Satellites in communication - class-V

A crosslink is when satellites communicate directly with eachother, instead of communicating with a ground station which, in turn, communicates with other satellites. Frequencies that are quickly attenutated in the atmosphere are commonly used, making the link undetectable and unjamable from the ground.

$Answer:-$ B

A communication satellite works like a relay station where signals transmitted by the ground stations are picked up by the satellite's receiver antennas(uplink), the signals are filtered, their frequency changed and amplified, and then routed via the transmit antennas(downlink) back down to Earth.

The area of earth covered by radio beam from a satellite dish (transponder) is called satellite footprint.

Terrestrial communication has two stations with repeaters to strengthen the signal.

A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth.

The direction of transmission and receiving are both fixed in ground wave propagation, sky wave propagation and space wave propagation. However in satellite communication, the antennas are oriented in the required direction depending upon the position of satellite, so that the beam is aligned with the satellite.

The wavelength lambda is calculated using the formula lambda = c / f, where c is the speed of light (3 * 10^8 m/s) and f is the frequency (30 MHz = 30 * 10^6 Hz). Thus, lambda = (3 * 10^8) / (3 * 10^7) = 10 meters.

TV-signal ( $30: MHz$ - $300:MHz$) are not reflected by ionosphere. Therefore, sky-wave propagation is not possible and similarly ground and space wave is also not feasible. Hence, they are transmitted to artificial satellite from where they are transmitted back to the Earth.

We uses three geostationary satellites placed at the vertices of an equilateral triangle, then the entire earth can be covered by the communication network, each satellite covers $1/3$ of the globe.

A space wave travels in a straight line from transmitting antenna to the receiving antenna. Space waves are used for line of sight communication as well as satellite communication.

Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus it is in contrast to on site observation. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth.
Hence, the statement is true.

$V=\sqrt { \cfrac { GMe }{ r }  } \ \therefore V\propto \cfrac { 1 }{ \sqrt { r }  } $

Using Kepler's Third Law, T^2 is proportional to R^3. For a geostationary satellite, T is 1 day. For the moon, T is approximately 29 days. Thus, (Rm/Rs)^3 = (Tm/Ts)^2 = (29/1)^2, which means Rm/Rs = (29^2)^(1/3) = 29^(2/3).

Older satellite used $H-$ batteries.

a) Lunching of Apple - $1$ April, $1976$