|
Microfabricated atomic frequency references are expected to have a wide range of commercial and military applications ranging from wireless communication systems to global positioning. In some sense, we probably don't even know what the most critical applications are likely to be at present. However some important examples of how highly miniaturized, battery-operated atomic clocks might be used are given below. 1. Synchronization of communication networks With the recent growth of portable, high-bandwidth communication devices such as cellular telephones, personal desktop assistants, and the wireless internet, information distribution to remote locations is becoming increasingly important. In order to accomplish efficient and error-free digital data transfer between a large number of distributed nodes, all nodes must be synchronized; that is, all local clocks must run at exactly the same speed. Most often, this is done using a master-slave architecture in which timing signals from a single, precise clock (such as those on the GPS satellites) is transmitted to all network clocks at regular intervals to correct their time. However, master-slave architectures are highly susceptible to interruption of the master synchronization signal through environmental disturbance or intentional jamming. The more precise the local clocks installed at each node are, therefore, the longer the network can operate in the absence of synchronization to a master clock. The current generation of commercial compact atomic frequency standards are about the size of a cigarette pack and dissipate several watts of power while operating. They were developed primarily for synchronizing cell-phone networks, and are currently being installed in cell-phone base stations throughout the world. Microfabricated atomic clocks could extend this type of synchronization application to portable devices such as the cell phones themselves, enabling, for example, a higher degree of encryption than could be obtained otherwise. Networks of computers could also be synchronized over long periods without update if a low-cost, highly precise clock were available. 2. Anti-jam global positioning receivers The US global positioning system transmits two primary signals that can be used for position location. One, labeled the C/A code, is primarily for civilian use. The other, labeled the P(Y) code, is used primarily by the military. The GPS signals are extremely weak and can be interrupted if extraneous microwave power is present at the location of the GPS receiver. This extraneous power might be generated by accident, for example from malfunctioning electronics, or intentionally, by an adversary intent on disrupting the system. Since the P(Y) code is broadcast over a much larger frequency bandwidth, it is generally much less susceptible to jamming through this kind of RF interference. However, in order to acquire the P(Y) code, most military GPS receivers must first acquire the C/A code in order to determine a rough estimate of the time. The reason for this is that the P(Y) code is extremely long – it repeats itself only every week – and therefore a correlator attempting to match the code being received from the satellite to an internal known code will take a very long time (hours, possibly) before it finds a match if it doesn't know the time. If the time is known to with 1 ms, the acquisition of the P(Y) code is essentially instantaneous, however. Since a military GPS receiver must first acquire the C/A code in order to find out the time, the advantage with respect to jamming offered by the P(Y) code is lost: if the C/A code is jammed, even a military receiver won't work. However, if such a receiver had a very precise local clock, accurate to 1 ms over many days, it could determine the time without having to acquire the C/A code at all and therefore lock to the P(Y) code directly. Current low-power quartz oscillators are unable to maintain this timing stability over periods long enough to be useful in this application. Microfabricated atomic clocks could therefore have critical utility to military positioning and timing. Technology Transfer from NIST to US Industry Because of the large potential market for microfabricated atomic clocks, one of the goals of the NIST microfabricated clock program is to provide knowledge and measurements that will help U.S. industry manufacture these devices in the future. Technologies developed at NIST with the microfabricated atomic clock program are transferred to industry through a variety of channels. The primary channel is publication of results in scientific and engineering journals and conference presentations. In some cases, the technology is patented and is available for licensing, either through the NIST Office of Technology and Partnerships or, for some work, through the University of Colorado Technology Transfer Office. References:
H. Fruehoff, 'Fast "direct-P(Y)" GPS signal acquisition using a special portable clock,' Proc 33rd Ann. Precise Time and Time Interval (PTTI) Meeting, 359-369, 2001. J. Vig, "Military applications of high-accuracy frequency standards and clocks," IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, 40, 522, 1993. J. A. Kusters and C.A. Adams, "Performance requirements of communication base station time standards," RF Design, 28-38, May, 1999. |
||||||
