  In many applications of frequency estimation, the frequencies of the signals
are so high that the data sampled at Nyquist rate are hard to acquire due to
hardware limitation. In this paper, we propose a novel method based on subspace
techniques to estimate the frequencies by using two sub-Nyquist sample
sequences, provided that the two under-sampled ratios are relatively prime
integers. We analyze the impact of under-sampling and expand the estimated
frequencies which suffer from aliasing. Through jointing the results estimated
from these two sequences, the frequencies approximate to the frequency
components really contained in the signals are screened. The method requires a
small quantity of hardware and calculation. Numerical results show that this
method is valid and accurate at quite low sampling rates.
