  The influence of the laser frequency drift on the operation of
phase-sensitive optical time-domain reflectometry ($\Phi$-OTDR) systems is
considered. Theoretical results based on a new numerical $\Phi$-OTDR model
demonstrating the influence of the laser frequency instability on a signal are
reported. This model is verified based on experimental data. It has been used
to calculate the signal-to-noise ratio (SNR) of the system for different
parameters of the laser source stability. As a result, quantitative
requirements for lasers used in $\Phi$-OTDR systems are formulated.
