Radars are a form of echo‐location technology designed to detect and locate objects. Radars operate by transmitting radio signals anywhere between 2 MHz and 300 GHz and recording returns of the signal from reflecting objects. The information gathered about a remote object includes its range from the radar, its radial velocity, its angular direction with respect to the radar, and its size and shape. There are many applications of radar, including air traffic control, surveillance, and military operations.
The deployment of GPS shifted the focus in navigation systems research toward GPS‐integrated solutions. The advancement of computer technology shifted the focus of navigation systems of the time toward digital solutions. As time went on, the need for navigation solutions for environments where GPS is denied or unavailable became more apparent. The development of particle filters and other Bayesian estimators which took advantage of small, powerful computers allowed for processing of radar data in real time. This in turn generated a renewed interest in using Synthetic Aperture Radar (SAR) and other advanced techniques as a source of position information when GPS is unavailable. SAR meanwhile was becoming more feasible with the advancement of computer technology. Figure 42.4 shows the terrain imagery that could be generated in the 1980s by post‐processing data after the flight was complete. Today, the availability of ever‐advancing high‐performance computing platforms in embedded and smaller form factors has enabled the exploration of new options in radar navigation.