JP3001405B2 - Interferometric SAR method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a remote sensing system, and more particularly to an interferometric SAR.
About the method.

[0002]

2. Description of the Related Art Conventionally, in a remote sensing system for remotely measuring the earth and planets from a platform such as an aircraft or an artificial satellite, an observation device is mounted on the platform, and data measured by the observation device is separately installed. In the receiving processing system, a method of processing and analyzing is adopted.

[0003] There are various types of observation devices mounted on a platform. Among them, SAR (synthetic aperture radar) is used for a wide range of purposes.

[0004] In the SAR, radio waves are radiated on the ground sideways with respect to the traveling direction of the mounted platform.
By receiving the reflected wave from the ground, two-dimensional data for reproducing a ground image can be obtained.

The SAR is disclosed in Japanese Patent Application Laid-Open No. 62-3 / 1987.
676 and JP-A-62-8081.

[0006] Elevation data of terrain is observed using SAR. The method of observing the elevation data (three-dimensional data) of the terrain using the SAR is called a cross-track interferometry SAR.

FIG. 9 is a diagram showing the concept of the cross-track interferometry SAR.

[0008] Cross-track interferometry S
AR is composed of two SARs 101 and 102 as shown in FIG.
And the same target 104 was observed in SAR1
Two SAs observed in each of 01 and 102
The altitude Ht of the target 104 is measured by the interference of the R image.

The cross-track interferometry SAR is disclosed in JP-A-63-262578.

In a cross-track interferometric SAR using an airborne SAR, two SAR antennas are mounted on the same body, radio waves are radiated to the ground from only one antenna, and received waves are received by both antennas. The cross-track interferometric SAR is realized by processing the two SAR images received and received by both antennas.

In a cross-track interferometric SAR using a satellite-borne SAR, a recursive orbit is used, and the same target is observed twice to obtain two images SAR. Examples of cross-track interferometry SAR using regression trajectory are:
There are SAR onboard Japanese satellite "Fuyo" and SAR on European satellite EERS-1.

However, in a cross-track interferometry SAR using a satellite-borne SAR, since the number of days to return is several days to several tens of days, the time of the first observation is
The target may change before and after the second observation, in which case the observation accuracy will decrease, and the observation efficiency will decrease due to long return days and high dependence on orbital conditions. There is a problem.

As means for solving the above-mentioned problems, the following are considered.

(1) A plurality of satellites on which SARs are mounted are arranged in close parallel orbits, and an interferometric SAR is realized by simultaneously observing the same target.
Here, when irradiating the target with radio waves, 1
Only one SAR is used, and reception of a reflected wave from the target is performed using a plurality of SARs.

(2) Interferometry SAR is realized by arranging a plurality of satellites equipped with SAR in the same orbit close to each other and observing the same target almost simultaneously. Here, in irradiating the target with radio waves and receiving reflected waves from the target, each S
In the AR, the S at the time of observation for the observation area generated by the rotation of the earth during the time difference of the observation in each SAR
A cross-track interferometry SAR is realized by utilizing a difference in AR position.

Another thing observed using the SAR is the moving speed of a moving object. Note that a method of observing the moving speed of a moving object using the SAR is referred to as along track interferometry SAR.

FIG. 10 is a diagram showing an along-track interferometry SAR, where (a) is a conceptual diagram,
(B) is a diagram showing a measurement principle.

As shown in FIG. 10, in the along track interferometry SAR, first, the target 114 is observed in the SAR 111, and after a short time Δt, the target 114 is observed in the SAR 112 and observed in each of the SARs 111 and 112. By measuring the phase difference of the target 114,
The moving speed of the target 114 parallel to the line of sight with respect to the SARs 111 and 112 is measured. In addition,
In irradiating the target 114 with radio waves and receiving an SAR image from the target, the SARs 111 and 11 are used.
2 are performed.

The above-mentioned cross-track interferometry SAR and along-track interferometry SAR are collectively referred to as interferometry S
Called AR.

[0020]

However, the conventional SAR as described above has the following problems.

(1) In a SAR using a low frequency or a SAR on a satellite, it is necessary to increase the distance between the antennas of a plurality of SARs. Therefore, when a plurality of SARs are provided on the same platform, Is limited in size, and a small platform cannot be used.

(2) When a plurality of platforms are used, it is difficult to accurately measure the relative position between the antennas of the SAR mounted on the platforms, so that the measurement accuracy of the interferometric SAR is reduced.

(3) As a characteristic of the SAR, it is necessary to widen the beam width of the radiated radio wave.
R, along track interferometry SAR
, Due to its characteristics, it is necessary to operate a plurality of SARs in close proximity and operate simultaneously. Therefore, the beam irradiation range of each SAR overlaps, signal waves from other SARs are mixed, and the SAR after data processing is used. The image quality is degraded.

(4) Since the reception level is very high in the nadia echo (reflected radio wave from directly below), the reception timing of the nadia echo usually does not overlap with the reception timing from the observation area in a satellite-mounted SAR. In the SAR mounted on an aircraft, the reception timing of the nadia echo does not overlap with the reception timing of the observation area.

However, SAR on multiple platforms
In the interferometric SAR, the received wave from the observation region of one SAR is mixed with the nadia echo due to the transmitted wave of another SAR, and the image quality of the SAR image may be significantly deteriorated.

By the way, since the SAR mounted on a satellite is for space use, there are severe restrictions on weight and power consumption. Therefore, it is not appropriate to increase the weight and the power consumption particularly in a satellite-mounted SAR.

The present invention has been made in view of the above-mentioned problems of the prior art, and it is possible to prevent the deterioration of the image quality of a SAR image without significantly increasing the weight and power consumption. It is an object of the present invention to provide an interferometric SAR system using a SAR mounted on a plurality of platforms that can be used.

[0028]

According to the present invention, there is provided an interferometry SAR system for remotely measuring a target by using a SAR mounted on each of a plurality of platforms close to each other. With multiple GPS satellites that emit reference radio waves and installed on the ground,
Measuring the own position by using the GPS satellite, the self
And a DGPS reference station for providing these positions as position information serving as a reference for precise measurement of the relative positions of the SARs.

[0029] One of the SARs is connected to another SA.
A SAR other than a SAR including a transmitter and an antenna for transmitting a common original signal for establishing coherency of a transmission wave to R and for synchronizing transmission and reception with respect to R Is characterized by comprising a receiver and an antenna for receiving the original signal.

Further, each of the SARs operates based on the reference radio wave.

The chirp modulation in each of the SARs is characterized in that one of the adjacent SARs is up-chirp and the other is down-chirp.

(Operation) In the present invention configured as described above, since the respective positions of the SAR are accurately calculated using the GPS satellites, the interferometry SAR with excellent measurement accuracy is implemented. .

Since each of the plurality of SAR transmission waves is formed based on the original signal, the SAR
Coherency between them is established.

Further, since each of the plurality of SARs performs a synchronous operation based on the original signal or the reference signal, the reception timing of the SAR's own nadia echo and the reception timing of the reflected wave from the observation area are different in advance. If it is set, the Nadia echo by another SAR will not be mixed into the reflected wave from the observation area of the SAR itself.

Further, one chirp modulation of adjacent SARs is set as up-chirp, and the other chirp modulation is set as down-chirp, so that the level of unnecessary waves mixed into the reflected wave from the observation area of the SAR itself is suppressed. Is done.

[0036]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a diagram showing a first embodiment of the interferometry SAR system of the present invention.

As shown in FIG. 1, this embodiment is mounted on each of a plurality of platforms (not shown), and performs SAR1, SAR2, and SAR for remote measurement of the target 4.
3, a plurality of GPS satellites 5 emitting reference radio waves, and a reference installed on the ground, measuring its own position using the GPS satellites 5, and calculating the precise positions of the SAR1, SAR2, and SAR3. D given as location information
And a GPS reference station 6.

In the interferometric SAR system configured as described above, accurate positions of SAR1, SAR2, and SAR3 are calculated using the GPS satellite 5 and the DGPS reference station 6, and the calculated position information is used. Is processed based on the data measured in SAR1, SAR2, and SAR3.

In this embodiment, the number of SARs is 3
However, the present invention is not limited to this.

(Second Embodiment) FIGS. 2A and 2B are diagrams showing a second embodiment of the interferometry SAR system of the present invention, wherein FIG. 2A shows the whole principle, and FIG. Is S
It is a figure showing the operation principle of AR.

As shown in FIG. 2, this embodiment includes SAR11 and SAR12 mounted on each of a plurality of platforms (not shown) for remote measurement of a target 14, and an original signal is transmitted from SAR11 to SAR12. Sent.

Here, the SAR 11 is provided with a transmitter 11b and an antenna 11c for transmitting the original signal,
The SAR 12 has a receiver 12b for receiving the original signal.
And an antenna 12c.

In the interferometric SAR system configured as described above, a common source signal is transmitted from SAR1 to SAR2 for establishing coherency of a transmission wave and for synchronizing operation.
2 and a synchronous operation is performed.

In addition, this allows the SAR of different frequencies
The interferometry SAR can be realized even between them.

(Third Embodiment) FIG. 3 shows an interferometry SA using a plurality of SARs arranged in close proximity.
It is a figure showing R.

As shown in FIG. 3, when a plurality of SARs are arranged close to each other, the beam irradiation range 21b of the SAR 21 and the beam irradiation range 2 of the SAR 22
2b, the SAR 21 is irradiated with the reflected wave 21c of the radio wave emitted from the SAR 21 together with the reflected wave 21c from the SAR 22, and the beam irradiation range 21
The reflected wave 22c reflected at b is received.

Here, in the SAR 21, the reflected wave 2
Reference numeral 2c denotes an unnecessary wave, and the quality of a reproduced image is deteriorated by receiving the unnecessary wave.

FIG. 4 shows the interferometry S of the present invention.
It is a figure which shows 3rd Embodiment of AR system, (a) is a schematic diagram of the whole, (b) is a figure which shows the structure of SAR shown to (a).

As shown in FIG. 4, this embodiment is mounted on each of a plurality of platforms (not shown), and performs SAR 21 and SAR for remote measurement for a target (not shown).
22 and a plurality of GPS satellites 25 that emit reference radio waves.
As shown in (b), a GPS receiver 33 that receives a clock signal 31 transmitted from a GPS satellite 25, and a calculator 34 that calculates a synchronization signal 35 from the clock signal 31
And a GPS device 37 that calculates position information 38 of the SAR itself from the clock signal 31 and a SAR device 36 that performs remote measurement for the target based on the synchronization signal 35. An antenna 32 for receiving is provided.

In the interferometric SAR system configured as described above, the SAR 21 and the SAR 2 are transmitted using the clock signal 31 transmitted from the GPS satellite 25.
2 are synchronized in the operation with the SAR 21 and the SA.
The remote measurement for the target in each of R22 is performed at the same timing.

FIG. 5 is a diagram showing the SAR operation timing in the interferometry SAR system shown in FIG.

As shown in FIG. 5, in this embodiment, S
The timing of receiving the nadia echo in each of the AR 21 and the SAR 22 is the same. Thereby,
If the reception timing of the own Nadia echo and the reception timing of the reflected wave from the observation area are set differently in advance, the Nadia echo by the SAR placed close by will be reflected from the own observation area. It does not get into the waves.

In the present embodiment, when DGPS is used to determine the position of the platform, a GPS receiver can be used. Therefore, the only hardware that must be added is an arithmetic unit that generates a synchronization signal. And the increase in power consumption is small.

In the second embodiment, the SAR
When transmitting an original signal for establishing coherency between the two, a synchronous signal can be generated based on the original signal, and a synchronous operation can be performed.

(Fourth Embodiment) FIGS. 6A and 6B are diagrams showing chirp modulation characteristics of SAR transmission waves, where FIG. 6A shows up-chirp modulation and FIG. 6B shows down-chirp modulation. It is.

In the present embodiment, the chirp modulation characteristics of the transmission waves between adjacent SARs are reversed. That is, with reference to FIG. 3, the transmission wave of the SAR 21 is up-chirp-modulated (the frequency increases with time).
Then, the transmission wave of the adjacent SAR 22 is down-chirp-modulated (the frequency decreases with time). Also,
Conversely, the transmission wave of the SAR 22 may be down-chirp-modulated, and the transmission wave of the adjacent SAR 21 may be up-chirp-modulated.

FIGS. 7A and 7B are diagrams showing received waves when the chirp modulation characteristics of the transmitted waves between adjacent SARs are reversed, and FIG. 7A is a diagram showing a state before signal compression, and FIG.
FIG. 4 is a diagram showing a state after signal compression.

Here, the pulse compression of the SAR is performed on the received wave and the reference wave (the chirp modulation characteristic thereof is opposite to the chirp modulation characteristic of the transmission wave (the same as the chirp modulation characteristic of the reception wave)).
When the correlation processing is performed, the principle that the energy of the received wave that has been temporally spread is concentrated at one point is used.

Therefore, as shown in FIG.
If the chirp modulation characteristics of the transmission waves of the Rs (SAR21, SAR22 (see FIG. 3)) are reversed, a signal that is a reflected wave of the SAR21 (see FIG. 3) due to the radiated radio waves when the received wave is subjected to pulse compression. The wave 41a becomes the emphasized signal wave 41b because the chirp modulation characteristic is opposite to the chirp characteristic of the reference wave in the pulse compression.
The unnecessary wave 42a, which is a reflected wave of the radiated radio wave of the AR 22 (see FIG. 3), has the same chirp characteristic of the reference wave in pulse compression, has no correlation, and becomes an unnecessary wave 42b which is noise without being emphasized.

On the other hand, when the chirp modulation characteristics of the transmission waves of the adjacent SARs are the same, the unnecessary waves are also emphasized.

As described above, if the chirp modulation characteristics of the transmission waves of adjacent SARs are reversed, the signal wave after compression becomes smaller than the case where the chirp modulation characteristics of the transmission waves of adjacent SARs are the same. The ratio between the level of 41b and the level of unnecessary wave 42b increases by the compression ratio of pulse compression.

In the polarimetric SAR by applying the above-described technique, the chirp characteristics of the vertically polarized wave and the horizontally polarized wave which are transmitted alternately are reversed, so that the transmission waves having different polarizations can be used. Unwanted waves can be suppressed from being mixed.

FIG. 8 is a diagram showing an example in which the interferometry SAR system in the first, third and fourth embodiments is applied.

As shown in FIG. 8, this embodiment is mounted on each of a plurality of platforms (not shown) and performs SAR 51, SAR 52,
SAR 53 and a plurality of GPS satellites 55 emitting reference radio waves
And a DGPS reference station 56 which is installed on the ground, measures its own position using a GPS satellite 55, and gives it as position information serving as a reference for calculating each precise position of the SAR 51, SAR 52, and SAR 53. ing.

[0066]

Since the present invention is configured as described above, it has the following effects.

According to the first aspect, the SAR
Is accurately calculated using the GPS satellites and the DGPS reference station, so that the measurement accuracy in the interferometric SAR system can be improved even when the SAR is mounted on each of a plurality of platforms.

[0068] This also allows the use of a small platform.

According to the second aspect, since the operation in each of the plurality of SARs is performed based on the original signal generated from one SAR, the plurality of SARs can be used.
, The coherency between the transmission waves of the SARs is established, the measurement accuracy of the interferometry SAR can be improved, the synchronization operation of the transmission and reception timings is established, and the mixing of nadia echoes by the transmission waves of other SARs can be avoided. In particular, interferometric SAR between SARs having different frequencies can be realized.

According to the third aspect, since the operation in each of the plurality of SARs is performed based on the reference radio wave emitted from the GPS satellite, the same effect as the synchronous operation in the second aspect is obtained.

According to the fourth aspect of the present invention, one of the chirp modulations of the adjacent SARs is up-chirp and the other chirp modulation is down-chirp. Unnecessary wave level can be suppressed.

With the above effects, it is possible to prevent the deterioration of the image quality of the SAR image without increasing the weight and the power consumption much.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of an interferometric SAR system of the present invention.

FIGS. 2A and 2B are diagrams showing a second embodiment of the interferometric SAR system of the present invention, wherein FIG. 2A is a diagram showing the overall principle, and FIG. 2B is a diagram showing the operating principle of the SARs; .

FIG. 3 is a diagram showing an interferometric SAR using a plurality of SARs arranged in close proximity.

FIGS. 4A and 4B are diagrams showing a third embodiment of the interferometric SAR system of the present invention, wherein FIG. 4A is a schematic diagram of the entire structure, and FIG. 4B is a diagram showing the structure of the SAR shown in FIG. It is.

5 is a diagram showing SAR operation timings in the interferometry SAR system shown in FIG.

6A and 6B are diagrams illustrating chirp modulation characteristics of a SAR transmission wave, wherein FIG. 6A is a diagram illustrating up-chirp modulation, and FIG. 6B is a diagram illustrating down-chirp modulation.

FIG. 7 is a diagram illustrating received waves when the chirp modulation characteristics of the transmitted waves between adjacent SARs are reversed.
(A) is a figure showing a state before signal compression, and (b) is a figure showing a state after signal compression.

FIG. 8 is a diagram illustrating an example in which the interferometry SAR method in the first, third, and fourth embodiments is applied.

FIG. 9 Cross-track interferometry SAR
It is a figure showing the concept of.

FIG. 10 Along track interferometry S
It is a figure which shows AR, (a) is a conceptual diagram, (b) is a figure which shows a measurement principle.

[Explanation of symbols]

1-3,11,12,21,22,51-53 SA
R 4,14,54 Target 5,25,55 GPS satellite 6,56 DGPS reference station 11a, 12a SAR signal 21b, 22b Beam irradiation range 21c, 22c Reflected wave 31 Clock signal 32 Antenna 33 GPS receiver 34 Arithmetic unit 35 Synchronization signal 36 SAR device 37 GPS device 38 Position information 41a, 41b Signal wave 42a, 42b Unwanted wave

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G01S ⁷ /00-7/42 G01S 13/00-13/95

Claims (4)

(57) [Claims] 1. An interferometric SAR system for remotely measuring a target by using a SAR mounted on each of a plurality of platforms close to each other, comprising: a plurality of GPS satellites for emitting reference radio waves; the measuring its own position using GPS satellites, give this own position as position information as a reference for precise measurement of the relative positions of and how the SAR DGP
Interferometry S by SAR mounted on a plurality of platforms, comprising: an S reference station;
AR method. 2. The interferometric SAR system using a SAR mounted on a plurality of platforms according to claim 1, wherein one of the SARs is for establishing coherency of a transmission wave with another SAR and for transmitting and receiving. SAR other than the SAR including the transmitter and the antenna for transmitting a common source signal for the synchronous operation of
Is an interferometric SAR system using SAR mounted on a plurality of platforms, comprising a receiver and an antenna for receiving the original signal. 3. The interferometric SAR system using a SAR mounted on a plurality of platforms according to claim 1, wherein each of the SARs operates based on the reference radio wave. Interferometric SAR method using SAR. 4. The interferometric SAR system using SARs mounted on a plurality of platforms according to claim 1, wherein the chirp modulation in each of the SARs is one of chirps of adjacent SARs. SA mounted on multiple platforms, characterized in that the modulation is up-chirp and the other chirp modulation is down-chirp
R interferometry SAR method.