Abstract
This paper introduces briefly the CCD
Camera for the China/Brazil Earth Resource
Satellite (CBERS) and its design and
technical features. It is scheduled to space
along with launching of CBERS on October
1999.
Introduction
CCD Camera is one of main instruments in
China/Brazil Earth Resource Satellite
(CBERS). It is used to take the earth ground
scenes photographed by push-broom
scanning, on 778km sun synchronous orbit,
simultaneously obtaining blue, green, red and
near infrared band images and one
panchromatic image. The ground pixel
resolution of each band is 20m. MTF at
characteristic spatial frequency (38.5lp/mm)
is 0.25. Swath width is 113km.The imager
has about 6000 effective pixels the global
coverage period is 26 days. There is a side-looking
reflector mounted in front of the lens.
The spindle set of reflector paralleling with
flight direction. It can be turned an angle
around its axis, so that photographing range
on earth will turn to left-wards or right-wards
32 degrees. When necessary, the ground
photographic range can be adjusted to widen
the recycle observing capability over special
interesting area where is taking place natural
disaster. The range of side-looking area can
reach 1100km; Maximum repeated cycle is 5
days. It causes the camera have some
function of stereo-triangulation. Data
transmitter of the Camera transmits the signal
to the ground station by 8bit encoded data.
The designed lifetime of the camera is two
years. The spectral range of every band of the
CCD Camera is at below. (Table 1.)
The main subsystems of the CCD camera
consist of optical system, beam splitting
system, CCD devices and CCD imaging
electronic circuit etc. The auxiliary
subsystems of the CCD camera are side-looking
reflector, relative calibration
mechanism, focusing device, temperature
controller with their structures, control
circuits, remote control and remote
measurement circuits and interfaces, and
power units etc. They are divided into four
packages mounted on the interior of the
satellite, that is the body of camera, optical
and mechanical control box, temperature
controller, and CCD imaging circuit. Fig 1 is
picture of optical path of the CCD Camera.
Fig 2 is block diagram of the CCD Camera
system.
Table 1. CCD Camera Design Description
| |
Band |
Wave length µ m |
Radiance
w/m2·sr |
S/N
db |
| Multispectral band |
|
|
Max |
Min |
Max |
Min |
| B1 |
0.42~0.52 |
28.7 |
4.6 |
48 |
32 |
| B2 |
0.52~0.59 |
30.1 |
3.7 |
50 |
31 |
| B3 |
0.63~0.69 |
25.9 |
2.4 |
48 |
26 |
| B4 |
0.77~0.89 |
35.6 |
2.7 |
52 |
29 |
| Panchromatic band |
B5 |
0.51~0.73 |
55.6 |
9.0 |
53 |
37 |
Figure 1. The Picture of the Optical Path
Fig 2 Block diagram of CCD Camera
The lens is the core of the camera for
absorbing light energy of the ground scenes
imaging in focus plane. The Camera uses
Fairchild Company CCD143A Array, the
diminution of CCD pixel is 13µm*13m.
According to satellite flying height, ground
pixel resolution, photographic swath and s/n
ratio, it is obtained that the focus length is
520mm, angle of view 8.4
° , relative aperture
1/4. For these requirements, It uses 8 pieces
of lenses to form a Double Gauss
Transmission Lens. As multi-spectral earth
resource satellite camera, there exist a higher
quality demands to the lens. The MTF of
every band at CCD characteristic frequency
for fitted up lens of the camera which
measured by Ealing optical instrument
reached 0.60. In the front of the lens, there is
a protect lens of quartz optical glasses to
guarantee the lens out of the affection of
space radiation particles and to reduce the
affection of environmental temperature.
