FlightAware Discussions

Understanding RTL_TEST -P results..

OK, so I’ve run RTL_TEST -P on both of the dongles that I have on hand. Can someone explain the results and what I am suppose to do with them to correct any error factor?

Dongle #1 tested for 30 minutes.
cumulative PPM: ~87

Dongle #2 tested for 30 minutes.
cumulative PPM: ~92

And in both case when I exited, Samples per million lost: 0

Much thanks in advance, Tom

You’ve done it right. Those are your PPM correction values. Plug them into dump1090. If you’re using dump1090-mutability, you can use ‘sudo dpkg-reconfigure dump1090-mutability’ or just edit /etc/default/dump1090-mutability and plug in the numbers.

Thank you very much, Tom.

Where to put these numbers if I use piaware image?

I believe I added the argument --ppm 87 to fadump1090.sh file located in /etc/init.d directory. Sorry, I switched to dump1090-mutability, so I cannot confirm or post a copy of the edited line in the file, which would have been more helpful. Perhaps someone else will chime in as well.

Thanks, it worked.

I’ve read somewhere that the proper value to pass to dump1090 is the negative of the value reported by rtl_test, so that if it reports 76, you should use -76 with dump1090. Can anyone verify?

Ran rtl_test on several dongles. Results varied, most were negative, one was positive. I plugged in the values resulted from rtl_test. Never tested the opposite signed value tho’.