SAR Concepts

Chapter 2

SAR CONCEPTS

‘Synthetic aperture length’ is the distance travelled by the sensor while the target is being illuminated by the radar beam as shown in fig 2.1. It can also be described as the distance travelled by the radar placed on the platform which is moving at a constant velocity.

The radar transmits a series of pulses towards the target or the target in the area of interest and receives the reflections from the target. An array of all the received reflections during the flight is stored in the memory which is known as the raw SAR data. After the raw data is collected, signal processing is done on the raw data focusing on the target to produce a high resolution image of area of interest. Fig 2.2 shows the mode of operation of Synthetic aperture radar.

Consequent sections in this chapter will explain the raw data collection and processing as a two dimensional problem, which includes the transmission and reception of EM waves, and also the processing steps involved to transform raw SAR data to an image.

2.1 SAR as 2D problem

The output SAR signal or the raw SAR data before signal processing is a two dimensional signal with the two dimensions as:

Range direction (fast time)

Azimuth direction (slow time)

The EM wave which are transmitted from the sensor’s transmitter to the target or point scatterer is a function of time, say T_x (τ). The reflected wave received at the sensor’s receiver is a scaled and shifted version of the original wave.

R_x (τ)=AT_x (τ-τ_0)

Now assume this function R_x (τ) is also a function of another variable say η, this results in a function R_x (τ,η). In SAR η is defined as the azimuth time or slow time and is controlled by the motion of the platform. The azimuth direction ...

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... used in the range direction, the azimuth resolution is the inverse of the bandwidth multiplied by 0.866, p_a=0.866/(BW_a ) where the azimuth bandwidth is represented by BWa. In distance units this is multiplied by the speed with which the beam moves on the ground and the squint angle at the beam centre. This result in

ρ_a=(0.866V_g cosθ_(r,c))/(BW_a )

According to Cumming and Wong [1], , the azimuth bandwidth is given as

BW_a=0.866(2V_s cos⁡〖θ_(r,c) 〗)/L_a

The azimuth resolution calculated here is indeed the cross range resolution but since low squint is assumed, the cross range and azimuth vectors are assumed parallel; the cross range vector is the direction perpendicular to that of the radar's line of sight [1].

The resulting azimuth resolution of the SAR system is given by [1]

ρ_a=L_a/c

Where L_a is the antenna length in azimuth direction.