CHANDRAYAAN-2 MISSION

CHANDRAYAAN-2 MISSION OVERVIEW 

The Chandrayaan-2 (“चन्द्रयान-२” in Sanskrit) spacecraft is an advanced version of the flight proven Chandrayaan-1 orbiter spacecraft. Chandrayaan-2 is a twomodule configuration spacecraft comprising of the ‘Orbiter Craft’ and the ‘Lander Craft’. The Orbiter Craft Module structure is a three-metric ton category bus structure made of a central composite cylinder, shear webs and deck panels. It was developed by Hindustan Aeronautics Limited (HAL) and delivered in June 2015 to ISRO Satellite Center where the other spacecraft subsystems and payloads were built onto the structure.7 The Chandrayaan-2 mission is aimed at placing an Orbiter around the moon and sending a Lander module with a Rover to the surface of the moon. Chandrayaan-2 will be launched by a Geo-Stationary Satellite Launch Vehicle (GSLV-MKII) during the first quarter of 2018..

CHANDRAYAAN-2 MISSION FFETURE

The primary objectives of Chandrayaan-2 mission are to design, realize and deploy a Lunar Lander module capable of soft landing on a specified lunar site and deploy a Rover to carry out in-situ analysis and to carry out remote sensing analysis of lunar surface to enhance the scientific objectives of Chandrayaan-1 with improved resolution through the orbiter science payloads observation. The Lander module with the Rover is to be de-orbited from 100 km circular orbit and has to descent to the moon surface at the identified site using the liquid engines for braking. Chandrayaan-2 orbiter is planned to be operational for two years while the Lander-Rover module is expected to be operational for only 14 Earth days.

CHANDRAYAAN-2 MISSION ORBITER

The Chandrayaan-2 Orbiter and the Lander will be stacked together and will be injected into an “Earth Parking Orbit”. After going around the Earth several times, the Orbiter will be inserted into an extremely elliptical Lunar orbit, which will be reduced to 100 km over the surface of the moon after a few Earth Bound Maneuver (EBN) orbits. The orbiter will carry the Lander, with the Rover on board, from Earth orbit to Moon orbit. The orbiter will survey the landing site before deploying the lander. 

CHANDRAYAAN-2 SPACECRAFT ARCHITECTURE

Chandrayaan-2 Orbiter Craft is built around a cuboidal structure and houses the propulsion tanks and the separation mechanism of the launch vehicle at one end and lander at the other end. The Orbiter decks have the different housekeeping systems of the Spacecraft. The Solar array consists of two solar panels which are stowed in the launch configuration and deployed on separation to provide the power required for the Orbiter Craft during different phases around the earth and the moon. Lithium Ion battery provides the power support during eclipse and peak power requirements of the spacecraft. Orbiter is a three-axis body stabilized spacecraft with reaction wheels which provide a stable platform for imaging. Thrusters are present for momentum dumping and attitude corrections. A bipropellant liquid engine is used to raise the orbit of the composite from earth parking orbit to 100km lunar orbit. The attitude and orbit control electronics receive the attitude data from the star sensors and the body rates from the Gyro’s for S/C control. The other sensors used for spacecraft control are Sun sensors and accelerometers. The telemetry system provides health information of the spacecraft while the tele-command system handles the command execution and distribution. The different payloads on the Orbiter are interfaced to the base band data handling system for formatting and recording in solid state recorder for play back later. The RF system consists of a S band TTC transponder and X band transmitter for Payload data transmission to Indian Deep Space Network (IDSN) station. The payload data is transmitted through a X-band dual gimbal antenna which will be pointed to the ground station.

Chandrayaan-2 Lander structure is a truncated pyramid around a cylinder which houses the propellant tank and the interface for the separation mechanism of Orbiter. The vertical panels have solar cells while the stiffener panels house all the electronic systems. The lander leg mechanism (four nos.) provides stability upon landing on different terrains. The body mounted solar panels provide the power for the different systems during the mission in all phases. In addition, lithium ion battery supports the power requirements during eclipse and the lander descent. The Control electronics provide the interface to all the sensors and the actuator drives. The sensors are configured for inertial navigation from separation to the end of rough braking and the absolute sensors determine the position and velocity with respect to the landing site to guide the lander beyond the rough braking phase to the identified site. The lander Navigation guidance and control will be autonomous from separation onwards and must ensure a precise, safe and soft landing on the lunar surface. The braking thrust for decelerating the lander is provided by four nos. of liquid engines. The attitude of the lander is maintained with eight nos. of thrusters. The lander leg mechanism ensures that the energy at touch down is absorbed and all the lander systems are integral and stable for further conduct of payload deployments and science on moon. Each leg consists of a telescopic leg assembly with crushable damper material in the leg and foot pad. Extensive analysis and tests are done for the lander leg mechanism to ensure stability under extreme terrain conditions and terminal velocity. The TTC communication between the Lander – IDSN is in S band and the payload data is transmitted by a high torque dual gimbal antenna. The Lander has a TM-TC data handling system with inbuilt storage. The Chandrayaan-2 Rover is stowed in the lander during launch and upon landing the ramps are deployed and Rover starts its journey on the lunar surface. The Lander payloads will be deployed on landing.

Chandrayaan-2 Rover is a six-wheeled mobility system with the objective of performing mobility on the low gravity & vacuum of moon and in addition conduct science for understanding the lunar resources. The design of the Rover is based on the well-proven space rover “Sojourner” that was deployed by NASA for the exploration of Mars in July 1997. Rover chassis houses all the electronics and has two navigation cameras to generate stereo images for path planning. The deployed solar panel provides the power during the mission. The rocker bogie mechanism along with the six wheels ensure a rugged mobility system over obstacles and slopes along the identified path for exploration of the region. The Rover communicates to the IDSN via the Lander. The two Rover payloads conduct science on the lunar surface.

 The Lander module operations from separation to touch down shall be carried out by a closed loop NGC system. The Inertial Navigation System (INS) alone will not be able to meet the stringent touchdown requirement of < 5 m/s in vertical and horizontal velocity. The unbounded error growth in the INS with time is corrected with the help of other absolute external measurements. An integrated navigation system consisting of an INS, star tracker (2), altimeter (2), velocimeter (2) and image sensor (2) will be utilized. The initial attitude of IMU at de-boost is determined using star tracker. The accelerometer and gyro drifts are also updated before the first burn. The state vectors are established using Deep Space Network (DSN), Orbit Determination and ground uplink and transferred to the INS system. The lander NGC will be active before the separation from orbiter itself. The INS after updating the state vector is used for the first burn. During the long coast phase also, the attitude and gyro drifts are updated using star tracker. The accelerometer bias also is updated during the long coast phase. The INS state vector is used during the second burn. During the vertical descent phase, radar altimeter is used for the height information. Doppler velocity sensor is primarily used to measure the horizontal velocity in the terminal landing phase to ensure safe landing with a touch down velocity of < 5 m/s. Vision aiding or terrain sensor using CCD camera is used to get the image of lunar surface to avoid the obstacles and re-targeting the landing surface.

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