During the powered descent phase, the lander shall use an Inertial Navigation System

ISRO's Chandrayaan 2 Moon Landing: Lander can communicate with Indian Deep Space Network at Byalalu near Bangalore and also with the Orbiter above it

by Milind Kulshreshtha

Chandrayaan-2 is gradually moving closer to the Moon for the penultimate orbital rendezvous with it and facilitate scheduled 07th Sep 19 landing of Vikram (Lander) on the Lunar surface. ISRO’s trans-Lunar injection of Chandrayaan-2 forms part of a well-planned Transfer approach, wherein the spacecraft’s low Earth circular orbit is translated to a highly eccentric orbit to make it elliptical. Using this ‘slingshot’ technique, once near Moon, Chandrayaan-2 shall enter the Lunar sphere of influence and spacecraft shall start orbiting the Moon by 20th Aug 19.

The Lander Vikram has a unique four-legged body design (unlike the well known usual space rocket shape) since the Lander is brought to activation only after Chandrayaan-2 reaches Space, where there is an only vacuum and no atmospheric resistance to worry for aerodynamics. In Space, as Chandrayaan-2 is travelling, it experiences multiple varying external forces as ‘everything with a mass exerts a gravitational force, larger the mass, larger is this force’. To calculate these force vector components w.r.t. Celestial bodies, an accurate Solar System Model exists with ISRO. A force is also being generated as the spacecraft velocity changes (Kinetic Energy principles) as part of Orbit variations being affected by ISRO. Further, even solar winds exert some force on the spacecraft. Thus, in order to maintain its desired trajectory, spacecraft uses Celestial Navigation system for station keeping and to maintain the course.

Terrifying Minutes: Once Chandrayaan-2 is in the designated Moon orbit, the Lander unit shall undock and separate from the Orbiter at a predetermined position. The Lander itself weighs 1,471 Kg and has an additional 27 Kg Pragyan rover in its belly. Once Lander capsule is decoupled and is at a safe distance from the Orbiter, Lander’s Descent Orbit Insertion stage fires in to reduce its speed and altitude so as to further undertake the Powered Descent initiation. Thus, the final 30 km journey towards the Lunar surface shall be covered using a sequence of powered manoeuvre in about 15 minutes. Here, the Lander’s on-board rocket thrust mechanism is used in a highly controlled manner and thruster operations is a complex combination of Navigation, Guidance, Controls, Propulsion, Sensors and various other operational constraints. It works on various feedback mechanisms received for Roll, Pitch, Yaw, Altitude, Speed etc. Through Simulation technique, ISRO has already arrived at the most suitable Lunar surface approach trajectory for the Lander unit for various scenarios. To assist in the descent, Radio signal Doppler measurements based on a highly accurate atomic clock and received from radio telescopes of the Indian Deep Space Network shall display the accurate speed and distance parameters of the Lander.

Only when all Flight Controller parameters are achieved as a ‘Go’, Lander downward journey to Lunar surface shall commence. Too high speed of Lander can result in the possible risk of it’s crashing into the Lunar surface or, even a small altitude variance at the time of Lander descent can cause it to miss the landing site.

Soft Landing: During the Powered Descent phase, the Lander shall use an Inertial Navigation System, comprising of specialised instruments like Gyroscopes, Accelerometers and other special sensors like Radars etc. The ‘drift’ error for the instrument would be compensated for by the use of optical sensors (similar to a Sextant principle) and numerous other corrections. The cameras fitted on-board the Orbiter and Lander units too shall provide valuable real-time optical assessments. The Lander’s downward-looking camera shall verify the Lunar surface landmarks during the final touch downstage and check for any undesired obstruction or an artefact in the landing zone. The closing distance to Moon surface shall be reported by the downward-looking radar. The Lander legs also have proximity sensors to cut off a rocket at the moment of Lander sit down. The four legs of Lander guarantees a safe landing on uneven terrain also and has shock absorbers fitted. The soft landing shall be considered completed when a vertical sit-down of the Vikram on foot pads of four legs is achieved. Lunar dust is expected to rise around the Lander during this touchdown but due to the absence of any significant atmosphere, dust particles only below the rocket exhaust shall be disturbed. It is for such involved reasons that Dr Sivan had mentioned earlier that “There will be 15 terrifying minutes for scientists once the lander is released and is hurled towards the South Pole of the Moon”.

The Lander is wrapped in a golden coloured Multi-Layer Insulation polymer to protect its sensitive electronics as their operational parameters are highly sensitive to environmental conditions. The Lander is also fitted with solar panels for energy generation for sustenance on the Lunar surface. The Lander itself has multiple arrays of scientific probes fitted for:-

(a) Radio Anatomy of Moon Hypersensitive ionosphere & Atmosphere experiment to study Lunar ionosphere which is a dynamic plasma environment as per Solar conditions

(b) Chandra’s Surface Thermophysical experiment to measure temperature gradient and thermal conductivity on the Lunar surface using probe inserted up to 10cm on Lunar bedrock

(c) An instrument for Lunar Seismic Activity is a tri-axial seismometer to measure Lunar quakes

The Lander shall also release a slide to launch the six-wheeled rover, Pragyan on the Lunar surface for exploration for up to 500m from the Lander. Lander can communicate with Indian Deep Space Network at Byalalu near Bangalore and also with the Orbiter above it. Further, Lander is designed for one-to-one communication with Rover and act as the relay for rover information. Vikram is intended to operate for one Lunar day (equivalent to 14 Earth days) and is meant to be left on the lunar surface after completion of its active life.

With the Vikram’s soft touch down scheduled by 07th Sep 19, India shall become the fourth nation to successfully land a spacecraft on moon i.e. after the USA, Russia (erstwhile USSR) and China (Israeli lander attempt to land on the lunar surface earlier this year was not successful). This historic achievement is poised to trigger India’s golden the era of Moon research and Space exploration for the rest of the 21st Century.