The core technology of GPS
: precise timekeeping and atomic clocks
The operating principle of the GPS system is actually very simple: each satellite is continuously transmitted containing its position and accurate to a billion A digital radio signal with a time of a fraction of a second. The GPS receiver receives the signals from the four satellites and calculates its position on Earth with an error of only a few hundred feet. The receiver compares the reception time with the satellite launch time, and calculates the distance away from the satellite from the difference (the speed of light is 186,000 miles per second, if the satellite launch time is one thousandth of a second later than the reception time, then accept the The device is 186 miles away from the satellite). By comparing this time with the time of three other satellites whose positions are known, the receiver can determine the latitude, longitude and altitude.
It can be seen from the above discussion that precise timing and its timing tools play an important role in the entire GPS system.
Speaking of the atomic clock, it was originally created by physicists to explore the nature of the universe; they never imagined that the technology could one day be used in global navigation systems.
According to the basic principles of quantum physics, atoms absorb or release electromagnetic energy according to the energy difference of different electron arrangements, that is, the energy difference of different electron layers around the nucleus. Here the electromagnetic energy is discontinuous. When an atom transitions from an ‘energy state’ to a lower ‘energy state,’ it releases electromagnetic waves. This electromagnetic wave characteristic frequency is discontinuous, which is what people call the resonance frequency. The resonance frequency of the same kind of atom is fixed, for example, the resonance frequency of cesium 133 is 9192631770 cycles per second. So cesium atoms are used as a kind of metronome to keep highly accurate time.
In the 1930s, Rabbi and his students studied the fundamental properties of atoms and nuclei in a laboratory at Columbia University. It was here that they took a valuable first step towards building clocks based on this atomic timer. In the course of his research, Rabi invented a technique known as magnetic resonance. With this technique, he was able to measure the natural resonance frequencies of atoms. For this he also won the Nobel Prize in 1944. That same year, he was also the first to ‘discuss the idea’ (said his students) that these resonant frequencies were so accurate that they could be used to make high-precision clocks. He also specifically proposed exploiting the frequencies of so-called ‘hyperfine transitions’ of atoms. Such hyperfine transitions refer to transitions between two states with subtle energy differences caused by different magnetic interactions between nuclei and electrons.
In this clock, a beam of atoms in a particular ‘hyperfine state’ travels through an oscillating electromagnetic field. When the atom’s hyperfine transition frequency is closer to the oscillation frequency of the magnetic field, the more energy the atom absorbs from the magnetic field, resulting in a transition from the original hyperfine state to the one state. Through a feedback loop, one can adjust the frequency of the oscillating field until all atoms have completed the transition. An atomic clock is a metronome that uses the frequency of the oscillating field, which is exactly the same as the resonant frequency of the atoms, to generate time pulses.
The work of two pioneering scientists laid the groundwork for the development of the GPS: Left: Rabi’s research into the fundamental properties of atoms and nuclei led him to invent the technique of magnetic resonance, the first The advent of an atomic clock laid the groundwork. Right: The Rabbi’s former student Norman Ramsay laid the groundwork for the development of the cesium beam ‘fountain’ clock. He also invented the hydrogen maser instrument, which redefines the concept of time recording.
The Rabbi himself did not go deep into the work of making such a clock, but other researchers continued to work on improving the idea and technique. In 1949, research by Rabbi’s student Norman Ramsay showed that a more accurate clock could be obtained by passing an atomic beam through an oscillating field twice. For this, Ramsay won the Nobel Prize in 1989.
Currently commonly used high-precision timekeeping tools are manufactured by using the energy-level transition vibrational frequencies of cesium atoms. Such atomic clocks are usually accurate to 1??10-13 seconds per day, or one second in 300,000 years.
Ordinary clocks must rely on a fixed vibration frequency when measuring time. The frequency of the balance wheel of a mechanical watch is 5 or 6 times per second, and the frequency of a tuning fork clock is hundreds to thousands of times per second. The vibration frequency of a quartz watch (quartz oscillating type) is generated by the vibration of a tiny quartz plate, and its fixed vibration frequency is 32,000 times per second. The cesium atomic clock vibrates at a frequency of 9.19??109 times. The higher the vibration frequency, the more precise the timekeeping. The cesium atomic clock is the most accurate timekeeping instrument at present. In addition to cesium, atomic clocks can also be made with the vibrational frequencies of energy level transitions of other atoms. Earthquakes can also be predicted using atomic clocks. If the speed of radio waves or lasers is known, the distance between two points can be calculated as long as an atomic clock is used to measure the time it takes to get from one point to another. Using this principle, small changes in the earth’s surface can be measured. Before an earthquake, changes in the Earth’s crust occur first, mainly in tiny stretches of the surface (just a few meters over a distance of several hundred kilometers). The use of atomic clocks and artificial synchronous satellites can accurately measure the extent of surface extension, so as to effectively predict earthquakes. At present, there are two such earthquake prediction stations in the world, one in California, USA, and one in Munich, Germany.
After World War II, the US National Bureau of Standards and the UK’s National Physical Laboratory worked together to develop atomic time standards based on atomic resonance research done by Rabi and his students. Louis Essen and John Parry of the National Physical Laboratory built the world’s first atomic clock, but the instruments needed for the clock took up an entire room. Another old colleague of Rabbi’s, Gerald Zachirias, from MIT, also tried to improve the atomic clock into a practical device. Zakirias plans to build what he calls an ‘atomic fountain,’ an envisioned atomic clock so precise that it could be used to study the effect of gravity on time as mentioned by Einstein. In practice, he built atomic clocks on a smaller scale that could be pushed from one lab to another. In 1954, Zakirias joined Panasonic Corporation in Malden, Massachusetts, to manufacture atomic clocks for commercial purposes in portable instruments. Two years later, the company built the first commercial atomic clock, the ‘Atomichron,’ and sold fifty of them over the next four years. The atomic clocks we use in GPS systems today are all derived from the Atomichron.
In 1967, due to the abundant achievements in the study of atomic clocks, people redefined the second, that is, according to the oscillation frequency of the cesium atom. Today’s atomic clocks are accurate to within one second every 100,000 years.
Meanwhile, physicists continue to experiment with new approaches to atomic clocks, the idea of atomic resonance proposed by Rabi and his students. In addition to using magnetism, another technique uses a phenomenon called ‘optical pumping’ to pick out atoms at different energy levels that can be used for timing. The technique forces all atoms into a beam to achieve the desired state. Alfred Kasler from the Ecole Normale Sup??rieure in Paris won the Nobel Prize for this. Many atomic clocks today use optically pumped rubidium atoms instead of cesium atoms. Rubidium clocks are smaller and much cheaper than cesium clocks, but not as accurate.
Another atomic clock is the hydrogen maser. The hydrogen maser began with the study of molecular structure by Charles Downes and colleagues at Columbia University in 1954. Downes also shared the 1964 Nobel Prize in Physics for this. A maser, the predecessor of the laser, is a microwave instrument that generates a signal by direct radiation of atoms or molecules. The prototype of Downs’ maser used ammonia molecules, and Ramsay and his colleagues at Harvard invented a maser using hydrogen in 1960 and produced an extremely accurate atomic clock.
In 1967, due to the abundant achievements in the study of atomic clocks, people redefined the second, that is, according to the oscillation frequency of the cesium atom. Today’s atomic clocks are accurate to within one second every 100,000 years. The main standard time in my country refers to the atomic clock recently installed by the National Institute of Standards and Technology (NIST-7). Its accuracy is expected to be within one second every three million years.
Cesium beam clocks, hydrogen maser clocks and rubidium clocks have played an important role in space for decades, either installed on satellites or in ground control systems inside. The satellites of the GPS system must ultimately rely on these cesium clocks, similar to those conceived by the Rabbi sixty years ago.
In 1993, 20 years after the Pentagon conceived the GPS system, with the launch of the twenty-fourth satellite, the GPS system finally became a practical system. The U.S. Air Force operates the satellites and monitors them from five ground stations around the world. The collected data is sent to the Air Force’s Joint Space Operations Center in Colorado for analysis, which transmits the latest data back to each satellite daily to correct clock and orbit data.