SAR radar system

• US 6,441,772 B1
• Filed: 06/18/2001
• Issued: 08/27/2002
• Est. Priority Date: 12/18/1998
• Status: Expired due to Term

• Current Assignee: Totalforsvarets Forskningsinstitut (FOI)
• Original Assignee: Totalforsvarets Forskningsinstitut (FOI)
• Inventors: Ulander, Lars, Hellsten, Hans
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US09/857,172
• Time in Patent Office: 435 Days
• Field of Search: 342/25, 342/190, 342/191, 342/192, 342/193, 342/194, 342/195, 342/196
• US Class Current: 342/25.00R
• CPC Class Codes: G01S 13/9017 with time domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 13/9029 specially adapted for moving target detection within a single SAR image or within multiple SAR images taken at the same time

Abstract

The present invention relates to a radar system which comprises a platform which moves over a number of objects. The number of objects can be very large and the objects can appear in the form of, for example, a ground surface. The platform supports radar equipment which reproduces the objects by means of synthetic aperture technique (SAR) via at least one antenna without requirements as to directivity or fractional bandwidth. Moreover, the movement of the platform is, during the recording of data for a SAR image, essentially rectilinear and uniform. The invention is characterized mainly in that it comprises a signal-processing device which records received radar echoes from each transmitted radar pulse and records or calculates the position of the used antenna or antennae, and which calculates a one-parameter quantity of two-dimensional SAR images as a function of two image co-ordinates where the parameter is the relative speed. Here use is made of the fact that each object, i.e. a radar echo with certain image co-ordinates, is reproduced at a maximum ratio of desired to undesired signal for a predetermined value of the relative speed parameter, which value is established to be the magnitude of the relative velocity vector between the object and the platform. In the calculation, the signal-processing device backprojects radar raw data in a hierarchical scheme, where each level is based only on the immediately preceding one, and where the summation of radar raw data occurs in the form of subapertures having a gradually increasing length.

Claims

1. A radar system comprising a platform which moves over a number, which can be very large, of objects, e.g. in the form of a ground surface, and supports radar equipment which reproduces the objects by means of synthetic aperture technique (SAR) via at least one antenna without requirements as to directivity or fractional bandwidth, the movement of the platform, during the recording of data for a SAR image, being essentially rectilinear and uniform, characterised in that it comprises a signal-processing device which records received radar echoes from each transmitted radar pulse and records or calculates the position of the used antenna or antennae and calculates a one-parameter quantity of two-dimensional SAR images as a function of two image co-ordinates where the parameter is the relative speed, use being made of the fact that each object, i.e. a radar echo with certain image co-ordinates, is reproduced with a maximum ratio of desired to undesired signal for a predetermined value of the relative speed parameter, said value being established to be the magnitude of the relative velocity vector between the object and the platform and, where the signal-processing device, in connection with the calculation, backprojects radar raw data in a hierarchical scheme, where each level is based only on the immediately preceding level, and where the summation of radar raw data occurs in the form of subapertures having a gradually increasing length.

2. A radar system as claimed in claim 1, characterised in that the calculation occurs with increments of the relative speed parameter, which decease essentially quadratically with the length of the subaperture.

3. A radar system as claimed in claim 2, characterised in that for an image consisting of N×

N pixels, derived starting from at least N radar pulses and at least N range samples for each radar pulse, preprocessed to a common sample spacing, the aperture synthesis processing first occurs with respect to subapertures based on K₁ radar pulses, which gives a low-resolution image with a constant radar amplitude over resolution calls consisting of N/K₁×

N pixels of the finest resolution that is intended to be achieved, use then being made of the fact that the reflectivity of each resolution cell at low resolution is an N/K₁-element-long vector, where each vector element is the reflectivity of a subaparture since a plurality of apertures give reflectivity values for the same resolution cell at low resolution, and use is made of the vector elements as radar data to combine K₂ apertures in groups to produce a more finely resolved image with a constant radar amplitude over N/(K₁K₂)×

N pixels of the finest resolution, whereupon this process continues iteratively until the finest resolution is achieved.

4. A radar system as claimed in claim 1, characterised in that it uses radar frequencies below 1 GHz.

5. A radar system as claimed in claim 1, characterised in that it linearly combines the signals from at least two antennae on the platform so that static objects are eliminated and moving ones are detected.

6. A radar system as claimed claim 5, characterised in that the signal-processing device in the calculation backwardly projects radar raw data in a generalised hierarchical scheme where the elimination of static objects and the detection of moving objects occur on each level.

7. A radar system as claimed in claim 5, characterised in that it uses at least three antennae on the platform and linearly combines the signals from at least two of the antennae, so that the bearing on moving objects can be determined by means of range differences between SAR images recorded with different antennae.

8. A radar system as claimed in claim 7, characterised in that it uses at least four antennae on the platform and eliminates interference signals using a known method.

9. A radar system as claimed in claim 1, characterised in that it linearly combines the signals from at least two antennae on the platform so that the bearing on moving objects can be determined by means of range differences between SAR images from different antennae.

10. A radar system as claimed in claim 9, characterised in that the signal-processing device in the calculation backwardly projects radar raw data in a generalised hierarchical scheme where the bearing on moving objects is determined on each level.

11. A radar system as claimed in claim 5, characterised in that it transmits a pulse sequence which is divided into at least two frequency steps, that it carries out different types of measuring, such as some of the types basic SAR imaging, detection of moving objects, determination of bearing and suppression of interference, during different frequency steps and optimises, for each frequency step, the number of antenna channels and the resulting allocation of bandwidth per antenna channel on the secondary condition that the Nyquist criterion for sampling of the Doppler signal is maintained, and minimises the number of frequency steps.

12. A radar system as claimed in claim 1, characterised in that the antennae are mounted in a translation-invariant and symmetric antenna system in front of or behind the platform.

13. A radar system as claimed in claim 12, characterised in that the antenna system comprises 5 broadband radar antennae arranged successively in a boom, the two outermost antennae being transmitting elements and the three inner ones receiving elements.

14. A radar system as claimed in claim 12, characterised in that it uses two antenna systems, one covering the frequency range 200-X MHz and the other X-800 MHz, and 350 MHz<

X<

550 MHz.