This study aims to experimentally verify the principle of constant velocity of light and Galilean relativity. Special relativity states that the velocity of light is constant to all observers, but there has been no study that had experimentally verified the principle of constant velocity of light as the velocity of the light source increases with respect to the observer. This paper aims to experimentally verify the principle of a constant velocity of light by using the velocity of the image data signal sent by Voyager 1 to Earth. The experiment is performed by dividing the distance of the light path between Voyager 1 and Earth by the delay time for the signal from Voyager 1 to reach Earth. If the principle of the velocity of light is correct, the velocity of the signal will be the same as the velocity of light in the vacuum. But if Galilean relativity is correct, the velocity of the signal will be the velocity of light subtracting the velocity of Voyager 1. In this study analyzing the velocity of the signal shows that the velocity of the signal does not match the velocity of light in the vacuum. Furthermore, the velocity of the signal decreases in inverse proportion to the increase in the velocity of Voyager 1, suggesting that the velocity of Voyager 1 affects the velocity of light. This phenomenon can be an example of Galilean relativity being applied to the velocity of light.