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)