite.

Fig.3:

HRS instrument fully integrated (MLI not shown)

Fig.4:

HRS exploded view

Timescale

Decision to implement HRS mission on SPOT5 was finalised in the very beginning of year 1999 with the major condition that HRS development should not delay the overall satellite timescale. This resulted in an overall timescale for HRS instrument development of 2 years.

Development logic

Given the available time, it was necessary to start in parallel units and instrument studies during the early engineering phase and to select a mono model philosophy, direct manufacturing of proto-flight model. Therefore, the instrument development was split in three phases, as illustrated on Fig.6:

Fig.6:

HRS development logic

The early engineering phase was an essential step for the success of the HRS challenging development. Indeed, to speed up the instrument definition, concurrent engineering was implemented with joint teams working in parallel to early identify design drivers and freeze the architecture, the design and the interfaces of the overall instrument. This phase has led to develop in parallel all mathematical models, to run simulations and establish budgets, each industrial team being fed in quasi real time by the others’ study outputs. For the same schedule constraints reasons, bulk of HRS instrument electronics are made of Mil standard parts, with adequate screening. Extensive use of Field Programmable Gate Array (FPGA) components for digital electronics allowed simple, late and quick design evolution. At satellite interface level, full « High-Rel » standard components are used.

Verification logic

Combination of analyses, simulations and tests, HRS verification and qualification programme was successfully held in 2000. However, the full qualification and the instrument flight worthy ability was only pronounced in 2001 after successful EMC qualification performed at S/L level.

Fig.7:

HRS in vacuum chamber during TB/TV tests

The verification programme, done at instrument level, followed the following sequence:

Fig.8:

HRS verification sequence

Verification results

Geometrical performances

After a characterization of reference frames transfer matrices, the alignment stabilities (geometrical performances) were trend during the instrument verification sequence (before and after mechanical and thermal environment).

The results, illustrated on Table3, highlighted excellent stability performances that allow for automatic data reduction at customer level.

Table3:

Alignment stabilities verification

Radiometric performances

After video chain calibration, mainly dedicated to gain and phase settings, the functional radiometric behaviour was first measured, especially:

Then, the radiometric coefficients (equalization and dark coefficients) necessary to compensate for telescope and detection chain non-uniformity were characterized and delivered to the customer. Finally, the radiometric performances measured at ambient and under vacuum, exhibited excellent results in all domains.

Table4:

Radiometric performances verification results

As illustrated on Fig.9, the noise under different brightness, main instrument radiometric performance, is well within the specifications

Fig.9:

Noise versus brightness

HRS detection chain is linear over the large dynamic range (see Fig.10).

Fig. 10:

Relative linearity@ G3

Optical performances

Spectral response, straylight, polarisation, distortion and Modulation Transfer Function (MTF) are the main optical performances, which have driven the instrument design. Measured at ambient and under vacuum (especially MTF), these performances are illustrated on Table4.

Table4:

Optical performances verification synthesis

HRS mission and elevation accuracy are very dependent of instrument MTF performances. Therefore, during the instrument verification sequence, a specific attention was paid to this critical parameter that includes the MTF over the Field of View at best focus and the defocus performances. Although well within the specification for both cameras, a better MTF performance is observed on camera2 (Fig.12) compared to camera1 (Fig.11). This is only due to the dioptric telescope lenses alignment procedure, largely improved on the second camera to reduce the residual astigmatism. Therefore, complying with MTF requirement means a defocus stability of + 30 μm on camera 1 (fore camera) and + 45 μm on camera2 (aft camera).

Fig.11:

Fore camera MTF over the Field of View

Fig.12:

Aft camera MTF over the Field of View

So, it is clear that focus alignment and stability and, consequently the thermal control of the cameras, are key issues for the HRS mission performances. Therefore, large efforts were spent to master the defocus stability performance.

Thanks to optical simulations, a defocus modelling was first established:

with:

Δf_i: Initial focus adjustment @ 0°C

S_θo: Sensitivity to camera mean temperature

S_Gl: Sensitivity to camera longitudinal gradient

S_θB: Sensitivity to baffle temperature

S_θhs: Sensitivity to heat sink of the observed scene

S_θhsw: Sensitivity to orbital variation of heat sink

θ_o: Camera mean temperature

G_l: Camera longitudinal gradient

θ_B: Baffle temperature

θ_hs: Heat sink of the observed scene

θ_hsw: Orbital variation of heat sink

Then, to correlate the model and predict flight performances, defocus sensitivity cases were introduced during TB/TV test. These specific technological cases performed on both cameras allowed estimating the model coefficients with defocus accuracy better than 4 μm over the whole campaign:

Referring to Fig. 11 and 12 and the estimated in-orbit defocus, [-15 μm; +11 μm] at 1σ and [-28 μm; +24 μm] at 2σ, the MTF performances at 2σ are:

Voltage, current and power

In-orbit HRS voltages, currents and power consumptions, identical to predictions and on-ground measurements, exhibit stable behaviour, demonstrating the good health of the instrument

Fig.13:

HRS power consumption on BNR/CU in imaging and stand-by modes

Temperatures

The in-orbit thermal control is performing as expected. The only unexpected behaviour concerns the CCD proximity electronics slightly hotter than predicted due to a higher satellite interface on –Xs side. Nevertheless this result has no performances impact.

Fig.14:

In-orbit HRS electronics temperature (black) versus operational (white) and non-operational (grey) temperature ranges

As expected, no long-term thermal evolution is observed on cameras, the main contributor remaining the orbital environment:

Fig.15:

In-orbit telescope temperature

Fig.16:

In-orbit telescope longitudinal gradient

In-orbit defocus estimation

Considering the correlated defocus model established during on-ground verification, the estimated in-orbit defocus, illustrated on Fig.17, is:

Fig.17:

In-orbit HRS instrument estimated defocus

The orbital stability, better than expected (+ 7 μm compared to + 14 μm), can be explained by the inaccuracy of the defocus model heat sink coefficients estimation, inaccuracy due to the impossibility to simulate on-ground the real heat sink (thermal shroud and optical ground support equipment disturbing the representativity of the observed scene during TB/TV).

In-orbit MTF estimation

From the estimated in-orbit defocus, the worst-case MTF over the field of view is:

much better than the required 0.18 specification.

The analysis of the in-orbit MTF (Fig.18 and Fig.19) demonstrates the good focus adjustment. Indeed, The focus setting is found close to the optimum (-2 μm on fore camera and -6.5 μm on aft camera).

Fig.18:

In-orbit MTF with actual and ideal defocus (fore camera)

Fig.19:

In-orbit MTF with actual and ideal defocus (aft camera)

Over the 1.5-year in-orbit lifetime, all parameters are stable and exhibit excellent performances.