Wednesday, 22 October 2008

Chandrayaan-1: How Ground Segment Receives The Signals From the Lunar Craft


October 22, 2008
By Syed Akbar
Hyderabad, Oct 21: If sending Chandrayaan-1 into the lunar orbit is a Herculean task, deciphering the radio signals that are sent back to the earth from the spacecraft is equally challenging.
The radio signals, beamed back by Chandrayaan-1 to the master control room in Bengaluru 4,00,000 km away from it, become quite weak. ISRO scientists will have to adopt special methods to enhance the signals to decipher the message from the first-ever lunar orbitor sent by India.
"During the various phases of its flight, Chandrayaan-1 spacecraft will send detailed information about its health to Earth through its transmitter. At the same time, the spacecraft will be ready to receive radio commands sent from Chandrayaan-1 Spacecraft Control Centre instructing it to perform various tasks. Besides, the spacecraft receives, modifies and retransmits the radio waves sent by ground antennas in a precise way. This plays a crucial role in knowing its position and orbit at a particular instant of time. All these happen at 'S-band' frequencies in the microwave region of the electromagnetic spectrum.," according to an ISRO
communiqué.
As Chandrayaan-1 orbits the Moon, the spacecraft sends valuable imagery and other scientific information to Earth through X-band (at a higher frequency compared to S-band), which also lies in the microwave region But, such information is transmitted through radio at a very low power of a few watts. Thus, radio signals carrying that
precious information become extremely feeble by the time they travel 4,00,000 km from the Moon and reach ISRO's ground station in Bengaluru back on the earth.
The Ground Segment of Chandrayaan-1 performs the crucial task of receiving the radio signals sent by spacecraft.
It also transmits the radio commands to be sent to the spacecraft during different phases of its mission. Besides, it processes and safe keeps the scientific information sent by Chandrayaan-1 spacecraft.
ISRO scientists are armed by a number of equipment to decode the message relayed back by Chandrayaan-1, howsoever weak they are. The ground segment of ISRO includes the Indian Deep Space Network, Spacecraft Control Centre and Indian Space Science Data Centre.
"Deep Space Network performs the important task of receiving the radio signals transmitted by Chandrayaan-1 spacecraft that become incredibly feeble by the time they reach the earth. Besides, it can send commands to the spacecraft at a power level of up to 20 kilowatts," the Chandrayaan pre-launch communiqué said.
IDSN consists of two large parabolic antennas, one with 18 m and the other 32 m diameter at Byalalu, in the outskirts of Bengaluru. Of these, the 32 m antenna with its 'seven mirror beam wave guide system' is indigenously developed. The 18 m antenna can support Chandrayaan-1 mission, but the 32 m antenna can support
Chandrayaan-1 and any spacecraft mission further deep into space.
During the initial phase of the mission, besides these two antennas, other ground stations in Lucknow, Sriharikota, Thiruvananthapuram, Port Blair, Mauritius, Brunei, Biak (Indonesia) and Bearslake (Russia) as well as external network stations at Goldstone, Applied Physics Laboratory in Maryland, Hawaii (all three in USA), Brazil and Russia support the mission.
The Spacecraft Control Centre, located near ISTRAC campus at Peenya, North of Bengaluru, is the focal point of all the operational activities of Chandrayaan-1 during all the phases of the mission. Commands to be transmitted to Chandrayaan-1 spacecraft to maintain its health as well as to make it perform various tasks originate from here.
Experts specialising in various spacecraft subsystems as well as spacecraft mission operations personnel are stationed at SCC.

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