Quality analysis plug-in

The Quality analysis plug-in provides tools for the measurement of IRF parameters such as geometric resolution, PSLR, SSLR, ISLR IRF shape and tools for statistics and radiometric analysis.

GSDView Analysis Menu

IRF Shape Analysis

IRF Shape Analysis tool performs measurements of characteristic parameters of the Impulse Response Function of a single point scatterer. The user selects an area of the image and the tool automatically searches the local peak and performs measurements on it. The results window contains two tabs:

• the Time domain target analysis
• the Frequency target analysis

Time domain target analysis

The time domain target analysis tab reports the results of the IRF shape analysis in the time domain. The GUI is organized in three main areas:

• target visualization area
• IRF shape measures table
• time domain target analysis tools panel

IRF Shape Analysis (time domain)

Time domain target visualization

The upper part of the GUI is dedicated to the visualization of the target. Three views of the target are shown:

• the upper left box shows the zoomed portion of the target - target image,
• the lower left box shows the range cut
• the upper left box shows the azimuth cut.

Target visualization (in time domain)

On the left side the color bar gives the possibility to stretch the image visualization. The stretching operation can be performed selecting the desired range of pixel values which are used to build up the custom look-up table. Once a range of the color bar is chosen, the image target is stretched accordingly and the color bar is zoomed on it. A further zoom and relative stretching is possible using the mouse wheel or another range selection. It is possible to return to previous zoom levels using Control+Arrows or to scroll the color bar using right mouse button.

The target image can be zoomed and scrolled using the mouse:

• zoom: it can be performed with the mouse wheel or by pressing the z keyboard key for a few seconds. In this second case the interested area can be selected. It is possible to return to previous zoom levels using Control+Arrows keyboard keys.
• scrolling: it is performed using the left button of the mouse.

IRF shape measures

On the lower right panel, the IRF shape measures have been collected into a table.

IRF shape measure table

The IRF shape measures computed over the target cuts are:

• main peak power: measure of the local peak power in dB
• peak row,col: absolute image coordinates of the local peak
• slant range resolution: resolution of the local target in range direction (slant projection) measured in pixels and in meters
• azimuth resolution: resolution of the local target in azimuth direction measured in lines and in meters
• ground range resolution: resolution of the local target in range direction (ground projection) measured in meters
• ISLR: Integrated Side Lobe Ratio in dB for range and azimuth cuts
• PSLR: Peak Side Lobe Ratio left and right in dB for range and azimuth cuts
• SSLR: Spurious Side Lobe Ratio left and right in dB for range and azimuth cuts
• IRF Shape -6dB/-3dB for range and azimuth cuts
• IRF Shape -10dB/-3dB for range and azimuth cuts

Time domain target analysis tools

On the lower left panel, three tabs contain additional tools.

The Options tab contains:

• Stretching options: the check-box tries to improve stretching automatically, the button Reset clears up the previous stretching changes.
• Target cut options: it gives the possibility to change the plotting scale of the range/azimuth cut (linear/logarithmic) and defines the orientation of the azimuth cut (vertical/horizontal).

Time domain target analysis tools

The Correct Target Angle tab contains the target rotation tool. It is used to rotate the target inspection tool setting a rotation angle. Once the angle is selected, a red cross appears in the image box and helps the user to choose the right angle value; the Start/Stop button refreshes the evaluation of the IRF Shape analysis over the rotated target. The Reset button restores the target rotation.

Correct target angle

The Target Localization Error tab contains the localization error tool that is useful in case the user already knows the real geographic coordinates (latitude, longitude and height) of the target and evaluates the difference with the estimated ones.

Target localization error

Example of angle correction

The target angle rotation tool can be useful in case the target to be measured is rotated with respect to the row/col grid. Trying to measure a point scatterer in a geo-coded product, for example, usually requires an image rotation. Once the analysis over the portion containing the rotated target is performed and the analysis panel is displayed, the user can select the target angle correction tab and click on the Start button. At this point a red cross is displayed over the image and the user can adjust the rotation in order to achieve a correct measurement of characteristics parameters of the point target IRF.

Target before rotation

Once the red cross is at the desired angle the user can press the Stop button and the tool performs image rotation, azimuth and range cuts, redraws and re-computes the IRF shape parameters.

Target after rotation

Note

In case one wants to measure a point scatterer in a SAR image acquired in squinted mode it might be necessary to perform range and azimuth measurements with two different rotation angles:

• zero degrees for measurements in range direction
• a rotation angle equal to the squint value to perform measurements in azimuth direction

Example of target localization error measurement

The computation of the localization error can be performed in case the target position is known and the localization error results to be the difference between the user provided coordinates and the measured ones.

A typical use-case is localization error measurement over calibration sites where corner reflectors and/or active transponders are installed controlling their position with GPS instruments.

Once located the target to analyze the user can select a small area around it and activate the tool for point scatterers analysis (Analysis->IRF Shape Analysis).

Rotated target of a geo-coded product

Using the Correct Target Angle tool, it is possible to adjust the image rotation to match the target orientation.

Angle correction of the rotated target

In the case of the figure Localization measures of the target after angle correction, the selected target is a real transponder with the following GPS coordinates: (40.68737N, 16.54415E). Using the Target Localization Error tool, it is possible to insert the GPS coordinates and compute the localization error.

Localization measures of the target after angle correction

Frequency analysis of point scatterers

As part of the point target analysis the possibility of performing the spectral analysis of the portion of image around the selected target is also provided.

The main purpose of this kind of analysis is to allow the user to check the correctness of the image interpolation performed before IRF measurements.

The interpolator uses an algorithm based on zero padding in Fourier domain. The critical part of the process is to take into account the Doppler centroid position. An error in Doppler centroid estimation or imprecise metadata can lead to a bad computation of the zero insertion point and produce a distorted (oversampled) image. In this case measured IRF parameters result to be worse than actual ones due to the interpolation artifacts so it is important to detect this kind of error, if any, during the analysis.

IRF Shape Analysis (frequency domain)

The tool has six box displaying various data characteristics. From the upper left corner:

• the bi-dimensional spectrum (amplitude)
• the averaged range spectrum envelope
• the averaged azimuth spectrum envelope
• the 2D view of range spectrum (amplitude)
• the 2D view of azimuth spectrum (amplitude)
• table reporting the interpolation factors in azimuth and range direction.

All images can be zoomed and scrolled. Each image has a checkbox which can be used to automatically improve the stretching. The stretching of the two 2D views can also be improved manually by scrolling up and down the corresponding envelope plot.

Spectral Analysis

The Spectral Analysis tool evaluates the bi-dimensional, range and azimuth spectrum over the selected portion of the image. The tool window shows six views as result. Starting from the upper left corner:

• image box containing the selected portion of the image
• averaged range spectrum envelope
• averaged azimuth spectrum envelope
• bi-dimensional spectrum of the selected portion of the image (lower left box)
• 2D view of range spectrum: module of 1-d FFT in range direction of the data matrix
• 2D view of azimuth spectrum: module of 1-d FFT in azimuth direction of the data matric

All images can be zoomed and scrolled as in target analysis, using the mouse and special keyboard keys. Each image has a checkbox which can be used to automatically improve the stretching. The stretching of both 2D views can be improved manually by scrolling up and down the corresponding envelope plot.

Spectral analysis

Statistics and Histogram

The Statistics tool evaluates the most common statistic values over the selected portion of the image.

Note

currently there is a size limit to the analyzable image area.

The tool window shows the graphic view of the histogram and a table with computed statistics:

• size of the selected area
• minimum
• maximum
• mean
• standard deviation
• coefficient of variation
• radiometric resolution
• equivalent number of looks

Statistics and Histogram

Raw Data Quality Analysis

The RAW data quality analysis tool performs the computation of statistics, histogram, missing line check and replica analysis.

Note

It must be underlined that the RAW data analysis is limited to COSMO-SkyMed products.

The kind of analysis actually performed depends on the HDF5 dataset selected by the user in the Level 0 product. Once selected a dataset the tool displays the corresponding GUI.

Parameters

The main panel of the RAW data analysis tool accepts three parameters:

Filename:the full path of input file. Group:/S01/START, /S01 or /S01/STOP. The default value is /S01 Dataset:B00x, CAL, NOISE, QLK or REPLICA. The default value is B001

Statistics tab

Statistics and histogram computation is possible on B00x datasets. In the upper left part of the tool window, the user can set the data portion of data to be analyzed. The analysis starts with a simple click on the button located in the middle of the panel. The computed statistics are shown in the form of a table on the right side of the GUI panel. Shown statistics are:

• mean: computed over the I/Q component of the selected portion
• standard deviation: computed over the I/Q component of the selected portion
• gain imbalance : computed over the selected portion
• non orthogonality : computed over the selected portion (degree)
• orthogonality : computed over the selected portion (degree)
• normality factors: computed over the I/Q component of the selected portion.

Moreover, for each of the above mentioned quantities, it is possible to perform the comparison with annotated values using the combobox (for parameter selection) and button above the parameters table.

Below the statistics section, histograms of the In Phase and In Quadrature components of the selected area are plotted. The bin interval over which the histograms is computed is retrieved by the Analog to Signal Reconstruction Level metadata annotated in the RAW product. It is the look-up table to be used to convert the quantized integer values of the RAW data into mV floating values.

RAW Data Analysis (statistics)

Missing Lines Check tab

In case the B00x subdataset is chosen, the missing line section is enabled; this tool searches for missing lines over the selected area of the RAW data. The tool shows also the annotated missing lines, so that it is possible to easily compare the results.

RAW Data Analysis (missing lines check)

Replica/Cal Analysis tab

The tool also provides the possibility to perform the IRF shape analysis over the replica/calibration lines.

The feature is activated if the user selects the REPLICA or CAL dataset in the main tool panel.

It is possible to choose among the available replicas or calibration lines. Each time the selection is changed the corresponding raw signal (replica or calibration line) and the compressed one (hopefully sinc shaped) are displayed.

The IRF shape analysis results are collected in a table (lower right corner), and can be compared to the annotated measures (table on the lower left corner).

RAW Data Analysis (replica analysis)

Note

It must be noted that the computed and annotated measures could be slightly different due to the different signal compression method used by the analysis tool with respect to the L0 processor.

In the spripmap case the RAW data analysis tool performs the chirp compression by means of an auto-correlation operation.

The COSMO-SkyMed L0 processor, instead, seems to perform a cross-correlation with a synthetic chirp replica in order to get the compressed signal. Of course achieved results mainly depend on the parameters used to generate the synthetic replica.