# A Review of Wireless Positioning Techniques and Technologies: From Smart Sensors to 6G
## 3 Wireless Technologies
### Radio Detection and Ranging (RADAR)  
The RADAR relies on the radio waves reflected by an object for remotely detecting its presence and speed. Its performance depends on the frequency, polarization, target properties, and distance from the target. The radio waves are commonly affected by dielectric objects, such as the Earth's surface, plants and water. Although it is commercially used for detecting aircraft, ships, spacecraft, etc., its coverage range depends on the minimum received power and visibility (RADAR horizon is defined by the Earth's curvature and the height of the target object). The object range is estimated by the time difference of the echo signal and the corresponding reference. For precise time measurements, various factors must be taken into consideration, such as the pressure and moisture. Additionally, for tracking, RADAR utilizes the Doppler shift, i.e., the change in frequency of a wave, with respect to the observer movement relative to the source. The newer and more advanced RADAR systems utilize MIMO antennas for angle resolution, so the angle-based measurements can also be utilized [87].  
Furthermore, the synthetic aperture RADAR (SAR) is used for the Earth remote sensing from airborne and satellite platforms by providing high-resolution 2D images not affected by daylight or weather conditions. Similar to the RADAR system, the SAR satellite transmitted pulse interacts with the Earth surface and only a fraction is back-scattered to the antenna [88]. SAR achieves finer spatial resolution by utilizing the motion of the RADAR antenna over a target region. The resolution is proportional to the size of the antenna aperture and inversely proportional to the distance between the illuminated area and the sensor. Centimeter or even millimeter accuracy can be achieved by comparing two or more complex RADAR images with slightly different positions or acquired at different times [88]. Moreover, the signal strength is affected by the ground properties, the distance between the antenna and ground, and the satellite view angle. Geometric distortions, such as foreshortening, layover, and shadows are also possible, depending on the satellite orbit, configuration, and acquisition parameters [89].  
Moreover, the frequency modulation continuous wave RADAR (FMCW RADAR) is a special type of RADAR sensor which radiates continuous transmission power similar to the simple continuous wave RADAR (CW RADAR). Their main difference is that FMCW RADAR can change its operating frequency during the measurement; thus the transmission signal is modulated in frequency or in phase. It is capable of measuring very small ranges to the target, as well as providing very high accuracy of range measurements [90], especially in harsh environments.  
In addition, RADARs have a variety of applications, including underwater. The echo sounding, or bathymetry, is the use of sonar for ranging, normally for determining the depth of water, and sound navigation and ranging (SONAR) is commonly used for exploring and mapping the ocean because sound waves travel farther in the water than RADAR and light waves. The multibeam sonar sends out multiple simultaneous sonar beams (or sound waves) at once in a fan-shaped pattern and is used for identifying seafloor types and morphology, whereas the single beam uses only one transducer to map the ocean floor [91].