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Aiming for the moon and beyond

  • Trajectory for Chandrayaan-2

Efficient tracking and maneuvering of Chandrayaan is crucial for the success of the mission. An efficient and low cost tracking algorithm from IIT Bombay could be a solution to this.

The recent success of the Mars Orbiter Mission (MOM), also called Mangalyaan was a milestone in space technology and research capabilities of the nation. Determined to continue its success streak, ISRO is planning its second lunar mission, the Chandrayan-2 to be launched by the end of year 2016 or in early 2017. Space missions are amongst the most technology and research intensive projects a country could take up. Chandrayaan-2 will comprise an orbiter, a lander, and a lunar rover. A number of extremely complex systems have to work in perfect synchronization to ensure a successful operation and execution of the project. Once the spacecraft reaches the outer space, satellite navigation becomes the most crucial part of the mission. The basic task of a satellite navigation system is the prediction of spacecraft position and velocity. Extensive research is employed to find novel methods to do this accurately in realtime, for efficient tracking of spacecrafts and initiation of steps for trajectory correction, if required.

A M.Tech. student Sanat Kumar Biswas and Prof. Hablani of Aerospace Department in IIT Bombay, took up this daunting challenge to develop a navigation system that could be put into use for the Chandrayan2 mission. Sanat’s project focuses on building algorithms to predict the trajectory of a spacecraft using the data collected by the ground stations (via spacecraft transmissions) while the spacecraft is in flight. The work started with the simulation of a cislunar trajectory, which the spacecraft would supposedly follow. The simulation of the trajectory of any object in space requires the knowledge of the nature and magnitude of the forces acting on it. The major forces identified for this specific problem were the gravitational forces of the sun, the moon and the earth and solar radiation pressure. Quantifying the influence of these forces was difficult because of their complex, nonlinear nature. Hence, a more complex model called J2 was used which even accounted for the the non-spherical and irregular shape of the earth and the moon, instead of assuming complete spherical shape.

After simulation of the trajectory, the next step was to run the simulations to obtain data similar to what would be received by the ground stations from the spacecraft. Such signals get distorted and corrupted with noise before reaching the ground stations, hence, the sources of distortion and error were taken into account as well and ground station data were generated via simulation. The work also incorporated estimation of the position and the velocity of the spacecraft; the two most essential parameters for navigation, from the data gathered by the ground stations. The authors also took into consideration the time delay between the broadcast of the message and its reception due to the finite speed of electromagnetic waves.

The results from this work were extraordinary. The final error in position and velocity at the time of arrival at the moon were within 30 kms and 1.5 m/s respectively. Although the error is low in view of the length of the trajectory, the authors believes scope for improvements exists and could be incorporated to make the algorithm application ready. These results were obtained without the use of any high end computational facilities. With higher computational power at disposal, more complex and accurate models can be incorporated into the algorithm.

(Presented in conference on ‘Advances in control, optimization of dynamic systems’, ACODS-2012, at Bangalore, India, DOI :10.13140/2.1.1492.6726)

Image courtsey : http://www.chandrayaan-i.com/index.php/chandrayaan-2/chandrayaan-2-how.html

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