On September 19, 2024, at 9:14 AM, the 59th and 60th Beidou Navigation Satellites soared into the sky aboard the Long March 3B rocket, along with the YZ-1 upper stage. This pivotal launch marks the successful completion of the Beidou-3 global satellite navigation system, with these final two satellites now in orbit.

What does this full deployment of Beidou-3 represent? Since the inception of the Beidou-1 project in 1994, what impact has it had on the global satellite navigation landscape?

**Clarifying the Role of the Final Two Satellites**

Some may be puzzled by the recent launch of Beidou satellites 59 and 60, believing the system was already complete. In fact, the core network for Beidou-3 was finalized back on June 23, 2020, with the launch of the 55th satellite. The satellites launched afterward—numbers 56 through 58, and now 59 and 60—have served primarily as backups. Though often regarded as secondary, these satellites are essential; they enhance system capabilities, expand service offerings, and support the stable operation and large-scale application of the navigation system while also testing technologies for future Beidou satellites.

Beidou is China’s independently developed satellite navigation system, named after the iconic celestial “Beidou,” which was historically used for navigation. Before Beidou, three major satellite navigation systems existed: GPS from the United States, GLONASS from Russia, and Galileo from the European Union. GPS, in particular, has become synonymous with satellite navigation, with many users unawares of its specific national origins.

Satellite navigation plays a crucial role in national security, transportation, emergency response, resource allocation, and advancements in aerospace technology. This necessity prompted China to pursue its own satellite navigation system, aiming to reduce dependency on foreign technology and the risks associated with it.

Since the late 20th century, China has worked towards developing a suitable satellite navigation system, gradually implementing a three-phase strategy: by the end of 2000, the Beidou-1 experimental navigation system was established for domestic use and key technology validation; by the end of 2012, the Beidou-2 system became operational across the Asia-Pacific; and by 2020, Beidou-3 achieved full global functionality.

**How Satellites Achieve Precise Positioning**

The core purpose of a navigation system is precise positioning. So, how do satellites determine the locations of people and objects?

Essentially, satellites broadcast their positions along with the current time, a signal picked up by user devices. By calculating the distance to each satellite based on the time delay of the signal, users can triangulate their precise position. To visualize this, picture three stakes in the sand and a blindfolded person walking randomly. They could ask friends how far they are from each stake, and by drawing circles based on those distances, the intersections would reveal their exact location.

Satellite positioning operates on a similar principle—sending signals that detail satellite coordinates and precise timing. When a user’s device receives the signal, it calculates the distance to the satellite based on the time it took for the signal to travel. By using signals from four satellites, users can determine their exact three-dimensional location.

**The Diversity of Beidou Satellites**

To ensure constant access to at least four satellites, a substantial number must be launched. Currently, the Beidou-3 system includes 35 satellites, including backups. The recently launched 59th and 60th satellites continue the numbering from the first Beidou-2 satellite launched in April 2007, representing the total 60 satellites launched across both Beidou generations.

Beidou satellites are categorized by their orbits. The first category includes Geostationary Earth Orbit (GEO) satellites, positioned 35,786 kilometers above the equator. These satellites synchronize their orbit with the Earth’s rotation, effectively hovering over a single spot. Just three GEO satellites can cover most of the globe, leaving out only the poles.

The second category, Inclined Geosynchronous Orbit (IGSO) satellites, trace a figure-eight pattern during their orbit. This configuration is particularly beneficial for users in northern latitudes, who may struggle to receive signals from GEO satellites due to geographical obstacles.

Lastly, Medium Earth Orbit (MEO) satellites orbit around 20,000 kilometers, making two passes around the Earth each day. Studies indicate that a constellation of 24 MEO satellites can effectively meet global navigation accuracy standards.

The Beidou-3 system comprises four GEO satellites, three IGSO satellites, and 28 MEO satellites, several of which serve as backups.

**Atomic Clocks and Enhanced Systems for Precision**

Precise timing is crucial for navigation satellites. Beidou satellites utilize onboard rubidium and hydrogen atomic clocks to achieve measurement accuracy of 0.5 nanoseconds per day. To put this in perspective, after 5.5 million years, the cumulative error would only amount to one second. This exceptional precision translates to a positional accuracy of about 15 centimeters daily, but to maintain such accuracy, regular synchronization with ground stations is necessary.

The navigation system also accounts for relativistic effects. As satellites move rapidly relative to the Earth’s surface, they may experience time dilation, impacting timing accuracy. To mitigate this, adjustments based on satellite altitude and regular synchronization with ground standards are critical.

Interference from the Earth’s atmosphere can cause measurement errors of up to several dozen meters. To address this, the Beidou system employs an augmentation approach that includes both space-based and ground-based components. Geostationary satellites broadcast correction data, while a network of ground stations provides real-time, high-precision positioning services.

**Next Generation Technologies Await**

China has pioneered innovative technologies during the Beidou navigation system’s development, one of which is the Beidou short messaging service, allowing brief text messages to be sent via satellite. This capability was vital during the 2008 Wenchuan earthquake when traditional communication systems failed, enabling first responders to share critical information through Beidou.

As of November 2023, the International Civil Aviation Organization’s revised Annex 10, which includes standards for the Beidou satellite navigation system, became effective. This advancement affirms Beidou’s global service capacity and will promote the system’s application in civil aviation.

Inter-satellite link technology has been crucial for extending Beidou’s global reach. This technology allows for direct communication between satellites, enabling them to autonomously update navigation data, thereby enhancing reliability and minimizing dependence on ground control.

Since becoming operational, the Beidou system has found extensive applications in transportation, agriculture, disaster response, and public safety sectors, significantly benefiting both the economy and society. Yang Changfeng, the design architect, recently announced a push to accelerate the development of the next-generation Beidou system, aiming for a more integrated, comprehensive, and intelligent framework by 2035.

The recently launched 59th and 60th satellites include upgraded atomic clock technology and new inter-satellite link terminals. Scientists will utilize these enhancements to further improve Beidou’s reliability and performance across navigation, timing services, and global short messaging capabilities. Additionally, these satellites will play a key role in testing new technologies for the next generation of Beidou.