# SPADEX

SpaDeX or Space Docking Experiment is a twin satellite mission developed by the Indian Space Research Organisation (ISRO) to mature technologies related to orbital rendezvous, docking, formation flying, with scope of applications in human spaceflight, in-space satellite servicing and other proximity operations.
SpaDeX campaign would consist of two modified IMS-1 class (220 kg) satellites, one would be Chaser and other being Target and both would be launched as co-passengers or auxiliary payloads. Both satellite would be injected into slightly different orbits.
The mission was launched from the Satish Dhawan Space Centre First Launch Pad aboard a dedicated Polar Satellite Launch Vehicle on 30 December 2024 at 16:30 UTC.

## Objectives

* Autonomous rendezvous and docking
* Verify the electrical power flow and establish duplicate controls between the docked spacecraft. Operating their payloads after undocking maneuver.
* Controlling one spacecraft with Attitude Control System of other spacecraft while in docked configuration.
* Formation flying
* Remote robotic arm operations in POEM-4.

## History

With preliminary studies done in 2016, Space Docking Experiment was approved by the Government of India with an initial funding of ₹10 crore, cleared in 2017. In June 2019, ISRO was looking for proposals to study remote robotic arm operation, rendezvous and docking related technologies on its PSLV fourth stage (PS4) orbital platform.
As of July 2022, Space Docking Experiment has been sanctioned ₹124.47 crore (equivalent to ₹132 crore or US$15 million in 2023) in funding.
Two 220 kg satellites named "Chaser" and "Target" will first be positioned at an altitude of 470 km with 55° inclination, where they will execute a number of precise maneuvers in order to rendezvous and dock. The "Chaser" and "Target" satellites, along with related docking technologies, were designed and implemented for the SpaDeX mission by UR Rao Satellite Centre (URSC) with assistance from Vikram Sarabhai Space Centre (VSSC), Liquid Propulsion Systems Centre (LPSC), Space Applications Centre (SAC), ISRO Inertial Systems Unit (IISU), and Laboratory for Electro-Optics Systems (LEOS).
In less than three months, Ananth Technologies finished the satellite assembly, integration, and testing for ISRO, delivering them to the U R Rao Satellite Centre (URSC). As per S. Somanath, docking is a crucial component of Chandrayaan-4 and that ISRO intended SpaDeX as a forerunner. Satellites for the SpaDeX mission were being further tested as of October 2024. All the testing process was to be finished by November 2024. The launch will take place on 30th Dec 2024. Both the planned Bharatiya Antariksha Station and Gaganyaan space flights depend on the mission's success.

## Description

The designations SDX01 and SDX02 are assigned to the "Chaser" and "Target" satellites, respectively though both are androgynous. The launch vehicle will create a 10–20 km inter-satellite separation between SDX01 and SDX02 by providing an incremental velocity between them. Following this drift arrest maneuver, Target and Chaser will have reached far rendezvous, when they are in the same orbit at the same speed but 20 km apart. In order to eventually link the two spacecraft, Chaser will then approach Target with increasingly shorter inter-satellite distances of 5 km, 1.5 km, 500 m, 225 m, 15 m, and 3 m. Electrical power flow will be established once docking and rigidization are completed successfully.
The Space Applications Centre developed a miniaturized high-resolution camera with photo and video capability carried aboard the SDX01. SDX02 is 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.
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. It has been patented by ISRO. In the near-Earth orbit, SDX01 and SDX02 will be moving at a speed of roughly 28,800 kmph. For autonomous docking, the relative velocities of SDX01 and SDX02 will be reduced to 0.036 kmph or 10 mmph 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.
Within ten to fifteen days following launch, the inter-satellite distances will be reduced. The Chandrayaan-4 mission won't require GNSS assistance from Earth if SpaDeX autonomous docking is successful. The final docking maneuver will take place on 7 January 2025.
The dual-motor actuation design will help for secure connections and accurate alignment at low speeds. The real-time alignment and navigation will be supported by proximity sensors, laser rangefinders, and rendezvous cameras. During docking operations, an Inter-Satellite Communication Link (ISL) will provide smooth data transmission, improving the system's dependability and autonomy.

## New process and technologies

* Instead of being assembled on the launch pad, the launch vehicle is assembled in a new PSLV integration facility and transported to the launch pad to reduce the lead-in time between missions.
* A low-impact, androgynous peripheral docking system with an approach velocity of 10 mm/s.
* 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. Charge-coupled device for 20 to 0.5 m range to capture docking event. Mechanism entry sensor from 8 cm to 4 cm to detect SDX01 entry into SDX02.
* Integrated processor for Relative Orbit Determination and Propagation (RODP) with differential GNSS-based Satellite Positioning System (SPS) for Positioning, Navigation and Timing (PNT). VHF/UHF transceivers to transfer and synchronize position vector and velocity data by Inter-Satellite Communication Link.
* 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.

## Challenges

ISRO will 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 will cost an additional ₹250 crore. Another problem is to manage lower inertia and build a compact, low-impact androgynous design compatible with modular spacecraft configurations. A tiny force or minor disruption can interfere with the docking process because of low mass and low inertia which could cause the satellite to deviate from its alignment.

## Future development

During the SpaDeX mission's post-launch briefing on December 30, 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.