Precision timekeeping is essential for spacecraft navigation. Most missions now rely on ground-based antennas paired with atomic clocks for this purpose. Ground antennas send narrowly focused signals to spacecraft, which, in turn, return the signal. NASA uses the difference in time between sending a signal and receiving a response to calculate the spacecraft’s location, velocity and path.
Some inefficiency plagues this method -- a ground station can only track one spacecraft at a time since it must wait for the spacecraft to return a signal. A tool to enable real-time decisions has been under development by NASA’s Jet Propulsion Laboratory for 20 years.
The Deep Space Atomic Clock (DSAC) is being designed to provide accurate onboard timekeeping for future NASA missions. The DSAC would eliminate the need for two-way tracking, since a spacecraft could use a signal sent from Earth to calculate position without returning the signal and waiting hours for commands from the ground. More precise maneuvering and adjustments to unexpected situations would be supported by the availability of timely location data and onboard control. Ground stations would also be able to track multiple satellites at once near areas like Mars, crowded with NASA science missions.
The small, low-mass atomic clock prototype is based on mercury-ion trap technology. The atomic clocks at ground stations in the Deep Space Network are about the size of a refrigerator. DSAC is about the size of a four-slice toaster, and could be further miniaturized for future missions.
A test flight of the DSAC is planned in 2018 as a hosted payload on General Atomic’s Orbital Test Bed spacecraft aboard the U.S. Air Force Space Technology Program (STP-2) mission. Researchers will use the navigation signals from U.S. GPS coupled with precise knowledge of GPS satellite orbits and clocks to confirm DSAC’s performance. The demonstration should confirm that the clock can maintain time accuracy to better than two nanoseconds (.000000002 seconds) over a day, with a goal of achieving 0.3 nanosecond accuracy.
Coupling DSAC with onboard radio navigation could ensure that future exploration missions have the navigation data needed to send humans back to the moon and traverse the solar system.
Technologies aboard DSAC could also improve GPS clock stability and, in turn, the service GPS provides to users worldwide. Ground-based test results have shown DSAC to be upwards of 50 times more stable than the atomic clocks currently flown on GPS. DSAC promises to be the most stable navigation space clock ever flown.