  GPS, an excellent tool for geodesy, may serve also particle physics. In the
presence of Earth's magnetic field, a GPS photon may be transformed into an
axion. The proposed experimental setup involves the transmission of a GPS
signal from a satellite to another satellite, both in low orbit around the
Earth. To increase the accuracy of the experiment, we evaluate the influence of
Earth's gravitational field on the whole quantum phenomenon. There is a
significant advantage in our proposal. While geomagnetic field B is low, the
magnetized length L is very large, resulting into a scale (BL)^2 orders of
magnitude higher than existing or proposed reaches. The transformation of the
GPS photons into axion particles will result in a dimming of the photons and
even to a "light shining through the Earth" phenomenon.
