A time service is a system or institution that provides accurate and synchronized timekeeping for a wide range of applications. From coordinating global financial transactions to ensuring the precision of scientific experiments, satellite timing are the backbone of countless industries and activities that rely on precise timing. This article mainly introduces satellite timing and its several application fields.

1.The History of Timing

The history of satellite timing is closely intertwined with the development of satellite technology and the quest for accurate timekeeping on a global scale. The journey began with the launch of the first artificial satellite, Sputnik 1, by the Soviet Union on October 4, 1957. While Sputnik 1 itself did not carry precise timing equipment, its launch marked the beginning of the space age and paved the way for subsequent advancements in satellite-based timekeeping.

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In the early years of space exploration, satellites primarily served scientific and military purposes. However, the need for precise timing capabilities became apparent as researchers and engineers sought to synchronize experiments, coordinate spacecraft operations, and facilitate communication across vast distances.

The concept of Global Navigation Satellite Systems (GNSS) emerged in the late 20th century, aiming to provide precise positioning, navigation, and timing services to civilian and commercial users worldwide. The Global Positioning System (GPS), developed by the United States Department of Defense, became the first fully operational GNSS, achieving initial operational capability in the 1980s. GPS utilized a constellation of satellites broadcasting precise timing signals synchronized with Coordinated Universal Time (UTC), enabling users to determine their precise position and time anywhere on Earth.

2.Satellite Timing and Other Methods of Timing

2.1 Shortwave timing

Timing is carried out using shortwave radio with wavelengths between 100m and 10m (frequencies: 3MHz to 30MHz).

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Take China as an example. In Lintong, Shaanxi, there is the headquarters of the National Time Service Center of the Chinese Academy of Sciences. It is responsible for the generation, maintenance, and dissemination of China Standard Time (Beijing Time).

The time signal transmitter of the National Time Service Center is located in Pucheng, Shaanxi. Shortwave radio stations here use frequencies of 2.5MHz, 5MHz, 10MHz, and 15MHz to continuously broadcast China's shortwave radio time signal, with the call sign BPM.

Shortwave time signals are transmitted via sky wave and ground wave. Ground waves can transmit up to 100 kilometers, while sky waves cover a radius of over 3000 kilometers, basically covering the entire territory of China, with timing accuracy at the millisecond level.

2.2 Longwave timing

Timing is carried out using longwave radio with wavelengths between 10km and 1km (frequencies: 30kHz to 300kHz).

The longwave radio station of the National Time Service Center of China has the call sign BPL, with a transmission frequency of 100kHz.

The ground wave range of longwave time signals is 1000-2000 kilometers, and the sky wave range is 3000 kilometers, basically covering inland areas and nearby sea areas of China, with timing accuracy at the microsecond level.

In addition, there are other methods of timing

Low frequency time code timing
Telephone timing
TV timing
Network timing
2.3 Satellite timing

People use navigation and positioning apps like Google Maps and Baidu Maps every day. We should also know that these apps are able to provide navigation and positioning because phones can communicate with satellites and use the services provided by satellites.

Navigation satellite systems that provide navigation and positioning services are called GNSS (Global Navigation Satellite Systems).

The GPS we are familiar with is the GNSS system of the United States and also the earliest GNSS system globally. The now-famous BeiDou is China's independently developed and constructed GNSS system. Other GNSS systems with global coverage capabilities include Russia's GLONASS and Europe's Galileo.

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Many people do not know that in addition to positioning and navigation, GNSS systems also have a very important function, which is—timing. The three core capabilities of GNSS, commonly referred to as PVT (Position, Velocity, and Timing) .

Then, how does GNSS achieve time synchronization?

Each GNSS satellite is equipped with an atomic clock. This means that the satellite signals sent contain accurate time data. By decoding these signals using specialized receivers or GNSS timing modules, devices can be quickly synchronized with atomic clocks.

Compared to the previously mentioned longwave, and shortwave, GNSS satellite timing has significant technical advantages.

GNSS timing has higher accuracy. Take BeiDou as an example. The time of the BeiDou Satellite Navigation System is called BDT. BDT belongs to atomic time and can be traced back to Coordinated Universal Time (UTC) of China's National Time Service Center, with a time difference control accuracy to UTC of less than 100 nanoseconds. For example, high precision positioning module ER-GNSS-M01 possesses timing accuracy of 10 ns.

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