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An Atomic Clock for Deep Space

NASA’s Orbital Test Bed satellite is scheduled for launch via a SpaceX Falcon Heavy on June 22, with live streaming here. Although two dozen satellites from various institutions will be aboard the launch vehicle, the NASA OTB satellite itself houses multiple payloads on a single platform, including a modular solar array and a programmable satellite receiver. The component that’s caught my eye, though, is the Deep Space Atomic Clock, a technology demonstrator that points to better navigation in deep space without reliance on Earth-based atomic clocks.

Consider current methods of navigation. An accurate reading on a spacecraft’s position depends on a measurement of the time it takes for a transmission to flow between a ground station and the vehicle. Collect enough time measurements, converting them to distance, and the spacecraft’s trajectory is established. We know how to do atomic clocks well — consider the US Naval Observatory’s use of clocks reliant on the oscillation of atoms in its cesium and hydrogen maser clocks. Atomic clocks at Deep Space Network ground stations make possible navigational readings on spacecraft at the expense of bulk and communications lag.

While GPS and other Global Navigation Satellite Systems (GNSS) use onboard atomic clocks, the technologies currently in play are too heavy for operations on spacecraft designed for exploration far from Earth. That puts the burden on communications, as distant spacecraft process a signal from an atomic clock on the ground. What the spacecraft lacks is autonomy.

A better methodology is something we have to develop as we look toward a future deep space infrastructure. Testing the miniaturization of atomic clocks and methods to harden them for operations elsewhere in the Solar System is the goal of the DSAC demonstrator mission, which points to a clock architecture that is considerably more efficient and also scalable.

Image: JPL’s Deep Space Atomic Clock will fly aboard the General Atomics Electromagnetic Systems Orbital Test Bed satellite as a hosted payload and launched in June as part of the U.S. Air Force’s Space Technology Program 2. Credit: NASA.

The Deep Space Atomic Clock will be the first atomic clock designed to fly aboard a spacecraft going beyond Earth orbit and, with a stability of better than one microsecond in a decade, it is also the most precise clock ever flown. Ground-based testing has shown that the DSAC is up to 50 times more stable than the atomic clocks on GPS satellites, losing only one second in nine million years. NASA considers it an enabling device for future on-board radio navigation.

It is the length of a second as measured by the frequency of light released by specific atoms that makes an atomic clock so precise as it records the vibrations induced in a quartz crystal. Key to the DSAC clock’s stability is the use of mercury ion trap technology. Contained within electromagnetic traps within the device, these ions are rendered less vulnerable to external forces like changing magnetic fields and variations in temperature than atoms currently in use.

Image: DSAC mercury ion trap housing with electric field trapping rods seen in the cutouts. This is where DSAC interrogates and measures the mercury ion resonance that is used to discipline a quartz crystal clock. Credit: NASA.

The distances involved in deep space operations force new technologies upon us, for communicating with atomic clocks on Earth to determine a spacecraft’s location not only takes time but also places an increased burden on our communication resources, another reason why we’re moving toward networking multiple spacecraft in orbital operations at places like Mars.

The plan is to test DSAC in Earth orbit for one year, with the goal of adapting it for future missions to deep space. Developed at the Jet Propulsion Laboratory, the device has been under development for 20 years, reducing the size of atomic clocks from those used at Deep Space Network ground stations — about the size of a refrigerator — to an object the size of a four-slice toaster, and one that can be further miniaturized depending on the needs of future missions.

You can see that we’re gradually upping the navigation service volume, considering that spacecraft near Earth don’t require an integrated atomic clock, being able to use existing global navigation services like GPS. These technologies, using multiple GNSS constellations, can get us out as far as geosynchronous orbit, and DSAC promises accurate navigation deep into the Solar System. Ahead lies X-ray navigation that keys off the oscillations of remote pulsars, a galactic positioning system that points to missions moving, one day, far beyond our Sun.


Comments on this entry are closed.

  • Robin Datta June 5, 2019, 15:42

    Spacefaring atomic clocks are another facet to the increasing autonomy mandated for remote systems by the constraints imposed through space and time by lightspeed. If technology progresses far enough, systems could become so remote that no control could. be exercised from home base.

  • Marc Millis June 6, 2019, 4:33

    FYI – I wondered about DSAC’s mass and power and found this: “The current best estimate (as of December 2013) of the DSAC Demonstration Unit’s (DU) size, weight, and average power are 29 cm x 26 cm x 23 cm, 16 kg, and 50 W, respectively. [Ely, T. (2014). Advancing navigation, timing, and science with the Deep Space Atomic Clock. In SpaceOps 2014 Conference (p. 1856).]

    • Bruce D. Mayfield June 6, 2019, 10:18

      It takes 50 watts!? That’s quite a large power load for something that needs to operate continuously for many years.

      • Alex Tolley June 6, 2019, 21:59

        50W is a just a small solar array of about 2000 cm^2. 2 panels on either side of the satellite and deployed to face the sun should be adequate to deliver that amount of power. Is the degradation rate of the panels the issue you had in mind?

        • Martin June 7, 2019, 18:16

          A small solar array 1 AU from sun, i.e. near Earth. Once you are far from Earth (around 40 AU for Kuiper Belt objects, 550 AU for using sun as gravity lens, or even beyond that for other missions …), solar panels become quite inefficient.

          • Bruce D. Mayfield June 7, 2019, 21:13

            That’s why I expressed concern. We are after all discussing “deep space” applications.

            • Alex Tolley June 8, 2019, 12:38

              The plan is to test DSAC in Earth orbit for one year, with the goal of adapting it for future missions to deep space.

              (emphasis mine)

              Therefore solar panels for the test should be fine.

              For deep space beyond Jupiter/Saturn, RTGs are the best option for powering such a device and. This Wikipedia entry shows that there are a number of existing options that fit the power output requirements. I am therefore not clear why the 50W power demand is such an issue. The only downside I can see is that if Pu238 is the chosen fissile source, there is very little available currently in the West.

  • Martin June 8, 2019, 21:24

    For traditional satellites or larger space probes, this would be good enough. For other projects, like starshot where the payload is in grams, this is still too power consuming and also too big at the moment … we need to miniaturize this a bit more for those applications or be able to do the mission with less presice clock.

    • Alex Tolley June 9, 2019, 10:59

      The tiny craft are like your cellphone, independent from the clock that would be part of the GPS satellites. I would argue the reverse is the case – that the satellites need to be high power to broadcast their positions and times so that the craft that must locate itself by those signals can use as small and low power a receiver as possible.