Intro:
Some of the Hackrf board use a bad performance crystal, it will cause some program can't work---such as GNSS(GPS) simulation.  Most GPS receiver can capture the satellite
when the doppler within 10KHz, but 10ppm bias of the crystal will cause  15Khz doppler.  And my hackrf board uses a 16ppm crystal :(  normal
Of course, I can replace the crystal or use a external clock. But for most people, add a ppm correct function to hackrf_transfer is useful.
How to use:
1, measure your crystal's error
You can use a precision  counter to measure it.
Connect the hackrf's "clock out" to the counter, get the frequency Fh, then ,
ppm = (Fh - 10000000)/10 
 
You can use a standard clock source such as OCXO, TCXO, or Atom Clock, and a counter
to measure the crystal error too.

2, add an argument "-C ppm " when you use hackrf_transfer.

Eg. , your  crystal error is -8ppm ( In other words , your clock is slower than the standard clock), then you should add "-C -8" follow the hackrf_transfer command.

Hope you can understand my poor English:)

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Thanks to Michael Ossmann;
Thanks to Jared Boone ;
Thanks to Benjamin Vernoux ;
Thanks to GPL.