INTRODUCTION
A satellite is an artificial object which is placed intentionally into an orbit of any natural satellite. Satellites are used for many purposes i.e. weather forecasting, digital transmission, scientific research and development etc.
In a communication context, a satellite is a specialized wireless transmitter/receiver that is launched by a rocket and placed in orbit around the earth.
A satellite can be natural, like the moon, or artificial (human made). So we can say that a satellite is an object that moves in a curved path around a planet.
Satellite can travel around planets or around stars such as our sun. All the planets are satellites around the sun.
Satellites which are far away from the surface of the earth can cover a wide area on the surface of the earth.
The moon is earth’s original, natural satellite, and there are many man-made (artificial) satellites for example: SPUTNIK, ECHO, and SYNCOM.
TYPES OF SATELLITES
There are several different types of satellites that are used for a variety of purposes.
A) On the Basis of Orbital Characteristics
B) On the Basis of Height
C) On the Basis of Application
ON THE BASIS OF ORBITAL CHARACTERISTICS
Geostationary Satellites
These are located at very high altitude of about 36000 km from the earth surface
Synchronous to rotation of the earth (west to east)
They seem stationary, relative to the earth surface
They revolve at speed of earth rotation (24 hrs)
They viewed always same portion of the earth
Mainly used for communication and metrological application
E.g- GSAT, INSAT, Kalpana1 etc..
Sun-Synchronous Satellites
Orbit plane near to pole / pole to pole (moves from north to south)
Placed at low altitude of 300-1000 km from the surface of earth
It is synchronous to the position of the sun
Satellite passes overall the places of the earth
Satellite takes 100 minutes to one revolution
It collect precisely information about earth (Resource satellites)
E.g- Landsat, Spot, IRS
Satellite orbit
Geostationary orbit
- High altitude (36,000km)
- Remains in same position above the Earth
Polar sun synchronous orbit
- Low altitude (200-1000km)
- Goes close to poles
ON THE BASIS OF HEIGHT
Geostationary Earth Orbit (GEO)
GEO is an orbit at an altitude of around 36,000 kilometers above the Earth’s surface. Satellites in GEO orbit at the same speed as the Earth’s rotation, so they appear to be stationary in the sky. These satellites are often used for communication and television broadcasting.
If a satellite should appear in fixed in the sky, it requires a period of 24 hours.
Geostationary satellites have a distance of almost 36,000 km to the earth. Examples are almost all TV and radio broadcast satellites, any weather satellites and satellites operating as backbones for the telephone network.
Objects in GEO moves around the earth at the same speed as the earth rotates. This means geostationary satellites remain in the same position relative to the surface of earth.
Advantages of GEO satellite
Three Geostationary satellites are enough for a complete coverage of almost any spot on earth.
Receivers and senders can use fixed antenna positions, no adjusting is needed.
GEOs are ideal for TV and radio broadcasting.
Lifetime expectations for GEOs are rather high, at about 15 years.
Disadvantages of GEO satellite
Northern or southern regions of the earth have more problems receiving these satellites due to the low elevation above latitude of 60 degree,
The transmit power needed is relatively high (about 10 W) which causes problems for battery powered devices.
These satellites can’t be used for small mobile phones.
Low-Earth Orbits (LEO)
LEO is an orbit at an altitude of around 100 to 2000 kilometers above the Earth’s surface. Satellites in LEO are closer to the Earth and therefore have a shorter distance to travel for communication.
However, they also experience more atmospheric drag and need to be replaced more frequently. Examples of LEO satellites include the International Space Station and weather satellites.
LEO satellites are much closer to earth than GEO satellites, ranging from 500 to 1,500 km above the surface.
LEO satellites do not stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.
Advantages of LEO satellite
Using advanced compression schemes, transmission rates of about 2,400 bit/s can be enough for voice communication.
A LEO satellite smaller area of coverage is less of a waste of bandwidth.
A LEO satellite’s proximity to earth compared to a Geostationary satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.
Disadvantages of LEO satellite
The biggest problem of the LEO concept is the need for many satellites if global coverage is to be reached.
The high number of satellites combined with the fast movement’s results in a high complexity of the whole satellite system
One general problem of LEO is the short lifetime of about five to eight years due to atmospheric drag and radiation from the inner Van Allen belt.
Medium Earth Orbit (MEO)
MEO is an orbit at an altitude of around 8,000 to 12,000 kilometers above the Earth’s surface. Satellites in MEO have a longer distance to travel for communication compared to LEO satellites, but they also experience less atmospheric drag and have a longer lifespan. Examples of MEO satellites include navigation satellites such as GPS.
A MEO satellite situates in orbit somewhere between 6,000 km to 20,000 km above the earth’s surface.
MEO satellites are similar to LEO satellites in the context of functionality.
Medium earth orbit satellites are visible for much longer periods of time than LEO satellites usually between 2 to 8 hours.
MEO satellites have a larger coverage area than Low Earth Orbit satellites.
Advantages of MEO Satellite
Using orbits around 10,000km, the system only requires a dozen satellites which is more than a GEO system, but much less than a LEO system.
Depending on the inclination, a MEO can cover larger populations, so requiring fewer handovers.
A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network.
Disadvantages of MEO Satellite
The satellites need higher transmit power and special antennas for smaller footprints.
A MEO satellite’s distance gives it a longer time delay and weaker signal than LEO satellite
High Earth Orbit (HEO)
HEO is an orbit that has a high degree of ellipticity, meaning it has a large difference between its closest and farthest points from the Earth. Satellites in HEO have a long period of time at a high altitude and a short period of time at a low altitude. They are often used for communication with polar regions or for military surveillance.
The High Earth orbit satellite is the only non-circular orbit of the four types.
HEO satellite operates with an elliptical orbit, with a maximum altitude (apogee) similar to GEO, and a minimum altitude (perigee) similar to the LEO.
The HEO satellites used for the special applications where coverage of high latitude locations is required.
ON THE BASIS OF APPLICATION
Communicational Satellites
These satellites are used to transmit and receive signals for telecommunications, such as phone calls, internet data, and television broadcasts.
These satellites are used for positioning, navigation, and timing (PNT) purposes, such as the Global Positioning System (GPS).
Satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few meters in real time.
Earth observation satellites
These satellites are used to collect data about the Earth’s surface and atmosphere, including images and other types of data, for a variety of applications, such as mapping, environmental monitoring, and weather forecasting.
Weather satellites
These satellites are specifically designed to collect data about the Earth’s weather and climate, including temperature, humidity, and atmospheric conditions.
Military satellites
These satellites are used by military forces for a variety of purposes, including communication, navigation, intelligence gathering, and surveillance.
Scientific research satellites
These satellites are used to conduct scientific research and experiments, such as studying the Earth’s atmosphere or the effects of space on human health.
Overall, there are many different types of satellites that serve a wide range of purposes, from practical applications like telecommunications and navigation to scientific research and military operations.
Characteristics of Satellite
Satellites have been launched for various applications and are placed at different altitudes. Moreover their weights are also dissimilar.
Provides connections between the Earth-based base station and the satellites sensors
The path followed by a satellite is referred to as its orbit.
The area imaged on the surface, is referred to as the swath ❖Just beneath point of satellite called nadir point