  The global positioning system (GPS) is composed of thirty one satellites
having atomic clocks with $10^{-15}$ accuracy on board and enables one to
calibrate the primary standard for frequency on the ground. Using the fact that
oscillators on the ground have been successfully stabilized with high accuracy
by receiving radio waves emitted from the GPS satellites, we set a constraint
on the strain amplitude of the gravitational wave background $h_{\rm c}$. We
find that the GPS has already placed a meaningful constraint, and the
constraint on the continuous component of gravitational waves is given as
$h_{\rm c}<4.8\times 10^{-12}(1/ f)$ at $10^{-2}\lesssim f \lesssim 10^{0}$ Hz,
for stabilized oscillators with ${\Delta \nu}/{\nu}\simeq 10^{-12}$. Thanks to
the advantage of the Doppler tracking method, seismic oscillations do not
affect the current constraint. Constraints on $h_c$ in the same frequency range
from the velocity measurements by the lunar explorers in the Apollo mission are
also derived.
