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Poster Session 2
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China Advanced Microwave Remote Sensor
2.2 Scatterometer unit
The main purpose of microwave SCAT unit
is to measure the backscattering coefficients
of sea surface, ice and snow and so on. Using
the backscattering coefficients, the ocean
surface wind vector can be retrieved. The
SCAT of CAMRS is a Ku band, dual-polarization,
dual-pencil-beam, and conically
scanned radar. Such a system has some
advantages, such as low transmitted power
requirement, wide sweeping width, simple
system configuration and so on, to the fan-beam
scatterometers of ERS1, ERS2, JERS,
and other spaceborne scatterometers in
operation.
The SCAT unit is composed of several parts
including antenna, transmitter and frequency
synthesizer, receiver, data processing and
control. Fig.3 is the diagram of the SCAT
unit.
System specifications
| Wind velocity range: |
2~30m/s |
| Wind velocity precision: |
2~20m/s: <2m/s; 20~30m/s: 10%rms |
| Wind direction range: |
0~360° |
| Wind direction precision: |
<20° |
| Backscattering coefficient precision: |
±1.0dB |
| Spatial resolution: |
50 km |
| Backscattering dynamic range: |
-40dB ~ +20dB |
| Polarization: |
VV, HH |
| Frequency: |
Ku-band: 13.4GHz |
| Platform altitude: |
800km, near-polar |
| Platform velocity: |
7.4556km/s |
| Orbit period: |
6043.4s(=1hr40min43.4s) |
| Antenna pointing precision: |
<0.2° |
Fig.3 Diagram of the Scatterometer
2.3 The multifrequency radiometers unit
The main purpose of microwave RAD unit is
to detect integrated water volume and
temperature measurement of stratosphere.
The advantage of microwave RAD at
118.75GHz is as follows: 1) for the same
size antenna, the space resolution of
118.75GHz is twice than that of 50-60GHz
microwave RAD. The space resolution is one
of the important characteristics of
meteorological remote sensing. 2) The
oxygen absorbing line is single and
symmetric at 118.75GHz, so the double side
band receiver can be designed and the noise
figure and the sensitivity of the receiver can
be improved. 3) A wide band receiver can be
designed at 118.75GHz. Only use one wide
band front-end, but multiple IF signal can be
gotten by band pass filters. So the cost of
hardware can be decreased. 4) The absorbing
line at 118.75GHz is far away from the other
oxygen absorbing line, so the effect of
overlap can be ignored.
Figure 4 is the block diagram of microwave
RAD unit. It includes antenna, receivers,
computer and power supply. They share one
antenna from 90-118.75GHz. So the
antenna’s weight and volume can be
decreased, and the scanning consideration
also can be simplified. The diameter of the
antenna is 20cm. Total power type is adopted
for the design of microwave RAD. The main
characteristics of microwave RAD unit are
listed in table 2.
Table 2 Characteristics of Microwave Radiometer Unit
| Ch |
Center freq. GHz |
Dynamic range K |
Scan angle |
D * cm |
TSYS K |
t* ms |
B * MHz |
Sensitivity T K |
| 1 |
90 |
2.7-320 |
±40° |
20 |
2263.0 |
180 |
2000 |
0.2 |
| 2 |
118.75
± 0.33 |
3610.0 |
180 |
200 |
0.6 |
| 3 |
118.75
± 0.65 |
3610.0 |
180 |
350 |
0.5 |
| 4 |
118.75
± 1.3 |
3610.0 |
180 |
500 |
0.4 |
| 5 |
118.75
± 2.0 |
3610.0 |
180 |
600 |
0.4 |
| 6 |
118.75
± 3.9 |
3610.0 |
180 |
600 |
0.4 |
D: Diameter; t : Integration time;
B: Bandwidth
Figure 4 Block Diagram of
Microwave Radiometer Unit
2.4 Computer Control and Data
Processing Unit
Computer control and data processing
unit(CCDPU) consists of two parts that are
instrument control unit(ICU) and module
data processing unit, showed by figure 5.
The module data processing unit comprises
ALT processing module, SCAT processing
module and RAD processing module.
CCDPU is designed to support incorporation
for the three modules, independent work for
each module, or incorporation for every two
of the three modules.
ICU is charge of onboard data
communication interface, mode control,
distributing satellite attitude parameter,
timing, and transferring measure and
monitoring data.
ALT offers AGC value connected with the
echo power, the evaluated slop value of
return wave leading edge, delay time
between the transmitting and receiving pulse,
and wave shape sampling value. SCAT
submits echo wave sampling value that can
be used to calculate wind vector. RAD
delivers different receiving paths sampling
value. Monitoring data of the instrument's
significant working point are also provided.
All these values are packed together and sent
to the ground station.
ICU can transfer remote instruction from
ground station to each module to change
their working mode and modify condition
parameters.
The rule to achieve modularized design
includes:
-
Easily reunite for single module, two
modules and three modules.
- Modularized design for hardware.
- Modularized design for software.
The general modularized software can be
achieved on account of the same CPU and
RAM for ALT, SCAT and RAD. All the time
sequence control and onboard
communication interface circuit for the three
modules are integrated in one chip based on
FPGA, and the corresponding CPU and
RAM are plugged in according to the
different modules' combination.
Figure 5 Computer Control and
Data Processing Unit
3.0 Conclusion
As it result from the CAMRS description,
the system design is at the state of art, from a
system and technological point of view.
Presently, an activity to evaluate the impact
of hardware on system performance is still
on progress.
4.0 References
- H.G.Liu, J.S.Jiang, Y.H.Chang,
X.L.Dong, J.Li, and K.Xu, “Spaceborne
Modularized Microwave Sensors with
High Functional Density”, Proceeding of
49th International Astronautical
Congress, IAF-98-B.3.03, October 1998
- J.S.Jiang, Z.F.Zheng, H.G.Liu, X.Zhang
and Z.F.Fan, “China Advanced
Microwave Remote Sensor,” Proceeding
of 47th International Astronautical
Congress, IAF-96-B.3.P107, October
1996
- D.B. Chelton, E.J. Walsh, and J.L.
MacArthur, "Pulse Compression and Sea
Level Tracking in Satellite Altimetry," J.
Atmospheric and Oceanic Technol.,
Vol.6, No.3, pp.407-438, June 1989.
- G.S. Brown, “The Average Impulse
Response of a Rough Surface and its
Application,” IEEE Trans. on Antenna
and Propagation, Vol. AP 25, No.1, pp.
67-74, January 1977.
- B.J. Lipa and D.E. Barrick, "Ocean
Surface Height-Slope Probability
Density Function from SEASAT
Altimeter Echo," Journal of Geophysical
Research, Vol. 86, No. C11, pp.10921-
10930, 1981.
- D.G.Long and M.W.Spencer, “Radar
Backscatter Measurement Accuracy for
a Spaceborne Pencil-Beam
Scatterometer with Scatterometer with
Transmit Modulation,” IEEE Trans. On
Geosci. & Remote Sens., 35(1), pp.102-114,
1997
- M.W.Spencer, C.Wu and D.G.Long,
“Tradeoffs in the Design of a
Spaceborne Scanning Pencil-Beam
Scatterometer: Application to
SeaWinds,” IEEE Trans. On Geosci. &
Remote Sens., 35(1), pp.115-126, 1997
- D.G.Long, “Wind Measurement
Resolution for a Scanning pencil Beam
Scatterometer,” IGARSS’94
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