| GISdevelopment.net ---> AARS ---> ACRS 1999 ---> Poster Session 2 |
The IRMSS for CBERS
Wang Huaiyi, Ma
Wenpo
(Beijing Institute of Space Machine and Electricity, Beijing , 100076)
Abstract(Beijing Institute of Space Machine and Electricity, Beijing , 100076)
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 |
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.
The scan line corrector consists of a pair of mirrors and associated servo mechanism. It has the function of correcting the effects of spacecraft motion by rotating synchronously with the scan mirror.
The on-board calibration system is used to correct the output changes of the scanner in flight. It includes internal calibrator and solar calibrator. The calibration precision is 3% for both the band to band and the channel to channel calibration. The absolute calibration accuracy is less than 10%.
The internal calibrator includes calibration lamp and calibration blackbody calibration are real time calibration that takes place during the scan-turn around interval. During that time a rotating shutter is driven to prevent the Earth flux from being incident on the focal plane and the flux from calibration lamp an calibration blackbody is reflected to the focal plane. The lamp calibrator has 4 operation states corresponding to different flux output. Each state lasts about 16 seconds.
The solar calibrator is designed to provide calibration reference with the Sun upon ground command. As the satellite passes over the north polar regions, the solar calibrator collects the solar flux and reflects it onto the Pan and SWIR bands detectors. The solar calibration also provides a check on the stability of the on-board lamp calibration. It is performed once every 13 day.
Each band employed an eight-element staggered detector array mounted along track. For the thermal band four of the eight detector are spare. The Pan band detectors are silicon photodiode. The photovoltaic Mercury Cadmium Telluride (HgCdTe) detectors were specified for the two SWIR bands and the LWIR band utilized photoconductive HgCdTe detectors. The detector size for all the four bands is 0.1 mm x 0.1 mm.
The radiative cooler provides pas;sive radiant cooling to the detectors on cold focal planes. It has two stages whose operating temperatures are 101 K and 148K respectively.
IRMSS has three focal plane assemblies corresponding to the pan band, the SWIR bands and the LWIR band. The Pan band is located on the warm focal plane. The SWIR bands and LWIR band are located on cold focal planes with cryogenic temperatures of 148K and 101 K respectively. The spectral band registration accuracy is less than 0.3 IFOV for Pan and SWIR bands, and less than 0.6 IFOV for the thermal band to the others.
The thermal control of IRMSS is achieved by both active and passive thermal control methods. The thermal controller controls the internal temperature of IRMS within the range of 18±3oC. The amplifier unit consists of main amplifier and multiplexer. The main amplifier is used to amplify the output signals of the pre-amplifier. The multiplexer is used to convert the parallel output signals of the main amplifier to a serial signal flow.
The main function of Radiative Cooler Controller is to control the decontamination of the radiative cooler by heating during the early phase of orbital flight and when contamination occurs again later. After first decontamination, the radiative cooler will open the cooler cover according to the telecommand. In this way, the normal radiative cooling state can be started.
The encoder is mainly responsible for converting the analog signal to digital with a 8-bit radiometric precision, encoding according to required format, and inserting necessary major frame synchronous signal and some auxiliary data. It was controlled by common computer. The data rate of the IRMSS is about 6Mbits/s.
IRMSS Technology Features
Although the IRMSS works on the same basic principle as the MSS and TM on board the LANDSAT satellites, as to the specific technology solutions, it differs from MSS/TM in many ways. For example, in the aspects of the short-wave infrared detectors, the focal plane assemblies, the relay optics, the on-board calibrators and the on-board computer, the IRMSS has its own technology features.
The IRMSS utilizes PV HgCdTe as the detectors of short-wave infrared bands. This makes it possible to cool the detectors with the first stage of the radiative cooler.
The radiation from the prime focal plane is divided into three parts by relay optics which corresponded to three focal planes. Two of them are cold focal planes and one is warm focal plane. The two cold focal planes are connected respectively with the first stage and the second stage of the radiative cooler.
Beside the function of on-board lamp calibration and blackbody calibration, the IRMSS has the function of solar calibration.
The IRMSS uses common computer to handle the on-board data.
Conclusion
It is a substantial technical challenge to develop the IRMSS based upon the late 1980s and early 1990s technology of China. In order to meet the challenge, trade-off design has been performed on the IRMSS. During the development of the IRMSS, all kinds of tests required for the IRMSS were conducted, which proved that its design is optimal. The assembly and test of the first IRMSS flight model have been finished and the final test results indicate that this sensor has met or exceeded its design specifications At this writing, the first IRMSS FLIGHT model is in the launching base waiting for launch.