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 _ { 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 _ { 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 .