Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
1980-05-27
2001-03-20
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C342S162000, C342S195000
Reexamination Certificate
active
06204799
ABSTRACT:
BACKGROUND
1. Field
This invention relates to the processing of synthetic aperture radar data and, in particular, to the processing of three dimensional bistatic synthetic aperture radar data.
2. Prior Art
In early radar systems, fine range resolution was obtained by transmitting a narrow pulse, while fine azimuth resolution was obtained by radiating a narrow beam. In more modern radars, pulse compression techniques or active linear FM techniques, commonly referred to as Stretch techniques, are used to produce a long pulse which can, through appropriate processing, provide the range resolution of a narrow pulse.
For example, in the Stretch technique, a linear FM signal is transmitted. The return signal is mixed with a sample of the transmitted signal to produce an output signal at a frequency which is dependent on the range of the target. The range resolution in the Stretch technique is a function of the bandwidth of the transmitted linear FM signal, or alternatively, the resolution may be considered dependent on the range to frequency scale factor and the degree to which the frequency of the output signal for a particular target can be ascertained.
A narrow beamwidth was previously obtained through the use of a large antenna; however, in a more modern technique known as synthetic aperture radar, or SAR, a small antenna can provide the effective aperture of a large antenna. This is accomplished by moving the antenna along a path, the length of which determines the synthetic aperture within the limitation imposed by the antenna beamwidth. The data received along the path is stored and later processed to produce a high resolution image. Conventional SAR radar and its processing are discussed in a number of publications, including
The Radar Handbook
, by M. Skolnik, McGraw-Hill, New York 1970.
A simple Stretch system is shown in
FIG. 1. A
transmitter
11
generates a linear FM signal which is passed through a coupler
12
, to a first antenna
13
, where it is radiated. The radiated signal is returned by a target
15
to a second antenna
14
, where it is passed to a mixer
16
. The mixer also receives a portion of the transmitted signal by way of the coupler
12
to serve as a local oscillator signal. The mixer output signal, at port
17
, often is converted to range information at port
19
by a Fourier transformer
18
, which is usually a pulse compression network, also referred to as a matched filter. A matched filter is a more general term, however, including any system for converting received radar signals to Stretch type frequency response signals.
The operation of the system of
FIG. 1
can be understood with the aid of FIG.
2
.
FIG. 2
is a graph containing a plot of a transmitted signal
24
, a received signal
25
, and an output signal
26
. The ordinate
22
represents frequency while the abscissa
23
represents time. The frequency axis is divided in two, with the upper portion being the normalized transmitted frequency, while the lower portion is the output frequency, which is calibrated in megahertz (MHz).
FIG. 2
is an example of the performance characteristics of a typical Stretch system. The transmitted signal varies linearly from 0.8 to 1.2 times the center frequency over a normalized period, while the received signal varies over the same frequency range, but is shown delayed by a time corresponding to the time for the signal to traverse the round trip radar path length to the target and back to the receiver. When these two signals mix in the mixer
16
, shown in
FIG. 1
, they produce a constant low output frequency such as a 4 MHz signal designated by drawing numeral
26
, in FIG.
2
.
The frequency difference between the transmitted and return signals is proportional to the delay which, in turn, is proportional to the target range. Therefore, an output signal representing a target at a fixed range will be at a frequency that is proportional to the range of the target. In a practical Stretch system, the mixer reference signal obtained via coupler
12
may be delayed a known amount to establish a reference range. The output signal for a target at that range will then be zero frequency.
A rudimentary SAR system is shown in FIG.
3
. In this Figure, antennas
31
,
32
, and
33
produce antenna patterns
34
,
35
, and
36
, respectively, illuminating a target
37
.
The antenna and antenna patterns shown in
FIG. 3
may be considered a simple phased array antenna system. The signals from each antenna may be phase shifted, weighted and then combined with the signals from the other antennas to produce an effective pattern
39
, which is narrower and higher in gain than any of the individual antennas. The narrow beamwidth of the effective pattern may then be used for improved angle resolution.
A SAR system is, in effect, a special case of a phased array system. Although a SAR system usually contains only a single antenna, this antenna is moved to simulate a number of antennas. For example, a single antenna of a SAR system could first be positioned at the location of antenna
31
. In this location, it would produce a pattern similar to pattern
34
. The signal received at this location containing target information is stored. The single SAR antenna is then moved to the location of antenna
32
and later to that of antenna
33
with the received data at each location being stored. The stored data is then weighted, phase shifted and combined to provide the same results which would have been obtained with three separate antennas.
In practical applications, the single SAR antenna is often located aboard an aircraft. The antenna is continually in motion rather than being shifted in incremental fashion from position to position; however, the transmission period is relatively short, making the distance moved per transmission period equally short. In this case, each transmission may be considered as occurring at a single location.
FIG. 8
illustrates the operation of a typical SAR in an aircraft
81
, flying along a flight path
86
. The SAR antenna is directed constantly to the side of the aircraft, as shown by directional arrows, such as arrow
82
. The radar beam
84
illuminates a swath on the ground
83
and at a particular instant illuminates an area, such as area
85
.
A commonly used method for recording SAR data is to print the data on photographic film, such as on the film shown in
FIG. 4
, using intensity modulation. In this Figure, a film
41
contains a series of lines, such as lines
42
and
43
. Each line contains data received as a result of a single transmitted pulse. If Stretch is combined with SAR the peaks of the sinousoidal return signals are recorded as short, dark portions along the line.
FIG. 9A
shows the coverage by a SAR radar of two targets. The radar beamwidth
91
illuminates two targets,
93
and
94
, in an area of illumination on the ground indicated by the swath
95
of width
92
. When the signals from the targets are recorded on film
99
via a series of range sweeps, such as range sweep
96
, they appear as shown in
FIG. 9B
, where signal history
98
corresponds to target
94
and signal history
97
corresponds to target
93
. The target histories are curved and cannot be both correct by simple geometric manipulation.
For a Stretch SAR, a single target at a constant range would produce a single frequency, such as the 4 MHz signal illustrated in FIG.
2
. This type of return signal would simply produce a series of constant size dots separated by a constant spacing. The dots correspond to the peaks of a 4 MHz signal. A return signal produced by a number of targets generates a more complex darkening of the line.
The data may be recorded on the film simply by displaying the received signal on an oscilloscope and focusing the display on the film. In such a system, the sweep speed corresponds to the rate at which the return signal is printed while the separation between the raster lines corresponds to the distance the antenna travels between pulses.
The data recorded in this form is in terms of frequency and time of receipt, rather t
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