Abstract
This paper introduces briefly the China-Brazil Earth Resources Satellite (CBERS). It mainly presents a design description of the Infrared Multispectral Scanner (IRMSS) for CBERS. The key technology features of the IRMSS are highlighted in this paper.

Introduction
The CBERS has been jointly developed by China and Brazil for more than ten years. It is scheduled for launch in 1999. It will carry three remote sensing instruments: the Infrared Multispectral Scanner (IRMSS), the CCD Camera and the Wide Field Imager (WFI). The CCD Camera and the WFI operates in the visible and near-infrared while the IRMSS mainly operates in the IRMSS mainly operates in the refrared. The three instruments are used together to provide image with different resolution and coverage. The IRMSS was developed by Beijing Institute of Space Machine and Electricity.

IRMSS Design Description
The IRMSS is a cross-track scanning imaging radiometer mainly operating in the infrared. It has one Panchromatic (Pan) band, two short-wave infrared (SWIR) bands and one long-wave infrared (LWIR) band. The spatial resolution of the Pan and SWIR bands is 78m in an orbit altitude of 778km while the LWIR band is 156m. Its swath is 120km, and the global coverage period is 26 days, Its key design characteristics are summarized in Table 1.

The IRMSS mainly include a scanner mainframe, an amplifier assembly, and encoder, a power supply assembly, a command & telemetry assembly, a radiative cooler controller and a thermal controller.

The scanner mainframe is composed of the mainframe structure, the linear scan mirror assembly, the scanning angle monitor, the telescope, the scan line corrector, the onboard calibration system, the relay optics, the focal plane assemblies, the pre-amplifiers and the radiative cooler. The IRMSS block diagram is illustrated in Figure 1.

Orbit Altitude	778km
Scanning method	Bidirectional cross-track
Aperture	250mm
F/No	F/4, F/2(L WIR)
Spectral bands(mm)And SNR (or NEDT)	0.50 ~090, 300 1.55~1.75, 100 2.08~2.35, 50 10.4~12.5, 1.2k
FOV	8.8°
Detector Size	0.1mm x 0.1 mm
Spatial Resolution	78m, 156m(LWIR)
MTF	0.4

Table 1: The IRMSS Characteristics


Figure 1. IRMSS Block Diagram
The mainframe structure is the structural backbone used to provide stable, rigid support for mounting and maintaining alignment of the other assemblies of the scanner mainframe. It was mainly made of titanium alloy and was lightweighted extensively.

The linear scan mirror assembly has the function of scanning in the cross-track direction by oscillating raround the short axis of the scan mirror. Its scan rate is 5.3908Hz and the scan angle is ± 2.5°. To meet low rotational inertia and high dynamic flatness and linearity requirements, beryllium was choosed as the material of the scan mirror. Further light weighting measures are taken to reduce the mass and inertia of the of the scan mirror. The scan mirror was suspended by two flex-pivots. A magnetic compensator was used to cancel the spring forces of the flex-pivots.

The scan angle monitor is used to measure accurately the relative position and direction of the scan mirror and provide synchronous signal for the Encoder. Its telemetry signal was used for ground image processing.

The telescope is a Ritchey-Chretien system with a 250mm aperture diameter and a focal length of 1000 mm. The telescope mirrors are made of fused silica glass and held by a structure made of Invar. The primary mirror was lightweighted.