Method for azimuth scaling of SAR data and highly accurate processor for two-dimensional processing of scanSAR data

• US 5,812,082 A
• Filed: 03/11/1997
• Issued: 09/22/1998
• Est. Priority Date: 03/13/1996
• Status: Expired due to Term

• Current Assignee: Deutsche Telekom AG
• Original Assignee: Deutsche Telekom AG
• Inventors: Mittermayer, Josef, Moreira, Alberto
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US08/816,044
• Time in Patent Office: 560 Days
• Field of Search: 342/25, 342/89, 342/162, 342/191, 342/192, 342/193, 342/194, 342/195, 342/196
• US Class Current: 342/25.00A
• CPC Class Codes: G01S 13/90 using synthetic aperture techniques , e.g. synthetic aperture radar [SAR] techniques
• CPC Class Codes: G01S 13/9011 with frequency domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 13/9017 with time domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 13/9056 Scan SAR mode

Abstract

In a method for azimuth scaling of SAR data without interpolation, raw SAR data in azimuth are multiplied with a phase function H₅ (f_a ;r_o), where f_a denotes the azimuth frequency and r_o denotes the range to a target point, and where a desired scaling factor is entered into the phase function. An azimuth modulation of the SAR data is subsequently adapted with the phase function H₅ (f_a,r_o) to that of a reference range, in a manner so that the azimuth modulation no longer depends on the range. In a last step of the process, a quadratic phase modulation is performed in the azimuth so that, in order to attain an azimuth processing with a very high phase accuracy, the azimuth frequency modulation becomes exactly linear.

Claims

1. A method for azimuth scaling of raw SAR data without interpolation, comprising the steps of:

multiplying the raw SAR data by a phase function H₅ given by a first equation ##EQU7## and where f_a is an azimuth frequency, r_o is a range to a target point, M is a wavelength, V_g is a ground velocity, V_s is a vehicle velocity, k_scl is a scaling Doppler rate, and r_scl is a scaling range, whereby the scaling range r_scl is within a range dimension of an image;

whereby azimuth modulation of the SAR data is made independent of the range r_o and is adapted to the scaling range r_scl ; and

whereby

azimuth frequency modulation becomes exactly linear and a quadratic phase modulation is performed instead of a hyperbolic phase modulation in the azimuth.

2. The method according to claim 1, including two-dimensional processing of ScanSAR data by a chirp scaling method and a SPECAN method, comprising:

(a) before performing the step of azimuth scaling, sequentially performing the steps of;

multiplying raw ScanSAR data for sidelobe suppression in a time-domain in the azimuth direction by a weighting function H₀ (3.2);

performing a first azimuth FFT (3.3) to obtain first signals;

multiplying the first signals by a chirp scaling function H₁ (3.5) given by a second equation

space="preserve" listing-type="equation">H.sub.1 (f.sub.a,t.sub.e ;

r.sub.o)=exp -jπ

k.sub.eff (f.sub.a ;

r.sub.ref)a(f.sub.a)(t.sub.e -t.sub.e,ref (f.sub.a)).sup.2 !(3);

performing a range FFT (3.6) to obtain second signals in the two-dimensional frequency-domain;

subjecting the second signals to a range compression and correction of a target range variation through multiplication (3.8) with a phase function H₂ given by a third equation ##EQU8## to obtain third signals;

performing a range IFFT (.3.9) on the third signals, multiplying the third signals by a phase correction function H₃ (3.11) given by a fourth equation ##EQU9## and subsequently multiplying (3.13) the third signals by a weighting function H₄ (f_a) corresponding to a reciprocal value of a determined antenna diagram; and

after performing the step of azimuth scaling, sequentially performing the steps of;

transforming back to a time domain with an azimuth IFFT (3.16) to obtain time data;

multiplying the time data obtained by a deramping function

space="preserve" listing-type="equation">H.sub.6 (t.sub.a)=exp jπ

k.sup.scl t.sub.a.sup.2 ! (8)

performing a second azimuth FFT (3.19) to obtain fourth signals; and

multiplying the fourth signals by a phase function H₇ given by a fifth equation ##EQU10## where f_a is an azimuth frequency, t_e is a time in range direction, r_o is a range to a point in time, k_eff is a modulation rate depending on the azimuth frequency and a reference range r_ref, a (f_a) is a linear scaling factor, f_e is a range frequency, c_o is the speed of light, λ

is a wavelength, V_g is the ground velocity, V_s is the ground velocity, k_scl is a scaling Doppler rate, r_scl is a scaling range, t_a is a time in azimuth direction, and T'"'"' is a total duration of the azimuth signal in the time-domain.