I am trying to optimise my piaware system and reading up on ppm finding conflicting information. If I run Kalibrate and get a result of -10.9ppm some are saying enter a ppm of -11. Others say that as this is an offset reslut you need to go the other way to compensate and enter +11. Any guru’s out there who actually know how the coding works and can comment?
As I understand it, kalibrate uses GSM mobile cell tower signals to determine ppm error, and since we are in a rural area with no nearby cell towers, I have calibrated all of my RTL2382u dongles on my laptop using a program such as SDR#. After plugging in the dongle and allowing plenty time for it to stabilize in frequency, I tune to a known frequency (I use NOAA radio here in the US or an FM station) and adjust PPM until the frequency is centered. Then I know the value I need to use with dump1090, for example. Apparently the offset is fairly linear so the frequency you use is not critical.
This is not a technical but a practical solution but is certainly accurate enough for ADS-B work.
ADS-B signals are so wide anyway that it doesn’t really matter if the dongle frequency is a bit off. Even 100ppm is only around 100kHz error, the signals are maybe 2MHz wide.
obj, would I be more correct in saying the gain settings are more critical than the ppm offset in respect of best SNR and also BER?