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Compact and Spherical Range Design, Application and Evaluation
Walter D. Burnside and Inder J. Gupta
The Ohio State UniversityElectroScience Laboratory
1320 Kinnear RoadColumbus, Ohio 43212
(614) 292-5747 and (614) 292-5951
Presented on September 21-22, 2005 for Raytheon (Tucson, AZ).
Course Outline
Basic Range Design Guidelines (Burnside) Compact Range Reflector Design (Gupta) Absorber Design and Layout (Burnside) Critical Range Evaluation (Gupta)
Second Half Day
First Full Day
R-Card Fences for Outdoor Ranges (Gupta) Summary of Range Design Issues (Burnside)
Inder (Jiti) Gupta
ElectroScience LaboratoryDept. of Electrical and Computer Engineering
The Ohio State University1320 Kinnear Road, Columbus, OH 43212
Phone: (614) 292-5951Fax: (614) 292-7297
Email: [email protected]
Critical Range Evaluation
What Range Evaluation Involves
Quiet zone field quality
–amplitude taper–phase uniformity–ripple in the fields–cross-polarization level
Stray signal source mapping
Quiet Zone Field Quality
• Direct measurement (field probing)– with a small antenna or a small sphere
• Indirect measurements1,2 (scattering measurements)– using a thin long bar
1van de Griendt et al., “Full characterization of the test zone fields using an RCS method,” AMTA, 19952B.L. Raghavachari, “Estimation of compact range test zone fields using a RCS method,” M.S. Thesis, The Ohio State University, 1998.
Direct Measurement
Advantage–No processing required
–Suitable for antenna as well as radar ranges
Disadvantages–Requires special equipment
–Depends on the probe quality
Indirect Measurements Advantages
– Very suitable for scattering ranges
– No special equipment required
– No spatial filtering
– Large dynamic range
Disadvantages– Not suitable for antenna ranges
– Data processing required
– Difficult to measure and isolate scattering from a long straight edge
Direct Method is Recommended
Stray Signal Source Mapping
Using field probe data
Using flat diagonal plate
Stray Signal Source Mapping Using Field Probe Data
Near Field Focusing
Beam Forming Technique
Direction of Arrival Estimation
Time of Arrival Estimation
Time and Direction of Arrival (TADOA) Spectra
Time Domain Near Field Focusing
Near Field Focusing Let the quiet zone be probed along a linear scan at M
points at the frequency of interest.
Then define
where h(m) is the field probe data,
(m) is a weighting function,
= sosin, z = socos.
F(,z) is called the Near Field Spectra.
2
1
( , )m
M j sm
m o
sF z m h m e
s
1
2 22 ,m ms z
Simulated Data Three incident signals. The desired plane wave (DPW) is incident from 0° and
has a SNR of 50 dB. The source of the second signal is 40′ from the center
of the scanner with = -20° and has a SNR of 20 dB. The source of the third signal is 20′ from the center of
the scanner with = 15° and has a SNR of 18 dB.
Simulated Data, 3 Incident Signals
Frequency = 2 GHz Frequency = 4 GHz
Near Field Spectra of the Simulated Data
Hamming Weights
Frequency = 2 GHz Frequency = 4 GHz
IMAGE USING NEAR FIELD FOCUSING IMAGE USING NEAR FIELD FOCUSING
Near Field Focusing (cont.)
The desired plane wave (DPW) limits the performance of the near field focusing technique.
One can estimate and then subtract the DPW from the field probe data to enhance the performance of the near field focusing.
The weighted average (smoothing) of the field probe data can be used to estimate the DPW.
Near Field Spectra of the Simulated Data
Hamming Weights, DPW Subtracted
Frequency = 2 GHz Frequency = 4 GHz
IMAGE USING NEAR FIELD FOCUSING IMAGE USING NEAR FIELD FOCUSING
Limited resolution in the down range direction (perpendicular to the scanner).
Computationally inefficient in that one has to calculate the function over the whole plane.
In general, not recommended.
Near Field Focusing (cont.)
Beam Forming Technique
Let so >>m, m = 1, 2…M
Then, and
This is called the Beam Forming Technique. Note that F needs to be calculated only as a function of
F() will have a maximum when there is a source along direction .
Near field sources will not focus properly.
2
sin
1
, ,m
M j
om
F z F s w m h m e
s inm o ms s
DOA Spectra Obtained Using Beam Forming Technique
Hamming Weights, Simulated Data
Frequency = 2 GHz Frequency = 4 GHz
DOA Spectra Obtained Using Beam Forming Technique
Hamming Weights, Simulated Data – Smoothed Data
Frequency = 2 GHz Frequency = 4 GHz
Beam Forming Technique (cont.)
At low frequencies, one may not have enough resolution to separate various signals in the angle domain
Two stray signals coming from the same direction but associated with different mechanisms cannot be differentiated.
Direction of Arrival Estimation The beam forming technique, as pointed out earlier,
basically estimates the DOA of the incident signals. One can use Capon’s method, MUSIC algorithm and its
variants, maximum likelihood estimator, etc. for DOA estimation.
For narrow band field probe data, the incident signals are correlated with each other
Stray signals do not have planar wavefront. The amplitude of the stray signal varies significantly over
the probed aperture. DOA estimation is carried out using a portion of the
probed aperture. At low frequencies where the probed aperture is already
small (electrically), these techniques do not provide any advantage over the beam forming technique.
Time of Arrival (TOA) Estimation
Let the quiet zone fields be probed over a frequency band at each probe location.
Then one can transform the frequency domain data at each probe location to time domain.
Inverse Fourier Transform (IFT) can be applied to the frequency domain data for time of arrival estimation.
Since the probe is a small antenna (point source), various peaks in the time domain will yield the relative TOA of the incident signals.
Illustrative Example
Probe data over 72” linear scan in 0.5” increments.
2-6 GHz frequency band.
Four incident signals.
DPW at 0° with 50 dB SNR.
Two signals incident from -20° with 15 dB and 20 dB SNR, respectively.
Another signal is incident from 15° and has 18 dB SNR.
The relative (with respect to DPW) TOA of the three stray signals at the center of the quiet zone are -10 nsec, 2 nsec and 3 nsec, respectively.
Simulated Data vs. Frequency
Four incident signals
Probe Location = 0.0 inch Probe Location = 18.0 inch
TOA Plots for the Simulated Data
Probe Location = 0.0 inch Probe Location = 18.0 inch
Hamming weights
Sinogram of the Simulated DataHamming weights
Time of Arrival Estimation (cont.)
Slopes of the various lines in a sinogram are linked with the location of the incident signal sources.
Let the source be located at (R,). Then the time of arrival at a probe location is given by
where c is the velocity of light in free space. Making the far field approximation,
1 22 22 sino
R R Rt t
c
sinot t
c
In practice, the magnitude of the probed data varies with frequency and its phase also displays non-linear variation.
Variations in magnitude are due to– gain of the feed antenna– gain of the probe antenna– losses in the cable and various connectors
Non-linear variation in phase can be due to – dispersion in the microwave components– phase center of the probe antenna – connectors
The probed data should be calibrated.
Time of Arrival Estimation (cont.)
Sinogram of the Experimental DataHamming weights, No calibration performed
Data Calibration for TOA Estimation
One needs another set of measurements for data calibration.
For field probe data, another set of measurements is not available.
We propose to use the DPW component of the probe data as the calibration signal.
Since the DPW is normal to the probe scanner, one can spatially smooth the probe data at each frequency to estimate the DPW component.
Then the DPW component at the center of the scanner can be used for the calibration of probe data at that frequency.
Sinogram of the Experimental Data after CalibrationHamming weights
Time and Direction of Arrival(TADOA) Spectra
Let the quiet zone fields be probed along a linear scan over a frequency band defined by (f1,f2). Then define
where h(f,) is the probed data at location , w(f,) is the weighting function, (1, 2) define the linear space over which the quiet zone fields are scanned, and
H(to, ) will have a local maxima if h(f,) contains a signal with time delay to and incidence angel
We will define |H(to, )|2 as the TADOA spectra.
2 2
1 1
2, , ,f j f t
o ffH t w f h f e d d
sin.ot t
c
(1)
(2)
TADOA Spectra
|H(t,)|2 involves the computation of a 2-D integration, and thus can be inefficient.
To increase the computational efficiency, one can use an FFT to carry out the integration over frequency.
(1) can be written as
where
(4) is the DOA spectra at frequency f.
2
1
2( , ) , of j f t
o fH t h f e df %
2
1
2 sin
, , , .f
jch f w f h f e d
%
(3)
(4)
TADOA SpectraCalibrated Data
Simulated Data Experimental Data
Time Domain Near Field Focusing (TDNFF)
Let the quiet zone fields be probed over a frequency band along a linear scan extending from l to h. Then define
where w(f) is a window function and H(f,) is the calibrated field probe data. Note that G(t) is the range time domain response at probe location .
Next, let
where w() is another window function, and
|I(0, z0)|2 is called the TDNFF Spectra.
2,l
fh j f t
fG t w f H f e df
0 0 0 0, , ,h
l
I z w G t z d
0 0 0 0 0 0
1, , , , , .t z R z R z
c
Experimental Range
30 meter long,Radar antenna height is 60 cm,Center of target zone is 3 meters above
ground,6-18 GHz frequency band,Six R-card fences,Fences are tilted 20° towards the feed.
A Drawing of the Experimental Test Range
A Photograph of the 30-meter Outdoor Range
Field Probe Data along the Vertical Scan. HP
No Calibration
No fences With fences
Field Probe Data along the Vertical Scan. HP
After Calibration
No fences With fences
Field Probe Data along the Vertical Scan. HP
No fences With fences
Stray Signal Source Mapping Using Field Probe Data
Various techniques for mapping stray signal sources in far-field antenna/RCS ranges were presented.
These techniques use the quiet zone field probe data at a single frequency or band of frequencies.
Relative merits and drawbacks of various mapping methods were also discussed.
It was shown that relatively simple processing of the field probe data can be used effectively for mapping of the stray signal sources.
Summary
Range Evaluation Using a Flat Diagonal Plate
A diagonal flat plate is positioned in the quiet zone and a portion of the chamber is scanned by rotating the plate.
The scattered fields from the plate are measured over the frequency band of interest at various plate orientations (rotation angle).
The stray signal response will peak up when the plate is oriented such that the stray signal is specularly reflected back in the DPW direction.
The measured scattered field data is processed to locate the sources of stray signals.
AdvantageVery suitable for scattering ranges.Strong to medium stray signals can be directly
identifiedLarge dynamic range
Plate SizeThe stray signals should be in the far zone of
the plate.
Range Evaluation Using a Flat Diagonal Plate
RCS Pattern of 1.25’ x 1.25’ Diagonal Plate
Mini Range
5’ Focal Length Frequency = 6 GHz
RCS Pattern of 1.25’ x 1.25’ Diagonal Plate
Mini Range with Absorber Wall
5’ Focal Length Frequency = 6 GHz
Diagnosis Tools
Raw data as a function of plate orientation at fixed frequency
Finite impulse response (TOA) at a set of aspect angles.
Inverse synthetic aperture radar (ISAR) images.
Range Evaluation Using a Flat Diagonal Plate
9’ x 9’ flat square plate.
Signal scenario consists of two signals.
One of the signals, referred to as the plane wave, is mono-statically scattered by the plate.
The other signal, referred to as the stray signal, is bistatically scattered by the plate.
The two signals are incident in = 45° plane.
The angular separation between the two signals is kept fixed at (40°).
The stray signal has fixed time delay with respect to the plane wave and is 70 dB below the plane wave level
Range Evaluation Using a Flat Diagonal Plate (Illustrative Example)
Scattered Fields of 9’ x 9’ Diagonal Plate
No Stray Signal -70 dB Stray Signal at 40°
UTD (Uniform Theory of Diffraction) Solution
Time Domain Response of 9’ x 9’ Diagonal Plate
No Stray Signal -70 dB Stray Signal With15 ns Delay
0.5 GHz to 1.5 GHz scattered field data
Time Domain Response of 9’ x 9’ Diagonal Plate
-70 dB Stray Signal With 5 ns Delay-70 dB Stray Signal With10 ns Delay
0.5 GHz to 1.5 GHz scattered field data
TOA Estimation Using Diagonal Flat Plate
The sinogram of the scattered fields from a diagonal plate provides an estimate of the direction of arrival as well as the time delay of the stray signals.
The time delay is the time difference between the plane wave and the stray signal assuming the return path of the scattered signals is the same
The time delay can be used to identify very weak stray signals if and only if the time delay is large
ISAR images are studied next.
ISAR Images of 9’ x 9’ Diagonal Plate
No Stray Signal Stray Signal With15 ns Delay
0.5 GHz to 1.5 GHz scattered field data10° to 30° aspect angle
ISAR Images of 9’ x 9’ Diagonal Plate
Stray Signal With 5 ns DelayStray Signal With10 ns Delay
0.5 GHz to 1.5 GHz scattered field data10° to 30° aspect angle
Example from the Ohio State University Compact Range
Scattered fields from a 3’ x 3’ diagonal flat plate.
2-18 GHz frequency band in 10 MHz frequency increments.
ISAR images using 20° aspect region with 0.1° angular increments.
The compact range reflector has an elliptical rolled edge.
Diagonal Plate in OSU Compact Test Range
Top View of the Range ISAR Image at 155°
Diagonal Plate in OSU Compact Test Range
Top View of the RangeWith 6” Sphere
ISAR Image at 155°With 6” Sphere
Measured scattered fields off a flat diagonal plate can be used effectively to evaluate the performance of a range.
The measured data can be used for stray signal source mapping.
Sinogram (time of arrival spectra at various plate orientations) can be used to identify stray signals with large time delays.
ISAR images obtained from the measured data are also effective in mapping stray signal sources
Sources of very weak stray signals can be mapped.
Critical Range Evaluation
Summary
Various methods for critical range evaluation were discussed.
These methods include probing of the quiet zone fields as well as measuring the scattered fields from specific targets (long bar, diagonal plate, etc.).
Many signal processing methods that can be used with the field probe data and/or scattered field data for range diagnosis were presented.
Summary
Critical Range Evaluation
References for Critical Range Evaluation
D.R. Koberstein, “Near field synthetic aperture imaging of probe data for scattering studies of the ElectroScience compact range,” M.S. Thesis, The Ohio State University, 1986.
E.K. Walton, “Compact range spurious signal mapping using probe data,” Technical Report 719627-12, The Ohio State University ElectroScience Laboratory, December 1987.
T.P. Delfeld and F.C. Delfeld, “Use of the MUSIC algorithm in the analysis of compact range field probe data,” AMTA’89, Monterey, CA, October 1989.
A. Moghaddar, E.K. Walton and W.D. Burnside, “Imaging of compact range stray signal sources using parametric modeling of the field probe data,” Technical Report 312884-21, The Ohio State University ElectroScience Laboratory, March 1990.
A. Moghaddar and E.K. Walton, “Imaging of low level signals in a compact range,” AMTA ’90, Philadelphia, PA, October 1990.
I.J. Gupta and W.D. Burnside, “Imaging of compact range probe data,” AMTA ’90, Philadelphia, PA, October 1990.
I.J. Gupta, “Performance of superresolution techniques in imaging compact range probe data,” AMTA ’91, Boulder, CO October 1991.
I.J. Gupta, T.-H. Lee and W.D. Burnside, “Study of the flat plates to map weak stray signals in compact ranges,” Technical Report 312496-4, The Ohio State University ElectroScience Laboratory, February 1991.
T. Lee, T. Clark, W. Burnside and I. Gupta, “Critical range evaluation using a diagonal flat plate,” IEEE Transactions on Antennas and Propagation, vol. 40, pp 966-974, Aug. 1992.
A. van der Merwe and D.J. Janse van Rensburg, “Main-beam reduction for compact range imaging,” IEE Proceedings, Part H, vol. 141, pp.461-463, December 1994.
I.J. Gupta and A. van der Merwe, “Compact range evaluation at low frequencies,” AMTA ’95, Williambsurg, VA, November 1995.
I.J. Gupta, E.K. Walton and W.D. Burnside, “Time and direction of arrival estimation of stray signals in a RCS/antenna range,” AMTA ’96, Seattle, WA, October 1996.
M.A.J. van de Griendt et al., “Full characterization of the test zone fields using an RCS method,” AMTA ’95, Williamsburg, VA, November 1995.
B.L. Raghavachari, “Estimation of compact range test zone fields using a RCS method,” M.S. Thesis, The Ohio State University, 1998.
T.D. Moore and I.J. Gupta, “Calibration of range probe data for stray signal analysis,” AMTA 2000 Philadelphia, PA, October 2000.
I.J. Gupta, “Time domain near field focusing to map stray signals in spherical ranges,” AMTA ’02, Cleveland, OH, November 2002.
I.J. Gupta, “Stray signal source location in far-field antenna/RCS ranges,” IEEE Antennas and Propagation Magazine, vol. 46, pp.20-29, June 2004.
References for Critical Range Evaluation