are designed to transmit microwave radiation either horizontally polarized (H) or vertically polarized (V). Similarly, the antenna receives either the horizontally or vertically polarized backscattered energy, and some radars can receive both. Thus, we have four polarization combinations: HH (likepolarized) for horizontal transmit and horizontal receive, VV -(like-polarized) for vertical transmit and vertical receive, HV - (cross-polarized) for horizontal transmit and vertical receive and VH - (cross-polarized) for vertical transmit and horizontal receive. Since various objects modify the polarization of the energy they reflect to varying degrees, the mode of signal polarization influences how the objects look in the resulting imagery.
SAR wavelengths and polarization combinations with respect to oil spill detection on the sea surface will be discussed in chapter 3 and 4.
2.2 Orbits and Coverage
Remote sensing satellites are often placed in polar sun synchronous orbits. A satellite in a polar orbit passes above or nearly above both poles of the planet on each revolution. It therefore has an inclination of (or very close to)90◦ to the equator. A satellite in a polar sun synchronous orbit will pass over a given latitude at the same time every day (different for ascending and descending passes). A sun synchronous orbit also makes it possible to operate at a constant angle between the satellite solar panels and the sun. This is why many radar satellites are put in sun synchronous orbits.
RADARSAT-1 is operating in an orbit 798 km above the Earth, circling from pole to pole in a sun-synchronous orbit with an inclination of 98.6◦. One orbit takes 100.7 minutes and the satellite has a 24 day repeat cycle [47]. ENVISAT carries 9 instruments, including the ASAR. ENVISAT also has a sun-synchronous orbit at an altitude of 800 km with an inclination of98◦. One orbit takes 101 minutes and ENVISAT has a 35 day repeat cycle [48].
Figure 2.3 and figure 2.4 show the number of images available for the North Sea in July 2004 for ENVISAT and RADARSAT-1.
Since orbit track spacing varies with latitude, the density of observations and the revisit rate are significantly greater at high latitude than at the equator. Coverage is also affected by the different swath widths. Current SAR systems are able to operate in different modes with different coverage and spatial resolution. Generally, high resolution modes cover smaller areas.
One method for increasing swath width is to use so-called ScanSAR imaging, where a swath widening (in range) can be achieved by the use of an antenna beam, which is electron-ically steerable in elevation. (The scene extension in azimuth is in principal only determined by the length of the observation period). Radar images are created by sequentially synthe-sizing images from the different beam positions. The area imaged by the different beams form sub-swaths. The principle of ScanSAR is to share radar operational time between two
or more separate sub swaths in such a way that full image coverage is obtained for each.
2.2. ORBITS AND COVERAGE 9
Figure 2.3: Coverage for RADARSAT-1 ScanSAR Narrow (300 km wide footprint) for the North Sea in July 2004 for one repeat cycle (1 July-24 July). It is assumed that all possible images are available (which is not usually the case in practice). Source: [43].
Figure 2.4: The number of images available for the North Sea in July 2004 for ENVISAT ASAR Wide Swath Mode (WSM) (400 km wide footprint). It is assumed that all possible images are available (which is not usually the case in practice). Source: [43].
Chapter 3
Scattering Mechanisms
In SAR imaging, there are several important factors that decide how strong a signal is reflected back from the target area. These factors can be divided into satellite system factors:
• the radar beam incidence angle
• the radar wavelength
• the polarization of the radar and ground surface factors:
• the roughness of the surface
• the geometrical structure of the surface
• the dielectric properties of the surface
• the wind speed
• the angle between the radar beam and the wind
The intention of this chapter is to provide an overview of target scattering mechanisms as a fundament for the discussion of oil spill detection in the following chapters of this thesis.
3.1 Surface Scattering
For flat terrain, the local reflection angle is the same as the incidence angle as shown in figure 3.1 a). Most of the incident energy will be reflected away from the sensor, resulting in a very low return signal. Rough surfaces will scatter incidence energy in all directions and
11
Figure 3.1: Scattering mechanisms. a) Reflection off a smooth surface. b) Scattering off a rough surface. (Adapted from [3]).
return a significant portion of the incident energy back to the antenna. This is illustrated in figure 3.1 b).
On the ocean surface it is the waves that make the surface rough. Whether the surface is perceived rough or not, depends on the wavelength of the SAR.
3.1.1 Bragg Resonance Model
The ocean surface is known to contain a spectrum of waves from short ripples of a few millimetres to waves hundreds of meters long. However, it is generally accepted that the dominating mechanism at work to support the backscattering is a type of Bragg resonance.
The particular application of the Bragg resonance model to the ocean surface, which is a complex summation of a wide spectrum of different wavelengths, requires the assumption that the Bragg mechanism is able to select just those waves that are in resonance. In terms of the ocean wavelength, λw, this means that:
λw = nλr
2sinθ, n= 1,2, ... (3.1)
defines the wavelength of the Bragg-selected waves. θ is the incidence angle and λr is the radar wavelength. (The dominant return will be for the wavelength where n = 1 [8]).
Note that to be selected by the resonance, the Bragg waves need to propagate toward or away from the look direction of the radar antenna. Equation 3.1 implies that the surface waves which influences the radar backscatter are those of comparable wavelength to the microwaves. It is the short gravity and capillary-gravity waves to which the radar responds directly. The Bragg condition also implies, for a given SAR, that the resonant surface waves will be shorter at more oblique incidence angles. This also relates to the general observation that the backscatter for a given sea state decreases with increasing incidence angle [36]
(the backscattered radar power is proportional to the spectral energy density of the Bragg waves and the spectral distribution decays at shorter wavelength), as will be discussed in section 3.3.
3.2. VOLUME SCATTERING 13