It’s About TIME —All of It


When GPS goes out, the chip-scale atomic clock may be coming to the rescue.


by Mr. John Delcolliano and Mr. Paul Olson A


ccurate time is crucial to our military. It enables all the warfighting functions of an expeditionary force: radio communications, network synchronization, information gath-


ering, weapon systems, manned and unmanned systems, maneuvers, fires, electronic warfare and all types of sensors.


In addition to relying on the positioning capabilities of the Global Positioning System (GPS), many Army sys- tems use GPS, which was developed by the U.S. military, for its highly accurate time. Tat’s because if you know where you were 10 seconds ago, you can determine where you are now based on very sophisticated calculations.


Timing is everything. GPS satellites, which have atomic clocks on board, send out signals at precisely timed inter- vals. On Earth, a GPS receiver calculates exactly how long it took to get the signal from the satellite to the ground. One measurement enables the receiver to determine the precise time of day, and three more triangulate the posi- tion of the GPS receiver on Earth.


However, more than 12,000 miles separate the Earth from the GPS satellites, leading to a fragile signal by the time it reaches the receiver. Tis makes GPS unreliable in some environments, such as dense forests or urban areas


with large skyscrapers, and vulnerable to jamming from enemies. Today GPS receivers use ordinary quartz clocks. During signal drop-outs, the clock drifts during short intervals, making reacquisition of the signal difficult. If a receiver had an additional source of accurate time—such as its own atomic clock—that would allow for easier and quicker GPS recovery.


VERY ACCURATE, VERY BIG Atomic clocks are recognized for their accuracy. But the typical atomic clock is rack-mounted, weighs 50 to 60 pounds and requires lots of power. Tey’re great for fixed- base tactical operation centers and large platforms such as ground and air vehicles, but not for dismounted Soldiers.


At the U.S. Army Communications-Electronics Research, Development and Engineering Center (CERDEC), the Positioning, Navigation and Timing (PNT) Division strives to enable true navigation, timing and total situ- ational understanding for the dismounted Soldier and commander in varying conditions, including degraded or GPS-challenged or -denied environments. Te division is part of the Command, Power and Integration Directorate (CP&ID) of CERDEC, home to the Army’s experts for Soldier and manned-unmanned ground platforms.


ASC.ARMY.MIL


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SCIENCE & TECHNOLOGY


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