A dual mode device (110) comprising a communications block (120) and a satellite positioning block (130), each comprising respective Base Band (BB) blocks (123, 132) and crystals or frequency sources (122, 1312). The communications crystal (122) provides a cellular clock signal (FS1) to the satellite positioning block (130), which monitors the difference in the frequencies the crystals produce (FS1, FS2), using Vernier interpolation to gain a resolution greater than one clock cycle. The positioning crystal (1312) is used independent of the cellular crystal to demodulate satellite signals. The positioning BB (132) tracks the demodulated satellite signals compensating any errors caused by the positioning crystal signals (FS2) to the demodulation, using the information resulting from the monitoring. The communications crystal is capable to network based calibration and the positioning BB can compensate for the fluctuation thus caused in the cellular clock signal (FS1).