Historical Evolution of Imaging Radar Sensors
Imaging radar is an application of radar which is used to create two dimensional images, typically of landscapes. It uses an antennna and digital computer storage tapes to record its images.
An imaging radar sensor is usually an active sensor, operating in the electromagnetic spectrum range of 1mm — 1m.
The beginning
Development of radar work began in the 1930s. However the idea of imaging radar reaches back to the patent on synthetic aperture radar (SAR) by John Wiley in 1954. From the early days of SAR when imaging was performed using photographic film technology, to modern times where enormous images can be formed on a single computer in practically no time with exquisite accuracy and resolution, there have been tremendous advances in the field. These advances have been, to some degree, motivated by the increasing availability of high-quality SAR data from the ever-expanding fleet of international airborne and space borne SAR systems in both the civilian and military sector.
The concept of SAR was recorded later on in 1951 by Wiley followed by the first SAR operation which took place in 1952. It was in 1974 when the concept of SAR interferometry for Earth observation was first proposed. Continuous experiments were conducted with airborne SARs until 1978 when the very first space borne SAR for earth observation on board the SEASAT satellite was launched into orbit. Although it suffered a short lifetime of 106 days, SEASAT-SAR was the pioneering mission that has led to SAR technology to both the present and the future status.
In 1991, the second space borne SAR following SEASAT –SAR was ERS-1 SAR. Within the 13-year interval between these two space borne SARs, tremendous work and effort was still being made to develop and experiment new techniques with airborne SARs and Shuttle Imaging Radar (SIR) series. The SIR-A mission took place in 19841 with an L-band HH-polarization SAR on board similar to SEASAT-SAR. The SIR-B mission later followed in the year 1984 with SAR operating at the same frequency and polarization as SIR-A, but varying incidence angles by a steered antenna.
This SIR missions continued and in 1994 SIR-C/X-SAR was launched in orbit. It was the first to operate at multi-frequency X-, C- and L-bands with a full polarimetric mode.
In 2000, the Shuttle Radar Topography Mission (STRM) carried X- and C-BAND main antennas and a second outboard antenna separated by a 60 m long mast. This interferometric system produced a digital elevation model (DEM) of approximately 80% of the land.
The heaviest satellite carrying SAR was ENVISAT which weighted approximately 8.2 tons; the Japanese ALOS was the second-heaviest with 3.85 tons. The main reason for the heavy weight is that the satellite carries many sensors on board. ENVISAT (ceased its mission in May 2012), for example, carried 10 optical/infrared and microwave instruments including ASAR (Advanced SAR). ALOS (ceased its mission in May 2011) carried PALSAR and two optical sensors.
Cosmo-SkyMed, TerraSAR-X, and SAR-Lupe are “sole-SAR” satellites, and as such, their weights are considerably reduced. As for the sole-SAR satellites of KOMPSAT-5 (Korea in 2012), Sentinel-1 (ESA in 2013), ALOS-2 (Japan in 2013), and AstroSAR-Light (ESA, in TBD), this trend may well continue in the future space borne SAR missions
Where are we now?
There has been an increase in the number of space borne SARs launched recently and further missions are being planned. The trend is that spatial resolution is becoming finer and the conventional single-polarization mode is becoming dual or full polarimetric modes. SAR systems are now capable of providing resolutions comparable to optical systems while operating in all weather and times, which are features of key importance for many applications.
References
(Bürgmann, RolandBürgmann, R., Rosen, P. A., & Fielding, E. J. (2000). Synthetic Aperture Radar Interferometry to Measure Earth’s Surface Topography and Its Deformation. Annual Review of Earth and Planetary Sciences, 28(1), Rosen, & Fielding, 2000)
(Franceschetti, of Photo-optical Instrumentation Engineers, of the European Communities. Directorate-General for Science Research, & Development, 1994)
(A. Ausherman, Kozma, L. Walker, M. Jones, & Poggio, 1984)
