SpaDeX or Space Docking Experiment is a twin satellite mission developed by the Indian Space Research Organisation (ISRO) to mature and demonstrate technologies related to orbital rendezvous, docking, formation flying, which will have future applications in areas such as human spaceflight, in-space satellite servicing and other proximity operations.[2]
SpaDeX consists of two modified IMS-1 class satellites weighing 220 kg each. During proximity operations one spacecraft acts as a Chaser and other acts as a Target.
After deployment, the two spacecraft then executed manoeuvres to bring them together again. The Chaser (SDX01) approached the target (SDX02) and then carried out precision manoeuvres to complete a successful docking.[5][6] With this success, India became one of the few countries in the world to have achieved a successful in-space docking using indigenous technology.
Objectives
According to ISRO, the SpaDeX mission had the following objectives:
To demonstrate an autonomous rendezvous and docking using the V-bar approach.[7]
To verify electrical power transfer between two docked spacecraft.[7]
To execute a composite spacecraft control using the Attitude Control System of another spacecraft while both are in a docked configuration.[8]
To act as independent satellites after undocking, and to operate their payloads for a period of two years.[9][10]
History
The research and development (R&D) needed to complete the project began with preliminary studies in 2016.[11] The Space Docking Experiment (SpaDeX) was approved by the Government of India with an initial funding of ₹10 crore, cleared in 2017.
As of July2022[update], SpaDeX was sanctioned ₹124.47 crore (equivalent to ₹132croreorUS$15million in 2023) in funding.[12]
For the SpaDeX satellites, Ananth Technologies provided Rendezvous Processing Units (RPU) and DC-to-DC converters. Additionally, the company provided 29 essential components for the PSLV-C60 launch vehicle, such as data acquisition units, transmitters, power modules, NavIC chip and control modules.[14][15]
In less than three months, Ananth Technologies completed the satellite assembly, integration, and testing for ISRO, and delivered them to the U R Rao Satellite Centre (URSC).[16] As per the previous head of ISRO, S. Somanath, docking is a crucial component for Chandrayaan-4 and that SpaDeX is intended as a demonstrator and forerunner.[17]
The Italian company Leaf Space, which offers ground segment-as-a-service (GSaaS) solutions, partnered with ISRO to create communication linkages with the POEM-4 platform and the SpaDeX satellites. It made telemetry, command operations, and ongoing SpaDeX mission telemetry monitoring possible.[24]
Description
The designations SDX01 and SDX02 are assigned to the "Chaser" and "Target" satellites, respectively, though both host docking devices capable of active or passive function.[9][25]
The launch vehicle deployed the spacecraft separately in order to create a separation between them of about 20km (12mi). After the spacecraft executed manoeuvres to prevent them from drifting further apart and then to begin the rendezvous operation.[26] The satellites were orbiting at about 28,400 km/h when they were 11km (6.8mi) apart.[27]
The Chaser then began to approach the Target, gradually lowering the separation between them to a distance of a few metres. After docking was completed, the mission planned to establish and demonstrate the transfer of electrical power between the spacecraft.[26]
The Space Applications Centre developed a miniaturized high-resolution camera with photo and video capability that was carried aboard SDX01. SDX02 was equipped with a Multi-Spectral Payload (MMX) for vegetation and natural resource monitoring. In order to properly plan the Gaganyaan missions, SDX02 has a radiation detector that will collect data on radiation levels in space.[28]
After separation, the two spacecraft will run their payloads for a period of two years.
The Bhartiya Docking System (BDS) was developed by ISRO based on the International Docking System Standard (IDSS) after unsuccessful attempts to import the docking technology from abroad.
For autonomous docking, the relative velocities of SDX01 and SDX02 were reduced to 0.036 km/h or 10 mm/s using retrorockets and a sensor suite. In contrast to the 24 motors used in IDSS, the BDS only uses two. The docking port on SpaDeX is 450 mm in diameter, whereas the docking port at the Gaganyaan and Bharatiya Antariksha Station will be 800 mm.[29][30][31]
The dual-motor actuation design was designed to help ensure secure connections and accurate alignment at low speeds. The real-time alignment and navigation was supported by proximity sensors, laser rangefinders, and rendezvous cameras. During docking operations, an Inter-Satellite Communication Link (ISL) provided smooth data transmission, improving the system's dependability and autonomy.[32] As soon as the satellites were under 5 km apart, they were able communicate via ISL to share orientation and location information.[11]
New process and technologies
Instead of being assembled and integrated at Vehicle Assembly and Launching Facility (VALF) and Mobile Service Tower (MST) respectively, the launch vehicle was assembled in a new PSLV Integration Facility (PIF) and transported to the launch pad to reduce the lead-in time between missions.[33][34]
A low-impact, androgynous peripheral docking system with an approach velocity of 10 mm/s was developed and employed.[32]
Laser rangefinder and corner cube retroreflectors for position vector and velocity in 6,000 to 200 m range. Rendezvous sensor for position vector in 2,000 to 250 m and 250 to 10 m range. Proximity and docking sensor for position vector and velocity in 30 m to 0.4 m range. An image sensor for 20 to 0.5 m range to capture the docking event. Mechanism entry sensor from 8 cm to 4 cm to detect SDX01 entry into SDX02.[7][33][28]Accelerometers for velocity measurement, and star tracker for attitude determination.[35]
Rendezvous and Docking Algorithms. The Rendezvous Simulation Lab to validate the algorithms through real-time simulation, the Docking Mechanism Performance Test for last stage of docking, and the Vertical Docking Experiment Lab to test the docking mechanisms under controlled settings.[33]
Timeline
Docking and undocking
On 30 December 2024, PSLV-C60 launched the SDX01 (Chaser) and SDX02 (Target) spacecraft into a 475-kilometer-high circular orbit with an inclination of 55°.[38]
Over subsequent days, ISRO increased the separation between the two spacecraft to greater than 20km. [39]
On 6 January, the docking manoeuvre was postponed from 7 January to 9 January after a need for additional validation through ground simulations was found. [40]
By 8 January, the two satellites had reduced their separation to 600 meters. To get the chaser satellite closer to the target, ISRO started a drift maneuver that moved it from 500 meters to 225 meters. However, the drift was greater than anticipated after the non-visibility period. The docking was thus was delayed for the second time.[41][42]
On 9 January, ISRO placed the chaser on a slow drift trajectory to bring it closer to the target. It was anticipated that the docking sequence would begin on 10 January, however this was subsequently delayed by several days.[43]
By 12 January, the SDX01 and SDX02 spacecraft were within 15 metres of one another. ISRO activated imaging sensors allowing the satellites to take pictures and videos of each other.[44]
On 13 January, an attempt was made by the chaser to approach within 15 meters and then 3 meters of the target. However, a delay in receiving signals from the proximity and docking sensors was observed during the inter-satellite distance reduction maneuver. For the SDX01 and SDX02 to correctly align throughout the docking process, these sensors were crucial. Because of the delay, an onboard safe mode was automatically activated to prevent any unintentional collision. This aborted the docking attempt and increased the inter-satellite distance between SDX01 and SDX02 to 8 km. [45]
On 16 January, a repeat of the docking attempt succeeded as the target captured the chaser for the first time. [46] ISRO also successfully managed to control the two satellites as a single stacked entity after docking.[47]
On 13 March, ISRO successfully undocked and separated the SpaDeX satellites.[48][49] The process began with an SDX02 extension, followed by the planned release of capture lever 3, the disengagement of the SDX02 capture lever, and the issuance of the de-capture command for both satellites.[50] All of the capabilities needed for rendezvous, docking, and undocking operations in a circular orbit have by this point been successfully proven by ISRO.[51] Both SDX01 and SDX02 subsequently orbited the Earth independently.[52]
ISRO plans to conduct the next series of experiments and another undocking/re-docking by 15 March 2025. The re-docking will take place automatically. Such experiments are only possible in a brief window of 15 days every two months.[53][54]
Additional experiments
On 28 March 2025, V. Narayanan verified that the first docking process had not finished transferring electrical power from one satellite to the other. The experiment was moved to the subsequent round because of a possible small misalignment of the power transmission ports in the first attempt. The SpaDeX satellites still have enough fuel to finish the experiment.[55]
The rolling experiment, which involved one satellite circumnavigating the other and returning to its original position while remaining in line of sight, was completed by ISRO on 28 March 2025.[56] Multiple software, sensors, positioning technology, ground station control over satellite maneuvering, etc. were all validated. The experiment will assist ISRO in comprehending the necessary steps to dock satellites from various orientations to a desired location. Additionally, it will assist ISRO in determining whether vertical docking is feasible.[57]
Challenges
ISRO did not undertake a trial mission for the space docking experiment due to financial constraints. The two satellites and associated equipment were built by ISRO for ₹125 crore, while the launch vehicle cost an additional ₹250 crore.[58]
As per media reports, SpaDeX is having technical difficulties with the undocking procedure that was supposed to take place on 16 January 2024. V. Narayanan announced on 29 January that ISRO is reviewing the undocking procedure because the agency does not want to undock and abandon the satellites. With 60–70% propellant remaining and the project expense being fully utilized, more docking, undocking, and power transfer experiments will be conducted.[59][60] V. Narayanan told PTI on 8 February 2025, that there are no issues. ISRO intends to take its time, studying, and organize additional experiments before trying the undocking procedure.[61]
Future development
During the SpaDeX mission's post-launch briefing on 30 December 2024, ISRO Chief S. Somanath declared that additional SpaDeX missions with greater size and complexity would be launched, showing larger docking systems, most likely for Gaganyaan and Bharatiya Antariksha Station.[62]
↑ Kumar, Santosh; S, Gouri; Srivastava, Vatsla (16 January 2025). "SpaDeX Mission: Revolutionising Space Exploration". Press Information Bureau. Department of Space, Government of India. Retrieved 18 January 2025.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
