Radar Imaging of the Ocean Waves

Mikhail B. Kanevsky

• Publisher: Elsevier Science
• Year: 2008
• ISBN:
  9780080932514
• Topic: Scienze fisiche
• Subtopic: Oceanografia

This book is dedicated to studying the ocean with radar tools, in particular, with space radars. Being intended mainly for the scientists preoccupied with the problem (as well as senior course students), it concentrates and generalizes the knowledge scattered over specialized journals. The significant part of the book contains the results obtained by the author.

• Systematically collects and describes the approaches used by different laboratories and institutions
• Deals with the physics of radar imagery and specifically with ocean surface imagery
• Useful for students and researchers specializing in the area of ocean remote sensing using airborne or space-borne radars, both SAR and RAR

Chapter Synthetic aperture radar

SAR images the scattering surface from large distances, particularly, from space when the target resolution cannot be achieved by enlarging the physical aperture of the radar antenna. Here we are speaking about resolution over azimuthal coordinate, as the range resolution just as with RAR can be reached by deploying either rather short pulses, or chirp pulses with further compression (see Chapter 1). Basic concepts of SAR Earth surface imaging are covered in Chapter 1; this chapter is devoted to the specifics of ocean surface random movement.

6.1. Preliminary Estimates

We have mentioned above that SAR as an ocean surface probing tool is not in any way equivalent to incoherent side-looking radar having hypothetically super high resolution. To obtain preliminary estimates of roughness influence on SAR imaging of the ocean surface we consider expression (1.4):(6.1)

where a(t) and a^SAR(t) are the complex amplitudes of backscattered field and of SAR signal, respectively; and Δt is the integration time (probing geometry is shown in Figure 1.1).

Having still the notion of SAR as a linear system it is easy to prove that the roughness-associated phenomena are the shift and expanding of the resolution cell.

In expression (1.8) for the complex amplitude of backscattered field(6.2)

we assume

, where v^rad is the radial (i.e., along the radar-look direction) component (against the probing direction) of point target (scatterer) velocity; the component is held positive if directed to radar and negative if directed from it. Then, if v^rad = const., then we get for SAR signal intensity:(6.3)

i.e. the point scatterer image is shifted against y⁰ by the range Rv^rad/V in either direction depending on the sign of v^rad. Thus, the image shifts along SAR carrier flight direction as the scatterer approaches radar and against as the scatterer moves away from radar.

We separate from Eqn. (6.2) the expressions for the Doppler frequency of the signal backscattered from the point scatterer:(6.4)

Again consider Eqns (1.13) and (6.3), and notice that in both the cases due to the matched filtration, the signals returned from the point scatterer focus in the image plane in the vicinity of the point with zero Doppler frequency. It means that the images of two nearby point scatterers having different radial velocities can be spaced apart rather far, specifically over the distance (R/V)Δv^rad, where Δv^rad is the difference between the radial velocities of the two scatterers. At the same time, the images of two or more spaced-apart scatterers can turn out to overlap in the image plane; this effect, as we shall see later, plays a significant role in SAR imaging of the ocean waves.

Let there be n point scatterers with variously valued and directed velocities as well as random and mutually non-correlated values of the reflection coefficient in the resolution cell. Then we substitute(6.5)

and obtain for the average intensity of SAR sign...