Deep Space Atomic Clock

National Aeronautics and Space Administration

Ground-based 

atomic clocks are 

the cornerstone of 

spacecraft navigation 

for most deep-space missions because of their use 

in generating precision two-way tracking measurements. These typically include range (the distance 

between two objects) and Doppler (a measure of 

the relative speed between them). A two-way link (a 

signal that originates and ends at the ground tracking antenna) is required because today’s spacecraft 

clocks introduce too much error for the equivalent 

one-way measurements to be useful. Ground atomic clocks, while providing extremely stable frequency and time references, are too large for hosting on 

a spacecraft and cannot survive the harshness of 

space. New technology is on the horizon that will 

change this paradigm. NASA’s Deep Space Atomic 

Clock (DSAC) project is developing a reducedsize mercury ion atomic clock that is as stable as 

a ground clock, small enough to be hosted on a 

spacecraft, and able to operate in deep space. 

Deep Space Navigation and Science 

with DSAC

In today’s two-way architecture, spacecraft are 

tracked by Earth-based antennas, and a team of 

navigators processes the data and uploads navigation commands to the spacecraft. By virtually eliminating spacecraft clock errors, DSAC enables a shift 

to a more efficient, flexible and scalable one-way 

tracking architecture that will benefit future navigaKey Facts

•	 Current ground-based atomic clocks are fundamental to deep space navigation; however, 

they are too massive to be flown in space. This results in tracking data being collected and 

processed here on Earth for most deep-space navigation applications.

•	 The Deep Space Atomic Clock (DSAC) is a miniaturized atomic clock that has been ruggedized for spaceflight. It will improve the precision of deep-space navigation, enable more 

efficient use of tracking networks, and — looking forward — serve as a key component to 

onboard deep-space navigation with radios.

•	 DSAC could be used to improve the performance of the Global Positioning System (GPS) 

and the accuracy of the navigation service it provides to the world.

tion and radio science. Here are some examples of 

how one-way deep-space tracking with DSAC can 

improve navigation and radio science that is not 

supported by current two-way tracking. 

1. Simultaneously track two spacecraft on a 

downlink with the Deep Space Network (DSN) 

at destinations such as Mars, and nearly double a space mission’s tracking data because it 

no longer has to “time-share” an antenna. 

2. Improve tracking data precision by an order of 

magnitude using the DSN’s Ka-band downlink 

tracking capability. 

3. Mitigate Ka-band’s weather sensitivity (as 

compared to two-way X-band) by being able 

to switch from a weather-impacted receiving 

antenna to one in a different location with no 

tracking outages. 

4. Track longer by using a ground antenna’s entire spacecraft viewing period. At Jupiter, this 

yields a 10 to 15 percent increase in tracking; 

at Saturn, it grows to 15 to 25 percent, with 

the percentage increasing the farther a spacecraft travels. 

5. Make new discoveries as a Ka-band—capable 

radio science instrument with a 10 times 

improvement in data precision for both gravity 

and occultation science and deliver more data 

because of one-way tracking’s operational 

flexibility. 

6. Explore deep space as a key element of a 

real-time autonomous navigation system that 

tracks one-way radio signals on the uplink 

and, coupled with optical navigation, provides 

for robust absolute and relative navigation.
JPL 400-1534, Rev. 1 6/13 NASA Facts

www.nasa.gov

For more information about the Deep Space Atomic Clock project, please 

see http://www.nasa.gov/mission_pages/tdm/clock/ 

Waveguide 

Window

Multipole Trap

Quadrupole Trap

Optical Window

Titanium Vacuum Tube

Figure 1. Drawing of the DSAC mercury-ion trap showing the traps and the titanium vacuum tube that confine the ions. The quadrupole trap is where the hyperfine transition is optically measured and the multipole trap is where the ions are 

“interrogated” by a microwave signal via a waveguide from the quartz oscillator.

Figure 3. DSAC on-orbit demonstration mission architecture.

Host Spacecraft

Commanding and 

Telemetry

Host 

Ground 

Network

DSAC

Investigation

Team

DSAC

Hosted

Payload

JPL GPS

Receiver and

Atomic Clock

International GNSS Service (IGS)

GPS Receivers and Atomic Clocks

Range and 

Phase

GPS Sat 1

GPS Sat 2

GPS Sat n

Figure 2. Preliminary design of DSAC with the outer cover removed, showing the 

internal components.

These benefits can be applied directly to missions being 

contemplated by NASA. For instance, a Mars orbiter would 

obtain direct benefit from DSAC with improved navigation and 

gravity science that combine the Ka-band and the simultaneous tracking benefits. If this orbiter were to aerobrake (i.e., use 

the Martian atmosphere to slow down to a final science orbit), 

one-way tracking on an uplink, coupled with an onboard guidance, navigation and control system, would make it possible 

to do this operation autonomously and yield a significant reduction in ground support operations. Studies of Europa flyby 

mission gravity science have shown that one-way Ka-band 

tracking with DSAC would result in a robust, weather-tolerant 

method for determining Europa’s gravity field. Any mission 

concept with DSAC would greatly simplify spacecraft-based 

timekeeping — it is expected that DSAC would incur no more 

than 1 microsecond of error in 10 years of operations. Looking to the future, DSAC coupled with an onboard navigation 

system would be fundamental to human explorers requiring 

real-time navigation data. 

Near Space Navigation with DSAC

Looking closer to Earth, DSAC is more stable and accurate 

than the atomic clocks that currently fly on the GPS satellites. As the Department of Defense plans to upgrade GPS to 

improve its performance and ensure the health of the nation’s 

clock industrial base that supplies GPS, DSAC is a key technology that can be used to meet these needs.

DSAC’s Technology

The DSAC technology uses the property of mercury ions’ hyperfine transition frequency at 40.50 GHz to effectively “steer” 

the frequency output of a quartz oscillator to a near-constant 

value. DSAC does this by confining the mercury ions with 

electric fields in a trap (see Figure 1) and protecting them by 

applying magnetic fields and shielding. This provides a stable 

environment for measuring the hyperfine transition very accurately and minimizes sensitivity to temperature and magnetic 

variations. Coupling this with the fact that the DSAC technology has almost no expendables enables development of a 

clock suitable for very long-duration space missions.

Demonstrating the World’s Most Stable Small 

Space Clock

The DSAC project is currently building a demonstration unit 

(see Figure 2) and payload that will be hosted on a spacecraft 

provided by Surrey Satellite Technologies US LLC, Englewood, Colo. It will launch in 2015 into Earth orbit. The DSAC 

payload will be operated for at least a year to demonstrate its 

functionality and utility for one-way-based navigation. Figure 3 

illustrates the on-orbit mission architecture, where the plan is 

to collect measurements between GPS and the host spacecraft using a GPS receiver connected to DSAC. The data will 

be processed on the ground by the DSAC team to validate the 

clock’s in-space performance.

DSAC is a cross-cutting technology that has the potential 

to change the “calculus” of spaceflight. As one of NASA’s 

Technology Demonstration Missions, a key objective of the 

project is to demonstrate DSAC’s performance and operability 

in space, and develop the technology to a point where it can 

be transitioned to industry for commercial production. DSAC’s 

space demonstration will have proven its flight worthiness and 

will radically reduce risk to future missions that adopt it.

The DSAC project is sponsored by the NASA Space Technology Mission Directorate and managed by NASA’s Jet Propulsion 

Laboratory in Pasadena, Calif.