In-depth Explanation: What is "Time Granting"?

Latest update time：2020-09-14

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What is the most precious thing in the world?

I believe many people’s answer is – “time”.

Yes, time is very important. In ancient times, countless sages warned us to cherish time and make good use of it. As the saying goes, "Time is money, but money can't buy time."

So, the question is, the ancients had neither clocks nor watches, so how did they know the time?

▉ “Respectfully remember the time of heaven and teach it to the people”

Everyone should remember that in costume dramas, a day is divided into twelve hours.

After nightfall, every hour, a night watchman would beat the gong, rhythmically beating the clappers and shouting, "It's dry, be careful of fire!"

Yes, if the ancients wanted to know the time information, they basically relied on "listening".

At that time, there was a group of "civil servants" who confirmed the time through tools such as gnomon and sundial, and then passed the time information to the surrounding residents through methods such as ringing bells in bell towers, beating drums in drum towers, and night watchmen beating the clock.

Around the emperor, there was a group of higher-ranking astrology experts who were responsible for observing the night sky, formulating the calendar, and guiding farmers to sow, fertilize, and harvest on time.

Historically, this behavior of establishing time standards and transmitting time information is called "respectfully recording the time and passing it on to the people", which is abbreviated as "授时" .

In foreign countries, this behavior is called Time Service .

▉ From calendar time to atomic time, the evolution of the time system

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 achieved industrialized 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: a must-have for British gentlemen in the 19th century

After entering the 20th century, the electronics industry developed rapidly, and battery-driven clocks, AC clocks, electromechanical watches, and quartz electronic clocks were introduced one after another. Clocks entered a new era of quartzization that combined microelectronics technology with precision machinery, and the daily error was gradually controlled within 0.5 seconds.

At the same time, human's understanding of time has entered a new stage, and the concept of "time system" has gradually been established .

The time system, also known as the time frequency standard, is simply how to measure time.

There are three common time systems:

Universal Time (UT) based on the Earth's rotation period
Ephemeris Time (ET) based on the Earth's revolution period around the Sun
Atomic time (AT) is based on the frequency of electromagnetic oscillations emitted by atoms inside matter (such as cesium atoms).

Universal time is not uniform, and the measurement accuracy of calendar time is low. Therefore, at the 13th World Conference on Weights and Measures in 1967, representatives of various countries voted to adopt atomic time to replace calendar time as the basic time measurement system. The length of a second in 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 internationally accepted standard time is called Coordinated Universal Time (UTC), also known as "World Standard Time" . It is a combination of atomic time and world time, based on the length of seconds in atomic time, and as close to world time as possible.

We all know that the earth is divided into 24 time zones according to longitude. Although China spans 5 time zones, it uniformly adopts "Beijing Time", which is "UTC+8".

Our country's time zone

▉ What are the methods of timekeeping?

The timekeeping tools and time systems have undergone tremendous changes, and the timekeeping methods must of course change accordingly.

The timing process is actually a communication process. Electromagnetic theory has changed communication and timing as well.

According to different electromagnetic wave frequencies and transmission methods, modern timing technologies are divided into the following categories:

1. Shortwave timing

Shortwave radio with a wavelength of 100m to 10m (frequency: 3MHz to 30MHz) is used for time synchronization.

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 is responsible for the generation, maintenance and broadcasting of China's national standard time (Beijing time).

The time station of the National Time Service Center is located in Pucheng, Shaanxi. The shortwave radio station here uses 2.5MHz, 5MHz, 10MHz, and 15MHz frequencies to continuously broadcast my country's shortwave radio time signs throughout the day with the call sign BPM.

Shortwave timing signals are transmitted via sky waves and ground waves. Ground waves can transmit 100 kilometers, while sky waves have a coverage radius of more than 3,000 kilometers, basically covering the entire country, with a timing accuracy of milliseconds.

Sky waves and ground waves

2. Long wave timing

Long-wave radio with a wavelength of 10km-1km (frequency: 30KHz~300KHz) is used for time synchronization.

The long-wave radio station of the National Time Service Center has a call sign of BPL and a transmission frequency of 100KHz.

The ground wave range of the long-wave timing signal is 1,000-2,000 kilometers, and the sky wave signal is 3,000 kilometers, basically covering my country's inland and offshore waters, and the timing accuracy is in the microsecond level.

3. Low frequency time code timing

Low-frequency time code timing is a special type of long-wave timing, which is suitable for regional standard time and frequency transmission.

The National Time Service Center adopts a continuous wave time code timing system technology with a carrier frequency of 68.5KHz.

Our common radio clocks/watches can receive this signal and automatically adjust the time with an accuracy of no more than 1 second in 300,000 years.

Radio-controlled watch

4. Telephone timing

The use of the telephone network to transmit standard time is called telephone timing.

For example, by dialing the National Time Service Center's service hotline through a dedicated telephone time code receiver, you can automatically obtain the standard Beijing time display and output with a timing accuracy of 10 milliseconds.

5. Television timing

Haha, this does not refer to the news broadcast at 7 p.m. every day.

You probably wouldn't have thought that CCTV actually "secretly" inserts time information provided by an atomic clock into its own TV signals. After receiving the TV signals, the user equipment can correct them and achieve timing with an accuracy of about 10 microseconds.

6. Network timing

This should be familiar to everyone. The NTP (Network Time Protocol) that we often use on our computers is the network time synchronization.

As long as the IP address of the target NTP server is set, the local computer can achieve time synchronization.

NTP Configuration Interface

7. Satellite timing

What we have introduced before 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 like Baidu and AutoNavi every day. As you all know, these apps can achieve navigation and positioning 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 now famous Beidou is a GNSS system independently developed and built by China.

Other GNSS systems that also have global coverage capabilities include Russia's GLONASS and Europe's Galileo.

In addition to the global satellite system, GNSS also includes some regional systems and augmentation systems.

Many people don’t know that in addition to positioning and navigation, the GNSS system has another very important function, which is timing.

The three core capabilities of GNSS are usually referred to as PVT, namely Position, Velocity and Time.

So, how does GNSS achieve timing?

Each GNSS satellite is equipped with an atomic clock. This means that the satellite signals sent contain accurate time data. These signals can be decoded through a dedicated receiver or GNSS timing module, and the device can be quickly synchronized with the atomic clock.

Compared with the long-wave, short-wave, network and other timing technologies mentioned above, GNSS satellite timing has obvious technical advantages.

First, GNSS timing is more accurate.

Take Beidou as an example. The time of Beidou satellite navigation system is called BDT. BDT is atomic time and can be traced back to the coordinated universal time (UTC) of my country's National Time Service Center. 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 an inherent coverage advantage.

Both long-wave and short-wave ground-based timing are limited by physical propagation distance. If there are environmental barriers such as high mountains, the propagation distance will be further reduced.

GNSS satellite timing is obviously much stronger in coverage, especially for ocean navigation and aerospace scenarios, GNSS satellite timing has obvious advantages.

▉ Application scenarios of timing services

Why do we need such a high-precision timing service? Is it just to facilitate online shopping?

of course not.

Given the physiological limits of human beings, millisecond-level accuracy is sufficient. High-precision timing such as GNSS is mainly used in high-tech fields.

Human competitive sports are generally only accurate to milliseconds.

The earliest demand for high-precision timing applications came from aerospace.

Aerospace vehicles often fly at extremely high speeds. Without precise time synchronization, the exact position of the vehicle cannot be confirmed.

Especially in scenarios such as space docking, if the time of the two spacecraft is not synchronized, the distance will be very different, and there will be huge errors in the flight attitude, which will eventually lead to serious accidents.

Space Docking

In addition to the scientific research field, as high-tech is gradually implemented in various industries, many systems closely related to our lives also have high-precision timing requirements, such as power systems, financial systems, and communication systems.

Why does the power industry require time synchronization?

It's very simple. We all use alternating current, and the direction of the current in alternating current changes with time. When different power grid equipment is connected to the grid, if the time is inconsistent, the peaks and troughs will be inconsistent, which will cause unnecessary energy loss at the least, or even a direct short circuit, destroying equipment, paralyzing the grid, and causing large-scale power outages.

Power grid equipment

The financial sector also relies on time synchronization.

Now we are all digital finance, all transactions are conducted through computers and the Internet. If the system time is not synchronized, it is likely to cause transaction failures and missed opportunities in the ever-changing market. Unsynchronized time may also be exploited by hackers, bringing security risks to the system.

The communication systems we are familiar with also rely on the support of high-precision timing.

The switching and roaming of communication base stations require precise time control, high synchronization accuracy, and sufficient stability. TDD time division systems represented by TD-LTE have higher requirements for time synchronization, and the system time synchronization requirement is ±1.5μs.

The 5G we are using now basically adopts the TDD time division multiplexing mode. In the process of high-speed data transmission, the time synchronization accuracy is extremely high. If the time between communication devices is not synchronized, it will affect the time slot and frame, and thus affect the normal operation of the business.

In addition to the above-mentioned industries, various fields including traffic dispatching, geographic surveying and mapping, earthquake prevention and disaster reduction, and meteorological monitoring all have rigid demands for high-precision time synchronization.

▉ High-precision timing module

At present, GNSS satellite timing has become the most popular and widely used timing method due to its advantages such as high timing accuracy, wide coverage and low implementation cost.

More and more industries are choosing GNSS satellite timing as their time solution. New GNSS timing modules are emerging one after another, and the shipment volume is increasing year by year, with a bright market prospect.

Take Quectel's L26-T and LC98S modules as an example.

Both modules are automotive-grade high-precision GNSS timing modules that support GPS, GLONASS, BeiDou, Galileo and QZSS multi-satellite systems, and integrate differential global positioning system (DGPS) and satellite-based augmentation system (SBAS) (including WAAS, EGNOS, MSAS and GAGAN), which can significantly improve the stability and accuracy of timing. Even in complex signal environments, they can provide high-precision, high-integrity precision timing services.

The existence of the Position hold mode greatly reduces the timing jitter of the module. It supports single-satellite timing, which can maintain high-precision timing even when there is only one visible satellite. The AGNSS function can also help the module significantly reduce the first positioning time.

The module undergoes rigorous reliability testing before leaving the factory to ensure that it can work normally in complex environments and provide precise timing services for global communication base stations, financial services, power systems, railway dispatching and other industry applications.

Okay, after saying so much, I think everyone should have a comprehensive understanding of timekeeping, right?

As the wave of digitalization continues to deepen, high-precision timing services will enter more industries and create more application scenarios. The importance of timing-related equipment and systems is becoming increasingly prominent, and they are gradually becoming an important national information infrastructure.

High-precision timing services will completely change the lives of each of us.

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