The propagation of light - or, more general - photons is influenced by a gravitational field. This leads to a run-time delay of electromagnetic signals in the gravitational field of the sun, when they are reflected from planets or satellites. The delay becomes all the larger if the signal gets closer to the mass and the larger the mass is.
This effect was first proven by Shapiro in experiments where a radar signal was sent near the mass of the sun and reflected from Venus. Later also satellites in the vicinity of Mars were used as reflectors (e.g. Mariner 6 and Mariner 7), and at last in summer 2002 the satellite Cassini.