A framework is presented for exploiting the frequency stability of non-GNSS signals to extend the coherence time of inexpensive GNSS receiver clocks. This is accomplished by leveraging stable ambient radio frequency signals, called “signals of opportunity,” to compensate for the frequency instability of the reference oscillators typically used in inexpensive handheld GNSS receivers. Adequate compensation for this frequency instability permits the long coherent integration intervals required to acquire and track GNSS signals with low carrier-to-noise ratios. The goal of this work is to push the use of GNSS deeper indoors or into environments where GNSS may be subject to interference.