[en] Low-frequency gravitational radiation, with wavelengths reaching or exceeding interplanetary distances, and with a mean energy density rho of the order of the critical cosmological density rhosub(c), generates a frequency-shift of order Δv/v approximately equal to 10-15h0(1/108km)(rho/rhosub(c))sup(1/2) in electromagnetic signals transponded by interplanetary spacecraft at a distance l from the Earth. (Auth.)