Abstract:
the principle of china imaging
altimeter has been introduced. Some key
issues on the system design, such as antenna,
baseline, observation angle, PRF, signal
bandwidth, compression in azimuthal and
range direction, and finally tracking algorithm
have been discussed, also some preliminary
results have been given.
I. Introduction
In recent two decades, the development
of microwave remote sensing technologies
and its applications have experienced a rapid
progress. Microwave sensors can provide one
or two-dimensional useful data. For example,
altimeter can provide one-dimensional height
profile of ocean surface; scatterometer can
provide two-dimensional wind vector on the
ocean surface; radiometer can provide one-dimensional
temperature or vapor profile of
the atmosphere and the ordinary SAR can
provide two-dimensional high-resolution
image. As the application of microwave
remote sensing becomes more and more wide
and more and more far-reaching, one-dimensional
or two-dimensional data (or
image) cannot fulfill the requirements. So,
three-dimensional data (image) is highly
demanded now. They are one of the
fundamental data for digital globe.
Up to now, three-dimensional image can
be obtained by interferometric SAR through
two approaches, one is repeat pass with single
antenna and another one is single pass with
two antennae. Aircraft borne interferometric
systems
[1-2]
using the second approach have
been developed several years ago, but the
second approach is difficult to realize in space
case.
Traditional radar altimeter has proven to
be very useful for the study of oceanic science
and oceanic environmental monitory, usually
however, it can not be used for land and
cannot be used to get image. In recent years,
several new conceptual imaging altimeters
have been proposed
[3-6]. They are called
scanning altimeter, or SAR altimeter etc.
In this paper, we give an introduction to
our preliminary research on imaging altimeter
(China Imaging Altimeter, CIALT)
[7], which is
supported by china high-tech research and
development program. The object of this
project is to develop a three-dimensional
imaging altimeter system, which can be,
applied both to ocean, ocean ice and land use.
Firstly, we introduce the principle, and then
we discuss about some key issues in the
design of CIALT.
II. Principle of CIALT
The principle of CIALT concludes as the
following four aspects:
1. Off-nadir looking
Off-nadir looking makes the swath more
wide compared with traditional nadir looking
and at the same time, the resolution in height
can transfer to resolution in ground range.
2. Using synthetic aperture signal pro-cessing
to obtain azimuthal resolution
Focused or unfocused synthetic aperture
signal processing can be applied to improve
the azimuthal resolution
3. Incorporation of pulse limited and
beam limited
When used in ocean case, pulse limited is
applied to obtain average sea level, significant
wave height. When imaging, beam limited is
applied to achieve wider swath. Interfero-metric
information is used to obtain the height
value corresponding to pixel, so three-dimensional
imaging can be realized.
4. Off-set center of gravity (OCOG)
tracking algorithm
When used in ocean ice and land, the
measurement of average height of the surface
and height tracking are realized through
OCOG algorithm. This accurate average
height value can be used as a benchmark
when using interferometric information to
obtain the pixel height.
Fig.1 Designed feed pattern
Fig.2 Designed antenna pattern
III. Key issues on the design of CIALT
In this section, we will discuss some key
issues on the design of CIALT, such as
antenna, baseline, observation angle, PRF,
signal bandwidth, compression algorithm in
azimuthal and range direction and tracking
algorithm.
1. Antenna design
In order to achieve more wide coverage,
elliptical beamwidth is preferable to adopt
unlike conventional altimeter in which circle
symmetrical beamwidth is usually used.
Obviously knifing parabolic antenna is one of
better candidate with high gain and
lightweight. Conical rectangular horn antenna
is appropriate to be used as feed. The
dimension of knifing dish antenna is 0.7m*2.0m.
Fig. 1 and Fig. 2 dipict the pattern of
feed and antenna respectively.
