Explainer: How ISRO will join 2 satellites orbiting at 28,800 kmph 470 km above earth – The Tribune-OxBig News Network

The countdown has begun for the launch of Indian Space Research Organisation’s (ISRO) pioneering Space Docking Experiment (SpaDeX), a critical venture involving the mating of two satellites in space that, if successful, will pave the way for setting up India’s space station, conduct future inter-planetary missions and propel it into a select global league of three such operators.

The two identical satellites are designated as ‘SDX01 Chaser’ and ‘SDX02 Target’.

Christened Bharatiya Docking System, the mission is scheduled to be launched into low orbit on board ISRO’s successful workhorse, the Polar Satellite Launch Vehicle (PSLV) from Sriharikota, off the coast of Odisha, on the evening of December 30.

During a recent visit to Chandigarh, Dr KV Sriram, a top ISRO scientist leading its Laboratory for Electro-Optics Systems (LEOS), had told The Tribune that a key spin-off of this technology would be extending the operational life of existing satellites. LEOS is among the ISRO centres involved in developing the satellites.

Space assets like geo-stationary satellites, he said, are very expensive but have a life of eight to 10 years when their propulsion units run out of fuel even though their other onboard systems and sensors are fully functional. Space-docking technology would enable replacing propulsion units repeatedly, enabling satellites to remain effective for many more years.

According to ISRO, SpaDeX is a cost-effective technology demonstrator mission for in-space docking using two small spacecraft. This technology is essential for India’s space ambitions such as Indian on Moon, sample return from the Moon, the building and operation of Bharatiya Antariksh Station (BAS), etc. In-space docking technology comes into play when multiple rocket launches are required to achieve common mission objectives.

The Chaser and Target, weighing about 220 kg each, will be launched by PSLV-C60, independently and simultaneously, into a 470 km circular orbit at a 55-degree inclination, with a local time cycle of about 66 days. The PSLV will be utilised to give a small relative velocity between the Target and Chaser at the time of separation from the launch vehicle. This incremental velocity will allow the Target spacecraft to build a 10-20 km inter-satellite separation with respect to the Chaser within a day. At this point, the relative velocity between the Target will be compensated using the propulsion system of the Target spacecraft.

At the end of this manoeuvre, the Target and Chaser will be in the same orbit with identical velocity but separated by about 20 km, known as ‘far rendezvous’. The Chaser will approach the Target with progressively reduced inter-satellite distances of 5 km, 1.5 km, 500 metres, 225 metres, 15 metres and 3 metres, ultimately leading to the docking of the two spacecraft.

After being injected into orbit, the two satellites will be moving at a speed of 28,800 kmph. Their relative velocities will be reduced to just 0.036 kmph. At the time of the low-impact docking, when the two satellites join to become a single space unit, the approach velocity will be 10 mm per second.

After successful docking and securing, electrical power transfer between the two satellites will be demonstrated before undocking and separation of the two satellites to start the operation of their respective payloads for the expected mission life of up to two years.

ISRO has developed a host of indigenous technologies for SpaDeX. These include docking mechanism, a suite of four rendezvous and docking sensors, power transfer technology, novel autonomous rendezvous and docking strategy, inter-satellite communication link for autonomous communication between spacecraft with inbuilt intelligence to know the state of the other spacecraft, GNSS-based novel relative orbit determination and propagation processor to determine the relative position and velocity of the other spacecraft, and simulation test beds for hardware as well as software design validation.

Because of its small size and mass, the mission is even more challenging due to the finer precision required for the rendezvous and docking manoeuvres compared to the mating of two large spacecraft, according to ISRO.

After the docking and undocking events, the spacecraft will be separated and used for designated application missions. A high-resolution camera is mounted on SDX01 and a miniature multi-spectral payload is mounted on SDX02, which will be useful for natural resource monitoring and vegetation studies. SDX02 is also carrying a radiation monitor which will measure radiation doses encountered in space to help generate a radiation database for space science studies with applications in human spaceflight.

The SpaDeX spacecraft were designed and developed by the UR Rao Satellite Centre (URSC), Bengaluru, with the support of Vikram Sarabhai Space Centre, Liquid Propulsion Systems Centre, Space Applications Centre, ISRO Inertial Systems Unit and LEOS. The full integration and testing of the satellite were carried out by Ananth Technologies, Bengaluru, under the supervision of URSC.