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Satellites: An Overview ​

Satellites are objects that revolve around a larger celestial body due to gravitational forces. They can be either natural or artificial (man-made). For instance, the Moon is Earth's natural satellite, while numerous artificial satellites have been launched by various countries.

A Brief History of Satellite Development ​

The fascination with celestial objects and their motion has existed since ancient times, with early models like Ptolemy's geocentric model suggesting all celestial objects revolved around the Earth. Later, Copernicus proposed a heliocentric model where planets revolved around the Sun, a theory supported by Galileo's astronomical observations. Isaac Newton then formulated the universal law of gravitation and three laws of motion, which explained planetary motion and Kepler's laws.

The era of artificial satellites began with Sputnik-1, the first artificial Earth satellite, launched by the USSR on October 4, 1957, which carried a radio transmitter. The first American satellite to relay communications was Project Score in 1958.

India's space program, spearheaded by the Indian Space Research Organisation (ISRO), was established in 1969 under the leadership of Dr. Vikram Sarabhai, with a vision to use satellites for national development.

  • India launched its first experimental satellite, Aryabhata, from a USSR launching facility on April 19, 1975.
  • This was followed by Bhaskara-I (June 7, 1979), India's first experimental remote sensing satellite.
  • After developing its indigenous launch vehicle, SLV-3, India successfully launched the Rohini-I satellite on July 18, 1980.
  • The INSAT series of satellites was initiated in 1983, revolutionizing telecommunications and broadcasting in India.
  • The PSLV (Polar Satellite Launch Vehicle) debuted in 1993, becoming a reliable workhorse for launching satellites into polar orbits, and the GSLV (Geosynchronous Satellite Launch Vehicle) was developed for heavier payloads.
  • Significant Indian missions include Chandrayaan-1 (2008), which discovered water molecules on the Moon, and the Mars Orbiter Mission (Mangalyaan) (2013), making India the first Asian nation and the first in the world to reach Martian orbit in its maiden attempt.
  • In 2017, ISRO set a world record by launching 104 satellites on a single PSLV-C37 rocket.
  • Chandrayaan-3 (2023) successfully landed on the Moon's south pole, making India the first country to achieve this feat.

Types of Satellites ​

Satellites are categorized based on their application and mass:

A) Based on Applications ​

  • Communication Satellites: These satellites are used for television, telephone, radio, internet, and military applications. They typically reside in geostationary orbits. Examples include the INSAT series and GSAT series.
  • Earth Observation (Remote Sensing) Satellites: These satellites monitor environmental changes, weather, land-use, and natural disasters by collecting information from a distance. They are often in low Earth or polar sun-synchronous orbits. Examples include the IRS series, Cartosat, RISAT, and Oceansat.
  • Navigation Satellites: They provide global or regional navigation services, such as GPS (Global Positioning System). India's independent system is NavIC (Navigation with Indian Constellation), formerly IRNSS. These are primarily launched in Medium Earth Orbits.
  • Scientific/Space Exploration Satellites: Used for astronomical observations, fundamental physics experiments, and deep space studies. Examples include AstroSat for multi-wavelength astronomy, Aditya-L1 for solar observation, and lunar/Martian missions like Chandrayaan and Mangalyaan.
  • Military/Surveillance Satellites: These satellites are used for reconnaissance, surveillance, intelligence gathering, missile tracking, and other defense purposes, including monitoring enemy troop movement and electronic communications. Examples include GSAT-7 (Rukmini) for the Indian Navy and GSAT-7A (Angry Bird) for the Indian Air Force, and EMISAT for electronic intelligence.
  • Experimental Satellites: These are used for testing new technologies or validating concepts. Examples include Aryabhata and Rohini.

B) Based on Mass ​

  • Large Satellite: >1000 kg (e.g., GSAT Series).
  • Medium-Sized Satellite: 500–1000 kg (e.g., INSAT-3DR).
  • Mini Satellite: 100–500 kg (e.g., IMS-1, Youthsat, SARAL).
  • Micro Satellite: 10–100 kg (e.g., Microsat built by ISRO).
  • Nano Satellite: 1–10 kg (e.g., Jugnu, INS-1A, INS-1B).
  • Pico Satellite: <1 kg (e.g., STUDSAT).

Understanding Satellite Orbits ​

An orbit is the curved path an object, such as a planet, moon, or satellite, follows as it moves around another object due to the force of gravity. An orbit is the result of a balance between two forces: gravitational force, which pulls the object towards the central body (acting as the centripetal force), and inertia (or the apparent centrifugal force), which is the object's tendency to move in a straight line at constant speed.

Key Orbital Terminology ​

  • Orbital Velocity: The speed at which an object must travel to stay in orbit, balancing gravity and its forward motion. It does not depend on the mass of the satellite but on the mass of the central body and the orbital radius. For a satellite close to Earth, this is approximately 8 km/s.
  • Orbital Period: The time taken by an object to complete one full orbit around its central body.
  • Perigee: The point in an orbit closest to the central body.
  • Apogee: The farthest point in an orbit from the central body.
  • Elliptical Orbit: An oval-shaped orbit where the object moves closer and farther from the central body. Kepler's first law states that planets move in elliptical orbits with the Sun at one focus.
  • Circular Orbit: A special case of an elliptical orbit that is perfectly round.

Orbits Categorized by Altitude ​

  • Low Earth Orbit (LEO):
    • Altitude: 160 km to 2,000 km above Earth’s surface.
    • Characteristics: Satellites in LEO complete one orbit in approximately 90 to 120 minutes. Their orbital velocity is higher than other orbits (e.g., ~7.8 km/s at 200 km altitude). They are subject to atmospheric drag, which requires occasional adjustments to maintain altitude.
    • Advantages: Low energy requirements for launches, fast communication with low latency, less powerful transmitters needed, easy servicing and replacement, and cost-effective to launch and maintain.
    • Disadvantages: Limited field of view (requires large constellations for continuous coverage), orbital decay necessitates re-boosting, high collision risk with space debris, and shorter lifespan due to drag.
    • Applications: Earth observation, disaster management, spy satellites and satellite imaging (e.g., Rohini Satellite, RISAT-2B, EOS-01). LEO is also home to space stations like the International Space Station (ISS) (at 400 km) and communication systems like satellite internet constellations (e.g., Starlink, OneWeb, Amazon Kuiper Project). Astronomical observations, such as those by the Hubble Space Telescope and AstroSat, also utilize LEO.
  • Medium Earth Orbit (MEO):
    • Altitude: 2,000 km to 35,786 km above Earth’s surface.
    • Characteristics: Orbital period ranges from 2 to 12 hours. Speed is lower than LEO but higher than GEO.
    • Applications: MEO is the standard orbit for Global Navigation Satellite Systems (GNSS) like GPS (~20,200 km), GLONASS, Galileo, and BeiDou. Some communication satellites also use MEO.
    • Advantages: Efficient for global coverage, lower latency compared to HEO, and cost-effective coverage as fewer satellites are needed than in LEO constellations.
    • Challenges: Satellites must be shielded against the Van Allen Radiation Belts, and perturbing forces require corrections for orbital stability.
  • High Earth Orbit (HEO):
    • Altitude: A geocentric orbit with an apogee farther than the geosynchronous orbit (35,786 km or 22,236 mi above Earth).
    • Characteristics: Orbital period is longer than 24 hours.
    • Types: Includes Geostationary Orbit (GEO), Geosynchronous Orbit (GSO), Geostationary Transfer Orbit (GTO), Highly Elliptical Orbit (HEO), and Near-Rectilinear Halo Orbit (NRHO).
    • Applications: Limited for communication and navigation due to delay but useful for global broadcasting or redundancy. Ideal for astronomical observatories and Earth science missions requiring a wide view or deep-space perspective. Also used for military surveillance and strategic communication.
    • Advantages: Provides an unobstructed view of Earth and deep space, ideal for astronomical observations, Earth monitoring, and continuous global coverage. Enables advancements in space exploration, astronomy, and global systems, with reduced interference from Earth's atmosphere.
    • Challenges: Long orbital periods make them unsuitable for missions needing frequent orbits, communication delay/latency, radiation exposure risks (lying outside Earth's magnetic field), and high launch costs with reduced payload capacity.
    • Special Case: Highly Elliptical Orbit (HEO): A type of HEO with a low perigee (as low as 2,000 km) and a high apogee (potentially beyond GEO), often used for high-latitude coverage (e.g., space telescopes to avoid Earth's shadow).

Specialized Orbits ​

  • Geosynchronous Orbit (GSO):
    • A satellite in GSO has an orbital period matching Earth's rotation (23 hours, 56 minutes, 4 seconds), completing one orbit per day.
    • The orbit can be inclined, not necessarily equatorial.
    • When viewed from the ground, the satellite appears to move in a figure-eight path (analemma).
    • Uses: Ideal for applications like communications or scientific observations without needing a fixed ground position. Weather satellites observing wide Earth areas may also use inclined GSO.
    • Examples: GSAT series, KALPANA-1, Bhaskara-I.
  • Geostationary Orbit (GEO):
    • A special case of geosynchronous orbit where the satellite orbits directly above Earth’s equator (0° inclination) and appears stationary relative to a specific point on the Earth's surface.
    • Arthur C. Clarke popularized the concept of geostationary satellites for global communications in 1945, which is why this orbit is sometimes called the "Clarke Orbit" or "Clarke Belt".
    • Characteristics:
      • Altitude: Approximately 35,786 km (or 36,000 km) above Earth's equator.
      • Direction: Satellite orbits in the same direction as Earth's rotation (west to east, or prograde).
      • Orbit: Must be circular and in the same plane as the equator.
      • Orbital Period: Matches Earth's rotation (23 hours, 56 minutes, and 4 seconds, or approximately 24 hours).
    • Applications:
      • Communications: Ideal for broadcasting TV, radio, and internet services, especially for areas without terrestrial networks. India's INSAT group of satellites are geostationary satellites widely used for telecommunications.
      • Meteorology: Provides real-time weather monitoring, tracking cyclones, measuring cloud properties, and predicting volcanic ash dispersion.
      • Navigation: Enhances Global Navigation Satellite Systems (GNSS).
      • Scientific Research: Some GEO satellites support Earth observation and data relay missions for deep-space probes.
    • Launch Requirements: Satellites are typically launched into a Geostationary Transfer Orbit (GTO) first, then use onboard propulsion to circularize their orbit at GEO altitude. Proximity to the equator for launch sites reduces inclination change and uses Earth's rotation for additional speed.
    • Advantages: Wide coverage (each satellite can observe up to 81° latitude and 77° longitude), fixed position relative to Earth eliminates the need for continuous antenna tracking on the ground, leading to cost savings. Only three satellites in GEO can provide coverage of the entire Earth.
    • Challenges: High launch costs compared to low orbit satellites, and fuel dependency for station-keeping and eventual relocation.
    • Difference between GSO and GEO: All geostationary orbits are geosynchronous, but not all geosynchronous orbits are geostationary. GEO is a subset of GSO, specifically optimized for constant monitoring of specific regions by having a 0° inclination (equatorial).
  • Polar Orbit:
    • Satellites in this orbit pass over the Earth's poles, traveling from north to south.
    • Altitude: Typically 200 km to 1,000 km (a type of Low Earth Orbit).
    • Applications: Earth observation (weather, climate monitoring, reconnaissance, disaster mapping), environmental and agricultural studies, and sometimes telecommunications (e.g., Iridium satellite constellation).
  • Sun-Synchronous Orbit (SSO):
    • A special type of polar orbit where the satellite’s path is synchronized with the Sun. This means the satellite always passes over the same region at the same local solar time (e.g., noon every day).
    • Altitude: Typically 600–800 km.
    • Characteristics: The inclination of the satellite's orbit remains constant with respect to the Sun, and the orbit gradually shifts (precession) to maintain this constant angle.
    • Applications: Ideal for consistent lighting conditions for imaging and data collection, making it useful for long-term monitoring of weather, forest fires, deforestation, flooding, and rising sea levels. This consistency allows for precise comparison of images over time.
    • Examples: Cartosat series, Oceansat series, SARAL.
  • Transfer Orbits:
    • Special orbits used to transition a satellite or spacecraft from one orbit to another using minimal energy.
    • Geostationary Transfer Orbit (GTO): A common elliptical transfer orbit used to move satellites from LEO or MEO to GEO. Communication satellites are often first launched into GTO and then transferred to GEO by their own propulsion system.
    • Hohmann Transfer Orbit: A highly efficient elliptical orbit used to transfer a spacecraft between two circular orbits, typically for interplanetary travel or different orbital altitudes with minimal fuel consumption.
  • Lagrange Point Orbits:
    • These orbits exist around Lagrange points of a two-body system (e.g., Earth-Sun or Earth-Moon). A Lagrange point is a stable location where the gravitational forces of two large objects balance out with the centrifugal force felt by a smaller object.
    • There are five Lagrange points (L1, L2, L3, L4, L5) for any pair of orbiting bodies.
      • L1, L2, and L3 are unstable equilibrium points, meaning objects can drift away with small deviations.
      • L4 and L5 are stable equilibrium points, ideal for satellite placement as objects can remain in orbit for long periods.
    • Applications:
      • L1 is used for solar observation, such as India's Aditya-L1 mission.
      • L2 is used for deep space observatories, like the James Webb Space Telescope (JWST).
  • Halo Orbit: A special type of three-dimensional, loop-like orbit that occurs around Lagrange points, particularly the unstable L1, L2, and L3. It provides stability and energy efficiency for spacecraft near these points.
  • Heliocentric Orbits: Satellites that orbit the Sun instead of Earth. Examples include NASA's Parker Solar Probe and the Kepler Space Telescope.
  • Graveyard Orbit: Also known as junk or disposal orbit, it is situated at an altitude of approximately 36,050 km (about 300 km above GEO). It is used to transfer inactive or non-operational satellites to mitigate the problem of space debris.

Satellite Launch and Composition ​

To launch a satellite into orbit, it is first lifted to a sufficient height (e.g., about 200 km for LEO) to minimize atmospheric friction. Then, it is given a horizontal push with the required orbital velocity (e.g., about 8 km/s for low Earth orbit). Rockets are used to generate thrust by burning propellants, and they often involve multiple stages where empty fuel tanks are discarded to reduce weight. Once the satellite achieves orbital velocity, it separates from the rocket and begins its mission.

Satellites are complex machines composed of various subsystems:

  • Payload Subsystem: Carries the instruments necessary for the mission's objectives (e.g., cameras, sensors, transponders).
  • Propulsion Subsystem: Provides the necessary thrust to adjust the satellite's velocity and orbit.
  • Structural and Mechanical Subsystem: Provides the physical strength and shape to withstand launch forces and space conditions.
  • Thermal Subsystem: Maintains temperature stability within the satellite's components.
  • Attitude and Orbit Control Subsystem (AOCS): Maintains the satellite's orientation and corrects its orbit as needed.
  • Electrical Power Subsystem: Generates (e.g., using solar panels), stores, and distributes power to all satellite systems.
  • Telemetry, Tracking, and Command (TTC) Subsystem: Acts as the communication link between the satellite and ground stations.