Geosynchronous Satellites

A geosynchronous satellite is an artificial satellite that revolves around the Earth in an orbit such that its orbital period matches the Earth’s rotation period. This means the satellite takes approximately 24 hours to complete one revolution around the Earth, synchronising its motion with the planet’s rotation. Geosynchronous satellites are fundamental to modern telecommunications, weather forecasting, navigation, and military surveillance systems, offering continuous coverage of a particular region of the Earth.

Concept and Principle of Operation

The term “geosynchronous” refers to the satellite’s ability to stay synchronised with the Earth’s rotation. According to Kepler’s third law of planetary motion, the orbital period of a satellite depends on its distance from the Earth’s centre. For a satellite to complete one revolution in 24 hours, it must orbit at a specific altitude of approximately 35,786 kilometres above the Earth’s equator.
At this height, the gravitational pull of the Earth and the centrifugal force due to the satellite’s motion are balanced, enabling the satellite to remain in a stable orbit. The orbital velocity required at this altitude is about 3.07 km/s.
If a geosynchronous satellite’s orbit is circular and lies directly above the equator, it is called a geostationary satellite. Such a satellite appears stationary to an observer on Earth because it remains fixed above one point on the equator.

Orbital Characteristics

A geosynchronous orbit possesses the following main features:

• Orbital Period: 23 hours 56 minutes (equal to Earth’s sidereal day).
• Altitude: Approximately 35,786 km from Earth’s surface.
• Inclination: May vary; if it is 0°, the orbit is geostationary.
• Shape: Generally circular, though slightly elliptical orbits can occur.
• Position: Directly above the equator for geostationary satellites, or inclined for inclined geosynchronous satellites.

Satellites in inclined geosynchronous orbits trace a figure-eight (analemma) path in the sky as viewed from Earth, while geostationary satellites maintain a constant position relative to the surface.

Types of Geosynchronous Satellites

1. Geostationary Satellites: These are a subset of geosynchronous satellites that orbit directly above the equator in a circular path. They remain fixed relative to a point on Earth, making them ideal for communication, weather monitoring, and broadcasting.
2. Inclined Geosynchronous Satellites: These satellites have orbits slightly inclined to the equatorial plane. They appear to move north and south of the equator in a daily cycle but still complete one revolution in 24 hours. Such orbits are often used when precise station-keeping over the equator is unnecessary or too costly.

Launch and Positioning

To place a satellite into geosynchronous orbit, it is first launched into a transfer orbit—typically a geostationary transfer orbit (GTO)—using a multi-stage rocket. From this elliptical orbit, the satellite’s onboard thrusters are fired at the apogee to circularise the orbit at the desired altitude.
The positioning of geosynchronous satellites is crucial to avoid interference. Satellites are separated by at least 2° of longitude, corresponding to about 150 kilometres at the orbital altitude. This ensures clear communication channels and reduces the risk of signal overlap.

Applications of Geosynchronous Satellites

Geosynchronous satellites are indispensable in several technological and scientific domains. Their continuous coverage of specific regions makes them ideal for:

• Telecommunication: Used extensively for long-distance telephone, internet, and television broadcasting. Communication satellites such as INSAT, INTELSAT, and Inmarsat operate in geosynchronous orbits.
• Weather Observation: Meteorological satellites like INSAT-3D and GOES provide real-time data on cloud movement, storms, and cyclones. Continuous observation helps in weather forecasting and disaster management.
• Navigation and Surveillance: Used in navigation systems and for monitoring military and strategic activities. Defence satellites in such orbits can provide consistent coverage of critical regions.
• Data Relay and Remote Sensing: Geosynchronous satellites facilitate data transfer between low-Earth orbit (LEO) satellites and ground stations, enabling seamless global data relay.
• Broadcasting: Direct-to-home (DTH) television services rely heavily on geostationary satellites, which transmit signals over fixed regions without the need for constant tracking.

Advantages of Geosynchronous Satellites

Geosynchronous satellites possess several advantages due to their high altitude and synchronised orbit:

• Continuous Coverage: A single satellite can cover nearly one-third of the Earth’s surface, enabling continuous monitoring or communication.
• Fixed Antenna Alignment: Ground-based antennas do not need to track the satellite’s motion, simplifying receiver design and reducing operational cost.
• Reliable Communication: Ideal for stable, long-term communications and broadcasting services.
• Wide Field of View: The high altitude allows observation of large geographical areas simultaneously, essential for meteorology and surveillance.

Limitations and Challenges

Despite their benefits, geosynchronous satellites face certain constraints:

• High Launch Cost: Reaching an altitude of over 35,000 km requires powerful launch vehicles, increasing mission cost.
• Signal Delay: The large distance causes a delay of about 0.24 seconds for a round trip, affecting real-time communications.
• Coverage Limitation: Polar regions cannot be effectively observed due to the equatorial orbit.
• Orbital Congestion: The geostationary belt is becoming crowded, making it difficult to allocate orbital slots and frequency bands.
• Maintenance and Degradation: Solar radiation, micrometeoroids, and radiation belts can degrade satellite components over time.

Examples of Geosynchronous Satellites

Several important satellites operate in geosynchronous orbits, serving communication, weather, and defence purposes:

• INSAT (India’s National Satellite System): Provides telecommunication, broadcasting, and meteorological services across India.
• GSAT Series (India): Used for advanced communication, internet, and tele-education services.
• GOES (Geostationary Operational Environmental Satellites – USA): Monitors weather patterns and atmospheric changes.
• INTELSAT (International Telecommunications Satellite): Offers global communication links for broadcasting and telephony.
• INMARSAT: Provides satellite-based maritime communication services.

Comparison Between Geosynchronous and Geostationary Satellites

Role in Global Communication and Research

Geosynchronous satellites have revolutionised global communication, enabling real-time connectivity across continents. They play a critical role in telemedicine, distance education, weather forecasting, and national security. Developing nations, including India, have utilised these satellites to extend telecommunication and broadcasting services to remote areas.