(Modified from https://github.com/khaytsus/direwolf-init)

trunk-recorder PPM calibration

Problem: RTL-SDR sticks the PPM varies as the temperature changes, and decoding P25 requires the PPM to be pretty close, probably within 2-3 or you'll have issues decoding.

Solution: Using a chosen frequency as a source, I use rtl_power to find the best signal and then figure out what the correct PPM is based on that, and use that new PPM.

Setup a cron job every hour to run ./get_ppm which will run the calibration script and update the trunk-recorder config file listed within the script with the new PPM value.

Files:

• findppm.pl - Perl script which determines the best PPM
• get_ppm - Script which updates ppm

Edit get_ppm with the path to your config file, and the reference frequencies / device IDs to use.