Atomic Clock project Will Bring Better Navigation, More Data for Exploration Missions

By Christopher Paul

Laboratory Mock-up of the Deep Space Atomic Clock

This is a laboratory mock-up of Deep Space Atomic Clock. It does not contain flight components, but is a rough representation of what DSAC will look like when it flies. Photo credit: NASA/JPL

A new NASA technology demonstration mission, the Deep Space Atomic Clock, or DSAC, promises to improve data gathering and navigation for probes sent to explore the solar system.

Most current spacecraft carry Ultra Stable Oscillators, bits of quartz that vibrate at a specific frequency when current is passed through them. But these oscillators have an inherent drift, making the spacecraft's clock too slow or fast. But scientists at JPL have been working for years to improve deep space timekeeping.

The current gold standard for timekeeping is the atomic clock, which measures the time it takes an atom to go from one energy state to another, a time which is known and constant. The GPS system flies with cesium-rubidium clocks that also have an extremely small drift, much smaller than any quartz-based clock.

But NASA's new project, DSAC, had developed a mercury-ion atomic clock with a drift of only 1/100 of that of the GPS constellation.

But why would time-keeping be important for exploratory spacecraft? There are several reasons.

One is that it improves spacecraft navigation. If the spacecraft knows exactly what time it is, it can execute course-correction maneuvers autonomously if they are programmed ahead of time. Currently, spacecraft must be commanded to execute such maneuvers from the ground.

Exact clocks would also improve the efficiency of the Deep Space Network, NASA's only method of keeping in touch with deep space exploration craft. Currently, the DSN must command a spacecraft to begin transmitting telemetry once it is in a position to do so. This decreases the amount of time the probe could be sending science data. If the spacecraft knew the exact time, it could therefore know that it was time to point towards Earth and begin transmitting telemetry, without being commanded by the DSN. This would also enhance DSN efficiency because while the DSN can receive multiple downlinks, it is limited in its ability to send uplinks to spacecraft. Using this method, fewer uplinks would be needed.

As an example, Dr. Todd Ely, Principal Investigator of the DSAC experiment cites a potential improvement to a Cassini-like mission that included an atomic clock. Suppose the round-trip light-time to Saturn is four hours. At present, the DSN would have to send a signal to Cassini to command it to begin transmitting. If Cassini had an atomic clock, it would know on its own that it was time to begin transmitting to Earth. So if the window for communication was 10 hours long, Cassini would be able to transmit all 10 hours, instead of having to wait for the ground command.

Artist's rendering of the Cassini spacecraft around Saturn. Image Credit: NASA

The DSN could be made even more efficient by switching from the modern X-band tracking, that is, sending signals to spacecraft on the X-band section of the radio spectrum, to the clearer Ka-band tracking. The X-band region of the spectrum is vulnerable to solar interference, degrading its tracking ability. Spacecraft already transmit their data to the DSN on the Ka-band, but the DSN relies on the X-band to track spacecraft. If the DSN could just listen for the spacecraft, which would know what time to begin transmitting, the telemetry would be one-way and all on the Ka-band, reducing interference.

Atomic clocks would also improve radio science data. Since radio science investigations rely on knowing exactly where the spacecraft is, the Ka-band tracking data sent from the spacecraft to the DSN would be superior to the X-band tracking now used. Knowing the exact location of the spacecraft would reduce the error in any radio science investigation.

JPL's work over the years has reduced the drift of mercury-ion clocks and made them small enough and light enough to fit on virtually any spacecraft. The demonstration clock is about only about 10 kg. Dr. Ely estimates that a mission-qualified clock would have a mass of only 5 kg.

The DSAC demonstration is scheduled to fly as a secondary payload on an Iridium-NEXT satellite, set for launch in 2015.