Aircraft or spacecraft based synthetic aperture radar

Details Aircraft or spacecraft based synthetic aperture radar Aircraft or spacecraft based synthetic aperture radar

2000-08-16

2002-05-14

Gregory, Bernarr E. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Synthetic aperture radar

C342S059000, C342S159000, C342S195000

Reexamination Certificate

active

06388606

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an aircraft or spacecraft based radar system with synthetic antenna aperture (SAR=Synthetic Aperture Radar) for imaging the earth's surface in such a way that an ambiguity suppression is provided by means of a minimum antenna directionality with side lobe suppression resulting in a swath illumination on the ground.
2. Prior Art
By now, numerous SAR systems are used worldwide to image the earth's surface. These systems may be implemented both mounted on an aircraft, as well as mounted on a spacecraft, e.g., on a satellite. In comparison to optical imaging, SAR systems are unable to use natural light sources, but must themselves illuminate the area to be imaged in the desired frequency range in a suitable manner.
In the known SAR systems this is achieved by using a single antenna for both the transmit and the receive operation. A SAR system of this type, which may also be referred to as a monostatic SAR system, requires a pulsed radar operation whereby the transmit pulses are limited in time, so that the time between every two successive pulses can be used for the receive operation. This radar mode has some principle-based shortcomings and limitations.
In the conventional monostatic SAR systems, as shown with the aid of an example in
FIG. 1
, the transmit pulses coming from a high power amplifier (HPA)
1
in the transmit branch are switched via an RF circulator
2
to an antenna
3
, and emitted from there into space toward the ground. The transmit pulses are first processed in the transmit branch by a digital chirp generator
4
and routed via two quadrature channels I and Q with respective filters
5
and
6
and respective mixing steps
7
and
8
to an adder
9
. Afterwards they are converted to the transmit frequency position by means of a mixing step
10
, which is furthermore operated with the frequency of a local oscillator
11
, after which they are filtered with the aid of an RF filter
12
. Afterwards they are routed to the aforementioned high power amplifier
1
. The radar echoes that arrive between successive transmit pulses are received by the same antenna
3
and switched via the HF circular
2
and a receiver protection circuit
13
in the receive branch to a low noise amplifier (LNA)
14
.
After pre-filtering in an HF filter
15
, mixing with the frequency of a local oscillator
16
in a mixing step
17
, filtering into the baseband by means of a filter
18
, sufficient amplification with the aid of an amplifier
19
, and analog/digital conversion by means of an analog/digital converter
20
, the standard processing methods for the SAR image generation are applied to the raw radar data that have been obtained in this manner.
The described SAR system of the monostatic type, which is known, e.g., from U.S. Pat. No. 4,866,446, and in comparable form for topographic mapping also from DE 37 12 065 C1, has a number of shortcomings, however.
The circulator that is necessary to use a single antenna results in losses and has only a limited decoupling between the transmit branch and the receive branch. In combination with the receiver protection circuit, which is therefore required, this leads to higher losses and a deterioration of the system noise factor. Also, the maximum peak transmitting power of monostatic systems is presently limited by the receiver protection circuit, which can be implemented only to a certain extent. Also of disadvantage are the complex power supply and the high EMC (electromagnetic compatibility) load on the total system, which results because high peak transmitting outputs become necessary in the pulse operation. The dimensioning of the transmit pulse length must take into consideration the requirements of the receive window. A reduction of the transmit pulse length, however, necessitates an increase in the transmitting power if a constant signal
oise ratio is to be maintained.
In the known monostatic SAR system, a limited flexibility of the total system furthermore results when special SAR operating modes are implemented, and when interferometric measurements are performed. Also, a monostatic SAR system is not capable of transmitting and receiving simultaneously. During the transmit operation a reception is not possible, and during reception of the radar echo no transmit pulse can be emitted. This limits the maximum time available for the scanning of a radar echo to a fraction of the transmit pulse interval. Since the transmit pulse interval, too, must not fall below a minimum value, which is dictated mainly by the resolution, a largest possible maximum image swath width exists, which is dependent mainly upon the required resolution and which cannot be exceeded.
For spacecraft based SAR systems, this maximum image swath width is approximately 8 to 20 km, depending on the incident angle, at a resolution of 1 m, or approximately 40 to 100 km at a resolution of 5 m.
The inability of the known monostatic SAR system to simultaneously transmit and receive furthermore means that those range domains cannot be imaged for which the time delay of the radar wave is an integral multiple of the transmit pulse interval. In order to still be able to image these range domains, the transmit pulse interval or the pulse repetition frequency (PRF) of the transmit pulses must be changed. However, during each such switching process, the end of the time delay of the radar wave must be waited for, which means system losses and which, ultimately, further limits the resolution that can be attained with the SAR system.
Since the incident angle and the angle of reflection are always the same in the known monostatic SAR system, and located on the same side of the perpendicular to the area to be imaged, the backscatter characteristics of the surface to be imaged can be measured only for identical incident and reflection angles, which means that a large part of the microwave characteristics of the surface to be imaged, therefore, cannot be registered by the known SAR system of the monostatic type.
It is true that a radar system with a synthetic aperture of the biostatic type is already known from JP 61-140 884 A, wherein a transmit antenna and a receive antenna are provided, which are physically separate and at least one of which, namely the transmit antenna, moves so that a relative movement results between the transmit antenna and the receive antenna. The transmit antenna, which is arranged on a moving platform, is located above the earth's surface; however, the receive antenna with its related reception and evaluation system is not. Instead, the receive antenna is located on the ground or on a ship.
The receive antenna which is mounted stationary on the ground or on a ship, receives the portions of the signals that are reflected from a specific target object, which come from the transmit antenna that moves above the earth's surface. The specific target object is also located above the earth=s surface, since a receive antenna mounted on the ground would not be able to receive meaningful reflection signals from a target object that is also located on the ground, for reasons of the usually present unevenness of the ground and the curvature of the earth's surface alone. This SAR radar system, because of its design with a receive antenna mounted stationary on the ground or on a ship, is therefore not suitable at all for imaging ground structures, and its bistatic characteristics also cannot be transferred to a monostatic SAR radar system intended to image the earth=s surface.
OBJECT AND SUMMARY OF THE INVENTION
It is the aim of the invention to create a SAR system for imaging the earth's surface that permits, without significant system losses and with a high attainable resolution, a greater flexibility both in the image arrangement, as well as in the suppression of the described ambiguities, and permits a noticeable increase in the swath width. Furthermore, the SAR system to be created should be easy to implement and also resul

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