Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
1999-06-28
2001-03-27
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C342S195000, C342S190000
Reexamination Certificate
active
06208283
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synthetic aperture radar system and a platform position measuring apparatus used in the same, and more particularly to a technique of increasing the measuring precision of a position of a flying body on which a platform is mounted.
2. Description of the Related Art
A synthetic aperture radar (SAR) is mounted on an artificial satellite and an aircraft and is used to obtain an image of a portion of a ground surface in a high resolution regardless of night and day and the weather of the ground surface portion.
FIG. 1
is a block diagram illustrating a basic structure example of a conventional synthetic aperture radar. Referring to
FIG. 1
, a synthetic aperture radar basic unit
10
and a position and attitude measuring apparatus
14
are mounted on a platform (not shown). In the synthetic aperture radar basic unit
10
, a pulse signal is generated by a transmitter
11
and is radiated as a electromagnetic wave for the ground from a transmission and reception antenna
12
. The electromagnetic wave is reflected on the ground surface and is received by the transmission and reception antenna
12
. The received electromagnetic wave is amplified and detected by a receiver
13
and is recorded on a recording medium (not shown) such as a magnetic tape in a complex data format by a data recording unit
15
.
A series of operations are repeated in a predetermined time interval of 1 msec. or at the frequency of 1000 Hz. Also, a position and attitude data of the platform measured by the position and attitude measuring unit
14
is also recorded by the data recording unit
15
together with the receive data by the synthetic aperture, radar basic unit
10
.
After the measurement, an image is produced from the recorded data by an SAR (Synthetic Aperture Radar) image reproducing unit
16
through an SAR image reproducing process which is well known. The SAR image reproducing process is described in, for example, the fourth chapter of “Remote Sensing for Resource Investigation: practical use series 5 Synthetic Aperture Radar (SAR)” by Yoshirou Iguchi (published from Resource Observation and Analysis Centers on Mar. 31, 1992, pp153-198).
When the fluctuation of the platform in position is large in an aircraft, a fluctuation compensating process is executed using the platform position and attitude data synchronous with a pulse signal in the measurement in the case of the SAR image reproducing process. Thus, it is necessary to prevent the degradation of a resolution of the reproduced image and the warp of the image due to the fluctuation of the platform.
The fluctuation compensating process is a process in which the variation of a phase of the reception signal is compensated or corrected based on the actual fluctuation of the platform, supposing that the platform flies on an ideal straight route at a uniform velocity. This process is well known. This process is described in, for example, “III. DATA PROCESSING” of “Repeat-Pass Interferometry with Airborne Synthetic Aperture Radar” by A. L. Grayet.al (IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, Vol.31, No.1, JANUARY 1993, pp.180-191).
In order to obtain an SAR reproduction image without the degradation of the resolution through the fluctuation compensating process, it is necessary to compensate the platform position and attitude data for the fluctuation of the platform in the measurement in precision of ⅛ or below of the measurement wavelength. Because the synthetic aperture radar system at present uses the wavelength from several cm to about tens of cm, the required detection precision of the fluctuation of the platform is from several mm to several cm.
Also, in recent years, a method of detecting a minute diastrophism to the extent of the wavelength of the measurement electromagnetic wave or below by unit of a differential interferometry type synthetic aperture radar is developed. This method is described in, for example, “Mapping Small Elevation Changes Over Large Areas: Differential Radar Interferometry” by A. K. Gabrielet.al (Journal of Geophysical Research, Vol.94, No.B7, 1989, pp.9183-9191).
In this case, it is necessary to detect the fluctuation of the platform in the precision higher than that of the detection of the minute diastrophism. The fluctuation compensating process is executed based on the detecting result. Therefore, it is required that the fluctuation of the platform is detected in the precision of about several mm for detection of the minute diastrophisms of several cm such as earthquake, volcanism, landslide, and land subsidence. In this way, it is very important to obtain the fluctuation data of the platform in a high precision, when a data is obtained by the synthetic aperture radar system.
Conventionally, as the position and attitude measuring unit which is mounted on the platform together with the synthetic aperture radar is generally used a global positioning system (GPS), an inertial navigation system or a hybrid navigation system of a combination of them. The position measurement precision is about several m in the hybrid navigation system, and about several cm in kinematic GPS using a carrier wave. Therefore, the above systems are insufficient in precision for the fluctuation compensating process in the synthetic aperture radar and the diastrophism detection in the differential interferometry type synthetic aperture radar.
A method of compensating for the fluctuation in the synthetic aperture radar and a method of measuring a position by the radar are described in Japanese Laid Open Patent Application (JP-A-Heisei 6-160515) to solve the above problem.
Next, the method of measuring the position of the platform on which the synthetic aperture radar is mounted will be described.
FIG. 2
is a diagram illustrating the conventional method of measuring the position of the platform using the radar.
Referring to
FIG. 2
, a reference numeral
1
denotes a radar platform, and a reference numeral
2
denotes a platform flight track. A reference numeral
3
denotes an measurement object area. Reference numerals
4
,
5
and
6
denote first, second and third repeaters. A reference numeral
7
denotes a phase compensation basing point. Here, the first repeater
4
, the second repeater
5
and the third repeater
6
are arranged in different positions.
FIG. 3
is a block diagram illustrating the structure of a conventional fluctuation compensating and position measuring system in a synthetic aperture radar system. Referring to
FIG. 3
, the system is composed of a synthetic aperture radar basic unit
50
, which is equivalent to the synthetic aperture radar basic unit
10
shown in FIG.
1
. In the synthetic aperture radar basic unit
50
is composed of a transmitter
51
, a transmission and reception switching unit
52
, a transmission and reception antenna
53
, a receiver
54
and a local oscillator
55
. The synthetic aperture radar system is further composed of a phase compensation reference signal generating unit
56
, a complex data multiplier
57
, an image reproducing unit
58
and a display unit
59
. Also, the synthetic aperture radar system is further composed of an inertial navigation system
60
and an antenna directional control unit
61
. Moreover, the synthetic aperture radar system is further composed of antennas
70
, transceivers
71
, relative distance calculating units
72
and a platform position calculating unit
73
. The antenna sends and receives a electromagnetic wave to and from a repeater. The transceiver
71
is connected with the antenna
70
. The relative distance calculating unit
72
is connected with the transceiver
71
and the inertial navigation system
60
and calculates a relative distance between the radar platform
1
and the phase compensation reference point
7
. The platform position calculating unit
73
determines the position of the platform from the relative distances calculated by the relative distance calculating units
72
.
The operation of the above-mentioned system will be descri
Miyawaki Masanori
Murata Minoru
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
Sotomayor John B.
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