Time, Clocks, Day

IN NO VA TION 10 12 or better. But over the long tern1 , its fre quency can drift by several parts in 10 11 per Time, Clocks, day. In order to keep two clocks using quartz crystal oscillators synchronized to l microsec and GPS ond . you wou ld have to reset them at least every few hours. The resonators used in atomic clocks have Richard B. Langley surpassed considerably the accuracy and sta bility of quartz resonators. University of New Brunswick ATOMIC RESONATORS An atomic clock contains an oscillator whose oscillati ons are governed by a particular erating the satellite's signals. But just what atomic process. According to the quantum pic is an atomic clock? Before we answer ture of matter. atoms and molecules exist in thi s question, let"s examine so me of the well-defined energy states. An atom that bas ic concepts associated with clocks and fa lls from a higher to a lower energy state timekeeping. emits radiation in the form of light or radio waves with a frequency that is directly pro THE QUARTZ CRYSTAL RESONATOR portional to the change in energy of the All clocks contai n an oscillator, which in atom. Conversely. an atom that jumps from turn contains a frequency-determining ele a lower energy state to a higher one absorbs ment called a resonator. A resonator is any radiat ion of exactly the same frequency. The "Innovation" is a regular column in GPS device that vibrates or osc illates with a well existence of such quantum jumps means that World featuring discussions on recent defin ed frequency when excited. such as gui atoms can be used as resonators to govern advances in CPS rec/mology and irs tar and viol in strings, pendulums. and quartz clocks precisely. given a way to tap their reso appl icatiom as well as on the fundamentals crystals. Once excited, the oscillations of a nance. Scient ists found such a way in the of GPS posirioning . In this issue 1ve present resonator slowly die out due to energy loss. late 1940s, using the ammonia molecule as a Tutorial 011 one of the underlying principles A perfect resonator has no energy loss and a resonator. However. although the ammoni a of GPS: the precise measurement of time. wi ll osci llate forever. molec ule was a very good resonator com This column is coordinared by Richard To be usefu l in an osc ill ator, a resonator pared to what had been prev iously used in Langley and Alfred Kleusberg of the must be sti mulated or energized repeated ly. clocks, certain problems resulted in the reso Deparrmenr of Sur veving Engineering at the This process is accomplished in a quartz crys nant frequency being somewhat unstable. So Universi(\· of New Brunswick. We welcome tal osciJlator by taking advantage of the piezo sc ient ists turned to the cesium atom and by your comments and suggestions of topics for electric effect: An extern al voltage applied the mid- 1950s had built the first ces iu m fi.aure columns. across opposite faces of a piece of quartz crys clocks. tal cut in a prescribed way causes the crystal The cesium atom, in its naturally occur Time is of the essence. This tatement. used to expand or contract depending on the po ri ng form. Cs 133. consists of a nucleus primaril y to characterize the clauses in cer larity of the voltage. The inverse is also true: containing 55 protons and 78 neutrons sur tain legal documents. also nicely describes forcibly deforming the crystal causes a small rounded by a swarm of electrons. The outer the nature of the Global Positioning System. electrical potential to develop. A crystal con most electron is in a shell of its own. Both The basic measurement a GPS receiver nected to an alternating voltage source wi ll vi the nucleus and this outer electron spi n on makes is of the ti me required for a ignal to brate. The vibrat ions. in turn. will generate their axes and, being charged. generate mag propagate from a particu lar satellite to the re an alternat ing voltage. These generated sig netic dipoles. ln other words. they act like ceiver. Multiplying this time interval by the nals interact with the applied voltage in such tiny bar magnets. The electron ·s magnetic di speed at which the signal propagates - the a way that the vibrations and the resultant cur pole is either parallel to the nucleus's dipole speed of light - converts it into a range or rent fl ow are at a maximum at a particular fre (both north poles pointing in the same direc di stance. Such a one-way ranging technique quency - the resonant freque ncy of the crys tion) or antiparallel. These two orientations requires accurate timekeeping in both the sat tal. which is determined by the size of the correspond to two energy states of the atom. ellite and the receiver. In this article. we' ll crystal and how it is cut. Frequencies range and transitions between these states form the investigate how this accuracy is achieved and from 10 kHz to well into the VHF range. basis of the cesium clock . examine some of the intricacies of time that Accuracy (how well the oscill ator can be A block diagram of a cesiu m clock is are important in GPS positioning. tuned to a specified frequency) and stability shown in Figure I . Cesium. a soft. silvery The high posit ion ing accuracy of GPS is (how well it stays on freq uency) determine metal . is heated in an oven to a couple of hun due. in part. to the use of atomic clocks the quality of an osc illator. The oscillator's dred degrees Celsius. Individual atoms boi l co control the generation of the signals stability is measured in terms of the relative off and pass through a magnetic state selec transmitted by the satellites. For redundancy, change in its frequen cy over a certain period tor, which defl ects atoms in the higher en each Navstar GPS Block II satell ite contains of time. A high-quality quartz crystal oscil ergy state. allowing only atoms in the lower four atom ic clocks, one of which is selected lator kept at a constant temperature by a min energy state to enter a microwave resonance by the spacecraft controllers to prov ide the iature oven has short-term stability (over pe cavity. Whi le in this cavi ty, which operates frequency and timing requirements for gen- riods shorter than about an hour) of a part in on the same principle as the home micro- 38 GPS WORLD No<embe r/ December 1991 INNOVAT I ON sium or rubidium clocks because they gener ally are more costly and less rugged. A ma Feedback ser has been employed in a sub-orbital rocket flight , but one has yet to be fl own in a satel lite. Plans to modify the Block II satellite qualification model for an Advanced Clock/ Ranging Experiment include a hydrogen ma Detector ser clock as one of the four clocks in the satel lite. JUST A SECOND - Before 1956. the fundamental unit of time Cesium Resonance cavity was the mean solar day. which is based on oven the rotation of the earth on its axis. But it Magnetic Magnetic was known for some time that the earth· s ro state selector state selector tation speed. and hence the length of the day. was not constant due to the action of ocean tides, atmospheric winds, and even mo Figure 1. Basic operation of a cesium atomic clock tions in the earth ·score. Although the larger wave oven. the cesium atoms are subjected form of a very low pressure gas. are con variations cou ld be measured and time scales to a radio signal synthesized from the output tained in a glass cell situated inside a micro corrected to account for them. it became nec of a quartz crystal oscillator. The frequency wave resonance cavity. A microwave signal essary to define a new fundamental unit of of this signal is very close to that of the en whose frequency is tuned by a feedback loop time. In 1956. the second. defined in terms ergy difference between the two states of the induces transitions. The short-term stability of the period of the earth· s orbit around the cesium atom described above. A certain num of rubidium clocks is almost as good as that sun. became the new standard. But this stan ber of atoms absorb this radiation and change of cesium clocks. In fact. over one-day av dard. known as the ephemeris second. was their state. eraging periods. the rubidium clocks in some short-lived. When the atoms leave the cavi ty, they of the Block I satellites had stabilities of 1.6 Because the frequency of the cesium reso again go through a magnetic gate . wh ich di parts in 10 13 or better. However. the fre nator is so stable, the development of the ce rects atoms in the higher energy state toward quency of rubidium clocks tends to wander sium clock made it poss ible to define the sec a detector. The detector produces an electri over longer periods. resulting in poorer per ond such that it cou ld be accurately realized cal signal whose intensity is related to the formance. Each Block II satelli te contains in a laboratory without resorting to a long se number of atoms it intercepts. This signal is two rubidium clocks in addition to the two ce ries of astronomica l observations.

Load more