Wavefront error estimation derived from observation of arbitrary unknown extended scenes

• US 5,350,911 A
• Filed: 04/09/1993
• Issued: 09/27/1994
• Est. Priority Date: 04/09/1993
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Rehfield, Mark J., Mount, Susan B., Ellerbroek, Brent L., Rafanelli, Gerard L.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): SHAMI, KHALED
• Application Number:
  US08/045,841
• Time in Patent Office: 536 Days
• Field of Search: 250/201.9, 359/849, 356/121, 364/525, 364/148, 364/150
• US Class Current: 250/201.9
• CPC Class Codes: G01J 9/00 Measuring optical phase difference devices or arrangements for controlling the phase of light beams G02F1/01; Determining degree of coherence; Measuring optical wavelength spectrometry G01J3/00

Abstract

An extended scene wavefront sensing apparatus and procedure that separates (deconvolves) scene effects from wavefront errors of an optical or similar system. The present wavefront sensing apparatus and procedure uses a point source wavefront slope sensor in scene scanning mode and estimates wavefront errors by using a cross-correlation or cross-coherence procedure that operates on the outputs of the point source wavefront slope sensor. A signal processing procedure employed by the point source wavefront slope sensor provides output signals corresponding to wavefront slopes at forward optics pupil locations geometrically projected to the location of the transmission and reflection measurement plane detector pairs. During the scanning process, each of the detector pairs (equivalent to a subaperture) measures the effects of the local unchanging wavefront error in the forward optical system and temporal variations due to the scanning scene. By cross correlating or cross-cohering each detector pair'"'"'s temporal difference with differences from selected reference detector pairs, the scene induced variations in the measurement are eliminated, thereby leaving the stationary wavefront error component of the measurement. The lead/lag time in the temporal differences output between detector pairs and reference detector pairs are then used to estimate the wavefront slope differences between equivalent subapertures. The wavefront slope differences are used in a reconstructor to generate wavefront errors or wavefront slope errors, except for an unobservable global tilt.

Claims

1. A cross-correlation, extended scene, wavefront sensing method for use with an adaptive system for determining and correcting system wavefront errors from observation of unknown extended scenes, and wherein the adaptive system comprises a receiver, detection means coupled to the receiver, and processing means coupled to the detection means for producing signals that are adapted to correct for system wavefront errors, said wavefront sensing method comprising the steps of:

processing measured intensity distributions derived from signals received by the receiver and detection means to produce a spatially filtered transmitted intensity and a spatially filtered reflected intensity;

differencing the measured intensity distributions to form intensity difference signals;

generating a temporal cross-correlation, between each subaperture of the receiver and detection means and a reference subaperture from the intensity difference signals;

matched filtering the cross-correlated signals;

comparing the output of all of the matched filters and selecting the value of lead/lag time corresponding to the matched filter with maximum peak output to determine the wavefront slope difference; and

reconstructing the wavefront slope differences to produce an error estimate that is adapted to correct for the system wavefront errors.

2. The cross-correlation, extended scene, wavefront sensing method of claim 1 wherein the step of reconstructing the wavefront slope differences produces a total wavefront slope error estimate that is adapted to correct for the system wavefront errors.

3. The cross-correlation, extended scene, wavefront sensing method of claim 1 wherein the step of reconstructing the wavefront slope differences produces a wavefront error estimate that is adapted to correct for the system wavefront errors.

4. A cross-correlation, extended scene, wavefront sensing method for use with an adaptive system for determining and correcting system wavefront errors from observations of extended scenes, and wherein the adaptive system comprises a receiver, detection means coupled to the receiver, and processing means coupled to the detection means for producing signals that are adapted to correct for system wavefront errors, said wavefront sensing method comprising the steps of:

processing measured intensity distributions derived from signals received by the receiver and detection means to produce a transmitted intensity, I_m+ (R, t), and a reflected intensity, I_m- (R, t);

differencing the measured intensity distributions to form intensity difference signals Δ

I_m (R, t), and differencing the intensity distributions to form intensity difference signals Δ

I_m (R, t);

generating a temporal cross-correlation, (Δ

I_m (R, t)Δ

I_m *(R, t)) , between each subaperture at locations R on a projected pupil of the receiver, and a reference subaperture at locations R'"'"' on the antenna from the intensity difference signals, and wherein the temporal cross-correlation is a function of the lead/lag time associated with a wavefront slope difference between cross-correlated subapertures, as defined by the equation,

space="preserve" listing-type="equation"><

Δ

I(R, t=0)Δ

I(R'"'"',t)>

.tbd.G(R, R'"'"', t-T)

where the lead/lag time, T, is given by

space="preserve" listing-type="equation">T=κ

Δ

S(R, R'"'"')

and where κ

is a constant that is a function of predetermined parameters of the adaptive system, S((R, R'"'"') is the difference between wavefront slopes at locations R'"'"' and R in a measurement plane, and G(R, R'"'"', t), is a function of pupil geometry, scene spatial frequency power spectral density, and a wavefront sensor filter function;

matched filtering the cross-correlated signals;

comparing the output of all of the matched filters and selecting the value of lead/lag time corresponding to the matched filter with maximum peak output to determine the wavefront slope difference; and

reconstructing the wavefront slope differences to produce an error estimate that is adapted to correct for the system wavefront errors.

5. The cross-correlation, extended scene, wavefront sensing method of claim 4 wherein the step of reconstructing the wavefront slope differences produces a total wavefront slope error estimate that is adapted to correct for the system wavefront errors.

6. The cross-correlation, extended scene, wavefront sensing method of claim 4 wherein the step of reconstructing the wavefront slope differences produces a wavefront error estimate that is adapted to correct for the system wavefront errors.

7. A cross-coherence, extended scene, wavefront sensing method for use with an adaptive system that corrects adaptive elements of a receiver of the system and measures and separates scene effects from system wavefront errors produced by the system, said wavefront sensing method comprising the steps of:

processing measured intensity distributions derived from signals received by the receiver to produce a spatially filtered transmitted intensity and a spatially filtered reflected intensity;

differencing the measured intensity distributions to form intensity difference signals;

performing a temporal Fourier transform on the differenced signals;

generating a cross-coherence signal, between each subaperture location and a reference subaperture location from the Fourier transformed signals;

generating optimal estimates (minimum error estimates) using the real and imaginary pans of the cross coherence signal;

computing an arctangent on the cross-coherence signal;

optimally estimating pairwise slope differences and weighting signals derived from the arctangent computation; and

reconstructing the wavefront slope differences to produce an error estimate that is adapted to correct for the system wavefront errors.

8. The cross-coherence, extended scene, wavefront sensing method of claim 7 which further comprises the step of removing pi ambiguities after optimally estimating the pairwise slope differences.

9. A cross-coherence, extended scene, wavefront sensing method for use with an adaptive system that corrects individual elements of an receiver of the system and measures and separates scene effects from system wavefront errors produced by the system, said wavefront sensing method comprising the steps of:

processing measured intensity distributions derived from the receiver to produce a transmitted intensity, I_m+ (R, t), and a reflected intensity, I_m- (R, t);

differencing the measured intensity distributions to form an intensity difference signal Δ

I_m (R, t);

performing a temporal Fourier transform on the differenced signals to produce Δ

I(R, t);

generating a cross-coherence signal, Δ

I(R, f)Δ

I*(R'"'"', f), between each subaperture at location R'"'"', and a reference subaperture at location R, from the Fourier transformed signals;

generating minimum error estimates using the real and imaginary parts of the cross coherence signal;

computing an arctangent on the cross-coherence signal to produce ##EQU2## optimally estimating pairwise slope differences and weighting the signals derived from the arctangent computation in accordance with the equation

space="preserve" listing-type="equation">Δ

S.sub.EST (R,R'"'"')=AΔ

(R, R'"'"', f); and

reconstructing the wavefront slope differences to produce an error estimate that is adapted to correct for the system wavefront errors.

10. The cross-coherence, extended scene, wavefront sensing method of claim 9 which further comprises the step of removing pi ambiguities after optimally estimating the pairwise slope differences.

11. Apparatus for use with an adaptive system for determining correction signals required to correct for system wavefront errors from observations of extended scenes with a receiver and a detector array, said apparatus comprising:

means for processing measured intensity distributions derived from signals received by the receiver to produce a spatially filtered transmitted intensity and a spatially filtered reflected intensity;

means for differencing the measured intensity distributions to form intensity difference signals;

means for performing a temporal Fourier transform on the differenced signals;

means for generating a cross-coherence signal, between each subaperture location and a reference subaperture location from the Fourier transformed signals;

means for generating optimal estimates (minimum error estimates) using the real and imaginary parts of the cross coherence signal;

means for computing an arctangent on the cross-coherence signal;

means for optimally estimating all pairwise slope differences and weighting the signals derived from the arctangent computation;

means for removing pi ambiguities; and

means for reconstructing the wavefront slope differences to produce an error estimate that is adapted to correct for the system wavefront errors.

12. Apparatus for use with an adaptive system that corrects individual elements of an antenna of the system and measures and separates scene effects from system wavefront errors produced by the system, said apparatus comprising:

means for processing measured intensity distributions derived from signals received by the receiver to produce a spatially filtered transmitted intensity and a spatially filtered reflected intensity;

means for differencing the measured intensity distributions to form intensity difference signals;

means for performing a temporal Fourier transform on the differenced signals;

means for generating a cross-coherence signal, between each subaperture location and a reference subaperture location from the Fourier transformed signals;

means for computing an arctangent on the cross-coherence signal;

means for optimally estimating all pairwise slope differences and weighting the signals derived from the arctangent computation;

means for removing pi ambiguities; and

means for reconstructing the wavefront slope differences to produce an error estimate that is adapted to correct for the system wavefront errors.