The Science of Synthetic Aperture Radar

February 1, 2006 By: Adrian Bohane

MDA Geospatial Services International's Adrian Bohane explains the technology behind — and benefits of — SAR sensors.

Synthetic Aperture Radar (SAR) satellites are lauded for their ability to provide all-weather, day-and-night imaging, an important capability for a variety of end-user applications. The evolution of progressively more advanced SAR sensors has elevated the technology to the point where, today, SAR data form the backbone of many operational programs, including ice mapping, oil monitoring, crop monitoring, ship detection, and surveillance of illegal fisheries.

Although its traditional benefits have moved radar applications far beyond the experimentation of a select few, the capabilities of the next generation of SAR satellites will far exceed the ability to peer through clouds. In addition to offering high-resolution imagery, the new SAR missions (RADARSAT-2 [see Figure 1], COSMO-Skymed, ALOS, and TerraSAR X) will provide highly advanced polarimetric datasets — a feature that many expect will have a much greater impact on practical applications.

Figure 1. The RADARSAT-2 satellite, set for launch later this year. (All images courtesy of MDA Geospatial Services International)

SAR polarimetry and its applications are still relatively new in the satellite industry. But with C-band and X-band SAR polarimetric missions on the horizon, the interest to discover just how beneficial this capability can be has spawned a number of studies and produced some very promising results.

Polarimetry Defined

Simply put, radar polarimetry depends on the:

1. Shape and orientation of the radar wave (relative to the Earth's surface)
2. Scattering of the radar wave by the Earth's surface
3. Reception of the scattered wave by an antenna.

SAR satellites continuously emit pulses of energy (in the form of radar waves) in a predetermined polarization state. For most sensors, the state of the wave is either horizontal (H) or vertical (V). Transmitting a wave of either polarization can generate a backscattered wave with a variety of polarizations. Microwaves transmitted by RADARSAT-1, for example, propagate in a horizontal plane, and although the waves are scattered in different polarizations based on the objects on the ground, the SAR sensor only detects the amount of energy that comes back in a horizontal plane. Therefore, it is an HH-polarized, or like-polarized, SAR satellite. Conversely, the ERS SAR sensors are vertically polarized, meaning radar waves are emitted vertical to the Earth's surface (see Figure 2).

Figure 2. For most sensors, a wave's state is either horizontal or vertical.

The advantage of polarimetric SAR sensors over the like-polarized systems is their ability to send and receive in H or V polarization states (for example, one can send an H-polarized wave and receive a V-polarized wave). The basic types of polarimetry are single-, dual- and quad-polarization. Configurations include:

1. HH or VV (single-polarized)
2. HH and HV, VV and HV, or HH and VV (dual-polarized)
3. HH, VV, HV, and VH (quadrature-, or "quad pol," polarization).

Dual-polarized sensors transmit in one state (for example, the H-state) and receive in two states (for example, H and V). This generates two images — one will be HH and the other HV (see Figure 3).

Figure 3. ENVISAT cross-polarized (HV) and co-polarized (HH) images. ENVISAT is a dual-polarized sensor.

From an information perspective, different scattering mechanisms (which are based on the polarization state of the scattered wave) imply different geophysical/biophysical properties of the Earth's surface. In quad-polarized imaging mode — a feature offered on RADARSAT-2 — the first radar pulse might be H, with reception of H and V. On the next pulse, however, V is transmitted, with subsequent reception of V and H. The process of alternating H and V transmit pulses, combined with reception of H and V on every pulse, produces four images: HH, HV, VH, and VV.

Unique to quad-polarization is the SAR sensor's ability to measure both the magnitudes and the phase difference between the channels, enabling users to synthesize any polarization they desire. Instead of just horizontal and vertical, for example, users could synthesize a wave at 45 degrees. By controlling the polarization of the incident wave and measuring the full polarization properties of the backscattered wave, users can tailor acquisitions to more specifically suit the problem they are trying to solve — whether it be to distinguish and map crop types, measure soil moisture, identify and map tree species, detect ships, monitor coastal zones, or determine and map sea ice.