The current international standard time is called Coordinated Universal Time (Universal Time Coordinated, UTC), also known as "World Standard Time". It is a combination of atomic time and universal time, based on the second length of atomic time, and is as close as possible to universal time in terms of time. The most commonly used modern timing method is satellite timing, and it is widely used in vehicle navigation, vehicle monitoring, traffic dispatching, shipping, communication base stations, power financial timing and other fields . So, what timing technologies are there in ancient and modern China and abroad? This article will take you to appreciate the timing and timing technology of ancient and modern China and foreign countries!
1. Standard
Guibiao is a simple and important astronomical instrument in ancient times. It consists of a vertical "watch" and a horizontal "gui". The ancients used the length of the shadow at noon to determine the solar terms and the length of a year.
2. Bell and Drum Tower
The ancients divided the night into five shifts, and each shift was an hour, which is two hours in modern times. The Bell and Drum Tower adopts the "copper pot hourglass + bell + drum" timing to tell the time, starting from the evening drum every day and ending with the morning bell.
3. Incense bell
Instruments that use burning spices to measure time appeared in the Northern Song Dynasty (960-1127) at the latest. Every once in a while, the incense will burn a thread, causing the metal ball to fall into the copper plate below and make a sound to report the time.
4. Observatory
The observatory is the earliest observatory in China, and its function is equivalent to the standard for measuring the shadow of the sun. The towering tower-like building is equivalent to a pole standing upright on the ground, and the "long embankment" just north of the platform is a "measuring ruler" used to measure the length of the sun's shadow.
5. Yanyou Hourglass
The whole instrument consists of a sun pot, a moon pot, a star pot, a receiving pot, a copper ruler with an hour scale, and a wooden floating arrow. The wooden arrow in the receiving pot is compared with the time scale on the copper scale to indicate the time at that time.
Historically, this kind of behavior of establishing time standards and transmitting time information is called "respecting the time and giving it to the people", abbreviated as "giving time".
In foreign countries, this behavior is called time service, that is, Time Service.
▉ Evolution of time system from ephemeral time to atomic time
From the 17th to the 19th century, with the continuous improvement of human mechanical technology, the watch manufacturing industry entered a period of rapid development and realized industrial production.
The rapid popularization of clocks and watches has gradually changed people's concept of time and promoted the development and progress of society.
Pocket watch - the standard equipment of British gentlemen in the 19th century
After entering the 20th century, the electronics industry developed rapidly, and battery-driven clocks, alternating current clocks, electromechanical watches, and quartz electronic clocks came out one after another. Clocks and watches have entered a new era of quartzization that combines microelectronics technology and precision machinery, and the daily error is gradually controlled within 0.5 seconds.
At the same time, human cognition of time has also entered a new stage, gradually establishing the concept of "time system".
A time system, also known as a time-frequency reference. To put it bluntly, it is how to measure time.
There are three common time systems, namely:
Universal Time (UT) based on the Earth's rotation period
Ephemeris Time (ET) based on the period of the Earth's revolution around the sun
Atomic time (AT) based on the electromagnetic oscillation frequency emitted by atoms (such as cesium atoms) inside the material
There is inhomogeneity in universal time, and the measurement accuracy of almanac time is low. Therefore, at the 13th World Conference on Weights and Measures in 1967, representatives of various countries voted to adopt atomic time instead of almanac time as the basic time measurement system. The second of atomic time is defined as the time unit of the International System of Units and is one of the basic units of the three major physical quantities.
The current international standard time is called Coordinated Universal Time (Universal Time Coordinated, UTC), also known as "World Standard Time". It is a combination of atomic time and universal time, based on the second length of atomic time, and is as close as possible to universal time in terms of time.
We all know that the earth is divided into 24 time zones according to longitude. Although China spans 5 time zones, we uniformly adopt "Beijing Time", which is the "UTC+8" time zone.
The time zone of our country
▉ What are the methods of timing
Great changes have taken place in timing tools and time systems, and of course the timing method must also change accordingly.
The timing process is actually a communication process. The theory of electromagnetism changed communication, and it also changed timing.
According to different electromagnetic wave frequencies and transmission means, modern timing technology is divided into the following categories:
1. Short wave timing
Use short-wave radio with a wavelength of 100m ~ 10m (frequency: 3MHz ~ 30MHz) for time service.
Take our country as an example. In Lintong, Shaanxi, there is a headquarters of the National Time Service Center of the Chinese Academy of Sciences. It undertakes the generation, maintenance and broadcasting tasks of my country's national standard time (Beijing time).
The time service station of the National Time Service Center is located in Pucheng, Shaanxi. The shortwave radio station here will use 2.5MHz, 5MHz, 10MHz, and 15MHz frequencies to continuously broadcast the Chinese shortwave radio time signal throughout the day, and the call sign is BPM.
The shortwave timing signal is transmitted by sky wave and ground wave. The ground wave can transmit 100 kilometers, and the sky wave can cover a radius of more than 3,000 kilometers, basically covering the whole country, and the timing accuracy is on the order of milliseconds.
sky and ground waves
2. Long wave timing
The long-wave radio with a wavelength of 10km-1km (frequency: 30KHz-300KHz) is used for time service.
The call sign of the long-wave radio station of the National Time Service Center is BPL, and the transmission frequency is 100KHz.
The ground wave action distance of the long wave timing signal is 1000-2000 kilometers, and the sky wave signal is 3000 kilometers, basically covering my country's inland and offshore waters, and the timing accuracy is on the order of microseconds.
3. Low frequency time code timing
Low-frequency time code timing is a special long-wave timing, which is suitable for regional standard time and frequency transmission.
The National Time Service Center adopts the continuous wave time code time service system technology with a carrier frequency of 68.5KHz.
Our common radio-controlled clocks/radio-controlled watches can receive this signal and automatically check the time. The accuracy can reach 300,000 years and the error does not exceed 1 second.
radio controlled watch
4. Telephone timing
Using the telephone network to transmit standard time is called telephone time service.
For example, through a dedicated telephone time code receiver, dial the service line of the National Time Service Center to automatically obtain standard Beijing time display and output, and the time service accuracy is 10 milliseconds.
5. TV timing
Haha, this does not refer to the news broadcast at 19:00 every day.
No one would have thought that China Central Television "secretly" inserted the time information provided by the atomic clock into its own TV signal. After the user equipment receives the TV signal, it can be corrected to realize timing with an accuracy of about 10 microseconds.
6. Network timing
Everyone should be familiar with this. NTP (Network Time Protocol, Network Time Protocol), which is often used on our computers, is network time service.
As long as the IP address of the target NTP server is set, the local computer can realize time synchronization.
NTP configuration interface
7. Satellite timing
What we have introduced above are all ground-based timing methods. Next, let’s take a look at the most popular space-based timing method, which is "satellite timing".
We use navigation and positioning apps such as Baidu and Gaode every day. Everyone should also know that the reason why these apps can achieve navigation and positioning is because mobile phones can communicate with satellites and use the services provided by satellites.
The satellite system that provides navigation and positioning services is called the GNSS system (Global Navigation Satellite System).
The famous GPS is the GNSS system of the United States and the earliest GNSS system in the world. The Beidou, which is now famous, is a GNSS system independently developed and built by China.
GNSS systems that also have global coverage capabilities include Russia's GLONASS (Glonass) and Europe's Galileo (Galileo).
In addition to global satellite systems, GNSS also includes some regional systems and augmentation systems.
Many people don't know that besides positioning and navigation, the GNSS system also has a very important function, that is-time service.
The three core capabilities of GNSS, usually abbreviated as PVT, are Position (position), Velocity (speed) and Time (time).
So, how does GNSS realize timing?
On every GNSS satellite, there is an atomic clock. This allows precise time data to be included in the transmitted satellite signal. With a dedicated receiver or GNSS timing module, these signals can be decoded, and the time synchronization of the device with the atomic clock can be quickly performed.
Compared with the long-wave, short-wave, network and other timing technologies mentioned above, GNSS satellite timing has obvious technical advantages.
First, the accuracy of GNSS timing is higher.
Take Beidou as an example. The time of the Beidou satellite navigation system is called BDT. When BDT is an atomic time, it can be traced back to the coordinated universal time UTC of the National Time Service Center of my country, and the time difference control accuracy with UTC is less than 100ns.
Comparison of timing accuracy of various timing methods
In addition to accuracy, GNSS satellite timing also has inherent coverage advantages.
Both long-wave and short-wave ground-based timing are limited by physical propagation distance. If it encounters environmental barriers such as mountains, the transmission distance will be further reduced.
The GNSS satellite timing is obviously much stronger in terms of coverage. Especially for ocean navigation and aerospace scenarios, GNSS satellite timing has obvious advantages.
Summarize:
GNSS satellite timing has obvious technical advantages - higher timing accuracy and wider coverage.
Information reference: www.skylab.com.cn/newsview-1015.html
Higher timing accuracy: Take Beidou as an example. The time of the Beidou satellite navigation system is called BDT. When BDT is an atomic time, it can be traced back to the coordinated universal time UTC of China's National Time Service Center, and the time difference control accuracy with UTC is less than 100ns; SKYLAB has a GPS timing module, and the Beidou timing module has a timing accuracy of 10nS, 15nS, and 20nS.
Wider coverage: Thanks to the signal coverage of GNSS satellites, in environments such as weak signals or alpine jungles without network coverage, especially for ocean navigation and aerospace scenarios, GNSS satellite timing can ignore the limitation of physical propagation distance, real-time Output time data for decoding by the GNSS timing module and quickly synchronize the time between the device and the atomic clock.