Passive RF sensors rely on antennas to receive and computers to analyze RF signals associated with communications between the GCS and the
UAV. Systems that use passive RF sensors as their primary sensor are sometimes referred to as ESM. Passive RF sensors analyze the radio signatures and modulations specific to UAS signals and are capable of identifying certain UAS models and manufacturers as well as locating the signal’s transmission origin–the UAV and/or the GCS.

Most C-UAS that rely primarily on passive RF sensors use libraries of known UAS radio signatures and compare detected signals to those in the library in order to classify or identify UAS. Signature libraries may be periodically updated to include additional UAS signatures and update existing signatures. Passive RF sensors can employ several different signal-processing methods to help locate the source of a UAS-associated signal. The more common methods are:

• Direction Finding (DF)
• Received Signal Strength Indicator (RSSI)
• Time Difference of Arrival (TDOA)
• Frequency Difference of Arrival (FDOA)

Reference- DHS Counter-UAS Technology Guide