During the launch and near-earth phase, continuous radio tracking data were acquired from two Viking spacecraft as they receded from earth. Analysis of this data yielded these values for the geocentric gravitational constant GM: 398600.5 ¿ 0.2 km3 s-2 for Viking 1 and 398600.65¿0.2 km3 s-2 for Viking 2. These values include the atmosphere of the earth and are consistent with the currently adopted speed of light (299792.458 km s-1).