The Indian Space Research Organisation (ISRO) successfully carried out the Space Docking Experiment (SpaDeX) on January 16, 2025, making India the fourth nation after the US, Russia, and China to achieve this historic milestone. Begun on December 30, 2024, the mission successfully launched the SpaDeX spacecraft aboard the Polar Satellite Launch Vehicle (PSLV)-C60 from Satish Dhawan Space Centre (SDSC) in Sriharikota. The two spacecraft involved in this mission are named SDX01, also known as the Chaser, and SDX02, referred to as the Target. Both of them weigh approximately 220 kilograms each. Since their launch, they have been travelling in space at a precisely calculated speed to ensure the success of the docking experiment. India used the indigenously developed Bhartiya Docking System to achieve this feat.
This ground-breaking mission highlights India’s technological capabilities in spacecraft rendezvous, docking, and undocking—essential skills for future advancements in satellite servicing, space station operations, and interplanetary exploration.
SDX01 is equipped with a high-resolution camera which is a miniaturised version developed by the Satellite Application Centre (SAC), ISRO.
SDX02 carries a Miniature Multi-Spectral Payload (MMX), developed by SAC/ISRO, which has four visible and near-infrared (VNIR) bands for natural resource monitoring and vegetation studies. Additionally, a Radiation Monitor (RadMon) on SDX02 will measure radiation dose in space, contributing to radiation database generation for future space missions.
Objectives of the Mission
The primary goal of the SpaDeX mission is to develop and showcase the technology required for the rendezvous, docking, and undocking of two small spacecraft, the Chaser and the Target, in a low-Earth-circular orbit. This achievement demonstrated India’s capability to execute precise manoeuvres in space, paving the way for future space missions that require such technology.
One of the key secondary goals has been to demonstrate the transfer of electric power between the docked spacecraft. This capability is crucial for future applications such as in-space robotics, where continuous power supply will be necessary. The mission also focuses on composite spacecraft control, which is essential for maintaining stability and manoeuvrability during the docking process. Finally, the mission aims to conduct payload operations after undocking, further proving the spacecraft’s versatility and operational readiness.
The mission will also test India’s capabilities in inter-satellite communication. During the docking and undocking phases, the spacecraft will be communicating with Earth station as well as with each other to share information about their position and velocity. Over the next two years, the cameras installed on the spacecraft will measure radiation in space and monitor natural resources on Earth.
Process of Docking
Docking is a very complex process that needs extreme precision and coordination. During the docking process, scientists carefully manoeuvred the spacecraft to reduce the distance between them which enabled them to successfully dock. Also, the two spacecraft had to be in the same orbit so that the Chaser could start to approach the Target. They successfully reduced the distance between the Chaser and the Target, first up to 15 metres and then to 3 metres. After the trial attempt, the spacecraft were moved back to a safe distance and they were in the process of analysing the data. At that point, their connectors were securely latched together.
In the next step, the two spacecraft were securely connected, creating an airtight passage that enabled the safe transfer of materials or crew. At this stage, the space docking was successfully completed. Now, the mission will conduct what is considered one of its most crucial experiments—transferring electrical power from the Chaser to the Target. This demonstrates that one spacecraft can be sent to service another in space. The experiment will then showcase the ‘undocking and separation’ of the two satellites.
The indigenous technologies developed to enable this docking mission include the following:
- A docking mechanism designed for secure and precise attachment of spacecraft
- A set of four rendezvous and docking sensors to ensure accurate positioning and alignment
- Power transfer technology for transferring energy between spacecraft
- An innovative autonomous rendezvous and docking strategy, developed to allow independent spacecraft manoeuvres
- An inter-satellite communication link (ISL) for autonomous communication between spacecraft, equipped with built-in intelligence to monitor the state of the other spacecraft
- A Global Navigation Satellite System (GNSS)-based Novel Relative Orbit Determination and Propagation (RODP) processor to calculate the relative position and velocity of the other spacecraft
- Simulation test beds for both hardware and software design validation and testing, ensuring the reliability of the mission’s technologies
Significance of the Mission
In addition, SpaDeX presented a greater challenge due to its small size and mass, requiring even finer precision for the rendezvous and docking manoeuvres compared to docking a larger spacecraft. This mission will serve as a precursor for the autonomous docking technology needed for future lunar missions, such as Chandrayaan-4, which will operate without the support of GNSS from Earth.
About the Mission
The docking mechanism is a low-impact, androgynous system, i.e., either of the spacecraft can function as the chaser (active spacecraft) during docking, with identical docking components for both the Chaser and Target spacecraft. It features a peripheral docking design, similar to the International Docking System Standard (IDSS) used by other agencies for human missions. The system is smaller and utilises two motors for extension, in contrast to the large IDSS with 24 motors. Multiple test beds were set up to stimulate and test the hardware and software, ensuring the docking kinematics and finalising the approach parameters.
The mission includes an advanced sensor suite with a Laser Range Finder (LRF) and Corner Cube Retro Reflectors for range determination from 6,000 to 200 metres. Rendezvous sensors provide relative position (x, y, z) data from 250 metres to 10 metres, while LRF also measures velocity. Proximity and docking sensors cover 30 metres to 0.4 metres, with laser diodes used as targets. A video monitor captures the docking event, and a mechanism entry sensor (MES) detects Chaser entry during docking. These sensors were calibrated and validated using multiple test beds.
Both SpaDeX spacecraft carry a differential GNSS-based Satellite Positioning System (SPS) to provide position, navigation, and timing solutions. A novel RODP processor in the SPS allows precise determination of relative position and velocity between the Chaser and Target. VHF/UHF transceivers enable GNSS data transfer between the satellites. Extensive testing, including closed-loop verifications, was conducted to characterise RODP performance.
Up to a five-kilometre inter-satellite distance (ISD), standard orbit maintenance, and attitude control algorithms used in ISRO’s LEO spacecraft are employed. For reducing the ISD and achieving docking, the V-bar strategy with n-Pulse, glideslope, and position and velocity guidance algorithms are used. These algorithms were translated into software, which was thoroughly tested and validated through multiple simulations, including digital, hardware-in-loop, onboard-in-loop, software-in-loop, and robotic testing.
After the docking and undocking events, the spacecraft will be separated and repurposed for application missions.
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