Dual Frequency Synthetic Aperture Radar (DFSAR) is an advanced remote sensing technology that uses a radar system to transmit and receive microwave signals at two distinct frequency bands simultaneously or independently to create high-resolution images of a target area. The foundational concept of Synthetic Aperture Radar (SAR) was invented by mathematician Carl A. Wiley in June 1951 at Goodyear Aircraft Company to solve the problem of achieving high image resolution without an impractically large physical antenna. SAR uses the motion of its platform (aircraft or satellite) to synthesize a much larger antenna, or "aperture," by processing the Doppler shifts in the radar echoes.
The key mechanism of DFSAR is exploiting the difference in the penetration capability of the two wavelengths. For example, the NASA-ISRO Synthetic Aperture Radar (NISAR) mission uses L-band (wavelength of 24 cm) developed by NASA and S-band (wavelength of 9.4 cm) developed by ISRO. The longer L-band waves penetrate deeper into the surface, allowing for the study of subsurface features like soil moisture, while the shorter S-band waves are more sensitive to surface roughness and canopy structure. This dual-frequency approach provides a more comprehensive understanding of Earth's features at two different penetration levels.
DFSAR connects directly to major Indian space initiatives, such as the Chandrayaan-2 orbiter, which carries a DFSAR payload that is the first fully-polarimetric SAR to study the Moon. This lunar DFSAR, operating in L- and S-band, has been used to investigate the possible presence of subsurface ice in the lunar South Polar Region. A significant recent change is the development of the NISAR mission, which incorporates the novel SweepSAR mode of data acquisition to achieve a wide swath of 240 km without compromising the azimuth resolution, and has a high repeat cycle of 12 days for deformation studies.