Like most waves in the electromagnetic spectrum, radio waves tend to reflect from various objects. This can be frustrating for someone trying to use things like GMRS or LoRa radios in dense cities, for example, but these reflections can also be exploited for productive uses. , the best known is radar. Radar has many uses, including weather forecasting and various military applications. Some software defined radio tools also allow you to track aircraft in real time using radar at home, like this DIY radar system.
Unlike active radar systems, which use a specific radio wave source and look for reflections, this system is a passive radar system that uses radio waves already present in the environment to track objects. A reference antenna is used to receive the target frequency, and in this installation a 9-element Yagi antenna is configured to receive the reflections. The radio waves each antenna receives are sent through a computer program that Yagi compares the two to identify the reflections of the reference radio signals he receives.
Although such systems do not contain high-performance active elements, they still require significant amounts of computing resources and some skill in identifying the data displayed by the software. [Nathan] alias [30hours] gives a fairly thorough overview of systems that can also recognize helicopters from other types of aircraft, and also use ADS-B surveillance systems as a health check. Radar can also be used to monitor other vehicles, like this 24 GHz radar module installed in modern passenger cars.