SAR Image Quality Assessment

A. Martínez

Description

Resumen
El análisis de calidad es una importante tarea desa-rrollo de métodos de procesado digital de genes SAR. En este artículo se presenta una metodología de análisis válida para blancos puntuales extensos. La utilidad de esta metodología se pone manifiesto en el estudio de dos casos prácticos: el enfoque correcto de las imágenes SAR y el efecto del ancho de banda procesado en las imágenes SAR
Abstract
Quality analysis is an important task in the devel-opment of SAR image digital processing methods. In this paper a methodology for the analysis of both point target and extended target in SAR images is presented. The utility of the proposed methodology is demonstrated in the study of two practical situa-tions: the correct focusing of images and the of the processed bandwidth on SAR images.

Sujets

• Calidad

Informations

• Publié par: erevistas
• Publié le: 01 janvier 1993
• Nombre de lectures: 137
• Langue: English

Extrait

Revista de Teledetección. 1993
SAR Image Quality Assessment
l 2A.Martínez and J.L.Marchand
1 Inisel Espacio, Av.Burgos 8 bis, 3. 28036 Madrid - Spain
2 On Board Data Handling Division. ESTEC - ESA P.O.Box 299, 2200 AG Noordwijk - The Netherlands


RESUMEN ABSTRACT
El análisis de calidad es una importante tarea desa- Quality analysis is an important task in the devel-
rrollo de métodos de procesado digital de genes opment of SAR image digital processing methods.
SAR. En este artículo se presenta una metodología In this paper a methodology for the analysis of both
de análisis válida para blancos puntuales extensos. point target and extended target in SAR images is
La utilidad de esta metodología se pone manifiesto presented. The utility of the proposed methodology
en el estudio de dos casos prácticos: el enfoque is demonstrated in the study of two practical situa-
correcto de las imágenes SAR y el efecto del ancho tions: the correct focusing of images and the of the
de banda procesado en las imágenes SAR; processed bandwidth on SAR images.

PALABRAS CLAVE: procesado SAR, calidad. KEY WORDS: SAR processing, quality analysis




The definitions of the main SAR quality parame-INTRODUCCIÓN
ters are given in the second section of this work.
The increasing interest of the Remote Sensing The methodology used to conduct the analysis is
Community on Synthetic Aperture Radar (SAR) presented in section three. Several examples of
images during the past twenty years has made SAR quality analysis are discussed in the fourth
Digital SAR Data Processing an active field of section. Finally, the conclusion are summarized.
research.
In contrast to optical imagery, SAR data have to SAR QUALITY PARAMETERS
be pre-processed in order to obtain an image. The
The SAR response to a point target, assuming focusing of SAR raw data is essentially a two
negligible background reflectivity and thermal dimensional problem. The reference function of
noise, is commonly referred to Impulse Response this correlation is the Impulse Response Function
Function (IRF). The analysis of the signature of a of the SAR system. The most straightforward way
point target in a SAR image allows the determina-
to perform the compression is directly in the 2D tion of several parameters that are related to the
time domain (Barber, 85). Due to the huge compu- SAR spatial resolution and the presence and im-
tational burden of this method, other approaches portance of undesired side lobe peaks. Also related
working in the frequency domain have been inves- with the SAR response to a point target is the am-
tigated in the past, being the most common proce- biguity level, which measures the energy of the
point target focused in different spatial points. dures the range-Doppler and the wave domain,
The radiometric resolution is obtained through approaches.
the study of the variations of pixel values within an The aim of these methods is to perform the 2D
homogeneous area; that is, the statistics of the correlation as two ID correlations. The main prob-
SAR response to a region with constant backscat-lems arise from the azimuth compression, as the
tering coefficient. input data for correlation are located along range
migration trajectories that are themselves Peak intensity and amplitude
dependent
A SAR image is fue result of coherently process-The main objective of the SAR quality analysis
ing retumed echo signals; thus, fue pixel values are methods to be presented in this paper is to provide
complex quantities. For most applications fue a tool for the study of the focusing performance of
representation of the magnitude of the image is different processing algorithms and the influence
enough. It is possible to use the modulus of fue
of key parameters in the quality of the final image. complex pixel (amplitude) as well as the squared
They can also play an important role when study- modulus (intensity). The peak intensity is fue
ing the influence of common post processing tech- maximum pixel value in fue main lobe of the im-
niques in SAR images, such as speckle filtering, pulse response function (if the image has been
geometric and radiometric calibration and data power detected). The square root of the intensity is
compression. the amplitude.
Nº 2– Noviembre 1993 1 de 7 A.Martínez and J.L.Marchand
Spatial resolution
The spatial resolution is the distance between the
points with intensities 3 dB bellow the maximum
intensity of the main lobe peak in the azimuth and
range directions. The definition of the - 3 dB
points is equivalent to the points with half fue
intensity of fue maximum:

[I ] = [I ] + 3max dB 3dB dB
10 log I = 10 log I + 310 max 10 3dB
3/10
I / I = 10 = 1.995262 ≈ 2max 3dB

It is sometimes useful to have parameters meas-
uring the width of the main lobe at differences
heights. We have made extensive use of the width
at one tenth of the intensity of the maximum (that
is, an intensity 10 dB bellow the maximum) and
algo of fue distance between the first two minima
Figure 1. Graphical representation of a SAR point target of the main lobe, both in range and azimuth direc-
showing several quality parameters. tions. See figure 1 for a graphical representation of
these parameters.
(Franceschetti,91) They define the normalized
Peak side lobe ratio integrated side lobe ratio, NISLR, as follows:

The peak side lobe ratio, PSLR, is defined as the +∞
ratio of the peak intensity of the most prominent Idxdy∫ (2) - ∞side lobe to the peak intensity of the main lobe. NISLR =I 10logmax 10 + ∞There are tour measures of the PSLR, correspond-
Idxdy- Idxdy∫ ∫ing to the two sides of the main lobe both in azi- -3dB
− ∞muth and range directions. Usually, the largest of

the two sides for a given direction is finally re-
(Guignard, 79) There are three different regions ported, and this parameter is expressed in decibels.
in fue IRF: the main bearn afea, which is 3 by 3
Unfortunately, there is not a simple procedure to
pixels centred on the maximum; fue guard band, decide wether a peak is a true secondary lobe or
which is formed by the 26 pixels surrounding the
not. For this reason, it is convenient to report the
main beam afea; and the side lobe afea, formed by two computed PSLR's corresponding to the first
a square of 99 pixels side, disregarding the inner 5
maximum (that closest to the main lobe) and to the
by 5 window. absolute one.

Idxdy- IdxdyIntegrated side lobe ratio ∫ ∫99x99 5x5 (3) =ISLR 10log10
Idxdy∫The integrated side lobe ratio, ISLR, is the ratio 3x3
of the power (energy) in the main peak to the total
(Holm, 91) Ratio of the power within a square power in all the side lobes (or vice versa, depend-
centred on fue maximum and twenty by twenty ing on definition). As for the PSLR, the ISLR is a
resolution cells, without considering an inner win-measurement of the relative importance of fue sirle
dow of three resolution cells side and the power in lobes with respect to the main lobe. This parameter
the second window. is usually expressed in decibels. There are several
definitions of the ISLR in the literature, with the
Idxdy- Idxdydifference in the adoption of the areas in which the ∫ ∫20x20 3x3 (4) ISLR =10log10 energy is integrated: Idxdy∫3x3(Sánchez, 91) ISLR is the ratio of the energy in-
side a rectangle centred on the maximum of the
(ESA, 90,91) Ratio of fue power within a square main lobe and side length equal to the - 3 dB width
centred on the maximum and ten resolution cells of the IRF to the rest of the energy of the IRF. In
side, without considering an inner window of two this definition, only one resolution cell is consid-
resolution cells side and the power in the second ered to have the energy in the main lobe.
window.
Idxdy (1) ∫ -3dBISLR =10log
10 + ∞ Idxdy- Idxdy∫∫10x10 2x2 (5) Idxdy- Idxdy ISLR =10log∫ ∫ 10 -3dB Idxdy− ∞ ∫2x2
2 de 7 Nº 2– Noviembre 1993 SAR Image Quality Assessment
The ESA' s definition will be used in this work,
along with a normalized ISLR defined as follows:

1
( Idxdy- Idxdy)∫ ∫10x10 2x2 (6) 96normISLR =10log10 1
Idxdy∫2x24
Ambiguity level
There are two types of ambiguities in SAR: the
range ambiguities, which results from the simulta-
neous arrival of different pulses at the antenna, and

the azimuth ambiguities. The range ambiguities are Figure 2. Point target analysis methodology
controlled via the PRF (Pulse Repetition Fre-
quency) selection. On the other hand, azimuth spurious influences of the surrounding environ-
ambiguities are produced by the finite sampling of ment. The point target should be as bright as pos-
the azimuth frequency spectrum at the PRF. sible, to improve the statistics. The preferable
During the processing of SAR raw data, a selec- targets are man-made transponders and reflectors,
tion of the frequency bandwidths has to be per- but they are not available in most of the images; in
formed. As the process is made in digital form, this situation, opportunity