Today, the global positioning system (GPS) has become the enabling technology of the communication, navigation, surveillance, and air traffic management (CNS/ATM) systems. However, the transmitting power of a GPS signal is extremely low and thus vulnerable to intentional or unintentional radio frequency interference (RFI). Therefore, this paper proposes a terrestrial GPS RFI monitor to identify a possible RFI source to reduce the impact on aviation operation. Each GPS RFI monitor includes a software receiver with either an automatic gain control (AGC) based or a modified energy based RFI detector in order to detect possible interference. The GPS RFI monitor also includes a camera to record the corresponding video stream simultaneously with a RFI event. If a GPS RFI device is present, the RFI monitor and camera will be triggered. Once the detector has been triggered, the associated intermediate frequency (IF) data and video stream are saved. This makes it possible to identify the characteristics of the possible GPS RFI device and to determine the time as well as the impact of the RFI event. In this paper, the recorded GPS IF signal is first processed to draw the time-frequency figures to evaluate the characteristics of the possible GPS interference, and then processed by a software-defined radio (SDR) receiver to assess the impact on positioning. The results from previous studies are compared with several RFI detectors whose detection schemes are presented in this paper as well.