Interferometric synthetic aperture radar (InSAR)

Synthetic Aperture Radar is an active remote sensing system with microwave using range frequency 0.3 GHz ~ 300 GHz (or expressed in wavelength is about 1 mm ~ 1m). Because it uses microwave that has the ability to penetrate the cloud, smoke and rain. This method is available for day-night time observation.

The SAR platforms (aircraft, space borne or satellite) are using side-looking radar system when fly along track in azimuth direction over area of interest (Fig. 2.1). The antenna of SAR is transmitting electromagnetic waves in range direction and receiving the backscattered signal from earth surface. The SAR image is constructed by the amplitude and the time delay of backscattered signal (echo) from the earth surface (Brown and Porcello, 1969). 

Figure 1. Geometry of Side-Looking Synthetic Aperture Radar (SAR)

SAR image contains complex number called Single Look Complex (SLC). Each pixel of SAR image contains information about amplitude (Fig. 2a) and phase (Fig. 2a) of echoes signal.

 

Figure 2 a) Amplitude of SAR images and b) Phase of SAR image

In Fig. 2.2a the bright area means strong power of echoes signal (namely building, road, etc). On the other hand, the dark area means low power of echoes signal, such as reflected by water, lake, or ocean. The pixel size is equal to range and azimuth resolution of SAR images.

 

 

2.1.1 Range and Azimuth Resolution

The Range is the direction perpendicular to flight path of the SAR platform. The minimum distance of two objects that can be identified and it’s called range resolution (Fig.3). Slant range resolution equation can be expressed as follow :

(2.1)

where c is speed of light and τ is pulse length of SAR transmitted signal. Coefficient of ½ because the pulse is round-trip (transmitted and received) of pulse wave (Brown and Porcello, 1969). Thus ground range resolution can be formulated as follow:

(2.2)

where θ is incidence angle of satellite beam to the normal direction of the Earth surface

 

Figure 3. Geometry of slant range resolution and ground range resolution

 

The azimuth resolution refers to the ability to resolve different objects in the azimuth direction or direction of flight of SAR platform. 

  

Figure 4. Geometry of azimuth resolution

 

The azimuth resolution of a radar system is a function of the antenna length (Brown and Porcello, 1969).

From Fig. 2.4 the diffraction resolution is :

(2.3)

and half-length of ground illumination can be expressed as follow :

(2.4)

Form a synthetic aperture that has a length of . The improved azimuth resolution is :

(2.5)

The azimuth resolution is independent of spacecraft height and improves as the antenna length is reduced.