Kalibrate, or kal, can scan for GSM base stations in a given frequency band and
can use those GSM base stations to calculate the local oscillator frequency
offset.

See http://thre.at/kalibrate 

Copyright (c) 2010, Joshua Lackey (jl@thre.at)

modified for use with rtl-sdr devices,
Copyright (c) 2012, Steve Markgraf (steve@steve-m.de)