The SpaDeX mission is a low-cost technology demonstrator mission that uses two tiny spacecraft launched by PSLV to demonstrate in-space docking. India’s space aspirations, including the construction and operation of Bharatiya Antariksh Station (BAS), Indian on the Moon, and sample return from the Moon, depend on this technology. When several rocket launches are necessary to accomplish shared mission objectives, in-space docking technology becomes crucial. India is moving closer to become the fourth nation in the world with space docking capability thanks to this mission.
The development and demonstration of the technology required for the rendezvous, docking, and undocking of two tiny spacecraft (SDX01, the Chaser, and SDX02, the Target, nominally) in a low-Earth circular orbit is the main goal of the SpaDeX mission. Secondary goals consist of:
Electric power transfer between the docked spacecraft is demonstrated, which is crucial for upcoming uses including payload operations after undocking, in-space robotics, and composite spacecraft control.
The two tiny spacecraft (approximately 220 kg each) that make up the SpaDeX mission will be launched independently and simultaneously by PSLV-C60 into a 470 km circular orbit at a 55° inclination and with a local time cycle of roughly 66 days. When the Target and Chaser spacecraft separate from the launch vehicle, a tiny relative velocity between them will be provided by the PSLV vehicle’s proven accuracy. The Target spacecraft will be able to create a 10–20 km intersatellite separation from the Chaser in a day because to this incremental velocity. At this stage, the Target spacecraft’s propulsion system will be used to adjust for the relative velocity difference.
Following this drift arrest maneuver, the Target and Chaser will have reached Far Rendezvous, where they are in the same orbit at the same velocity but separated by roughly 20 kilometers. The Chaser will approach the Target with increasingly smaller inter-satellite distances of 5 km, 1.5 km, 500 m, 225 m, 15 m, and 3 m, eventually resulting in the two spacecraft docking, using a similar technique of introducing and then compensating for a small relative velocity between the two spacecraft. Following a successful docking and rigidization process, the two spacecraft will show electrical power transmission before being undocked and separated to begin operating their individual payloads for the anticipated two-year mission life.
Furthermore, the rendezvous and docking maneuvers demand a greater level of precision than docking two big spacecraft, making SpaDeX considerably more difficult due to its modest size and bulk. This mission will serve as a prototype for autonomous docking, which will be required for further lunar missions like as Chandrayaan-4 without the assistance of Earth-based GNSS.
The docking mechanism is androgynous (the docking systems for the two spacecraft, Chaser and Target, are the same), low-impact (approach velocity is around 10 mm/s), and peripheral (the idea is similar to the International Docking System Standard used by other agencies for human missions). Compared to the IDSS (800 mm) on a hexapod with 24 motors, the mechanism is smaller (450 mm), has one extension degree of freedom, and requires two motors. To confirm and finalize the docking approach parameters, several test beds were set up to evaluate the hardware and software simulation of the docking kinematics.
The mission’s extra sensor suite consists of a Laser Range Finder (LRF) and Corner Cube Retro Reflectors, which can determine range (R) over a range of 6000 to 200 meters. The Rendezvous Sensors (RS) set is utilized between 2000 and 250 meters and between 250 and 10 meters. Relative position (x, y, z) is provided by RS, whereas velocity and relative position are independently determined by LRF.
The Proximity and Docking Sensor (PDS) measures velocity and relative location between 30 and 0.4 meters. RS and PDS use laser diodes (LDs) as targets. A video monitor is used in the 20 to 0.5 m range and it will capture the video of the docking event. A Mechanism Entry Sensor (MES) is
Both of the SpaDeX spacecraft, like all ISRO satellites in low-Earth orbit, are equipped with a differential GNSS-based Satellite Positioning System (SPS) that gives the satellites PNT (Position, Navigation, and Timing) solutions. Accurate measurement of the relative location and velocity of the Chaser and the Target is made possible by SpaDeX’s innovative RODP processor, which is integrated into the SPS receiver. Highly accurate relative states of the two satellites are obtained by subtracting the carrier phase measurements from the same GNSS satellites in both the Target and Chaser SPS receivers. By transmitting the GNSS satellite measurements from one satellite to the other, the VHF/UHF transceivers in both satellites facilitate this procedure. The RODP performance was characterized using hardware and software test beds, including closed-loop verifications.
Standard orbit maintenance and attitude control algorithms used in ISRO LEO spacecraft are applied up to an intersatellite distance (ISD) of 5 km. The V-bar strategy uses n-Pulse, Glideslope, and PV guidance algorithms to reduce the ISD between the satellites, hold at fixed ISDs to evaluate the sensors and software, and then docking because the spacecraft are in circular orbit and any change in velocity to the satellites will cause an orbital change. To accomplish the rendezvous and docking, these techniques were transformed into software. Several digital, hardware-in-loop, onboard-in-loop, software-in-loop, and robotic simulations were used to evaluate and validate these software solutions.
Following the processes of docking and undocking, the spacecrafts will be divided up and put to use for application missions.
SDX01 is equipped with a High-Resolution Camera (HRC) that has a 4.5 m IGFOV and a swath of 9.2 x 9.2 km (in snapshot mode) and 9.2 x 4.6 km (in video mode) from a height of 450 km. This is a scaled-down model of the SAC/ISRO surveillance camera.
SDX02 is equipped with a Miniature Multi-Spectral Payload (MMX), which was created by SAC/ISRO. This has a 25 m IGFOV with a 100 km swath from 450 km altitude and four VNIR bands (B1/B2/B3/B4) at 450 nm to 860 nm. Vegetation research and the monitoring of natural resources both benefit from the imagery.
SDX02 has a Radiation Monitor (RadMon) payload installed to measure the radiation exposure experienced in orbit. With applications in human spaceflight, this will aid in the creation of a radiation database for upcoming Total Ionization Dosimeter (TID) and Single Event Upset (SEU) observations for space science research.
With assistance from other ISRO centers (VSSC, LPSC, SAC, IISU, and LEOS), the UR Rao Satellite Centre (URSC) planned and built the SpaDeX spacecraft. ISRO ground stations and other externally rented ground stations will be used to manage the spacecraft during its orbital phase from ISTRAC. Under URSC’s supervision, the satellite’s complete integration and testing were completed at M/s Ananth Technologies in Bangalore. The spacecraft has now transitioned from URSC to SDSC and is getting ready for launch after passing all tests and permissions.
