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CESIUM ATOMIC CLOCKS

Only in the modern era of atomic clocks has timekeeping technology provided sufficient accuracy to allow the successful construction of the Navstar Global Positioning System. The evenly spaced timing pulses coming down from each Navstar satellite are generated by an atomic clock that contains no gears or cogs. It's extraordinary timekeeping abilities arise from the quantum mechanical behavior of certain specific atoms (cesium, rubidium, hydrogen), which tend to have a single outer-shell electron. Cesium atoms can exist in either of two principal states. In the high-energy state, the spin axis of the lone outer-shell electron is parallel to the spin axis of the atom's nucleus. In the low-energy state, the electron spins in a anti-parallel direction. For cesium, the energy difference between the two spin states corresponds to an electromagnetic frequency of 9,192,631,770 cycles per second. Thus, when a cloud of cesium gas is struck by radio wave oscillating near that particular frequency, some of the low-energy atoms will absorb one quantum of energy and, consequently, their outershell electron will flip over and begin spinning in the opposite direction. The closer the trigger frequency can be adjusted to 9,192,631,770 cycles per second, the more lowenergy electrons will reverse their direction of spin.

The heart of the cesium atomic clock is a voltage-controlled crystal oscillator - a small vibrating slab of quartz similar to the one that hums inside a digital watch. Small variations in the voltage feeding a voltage-controlled crystal oscillator create corresponding variations in its oscillation frequency. Any necessary adjustments are handled by a feedback control loop consisting of a cesium atomic clock wrapped around the quartz crystal oscillator.

A schematic diagram of the cesium atomic clocks carried onboard the GPS satellites is sketched in Figure 1. First solid cesium is vaporized at 100 degrees Centigrade and then it is routed through a collimator to form a steady stream of cesium gas, which, in its natural state, consists of an equal mixture of high-energy and low-energy atoms.

FIGURE 1. The low-energy atoms floating around inside the resonating chamber of this cesium atomic clock are hit with a radio wave as close as possible to 9,192,631,770 oscillations per second. Depending on the accuracy of that trigger frequency, larger or smaller numbers of low-energy atoms will absorb one quanta of energy to become highenergy atoms - which are subsequently converted into cesium ions by the hot-wire ionizer (bottom right). The resulting ion current automatically adjusts the frequency of the quartz crystal oscillator, which, in turn, creates more timing pulses and precisely controlled electromagnetic waves.

A selector magnet is then used to separate the cesium atoms into two separate streams. The high-energy atoms are discarded, the low-energy atoms are deflected into a resonating cavity with precisely machined dimensions were they are hit with radio waves generated by a voltage-controlled crystal oscillator coupled to a solid-state frequency multiplier circuit. The closer the trigger frequency is to 9,192,631,770 oscillations per second, the more outer shell electrons will be inverted to produce highenergy cesium atoms.

When the atoms emerge from the resonating cavity, they are again sorted by a selector magnet into two separate streams. This time the low-energy atoms are discarded. The high-energy atoms are deflected onto a hot-wire ionizer, which strips off their outer-shell electrons to produce a stream of cesium ions. The resulting current is then routed into a feedback control loop connected to the voltage controlled crystal oscillator whose oscillation frequency is constantly adjusted to produce new radio waves.

By adjusting the frequency to maximize the ion current and dithering the oscillator to make its frequency straddle the desired value of 9,192,631,770 oscillations per second, the frequency stability of the quartz crystal oscillator can be maintained within one part in 5 billion. Thus, the feedback control loop just described stabilizes the frequency of the quartz crystal by a factor of 10,000 or so, compared with a free-running quartz crystal with similar design characteristics.

RUBIDIUM ATOMIC CLOCKS

The rubidium atomic clocks carried on board the GPS satellites are, in many respects, similar to the cesium atomic clocks, but there are also important differences in their design. For one thing, the rubidium atoms are not used up while the device is keeping time. Instead, the atoms reside permanently inside the resonating chamber. The sensing mechanisms that monitor and adjust the clocks stability are also based on distinctly different scientific principles.

As the rubidium atoms linger inside the resonating chamber, they are impacted with electromagnetic waves whose oscillation frequencies are as close as possible to 6,834,682,613 oscillations per second (see Figure 2). As the transmission frequency is adjusted closer and closer to that precise target value, larger numbers of rubidium atoms will absorb exactly one quanta of energy. When they do, their spin-states automatically reverse to convert them from low-energy to high-energy atoms.

FIGURE 2. Unlike the cesium atomic clock, the atoms in a rubidium atomic clock remain always in the gaseous state. The trigger frequencies for the two devices are also different. For a rubidium atomic clock the trigger frequency is 6,834,682,613 oscillations per second. When the rubidium atoms inside the resonating cavity are hit with a trigger frequency as close as possible to that value, larger numbers of them are converted from low-energy atoms two high-energy atoms - that is, the spin axis of their lone outer shell electron is parallel to the spin axis of the nucleus. Successful inversions are monitored by shining a rubidium lamp through the resonating cavity. When larger numbers of rubidium atoms have a been converted to the high-energy state, the gaseous cesium in the resonating cavity is more opaque to rubidium light.

The rubidium atomic clock converges toward the desired frequency through a feedback control loop whose status is continuously evaluated by shining the beam of rubidium lamp through the resonating chamber. The gas inside the chamber becomes more or less opaque to rubidium light, depending on how many of the rubidium atoms inside have been successfully inverted. The intensity of the rubidium light passing through the chamber is measured by a photo detector, similar to the electric eye in a digital camera.

The output from the photo detector is fed into a set of solid-state integrated circuits rigged to make subtle and continuous adjustments to the frequency of the voltagecontrolled crystal oscillator. Pulses from the crystal oscillator, which vibrates at 5 million oscillations per second, are used in generating the evenly spaced C/A- and P-code pulses broadcast by the satellites. A portion of the output from the voltage-controlled crystal oscillator is also fed into a set of frequency multiplier circuits which generate the desired 6,834,682,613 oscillation-per-second frequency, which is, in turn, routed into the atomic clock's resonating chamber.

DEVELOPING ATOMIC CLOCKS LIGHT ENOUGH TO TRAVEL INTO SPACE

When the architecture for the Navstar navigation system was first being selected, many experts argued convincingly that the atomic clocks should remain firmly planted on the ground. The C/A- and P-code pulse trains, they believed, should be sent up to the satellites through radio links for rebroadcast back down to the users down below. This contention position was quite defensible because all available atomic clocks were big and heavy, power-hungry, an extremely temperamental.

The best available cesium atomic clocks operated by the National Bureau of Standards, for instance, were larger than a household deep-freeze, and they had to be tended by a fretful army of highly trained technicians. However, emerging technology soon produced much smaller and far more dependable atomic clocks. After years of intellectual struggle, the cesium and rubidium atomic clocks on board the Navstar satellites have turned out to be surprisingly small and compact. They also consume moderate quantities of electricity and can operate for several years without failure. The rubidium clocks carried aboard the Navstar satellites are roughly the same size as a car battery. Each one weighs about fifteen pounds. The cesium atomic clocks are a little bigger. They weigh thirty pounds each.

The earliest Navstar GPS performance specifications called for atomic clocks with fractional frequency stabilities of one part in 1 trillion. The fractional frequency stability of an atomic clock can be defined as the one sigma error pulse to pulse divided by the duration between pulses. An atomic clock with a fractional frequency stability of one part in 1 trillion is capable of keeping time to within one second over at interval of 30,000 years.

Although this performance specification may seem rather stringent, the first few spaceborne atomic clocks were two to five times more stable than required. Consequently, the specification goal was eventually raised to two parts in 10 trillion. The Navstar clocks have turned out to be surprisingly accurate and stable, but clock reliability problems plagued the first few GPS satellites. On the average, only five on orbit months went by before a satellite component failure occurred. Almost always it was an atomic clock component that failed. With intense design efforts, these problems were eventually brought under control so that, today, the probability that at least one of the four atomic clocks on the Block II satellite will still be operating at the end of its 7.5 year mission is estimated to be 99.44 percent.

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