China's DRO constellation & resonant orbits

I was reading about the first satellite laser ranging experiment at Earth-Moon distances [REF] and got very intrigued by the mission behind this achievement. This space mission has almost everything that an astrodynamics enthusiast loves, from near-failure after an upper stage malfunction, to a long-duration lunar weak capture trajectory and from exciting resonant and distant retrograde orbits [more info here] to cislunar navigation systems.

Mission timeline

In a remarkable display of resilience and innovation, China successfully established the world's first satellite laser ranging experiment at lunar-distance scales back in April 2025, marking a major technological breakthrough in deep-space exploration. To achieve that, they employed the first satellite constellation in a Distant Retrograde Orbit (DRO) around the Earth-Moon system, overcoming significant challenges along the way [REF].

The adventure of the DRO probes started back in March 2024 on top of a Chang Zheng 2C (Long March 2C) rocket (launch overview here). The DRO-A and DRO-B satellites didn't initially reach their intended trajectories, due to a malfunction of the Long March 2C upper stage, which failed at executing the trans-lunar injection (TLI) to bring the probes on their path towards the moon, thereby leaving them stranded in low Earth orbit [REF]. Given their limited on-board propellant, a creative "escape-plan" was needed, to place the probes in their proper trajectory. This was tackled using a weak capture, a fuel-efficient orbit insertion method, which used very precise trajectory adjustments and a large dose of patience (exchanging fuel with time) to place the probes around the moon using the moon's and sun's gravity.

According to information presented by the mission design team and shown here, the  emergency orbit correction took:

• 167 days

• 5x Perigee Kicks: Small engine burns at the satellites' closest approach to Earth gradually raised their apogee.

• 5x Trajectory Adjustments: Five major orbital maneuvers and additional trajectory corrections fine-tuned the path toward the Moon.

• 3x Gravity Assists: Utilized Earth's and Moon's gravity to aid in reaching the desired orbit

before placing the probes into a DRO on August 27th 2024. A similar "rescue" of a stranded probe was executed for the Japanese Hiten probe back in 1990, which was the first ballistic capture into lunar orbit (albeit not into a DRO) [REF].

The third member of the constellation, DRO-L, was launched a month before DRO-A and B into low Earth orbit, following a conventional sun-synchronus path.

On August 28th, the DRO-A and DRO-B vehicles separated from each other and subsequently captured images of one another. It was observed that the solar panels on DRO-A were bent nearly 90 degrees, while the arrays on DRO-B appeared to be suspended from wires. The reason for the damages appears to be a high spinning rate (~1.8 revolutions per second) due to the failure of the Yuanzheng-1 upper stage. Despite this damage, the power generated remained adequate for the operation of the vehicles. The satellites successfully established K-band microwave inter-satellite measurement links to DRO-L, creating an interplanetary satellite network independent of ground stations [REF]. 

DRO-A satellite after separation [REF].

DRO-B satellite after separation [REF].

With this mission, the Chinese Academy of Sciences' (CSU) has established a navigation system that enables auto-piloted satellites in the vast Earth-moon space, which is about 10,000 times larger than the traditional habitat of satellites – the Low Earth Orbit [REF].

In this location, the spacecraft are evaluating the characteristics of the unique orbit and testing technologies, including communication with DRO-L. The country intends to establish lunar navigation and communication infrastructure to support lunar exploration, and these satellites could contribute to these plans.

A diagram of the three DRO satellites and their orbits for the communication network [REF].

Mission objectives

China’s DRO-A and DRO-B mission is not just about technical demonstration—it’s part of a larger strategic blueprint to establish sovereign capabilities in cislunar space, which is the increasingly contested and geopolitically significant area between Earth and the Moon.

The mission is an attempt to demonstrate leadership in the field of cislunar infrastructure. As nations race to establish permanent lunar operations, including NASA’s Artemis and the Lunar Gateway, China aims to build an independent Earth-Moon infrastructure. In that pursuit, DRO orbits are ideal for long-duration, stable satellite deployment to support future navigation, communications, surveillance, and early warning systems.

When it comes to deep-space navigation, current navigation systems like GPS or BeiDou are not effective in cislunar space. These satellites orbit at ~20,200 km (MEO) above Earth and their antennas are designed to point toward Earth's surface, providing optimal overage up to about 3,000–4,000 km above Earth (and extending up to GEO), far away from the cislunar space which extends to nearly 400,000 km (the Moon's average distance). Even if a GPS signal can be detected in cislunar space, it's typically from the side lobes of the broadcast antenna — millions of times weaker than signals received on Earth, which makes acquisition and tracking difficult and unreliable without highly sensitive and specialized receivers. For autonomous deep-space navigation to be materialized, the presence and operation of constellations like the DRO-A and DRO-B would provide a massive support in space-based orbit determination and inter-satellite ranging, reducing dependence on Earth-based tracking and improving deep space mission flexibility.

At the same time, establishing space-to-space communication links (K-band microwave) is a step toward an independent lunar communication and PNT (Positioning, Navigation, and Timing) system. This is essential for future Chinese crewed lunar landings and robotic missions and underscores the general ambition of China to establish a permanent lunar presence. Specifically, during tests to interlink the satellites, the Chinese Academy of Sciences noted that K-band microwave measurement and communication links between the satellites and the ground were performed at a distance of 1.17 million kilometers. Additionally, the three satellites utilized three hours of in-orbit inter-satellite measurement data to determine their orbit at a precision level that would typically require two days of ground-based tracking [REF].

This is actually a big milestone. Inter-satellite measurement allows for direct communication and data exchange between the satellites in a constellation. Each satellite can precisely measure the distance and relative motion between itself and other satellites in the constellation, allowing them to quickly determine their positions with respect to each other, addressing challenges such as insufficient ground-based tracking and control precision.

The technology application is especially useful in scenarios where the satellites are in close formation or where relative positioning is important. A major benefit when it comes to this demonstration is that unlike ground-based tracking, where the measurement data has to be transmitted from the satellite to the ground station, processed, and then analyzed, inter-satellite links allow for data transfer in real time between the satellites. The measurement and processing can occur onboard the satellites, allowing them to update their orbital parameters immediately, without waiting for ground-based feedback. This real-time exchange of data significantly reduces the time required to determine an orbit, since the satellites can continuously update and refine their positions and velocities without waiting for ground stations to track them over extended periods of time.

Not to mention the increased precision compared to ground-based radar measurements, which are affected by atmospheric interference and signal delay. Ground-based tracking systems are influenced by atmospheric conditions (e.g., ionospheric effects, weather). These factors introduce errors in the measurements and can lengthen the time required to make precise orbit determinations. In contrast, inter-satellite measurements are not subject to atmospheric interference, meaning the data collected is inherently more accurate and timely.

Long term, a higher frequency of measurement updates and more data points will be possible with a larger number of satellites in the constellation, as multiple probes could be involved in measuring each other's positions. The more satellites are involved, the more data points are available to determine the orbits and the system can use multiple redundant measurements to improve accuracy. At the same time, satellites can perform continuous in-orbit updates to each other's orbital parameters. When satellites are communicating with each other in space, the measurement data can be constantly refined, leading to faster orbit determination. This contrasts with ground-based tracking, which typically relies on periodic measurement intervals.

Moreover, with their successful laser ranging demonstration in April 2025 at Earth-Moon distance, achieving centimeter-level accuracy, China was able to show that laser ranging works across cislunar distances (~400,000 km), paving the way for high-precision tracking of future Moon missions. Applications like this will be very useful for missions like NASA's LunaNet and ESA’s Moonlight plan to deploy communication and navigation satellites around the Moon. The ultimate goal is a seamless tracking and control of lunar landers, rovers, and eventually crewed missions and the feats of inter-satellite and ground-to-satellite communications are complementary to achieve it.

DROs and resonant orbits

Apart from the strategic and technological significance of the missions's achievements, the chosen orbits for the DRO-A and DRO-B probes are extremely interesting. After its separation from DRO-A, DRO-B seems to have entered a 3:2 resonant orbit, based on the mission schematics that can be found here. The description of the three probes in the same source, confirms that the DRO-B orbit consists of a "DRO trajectory and resonant trajectory":

Description of the three satellites in the constellation [REF]

What Are Resonant Orbits?

Resonant orbits are orbital trajectories for which the orbital period of a spacecraft maintains a simple numerical ratio with the orbital period of another body—typically the Moon in cislunar applications. For instance, a 2:1 resonant orbit would mean that the spacecraft orbits Earth twice for every orbit of the Moon. One can get more into detail and discern the differences between sidereal resonant orbits and synodic resonant orbits, but for the purposes of this post, we are referring to sidereal resonant orbits in the Earth-Moon system, as defined by Gupta in [REF].

One of the key motivations for emplying resonant lunar orbits, is that they enable predictable flybys or rendezvous opportunities with the Moon or with other spacecraft in cislunar space. This predictability is crucial for missions that require periodic lunar access, such as crewed operations, cargo transport, or lunar sample returns. Future logistical systems such as the Lunar Gateway and cislunar depots could rely on resonant orbit geometries to coordinate Earth–Moon transits, optimize delivery schedules, and maintain communications. Using resonant orbits supports cyclical operations, such as scheduled launches from Earth syncing with depot availability in lunar orbit. This extends also to flexibility when it comes to mission planning, especially focusing on launch and abort scenarios. If a mission must be delayed, resonant geometry helps ensure that subsequent opportunities remain aligned with lunar phases and energy minima. This enhances mission robustness and allows for safer abort scenarios with relatively low delta-V penalties.

A further key goal of the proposed infrastructure using resonant orbits is to create sustainable and reusable pathways in cislunar space. Resonant orbits offer long-term stability for staging platforms or relay satellites without requiring constant propulsion. For example, staging orbits near the Moon (e.g., DROs or NRHOs with resonant characteristics) serve as depots or transfer stations that spacecraft can regularly access. Along with stability, comes also performance, as resonant orbits can be used as part of low-energy transfer strategies like ballistic lunar transfers. These exploit the natural dynamics of the Earth-Moon system to reduce fuel requirements. By leveraging multi-body gravitational dynamics one can design transfers with minimal propellant usage—where resonant orbits play a crucial role in timing these dynamics effectively.

In the circular restricted 3-body problem (CR3BP), the presence of one periodic resonant orbit typically indicates the presence of a family of resonant orbits. Unlike resonant orbits in the two-body model, which are isolated solutions for a specific resonance ratio and eccentricity, families of resonant orbits in the CR3BP offer a variety of possible options. Although these orbits within a family span a range of periods and are thus only nearly resonant, they still display resonant behavior and characteristics, maintaining the utility of the fundamental sidereal resonant orbits.

Representative orbits in planar (grey) and three-dimensional families of p:q resonant orbits in the Earth-Moon system (rotating frame). From Vaquero et al. [REF].

Also, for certain sidereal resonant orbit families in the CR3BP, it is possible to identify members that, in addition to being nearly sidereal resonant with the Moon, exhibit lunar synodic resonance as well. This characteristic arises due to the orbits being nearly sidereal resonant, a characteristic that allows the family of sidereal resonant orbits to span a wide range of orbital periods in the rotating frame. Thus, the orbits retain favorable geometries and stability properties inherent to their sidereal resonant structure, with the added advantage of repeatability with respect to the orientation of the Sun in the Earth-Moon rotating frame. This property is particularly relevant for Space Situational Awareness (SSA) operations beyond the vicinity of the Earth, where illumination and visibility conditions are evaluated [REF].

In the context of cislunar infrastructure, resonant orbits are not just an academic curiosity—they are a critical enabler of sustainable, economical, and routine space operations. They help manage fuel use, improve schedule predictability, and integrate seamlessly with the complex gravitational environment of the Earth-Moon system.

3:2 resonant orbit family (left) with stability and period evolution (right) from Gupta 2024 [REF]. Note the resemblance to the orbital shape in the DRO A/B mission profile.

In this context, the 3:2 resonant orbit (meaning that for every 3 orbits of the spacecraft around Earth, the Moon completes 2 orbits) has been investigated as an enabler for efficient missions due to its stability, shadow avoidance and the fact they tour the L₃, L₄ and L₅ points [REF].

A possible orbital configuration for the DRO-B mission is illustrated the following animation. In the absence of available details on the orbital parameters, a Jacobi constant of 2.7396 was chosen, leading to an orbital period of roughly 55.27 days in the rotating frame (roughly twice the Moon's sidereal month), an Earth perigee of approximately 120,000 km and an Earth apogee of approximately 460,000 km.

Simulation of a 3:2 resonant orbit in the rotational (synodic) Earth-Moon frame and in the inertial frame. The circular restricted three-body problem (CR3BP) equations are used here.

DROs:

In contrast to the resonant orbits, the general concept of distant retrograde orbits (DROs) around the Moon is better documented [REF]. DROs are highly stable orbits due to their interactions with the Earth-Moon system's gravitational dynamics, particularly near the Earth-Moon Lagrange points. This stability allows spacecraft to maintain their orbits with minimal fuel consumption, making DROs suitable for long-duration missions and as staging areas for further space exploration. A DRO was the target of the Artemis I mission (for which I wrote a blog post here) as well as the orbit that Chang'e 5 used as part of its extended mission after dropping off the return samples [REF].

DRO's unique position allows spacecraft to enter Earth-Moon space with minimal energy expenditure while maintaining stable positioning and full-domain accessibility, making it a great selection for the infrastructure that China is building up for maintaining continuous human activities in the Earth-Moon sphere. [REF]

Especially for applications related to lunar navigation systems (essentially lunar GNSS), combining DROs with Halo orbits can provide comprehensive coverage of the Moon's surface. Specifically, DROs offer stable coverage of the lunar equatorial regions, while Halo orbits are more effective for polar areas. This combination ensures reliable positioning, navigation, and timing services across the entire lunar surface.

Similar to the resonant orbits, DROs can facilitate the development of lunar infrastructure by serving as hubs for communication relays, storage depots, or assembly points for lunar landers and other equipment. Their stable nature ensures consistent support for surface operations and exploration activities. In the future, they could also serve as advantageous locations for staging missions beyond the Moon, such as crewed missions to Mars. Their stability and relative ease of access make them ideal for assembling spacecraft, conducting system checks, and preparing for deep space travel.

The shape of a DRO with period of ~18.3 days (roughly a 3:2 resonance), is shown in the following plots. The minimum distance from the Moon is at ~90,000 km for this DRO, whereas its maximum distance lies at ~140,000 km. In the inertial frame, the DRO's perigee appears to be precessing over time, completing one full precession period in ~55 days (3x the orbital period) for this configuation. Note that the selected Jacobi constant / period leads to a closed loop in the CR3BP in the inertial frame, a property that not all DROs possess.

Sample DRO in the Earth-Moon rotational frame with L1 and L2 shown.

Sample DRO in the inertial frame.

When plotting both the DRO and the resonant trajectories with the 3:2 resonance together, one obtains a configuration which is very similar to the schematic of the DRO A/B mission profile:

Significance and Future Implications

The DRO mission is a fascinating one, both from an orbital mechanics perspective as well as due to its significance for deep-space navigation, autonomous spacecraft operations and space situational awareness in the cislunar domain. In terms of China's objectives, the successful recovery and deployment of DRO-A and DRO-B not only demonstrate technical prowess but also lay the groundwork for future lunar exploration, including crewed missions and the establishment of a lunar base. The technologies tested and validated through the DRO constellation are expected to play a pivotal role in supporting these ambitious goals. As more and more players continue to push the boundaries of what's possible in cislunar space, the world watches with keen interest.