# What Is A Satellite?

A2 A Level Physics

Date : 14/08/2017

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Subject : Physics

What is a Satellite?

The word satellite doesn t necessarily describe a mechanical box floating through space with solar panels attached to either sides. In more general terms, a satellite is any space-based object that orbits a planet. For instance, the moon is a natural satellite, whereas satellites launched to propagate communication and navigate are artificial, man-made satellites.

To place a satellite in a stable orbit around Earth, it needs to overcome Earth s gravitational pull and air resistance in its lower atmosphere. The velocity at which it escapes this pull is the escape velocity, which is around 7 miles per second. Even after escaping Earth, a satellite s tendency to remain in its orbit is contested by the Earth s constant pull of gravity.

The velocity to keep a satellite in orbit is much higher, and is known as the orbital speed, which is around 17,000 miles per hour. Its linear velocity pushes it outward, while gravity pulls it inward towards the planet. The satellite eventually settles in an orbit when these forces are balanced.

Satellites come in two types:

Polar: These satellites hover over the poles and monitor the Earth while it rotates continuously beneath them, so that eventually the entire Earth is subject to their scrutiny, as they remain in the same place.

Anatomy of a satellite Geosynchronous or Geostationary: These are attached to a particular location, as their rotation is synchronized with the Earth s rotation. These are used to monitor or communicate with the receivers in a particular location.

Anatomy of a satellite Attitude control:

Satellites must take precise measurements from their place in orbit without wobbling. This is why they are regularly stabilized, which is called attitude control.

Gyroscopic motion is utilized to stabilize the position of a satellite s cameras and its orientation in space with respect to the object that it s orbiting.

Without stabilization, a satellite might deviate from its path outward to space or towards the Earth, providing inaccurate results and rendering it unreliable.

Gyroscopes rotate up to 6,000 RPM for three-axis stabilization and around 60-70 RPM for a spin-stabilized cylindrical satellite.

Body or Bus:

The body of a satellite houses the necessary scientific equipment it possesses. It is designed specifically to carry them safely into space. Engineers must consider a number of different objectives while designing and developing the body.

The outer layer protects it from space particles or micrometeorites floating in space.

Anti-radiation materials that protect it from the Sun s harmful UV radiation.

The satellite must sustain a comfortable temperature for proper functioning and must conduct heat away from its equipment.

Structures to support and connect materials.

Another major factor is economics. Its development should be economical, as in, cost-effective regarding its expenditure, longevity and weight.

Communication:

The circuits responsible for communication are known as the satellite s transponders.

A satellite communicates by either transmitting or receiving signals.

Transmitter:

The transmitter is a combination of many individual circuits.

1. Power supply: To provide constant power to all the circuits in a transmitter for their functioning.

2. Oscillator: The oscillator circuit generates a radio frequency signal, which is a sine wave of constant amplitude. This wave is known as the carrier, as it is later combined with the information to be transmitted and literally carries it on itself.

3. Modulator: This circuit combines the carrier with the information to be transmitted by varying some parameter of the carrier, such as its amplitude or frequency.

4. Amplifier: Satellites also use amplifiers to amplify a weakened signal and retransmit it to other satellites.

5. Antenna: Finally, this amplified signal is passed to the antenna, which uses reflectors to spew and radiate the signal as radio waves towards the receiver.

The receiver intercepts the transmitted EM wave and extracts the information inside it to be used.

The receiver consists of circuits that mirror a transmitter s individual circuits.

1. Antenna: The antenna converges the radio waves through a reflector.

The waves are further converged by foci, passing through twists and turns to finally reach the circuits where they are processed.

2. Amplifier: As the signal travels through a medium, some of its energy is attenuated, which is why it s amplified again at the receiver s end.

3. Tuner: The receiver receives multiple signals on a variety of frequencies from different transmitters. A tuner is used to listen to a particular signal that you want to hear.

4. Detect: The receiver then demodulates the signal or extracts the required information from the carrier signal.

5. Amplifier: The information is amplified again at the end to strengthen it and make sure it s delivered with sufficient power.

Power source:

A satellite needs to operate 365 days a year, 24/7, revolving and collecting data. The most readily available source for power is the Sun, but even the Sun is screened by the Earth during eclipses. This is why solar cells are accompanied by high-performance batteries.

Command and Control Systems:

This is the brain of the satellite, and includes the Tracking, Telemetry and Control (TT&C) system, which monitors and controls all of the satellite s parameters, stores and analyses all the data, and governs its communication with one or more satellites.

The data consists of scientific information or telecommunication signals, as well as the satellite s position and health information.

How does it communicate?

Communication doesn t necessarily have to occur in the Radio spectrum. Your TV remote communicates with its set top box with infrared waves, while phones communicate with microwaves. The selection of a particular portion of the electromagnetic spectrum depends on many factors, such as the size of antennae, distance between the two participating devices and the obstacles between them.

Waves undergo a phenomenon called diffraction, which makes them deviate and travel around an obstacle.

Diffraction can only take place if a wave s wavelength is comparable to the obstacle s size.

The large wavelengths of radio waves allow them to be easily diffracted around buildings and mountains.

Radio waves transmitted by satellites are also reflected by the charged particles of the ionosphere.

This is extremely useful when two satellites on Earth need to communicate, but are not directly in line due to the Earth s curvature.

The transmitting antenna projects the waves at particular angles of incidence such that they are reflected by the ionosphere in the direction of the receiver.

Satellites can also communicate with the help of a space satellite.

Satellites hanging in space behave like a mirror, deflecting signals in the direction of the satellite receiving it.

Space satellites not only reflect signals, but amplify and re-transmit them, as they are weakened due to constant dissipation in the medium and absorption by the atmosphere.

Bandwidth and Transmission:

The radio spectrum comprises a range of frequencies on which information can travel.

However, communication only occurs on specific frequencies set by The International Telecommunication un ion based in Geneva, Switzerland.

The frequencies are grouped together to form bandwidths.

They can either be Narrow-band signals (kHz), which are used for limited services, such as paging and low-data communication, or Broadband signals (MHz), which are used for advanced communication, such as video transmission.

Transmission from a station on Earth to a satellite in space is called an uplink.

The satellite then amplifies it and reroutes this signal on a different frequency back to the Earth to one or more receivers.

This is called a downlink. The area covered by the radiation on Earth is called its footprint.

International Space Station Satellites are extremely useful devices that help us comprehend the incomprehensible geography of our planet.

Although satellites have lately gotten a bad reputation, as they allow nosy tech-giant advertising corporations and security organizations to keep us under constant surveillance and disrespect our privacy, they still connect us to people across the globe through our phone or computer screens, and help us reach those people by navigating us one turn at a time.

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