# Synchronization of Monostatic Radar Using a Time-Delayed Chaos-Based FM Waveform
## 2 Radar Systems and Stealth Counter: Overview  
Obviously, the design of radar systems mainly depends on the desired band and the purpose of applications in civil or military communications. The usual microwave radars are often used from 2 GHz up to 40 GHz, and among these are Ultra-Wide Band (UWB) radars. Meanwhile, the low-frequency radars frequently use lower bands less than 1 GHz, such as L-band (1-2 GHz), UHF, VHF, and HF bands. Other types are called LiDAR (Light Detection and Ranging) or Laser Radars. These radars use visible, ultraviolet, or near-infrared radiation to detect targets. Ultrasound and sonar systems, on the other hand, use acoustics (audio and speech) waveforms. Specifically, the relatively large antennas often use frequencies within a bandwidth of 270 MHz (as VHF (30-300) MHz) or 700 MHz (as UHF (300 MHz-1 GHz)) to achieve narrow beams that may experience interference as they propagate through the ionosphere. Small antennas, on the contrary, may be employed with weather radars in the S (2-4) GHz, C (4-8) GHz, X (8-12) GHz, K\({}_{\text{u}}\) (12-18) GHz, and K (18-27) GHz bands to produce narrow beams and good range resolution. Skywave and Ground wave radars are the two forms of Over-the-Horizon (OTH) radars, respectively. Ground-wave radars can identify targets at very long range, often between (100 and 1000) km. They also operate at low frequencies (below 3 MHz), and therefore they suffer from diffraction due to the surface of the earth. Longwave bands in HF band (3-30) MHz employ Skywave (shortwave), where the short wave depends on the ionosphere [2; 4; 5; 6; 7; 8; 9; 14]. On the other hand, the stealth counter has become an important topic in recent years to minimize the effectiveness of Radar Cross-Section (RCS) measures, and this requirement remains an open challenge in radar system design. Stealth is used to prevent detection systems under work by utilizing the electromagnetic spectrum, such as making the RCS small or through changing the shape of the aircraft, by using Radar Absorbing Material (RAM), or by utilizing the plasma which attenuates the radar energy. However, at high radar frequency, the attenuation effect of the plasma becomes greatest, while at VLFs it may be reflected by the plasma. Blessedly, most radar systems that aircraft are required to defend against operate in the X and K\({}_{\text{u}}\) frequency bands. However, the design aim of the Low Observable (L.O.) or stealthy aircraft in the X and K\({}_{\text{u}}\) bands is to reduce radar emission energy. At lower frequency bands, the RCS of L.O. aircraft must be increased, and the RAM use for the target is not so efficient in these bands. Therefore, this results in a serious interest in working on anti-stealth capabilities in radar systems [5; 6]. Notably, during the Kosovo war in 1999, for example, the old VHF (also known as low frequency) radar was utilized to shoot down an F-117 Nighthawk by Serb forces [8; 9]. Since 2013, several new radars for military and secure communications have been developed, including multitoned harmonic non-linear radar, US Navy Re-Locatable Over-the-Horizon Radar (ROTH), and OTH radar. Regardless of the target's altitude, they can identify stealthy and small targets at very long ranges. Although such radars have disadvantages, such as massive antennas, immobility, and a large quantity of clutter, they are nonetheless highly effective against stealth ships and stealth aircraft when deployed on the ground [9; 10; 14]. Additionally, the radar systems have also been linked with modern infrared for search and tracking (IRST) systems to provide anti-stealth capabilities in secure communications and the military. The IRST system actively scans the airspace for heat signatures, transmitting its IR emission, and passively detects and tracks the target based on IR emission, allowing the crew to attack without warning the target (i.e., first look then shoot).