INTRODUCTION

The partnership between Brazil and China for the development of satellites applied to remote sensing of the Earth started in 1988 through the CBERS program. So far, this program has already launched 3 units - CBERS 1, 2 and 2B. The next two satellites CBERS 3 and 4 are slight different from the previous launched and represent an evolution of the previous satellites. These satellites will carry four cameras on board. Two of the cameras (panchromatic and the IRMSS) are being built in China and the other two are being built in Brazil [1]. For these two Brazilian cameras (MUX and WFI), Opto Eletrônica S.A.[2] is in charge of their development, considering that this company participates in a consortium for the development of the WFI Camera (Wide Field Imaging Camera) and is responsible for the complete development of the MUX Camera (Multispectral Camera) and its Ground Support Equipment (GSE). The MUX-GSE equipments consists of an assemblage of equipments that perform on ground optical and electronic tests of this MUX camera. It includes a collimator, a scene simulator, an integrating sphere, a power supply, an oscilloscope, an image display system, etc. Fig. 1 exhibits some of the equipments of the MUX-GSE.

Fig. 1.

Some of the equipments of the Ground Support Equipments: collimator, integrating sphere, GSE controller, and image display system (Courtesy Opto Eletrônica S.A.).

THE MULTISPECTRAL CAMERA - MUX

The MUX Camera will image a ground swat of 120Km with 20m of resolution. This camera consists of three equipments: RBNA, RBNB e RBNC. Fig. 2. shows the RBNA equipment of the MUX Camera.

Fig. 2.

RBNA equipment (Courtesy Opto Eletrônica S.A.).

The RBNA is responsible for the image acquisition and is composed by the optical system (entrance mirror and lens assembly), optical housing and the focal plane assembly. The RBNB consists of the electronics responsible for the thermal control, the focus adjustment and the internal calibration system control. The RBNC is responsible for generating the CCD reading clocks, processing the CCD analog outputs to digital signal and encoding the data into a serial data stream. This data is transmitted to the satellite.

The sensor of this camera has four lines, each one with 1 x 6000 pixels of 13x13μm in size. Spectral thin films deposited over a window that covers the photosensitive elements of the CCD are responsible for the four spectral bands separation.

OPTICAL DESING

B. Optical System Characteristics

Fig. 3 presents the layout of the optical system developed. The optical system consists of a refractive system with 14 components: an entrance mirror, a window, 11 fixed lenses and a moving lens. The total length of the optical system is 653mm. The weight of the optical components (excluding the mirror) is about 4.7kg and the system is athermal in the temperature range from 0°C to 40°C. Fig. 3 exhibits the manufactured optical system aligned in its optical housing and during optical test.

Fig. 3.

Collecting lens system with focal adjust lens.

Fig. 4.

Collecting lens with focus adjust lens assembled in alignment stage.

C. Theoretical Performance

The Fig. 5 shows graphics of grid distortion (<0.107%) and graphics of lateral color at the mean temperature operation (17°C). Fig. 6 shows the graphics of the Modulation Transfer Function (MTF) for the four spectral bands all together at the overall temperature range (0°C, 17.5°C and 40°C) in the air and vacuum positions. It shows that the MTF is > 0.7 at 38.5lp/mm. Fig. 7 shows Through Focus MTF for all spectral range (from 450nm to 890nm) in the overall temperature range (0°C to 40°C) in the vacuum and in the air.

Fig. 5.

a) Grid Distortion, b) Lateral Color.

Fig. 6.

Modulation Transfer Function in the four spectral bands, all together, at 17.5°C, 0°C and 40°C in the vacuum and at 17.5°C in the air.

Fig. 7.

Through Focus MTF for the overall wavelength range in the temperatures: 17.5°C, 0°C and 40.0°C.

FLAT MIRROR

The entrance mirror must be manufactured with high superficial quality. The fabrication and mounting were performed at Opto Eletrônica S. A. Fig. 8 shows the schematic mounting draw and a picture of the mirror before it received the reflexive coating.

Fig. 8.

MUX Camera entrance mirror.

CONCLUSIONS

The MUX camera is being developed to allow high quality performance images. This camera is the first one of this type to be developed, manufactured, mounted and tested in this country. The project is now on the Qualification Model (QM). The previous phases allowed the refinement of the project by means of analysis and tests. This phase foresee environmental and optical tests.

The QM unit is already aligned and the optical tests are being taking place. The preliminary results were very satisfactory.