How are spacecraft like Mars Orbiter instantly controlled from Earth, crores of Km away? How do the images captured by the Orbiter’s camera reach the Earth in short time?
The communication (whether for controlling the spacecraft from Earth or image-transfer from the spacecraft to Earth) between a spacecraft in space and the Earth takes place through Radio Frequency (RF) electro magnetic waves typically in the frequency range of 300 MHz – 30 GHz (1m to 1cm ultra short wavelength as compared to 25, 31 or 49 metres in short wave radio transmission).
These waves travel at the speed of light (3 lakh Km per second). All inter-planetary missions like Mars Orbiter, which are millions of Km away from the Earth, take a very long time to send or receive data/control signals to/from the Earth.
As the distance between Mars and Earth keeps varying (due to rotation of both the planets around the Sun) in the range of about 55 million Km to 400 million Km, the travel-time for the RF signals can be even more than 22 minutes.
The electromagnetic wave from outside is able to penetrate the ionosphere surrounding the Earth and get to the earth and vice versa as its frequency exceeds the ionospheric plasma frequency.
Controlling the deep-space missions spacecraft, especially in emergency situations becomes a challenge as receiving the spacecraft-response to any of the ground commands takes double the time. The chosen orbit of the recently launched Indian Mars Orbiter Mission (MOM) will require maximum of about 42 minutes to know the spacecraft status in response to any control-command sent from The Earth. High level on-orbit autonomy, therefore, has been implemented in the MOM that will put the spacecraft into a safe mode in case of emergency till scientists on ground diagnose the problem and take corrective actions.
The signals received are very weak and hence require amplification. The antenna manages this because of its directivity. The larger the antenna the narrower the direction range from where the signal is picked up.
32m antenna at 6 GHz will pick up signals from a narrow cone of angle 0.1 deg only and used at the ground to control the deep-space missions/spacecraft and receive the data/images from them. India will use its own Deep-Space Mission network consisting of 18m and 32 m dish antennae to communicate with the MOM and has also an agreement with NASA to utilise its ground network facilities to retain the communication with the spacecraft when it goes beyond the capture range of Indian ground stations.
S.P.S. JAIN, Retd. Member, Engineering, Indian Railways, Greater Noida, Uttar Pradesh