How an Atomic Clock works and its use in GPS

The way that the atomic clock work amazes me. Let me explain the way the very first one worked.

Let’s take an example of a Jello. Tap on the Jello and it wiggles just like the swings of a pendulum. The oscillations of the Jello keeps time. Now Jello isn’t very good for this, but inside an atomic clock we have a trunk of quartz which is of similar shape that if we tap it which we do with a Joule of electricity it will oscillate some 5 Million times per second. It keeps time in 1 second in 90000 years which is fraction of the accuracy needed for an atomic clock. Quartz loses time because it slows down and needs to be nudged to restore its oscillations, that’s where the atomic part of the atomic clock comes into play. We use Caesium atoms to control this nudge very accurately. Every time the Quartz’s ion slows down just a tiniest bit, we give it tap with a Joule exactly at the same time, so essentially its oscillations never decay.

The way that the Atomic Clock Works:

 We use Caesium to do this. Atoms in pure Caesium exist in slightly two different states. A low energy form and the other with just a bit more energy. For atomic clock these two states have two properties critical in making a clock. First is that the can be separated by a magnet, and second that the lower energy atoms can be converted to higher energy Caesium atoms if we bombard it with the perfect radiation. Engineers tie the slowing down of the Quartz vibrations to the precise wavelength of the bombarding radiation to create a feedback loop. In an oven we heat the Caesium chloride to create a gaseous stream of Caesium ions. The stream contains both the low and high energy ions. We first flow it through a magnet, separating the two states, discarding the higher energy ones and passing the lower energy ones into the chamber. Inside the chamber we bombard the atoms with just the right wavelength radiation to make them jump to higher energy. As these gaseous ions leave the chamber, they pass through another magnet that directs the higher energy level ions into the detector this time detecting any lower energy ones. The detector converts the arriving ions into a current. The trick here is to tie that current to the Quartz oscillator. When the Quartz’s oscillations decay i.e., it slows down a little then the energy bombarding the Caesium ions in the chamber changes or stops. This tells the clock to zap the Quartz oscillator and correct the period of oscillation. It does this by applying the proper voltage that the Piezoelectric effect taps the Quartz and restore its oscillations. Thus, creating a clock that loses a second in many million years.

Use of Atomic clock in Global Positioning Systems(GPS)

 Our world needs Such accuracy. For example, the GPS or the Global Positioning System. The GPS consists of 24 satellites orbiting the Earth. A GPS receiver uses at least the position of four of these satellites to locate itself. One to correct the clock time on the receiver and three to locate its position. Here’s how it works. A signal is sent from the first satellite that contains the satellite’s location and the signal’s time of departure. The receiver then multiplies this travel time with the speed of light in order to calculate its distance from the satellite. With one such signal the device knows that is located in a sphere around that satellite with its radius equal to the calculated distance. So, it is the same calculation with the second satellite. The intersection of these two sphere narrows the location equal to the circumference of the circle. Then with the third satellite it reduces the location to a single point. Since the signal travel with the speed of light, error of even a millisecond may cause an error of a million feet or 300 kilometres. But with the atomic clock accuracy, the receiver can locate itself to an accuracy of about 3 feet!!!!

Most accurate Atomic Clock loses a second in almost 138 Million years.

World’s first commercially available chip scale Atomic Clock Symmetricom CSAC. It has the size of a coin.