On the December 22nd in 2011, ZY-1(02C) satellite was launched up successfully. PMS camera operates well and the image is good.
This paper gives the design and verification in the laboratory of the PMS camera. The test results show that the PMS camera satisfies the requirements.
ZY-1(02C) satellite was built by Chinese Academy of Space Technology (CAST), which was customized by Ministry of Land and Resources of China to satisfy the urgent demand of land remote sensing data.
The satellite will cover the earth except the northern and southern polar region. It can provide the dynamic remote sensing data of usage of land, minerals and other data for the Ministry of Land and Resources of China and other ministries of China.
On the December 22nd in 2011, ZY-1(02C) satellite was launched successfully. The mass of the satellite is about 2000 kg. The power of the satellite is 2400W. After launch, the satellite was adjusted to run on the 780Km sun-synchronous orbit with 98.5° inclination angle. The satellite has two Hyper Resolution cameras (HR camera) with 2.36m GSD and 57 Km swath and a Panchromatic/Multi-Spectral bands camera (PMS camera).
The satellite was the highest resolution satellite for civil use in China at the launch time. On the 23rd, the camera was powered on and the first image was transmitted successfully.
PMS camera is the next generation data-transferring multi-spectral bands optical payload developed for remote sensing by BISME. The older generation multi-spectral bands payload is CCD Camera which is installed in the CBERS 1/2/02B satellites (China Brazil Earth Resources Satellite). The CCD camera has one panchromatic band, 4 multi-spectral bands. The GSD of the five bands is all 20m at nadir.
SPECIFICAITIONS OF PMS
PMS camera is a linear push broom imager. It has one panchromatic band and three multi-spectral bands, gets 5m GSD of panchromatic band and 10m GSD of multi-spectral bands on the 780km orbit. The main performances see next .
DESIGN AND VERIYFICATION OF THE PMS
The camera concept is a push broom based on a catadioptric wide field telescope imaging simultaneously all the points of a line on the monolithic CCD array located in focal plane. The image is created thanks to the satellite motion. The focal plane of the camera is constitutive of four linear detector arrays corresponding to the four respective spectral bands.
The overall configuration of the camera is present below.
• Image with linear push broom modal to get a higher signal to noise (S/N).
• Use the 12,000 pixels linear CCD with size of 6.5 μ m ×6.5 μ m.
• Enhanced Schmidt catadioptric optical system to get high performance at the 77lp/mm nyquist frequency
• The video circuit composed of two main assemblies, the focal plane assembly ensuring the spectral separation and transforming the incoming optical signal in electronics, and the video circuit electronics, interfacing with the focal plane assembly and the satellite.
• The focal plane is adjustable to get a better image.
• To swing the swing mirror to get the ± 32 ° sight ability.
• The thermal control keeping the telescope within the close temperature domain. Passive and active thermal controls are required to main the average temperature within the ±2°C while minimizing the gradient down to 0.3°C.
• The service electronics which interfaces with the satellite and controls the camera mechanisms and its active thermal control.
Optical system Design and Verifcation
The optical system of PMS camera has a long focal length, small big F number and wide spectral. Table I gives the specifications of the optical system. It should get high image quality at the 77lp/mm nyquist frequency. Also the method of splitting the light into multibands and the register of multibands should be considered. Optical manufacturing and alignment are two of the most important restriction factors. All these factors add the difficulties to design the optical system. Balance all the factors and choose the catadioptric Schmidt optical system. The optical elements include: swing reflective mirror, window, 45 ° reflective mirror, spherical reflective mirror, lens and prism. There are two imaging channels, one for panchromatic band and the other for multibands. The prism is used to split the light into three multibands. Fig. 2 gives the drawing of the optical system. Fig. 3 gives the prism sketch. Fig. 4 gives the design result of the optical system. Fig. 5 gives picture of the telescope tested in the lab.
|Focal Length||1010 mm|
|Field of view – FOV||± 2.2°|
|Spectral bands||B1: 510–850nm; B2: 520–590nm;B3: 630–690nm; B4: 770–890nm;|
|Optical System MTF||0.45 at77lp/mm (B1 band) 0.65 at 38.5lp/mm (B2~B4 band)|
|Global MTF at the nyquist frequency||> 0.18 for B1 and B4 band, > 0.22 for B2 and B3 band.|
|Band-to-band registration||< 3.9 μm|
From the MTF curve above, the design MTF is almost coincide with the diffractive limit.
The Optical System Performance was estimated by using a ZYGO GPI/XP interferometer that works at the 632.8nm wavelength. The measurement of the optical Performance was performed only in this wavelength and it was used as a preliminary analysis that was carried out during the alignment phase.
Fig. 5 gives the test setup of the optical MTF.
From the Fig. 6, RMS of B1 band is 0.049 λ. The optical MTF is 0.47 at 77 lp/mm. The requirement is 0.45. The other three bands also reach the requirements.
A high quality optical system is designed, aligned and tested. The test results reach the requirements.
Design of the Opti-mechanical Body
From the design result of the optical system, the body is long and thin. It is difficult to draw the sketch. In order to have the sight ability of ±32°, the swing mirror needs to swing ± 16 ° off the track. The swing mirror is a flat and reflective mirror and it only provides the rotary direction. So the position accuracy is not very high as other optical elements. From the above analysis, we can get that the swing mirror and lens are separated mounted in the satellite. There is no fix connection between the two parts. Fig.7 gives the two separate assemblies. Fig.8 gives the whole opti-mechanical body.
The main function of the swing mirror is to swing ± 16 °. The main function of lens is to collect the light that reflected by the swing mirror and transmit the light into electrical signal. The separation mount also saves about 60kg mass. The mass of opti-mechanical Body is 218kg.
Design and Verification of Thermal Control
The thermal control of the camera consists of passive thermal control and active thermal control. Passive thermal control is composed multi-layer heat insulation materials, thermal control coat and reasonable heat emission plane. Active thermal control means the heating loops on the steer mirror, lens and focal plane. The function of active thermal control is to ensure the suitable work environment temperature of the optical mechanism main body with the thermal controller.
The requirements of the camera are above:
• The temperature range of the steer mirror 15°C~21°C;
• The temperature range of the lens: 16°C~20°C, the gradient except the window down to 0.3°C;
• The temperature range of the prism: 16°C~20°C, the gradient down to 0.7°C;
• The temperature of CCD: no more than 28°C.
In order to get a better image, there is a refocus mechanism with the prism, CCD and driving circuit boards. The prism and CCD will move some distance relative to the main body. But in order to keep the temperature of the CCD and prism, the heat generated by CCD should be dissipated. Conventional ways to dissipate the heat such as heat pipe or copper can not be used because they are fixed. Conductive rope is manufactured to solve this problem. The conductive is made of copper and has a high conductive efficiency. It is also like a rope and has freedom to move.
TEST RESULTS OF THERMAL CONTROL
|Assem by||Temperature Range||Temperature Gradient|
|Requirements||Test results||Requirements||Test results|
PMS SYSTEM TEST IN LABORATORY
Once the camera is built up, a series of tests will be conducted to verify the performance the camera.
System MTF Test
The MTF is of high importance on the evaluation of the spatial frequencies transferred from object to the image of an imaging optical system.
The MTF is equivalent to a combination of the MTF of optical system (MTFlens), CCD (MTFCCD), amplification and signal processing electronic (MTFelectronics)
The system MTF test is a very important phase. The system MTF was obtained by using an on-axis collimator (EFL =7000 mm) and a square target. This target is positioned in the focal plane of the collimator and then is moved in steps of 500μm, which allows for determining the CTF of the target image around a specific pixel in discrete steps of 4.2 μm. MTF is calculated by the equation 2.
Calibration test is to verify the radiometric quality of the camera. We can get the absolute calibration coefficients, relative calibration coefficients, linearity and S/N of the camera by the calibration test. A high steady and precision integral sphere is used to conduct the test. Fig.10 gives the test setup.
ABSOLUTE CALIBRATION COEFFICIENT
|Band||Test result of absolute calibration coefficient at default gain DN/(W.m-2-sr-1μm-1)|
The PMS camera of the ZY-1 (02C) satellite is a new developed generation payload. The payload has used the improved Schmidt catadioptric optical system. The design result of optical system reaches the diffractive limit. According the characteristics of the optical system, the opti-mechanical body is separated into two separate parts. The camera has experienced the test and experiment. The test result has reached the requirements. From the image transmitted by the satellite in the orbit, the image quality has reached the requirements.
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