Development of GMTI and GMTIm Techniques
The concept of radar can be traced back to the famous microwave experiment of German physicist Hertz at the end of 19th century. The principle of radar is to detect and locate the target by using the scattering of microwave. Radar was first applied in the World War II in the airplane and warship surveillance and detection. The range of a target is acquired by the time-delay of the received signal, and the direction of a target is acquired by the scan of the antenna [61-63]. In conclusion, the detection of moving targets are the origin function of radar.
As an imaging radar, SAR was firstly designed with only the function of imaging. The signal used in the stationary scene imaging of SAR is called the clutter in the traditional radar. However, the combination of radar imaging and moving target processing can acquire more information of the observation area.
With the advantage of all-time and all-weather, SAR system can provide both the high resolution radar image of the observation area and the location of small moving target such as military vehicles, mobile missile launcher, and artillery.
GMTI is an important function of modern radar system. This function was firstly applied in AN/APG-76, which is capable of real-time imaging of stationary scene and indicating moving targets [64, 65]. The U.S. land forces and air forces set up the Joint STARS, which played an important role in the Iraq War . Lynx system is boarded on I-GNAT, which can provide high resolution images and indicate moving targets with the slowest velocity of 2.98 m/s within the scope of 270°. HiSAR system is boarded on Global Hawk, which has both WAS mode and stripmap SAR/GMTI mode. The HiSAR system has a working range of 20110 km, observation swath of 37 km, and has a resolution of 6 m in the stripmap SAR/GMTI mode . Other typical airborne SAR systems with GMTI functions include MiniSAR, TESAR on Predator, EL/M2055, EL/M2060P, EL/M2040T, PAMIR, ASTOR. GMTI is also used on spaceborne SAR. The most famous SAR satellites are Lacrosse series of the U.S. and the RadarSat-2 of Canada. However, spaceborne SAR system cannot detect slow moving targets, which makes it not suitable for GMTI.
GMTIm is to obtain a 2-D high resolution of the moving target after it is detected. With the image of the target, the moving target can be further recognized and classified. However, in the existence of unknown motions, moving targets are smeared and dis-located in the stationary SAR image. GMTIm algorithms have to accurately estimate the motion parameters and eliminate the interference of system errors . Many researchers have devoted to research the GMTIm algorithms, but few have satisfying performances in real data processing.