HOW DO SCIENTISTS sitting in their control centres on earth talk to a satellite in deep space? How do they “drive” the satellite to its destination, on a surface several thousand kilometres away?
Once a satellite separates itself from its rocket, the first step in establishing communication is to know its location. For that, a simple radar is good enough. The radar sends a pulse, much like a torch illuminating a dark section of the room. The satellite reflects back that pulse, and thus, becomes “visible” on the radar. Scientists can track the movement of the satellite by adjusting the radar. The distance between the satellite and earth can be calculated from the time taken by the signal to hit the satellite and return.
Indian Space Research Organisation has two big antennae—the Deep Space Antenna-18 (DSN-18) and DSN-32 at Byalalu, Bengaluru, that were constructed during Chandrayaan-1 and are part of the Indian Deep Space Network (IDSN)—to communicate with its distant probes. Once the movement and position of a satellite are determined, scientists get down to the more complex task of calculating how far the satellite has to move, and in which direction and the speed for it to be placed in the next orbit desired, and how it should be oriented (facing away from the earth, for instance). Once these calculations are made, they then compute the amount of thrust required to be fired by the on-board engines to achieve the manoeuvre. The calculations have to be extremely precise, because even the tiniest inaccuracy could change the course of the satellite by a wide margin.
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All these calculations are converted into a programme, which is sent as an electronic signal to the satellite, via the deep space network. The satellite has got receiving antennae, through which it takes in the message, reads it and executes the instructions.
The probes are equipped to send their information back to their earth handlers in the form of electronic signals. On September 7 morning, ISRO was in constant touch with the Chandrayaan-2 lander, Vikram, through this mechanism, till the time the signals stopped.
Both the orbiter and the lander were programmed to send information directly to earth. The orbiter was able to take pictures of the landing site and relay the information to ISRO. The attempt to talk to Vikram is still on. Pragyaan, the rover, was only programmed to send information to Vikram, which would then relay it to earth.
It may not seem so to us, but space is noisy, with a slew of electronic signals all around. And, there are several electronic “ears” out there. So, scientists encode every message they send, which can only be understood by the satellite it is meant for.
Coding also keeps a satellite strictly under one command, preventing it from getting hijacked by someone else. Unless, of course, that someone else has got the codes. That, however, is for another day. For the present, it is reassuring to know that Chandrayaan-2’s orbiter will listen and talk only to ISRO.