Method to generate a three-dimensional image of a ground area using a SAR radar

• US 6,384,766 B1
• Filed: 12/20/1999
• Issued: 05/07/2002
• Est. Priority Date: 06/18/1997
• Status: Expired due to Term

• Current Assignee: Totalforsvarets Forskningsinstitut (FOI)
• Original Assignee: Totalforsvarets Forskningsinstitut (FOI)
• Inventors: Ulander, Lars
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US09/445,462
• Time in Patent Office: 869 Days
• Field of Search: 342/25, 342/159, 342/162, 342/189, 342/190, 342/191, 342/192, 342/194, 342/195, 342/196
• US Class Current: 342/25.00R
• CPC Class Codes: G01S 13/0209 Systems with very large relative bandwidth, i.e. larger than 10 %, e.g. baseband, pulse, carrier-free, ultrawideband
• CPC Class Codes: G01S 13/90 using synthetic aperture techniques , e.g. synthetic aperture radar [SAR] techniques
• CPC Class Codes: G01S 13/9023 combined with interferometric techniques
• CPC Class Codes: G01S 13/904 SAR modes

Abstract

The present invention relates to a method for generating a three-dimensional image of a ground area by means of a radar with a synthetic aperture, a SAR radar, which is supported by a platform moving in an essentially rectilinear manner. The method is characterized by the following steps.

Advancing the platform such that at least two images of the ground are created with great difference in the angle of illumination.

Transmitting radar pulses with a fractional bandwidth which is larger than or equal to 0.1, and using in the computations an aperture angle which is larger than or equal to 0.1 radians.

Detecting the reflected radar pulses with amplitude and phase.

For each pulse, measuring and storing the position of the antenna that transmits and the antenna that receives the pulse.

Computing a two-dimensional SAR signal per synthetic aperture.

Starting from the amplitude and phase of the two SAR signals as well as position data for the antennae, reconstructing a three-dimensional position description of the area relative to antenna position data.

Claims

1. A method for generating a three-dimensional image of a ground area by means of a radar with a synthetic aperture which is moving in an essentially rectilinear manner and includes at least two antennae, comprising the steps of:

creating at least two images of the ground with a large difference in the angle of illumination,

transmitting radar pulses with a fractional bandwidth which is larger than or equal to 0.1, and using in the computations an aperture angle which is larger than or equal to 0.1 radians,

detecting reflected radar pulses from said transmitting radar pulses, said reflected radar pulses having amplitude and phase,

for each transmitting radar pulse of said transmitting radar pulses, measuring and storing position data of the antenna transmitting and for each reflected radar pulses measuring and storing position data of the antenna receiving the reflected radar pulse,

computing a two-dimensional SAR signal having an amplitude and phase per synthetic aperture, and,

reconstructing a three-dimensional position topography of an area relative to said antenna position data, the topography being inverted from the measured SAR signals, determining a topography with a coarse resolution and with a limited depth of focusing using the amplitude of the SAR signals, and determining an unambiguous topography of a fine resolution using the phase of the SAR signals by comparing with a starting geometric signal model with the topography of the area and the position data of the antennae as input parameters.

2. A method as claimed in claim 1, and further comprising the step of:

advancing the platform such that at least two images of the ground are created with a great difference in the angle of incidence, which takes place from antenna positions that differ by a distance perpendicular to the platform path by the platform moving in time successively along two essentially parallel-displaced paths.

3. A method as claimed in claim 1, and further comprising the step of:

advancing the platform such that at least two images of the ground are created with a great difference in the angle of incidence, which takes place from antenna positions that differ by a distance perpendicular to the platform path by transmitting simultaneously radar signals from two antennae on the platform.

4. A method as claimed in claim 1, and further comprising the step of:

back-projecting the SAR signals to a three-dimensional computational grid along circles defined by the intersection between range cylinders and azimuth planes for each grid point, and

setting a value given by interpolation of the SAR signals to the range and azimuth positions of the given point.

7. A method as claimed in claim 1, and further comprising the step of:

filtering the SAR signals starting from the local inclination and the SAR measuring geometry such that the ground-projected resolution elements become identical.

8. A method as claimed in claim 1, and further comprising the step of:

carrying out the method at least at two center frequencies, which gives the position of different vegetation layers and/or ground layers.

9. A method as claimed in claim 1, and further comprising the steps of:

transmitting said radar pulses at least at one higher center frequency, and

receiving said radar pulses at least at one higher center frequency, wherein said transmission taking place with a normal bandwidth.

10. A method as claimed in claim 1, and further comprising the step of:

creating at least two images of the ground with a great difference in the angle of incidence, which takes place from antenna positions that differ by a distance perpendicular to the platform path by transmitting alternately radar signals from two antennae on the platform.

5. A method for generating a three-dimensional image of a ground area by means of a radar with a synthetic aperture which is moving in an essentially rectilinear manner, comprising the steps of:

creating at least two images of the ground with a large difference in the angle of illumination,

transmitting radar pulses with a fractional bandwidth which is larger than or equal to 0.1, and using in the computations an aperture angle which is larger than or equal to 0.1 radians,

detecting the reflected radar pulses with amplitude and phase,

for each pulse, measuring and storing the position of the antenna transmitting and the antenna receiving the pulse,

computing a two-dimensional SAR signal per synthetic aperture,

starting from the amplitude and phase of the SAR signals and position data for the antennae, reconstructing a three-dimensional position topography, of an area relative to antenna position data, the topography being inverted from the measured SAR signals starting from a geometric signal model with the topography of the area and the position data of the antennae as input parameters,

back-projecting the SAR signals to a three-dimensional computational grid along circles defined by an intersection between range cylinders and azimuth planes for each grid point,

setting a value given by interpolation of the SAR signals to the range cylinders and azimuth planes of the given point, and

maximising a vertical position in each horizontal position to determine in these horizontal positions a cross-correlation of the amplitude of the back-projected SAR signals.

6. A method as claimed in claim 5, and further comprising the step of:

determining a more accurate vertical position in each horizontal position by determining in each horizontal position the vertical position for which the back-projected SAR signals have the same phase value and which is closest to the previously determined vertical position.