Preliminary comparison between GPS and two-way satellite time transfer

The time scales of the US Naval Observatory in Washington, DC, and the Nation Bureau of Standards in Boulder, Colorado, were compared during the same time periods by the common view mode using the satellites of the Global Positioning System (GPS) and by the two-way mode using a US domestic communication satellite. Data collected over 3-month period showed residuals from a linear regression of 10 ns for the two-way technique and 30 ns for common-view GPS. The two-way technique achieved better than 500 ps of precision with less than 2 min of data.<>