What is a RTLSDR:
RTL-SDR (Realtek Software Defined Radio) is a type of software-defined radio that originally started as a project to develop a low-cost software radio. It is based on the RTL2832U chipset which was meant for DVB-T (Digital Video Broadcasting — Terrestrial) TV reception. However, it was discovered that this chipset could be modified to receive a wide range of frequencies, typically from 24 MHz up to 1766 MHz, making it a popular, inexpensive option for a wide range of radio enthusiasts.
These devices are USB dongles, initially intended for TV and radio reception and recording on computers. The RTL-SDR community has developed various software tools to use these dongles for a variety of applications, such as amateur radio, scanning, aviation and marine monitoring, weather satellite reception, and much more. The RTL-SDR has become popular in the hobbyist and amateur radio community due to its low cost and flexibility.