Slashdot Mirror
NASA To Test New Atomic Clock
edesio writes "Many satellites and spacecraft require accurate timing signals to ensure the proper operation of scientific instruments. In the case of GPS satellites, accurate timing is essential, otherwise anything relying on GPS signals to navigate could be misdirected. The third technology demonstration planned by NASA's Jet Propulsion Laboratory is the Deep Space Atomic Clock. The DSAC team plans to develop a small, low-mass atomic clock based on mercury-ion trap technology and demonstrate it in space."
Having once worked on GPS Satellite's clocking system, I was surprised that the AF was so against usage of atomic clocks phased-locked to crystals for accurate timing. Maybe the latest news about Galileo using atomic clock changed their mind?
... you might have heard about PHARAO, a caesium clock which is planned to fly on the ISS in 2013. Accuracy target is 1E-16.
ESA will get there first, with the Atomic Clock Ensemble in Space (ACES), intended for the ISS in 2013, which should be good to ~ 10**-16 and will include a test of relativity. I believe that this is the JPL clock, which is aiming at 10**-15 stability, and a 2015 launch. (Both are fairly low earth orbits, with the JPL clock intended for an Iridium satellite.)
So, the JPL effort is cool, and I would love to see one flown to Mars or truly deep space, but this is one case where the Europeans are in the lead.
Well, first, this is a NASA technology demonstration mission for a clock needed by JPL. That means that NASA is going to validate that this clock will work in space, so that JPL can use it where they operate (i.e., deep space, the Moon, Mars and beyond).
Second, yes, the very best modern clocks work differently with and without gravity. This is basically because the atoms used are so cold they are moving at human type velocities, and so gravity can't be ignored. The best terrestrial clocks are the fountains - take very cold atoms, moving at ~ 1 meter per second in a trap and shut off the trap. Some of the atoms (the ones that happen to be moving up) will ballistically go up, and then fall back down. (This is much like tossing your keys up 1 meter or so, and then catching them, except with single atoms.) The gravity is used to collimate the pulse of atoms going up and down, and (with timing the round trip) to select only the ultracold ones coming down. By timing the round trip, you can really select a particular set of velocities - the better constrained the velocity dispersion, the better constrained the clock read out.
NONE of that works in zero-G, and PHARAO (I am more familiar with this clock that the JPL Hg Ion one) is completely re-designed to use fountain-like ideas in a linear beam. I am not sure it would even work on the ground, and it definitely needs zero-g to meet its performance goals.
Were you a Payload Officer at 2 SOPS? And, are you referring to the Space or Ground segment? If you mean the backup clocks in the Mod, then that's slightly outside my knowledge. As far as the Space Segment, they've been reliably using Cesium and Rubium (atomic) clocks for over 3 decades. They don't want to change because it's known, reliable, etc. I was curious to note the newest generation of satellites dropped the 4th clock, and now launch with only 3. Since each clock is only usable a number of years (varies too much for me to generalize), I'd have thought they kept all four just in case the electrical system outlives normal design life, and you end up needing to go with a 4th clock when the 3rd one becomes too "deviant".
I think the GPS relevence is buried at the bottom of the article. Cesium and Rubidium clocks are both accurate to the nanosecond. That's just about as accurate as can be practical. The new atomic clock, however, they're saying is accurate LONGER. On GPS Satellites, the original satellites (Block I, II, IIA, IIR) launched with Cesium and Rubidium, 2 each. Usually you have one operational, sometimes one on "ready standby", and the other two off. As each atomic clock reaches the end of its mission-usable life, it's turned off. It become's "mission-unusable" (not a real term, I just made that up) when it's signal varies outside a normal window of acceptable predictability in terms of its output signal. There are design differences, such as Rubidium clocks have to stay within a tenth of a degree (F) in temperature stability (if memory serves correct). So, if they can create a clock that's more stable, for a longer period of time, this has huge potential for future GPS satellites. However, since we just awarded contract to Boeing the contract for IIF birds, with only 2 of 12 launched, it's going to be a very long time (decade at best) before you'd see this in a GPS satellite. Design life has also expanded from 7 to 12 years for each satellite (for point of reference Block I only had a design life of 3 years since they were R&D), so this pushes any usage even farther out since we're going to go longer before replacements need to be launched.