Abstract
This paper deals with the aim and the
application of the spaceborne China
Advanced Microwave Remote Sensor
(CAMRS). The aim of this project is to
develop the second generation China
Multimode Microwave Remote Sensor
(M
3RS) in which the dual frequency
altimeter, scanning scatterometer and multi-channel
millimetre-wave radiometers are
integrated together. Altimeter (ALT),
scatterometer (SCAT) and radiometer (RAD)
are all designed in module so they can be
combined or decomposed arbitrarily
according to different mission. This new
sensor can be used to sea surface,
atmosphere and land applications.
1.0 Introduction
Over the three decades, microwave remote
sensing has evolved into an important tool
for monitoring the atmospheres and surfaces
of planetary objects, with special emphasis
on observations of the planet earth. Sensors
are very important part of microwave remote
sensing and usually are divided into two
groups according to their modes of
operation: active and passive. Active modes
are those that provide their own source of
illumination and therefore contain a
transmitter and a receiver, while Passive
modes are simply receivers that measure the
radiation emanating from the scene under
observation. Active modes include imaging
radar, SCAT and ALT, and passive modes are
often referred to as microwave RAD. Active
sensors have been used extensively for
mapping geological structures and features.
Particularly in those parts of the world where
cloud cover presents a serious problem to
optical sensors. Active modes also can used
for other application areas, including
vegetation mapping, discrimination of sea-ice
types, measuring ocean wind speed and
direction, mapping soil moisture content and
snow water content, and land-use evaluation.
The CAMRS is a second generation M
3RS,
including scanning SCAT, the dual frequency
ALT and multi-channel millimetre-wave
RAD, in which active and passive modes are
working synergistically. The primary goal of
the sensor is to ocean and atmospheric
research. The ALT mode can provide geoid,
significant wave high and wind speed with
precision satisfying to user requirement.
SCAT mode will provide ocean wind field
data and the RAD can provide on
atmospheric data.
2.0 A brief description and key
specification of the system
The CAMRS consists of seven operation
modes and the mode combination can be
selected in order of user requirements. The
main modes are: ALT, SCAT, RAD,
ALT+SCAT, ALT+RAD, SCAT+RAD and
ALT+SCAT+RAD. The functional block
diagram has shown in fig-1.
2.1 The dual frequency altimeter unit
Satellite altimetry is devoted to the active
remote sensing of the ocean surface and
thereby represents an important new source
of measurements. These measurements are
sufficient to provide all-weather independent
observations of global topographic features,
thus contributing to the accurate mapping of
underwater features and the detection and
measurement of ocean current, tides, and
storm surges, as well as the monitoring of
wave height on a global basis. The radar
ALT is therefore an unique tool for studying
the role that oceans play in the Earth’s
climate system and for understanding
climatic change on a global scale.
The radar ALT operates in the pulse-width
limited mode, and use the full deramp
system of pulse compression. The basic
altimeter operation principle can be found in
(Chelton, 1989).
The radar ALT operates at C-band and Ku-band
respectively. The dual frequency design
allows for height measurement corrections
due to the ionospheric effects on the signal.
This system was designed to operate from a
space platform over the ocean with sea
conditions ranging from 0.5 to 20 meters in
significant wave height. The altitude
resolution is 4.5cm. The significant wave
height is 0.25m or <5%. The backscatter
coefficient is 0.4dB.
The radar ALT has several improvements
over previous China ALT.
- The dual frequency design allows for
height measurement corrections due to the
ionospheric effects on the signal.
- The new on-board processor
- Digital Chirp generated technology
- Solid-state transmitter
A summary of the radar ALT system
parameters is given in the table 1 and a
system block diagram is presented in figure 2
Table 1. Radar Altimeter System
Parameters
| Design parameter |
Main channel |
Common |
Secondary channel |
| Altitude range, km |
|
800 |
|
| Operative Frequency, GHz |
13.7 |
|
5.35 |
| Pulse Length, ms |
102 |
|
102 |
| Pulse Bandwidth, MHz |
320, 80, 20 |
|
100 or160 |
| Tx Peak Power, W |
5 |
|
20 |
| PRF, Hz |
1000~4000 |
|
400~1000 |
| Type of Antenna |
|
Parabola |
|
| Antenna diameter, m |
|
1.5 |
|
| Antenna Gain, dB |
44 |
|
36 |
Figure1. The Block Diagram of the
CAMRS
Figure2. The System Block Diagram of
the Dual Frequency Altimeter
