Tagged: close call

Open Source Close Call Monitor for RTL-SDR

Thank you to Fabian for writing in and sharing with us his newly released RTL-SDR close call monitoring software called "rtl-sdr-close-call-monitor". The software is open source and written in Python. It's main purpose is to log any signal peaks that appear within a defined frequency range. Over on the GitHub Fabian explains the software:

These scripts use an RTL-SDR device to detect peak signals on a user specified frequency range. The scripts can also make an automatic blacklist so that different sources of RF noise won't cause continuous false positives. There are two scripts provided as examples. The scripts can be used to monitor certain frequencies for a wide range of purposes such as:

  • In criminal investigations, a close-call RF signal monitor can be used to detect and track communication signals used by criminals. This can help law enforcement agencies gather intelligence and evidence, and even prevent future crimes from being committed. By analyzing the frequency and strength of signals emitted by communication devices, a close-call RF signal monitor can provide valuable insights into the movements and activities of suspects, allowing investigators to piece together a timeline of events and make informed decisions about how to proceed with a case.
  • In military operations, a close-call RF signal monitor can be used to identify and track enemy communication signals, providing valuable intelligence for strategic decision-making.
  • In scientific research, a close-call RF signal monitor can be used to collect and analyze data related to wireless communication systems, providing valuable insights for thesis projects and other research studies.
An excerpt of Fabian's close call monitoring Python code.

Creating an RF Proximity Alarm (Close Call) with an RTL-SDR

“Close Call” is a feature that some radio scanners have which notifies the user when there is a radio transmitter that is in the near vicinity (such as from a police radio). It works by detecting the strength of signals from near field emissions, and it requires a strong RF signal to trigger.

Over on the ar15.com forums, user seek2 wanted something similar to the “close call” feature, but didn’t want certain transmissions like APRS signals from hams driving by to set it off. He also didn’t want to be restricted to near field emissions, rather he wanted something that acted more like a squelch that would activate for strong signals only.

To implement this seek2 used an RTL-SDR dongle, together with the rtl_power spectrum scanning software. He outputs the signal strength data generated by rtl_power to a CSV file which is then piped into a tail -f terminal command in Linux which simply outputs the latest lines of the CSV file as it updates in real time. Then he uses a simple Python script to monitor the output and to set off an alarm and report strong signals when it see’s them. His script is also used to filter out reports from strong unwanted signals like APRS.

Below is a video showing an example of Close Call working on a Uniden hardware radio scanner for reference.

Uniden CloseCall© What is it? How does it work? How well does it perform?