MTF Analysis

The spatial response of a sensor is as important as its spectral and radiometric response. The spatial response can be determined in terms of the Point Spread Function (PSF) or the Modulation Transfer Function (MTF), in the frequency domain.

Usually, the procedures to validate the sensor on-orbit spatial response consists on:

• Modeling of the sensor pre-flight spatial response design specifications and laboratory measurements. These measurements will be used to develop a comprehensive, detailed PSF/MTF model based on its optical, detector and electronic components.
• Measurement of the sensor on-orbit spatial response from in-flight imagery.

The components that primarily influence the spatial response are the optics, the detectors and the signal processing electronics. To find the spatial response we typically model it at a higher level as three major components. The approach is to determine the overall MTF as a product of each component MTF.

Several techniques can be used to measure the PSF or its Fourier transform, MTF, from in-flight imagery. Two of them are:

• Using high contrast targets (lines, edges, etc).
• Comparison to high resolution image. In this case an image from sensor of higher resolution is used as reference.

The optical focus, combined with the A/D converter and any corrections necessary to compensate for internal detector problems, results in an image where the effective resolution (measured from in-flight imagery) will be different from the nominal resolution value. The greater the difference, the poorer the resulting image, resulting in a “blurring” effect. In order to determine the effective spatial resolution of a sensor, it is usual to define a parameter called EIFOV (Effective Instantaneous Field of View). The EIFOV is defined as a function of the MTF (modulation transfer function) of the sensor. EIFOV is defined as a function of frequency in which the MTF is equal to 50% of its maximum value. The EIFOV parameter enables a comparison between different sensors with similar nominal spatial resolution.

On going research: CCD imagery MTF analysis of the CBERS satellite.