GS Notes

Science and Technology Notes 02

10 min read

  • Geostationary Transfer Orbit (GTO)

    • Concept and Purpose: A Geostationary Transfer Orbit is an intermediate, highly elliptical orbit used to transition a satellite from a low-altitude parking orbit, typically a Low Earth Orbit (LEO), to a high-altitude Geostationary or Geosynchronous Orbit (GEO/GSO). This method is a practical application of the Hohmann transfer orbit, an orbital maneuver that uses two engine impulses to move a spacecraft between two coplanar circular orbits. It is the most fuel-efficient way to achieve a high-altitude orbit.
    • Orbital Characteristics:
      • Perigee (closest point to Earth): The perigee of a GTO is typically at the altitude of the initial LEO, usually between 180-250 km.
      • Apogee (farthest point from Earth): The apogee is set at the altitude of the target geostationary orbit, which is approximately 35,786 km above the Earth’s equator.
    • Maneuver Sequence:
      1. A launch vehicle, like India’s GSLV, first injects the satellite into a LEO.
      2. From the LEO, the rocket’s upper stage fires its engine at a precise point (perigee) to push the satellite into the elliptical GTO.
      3. The satellite then coasts upwards towards its apogee.
      4. Upon reaching the apogee, an onboard engine, known as the Apogee Kick Motor (AKM) or Liquid Apogee Motor (LAM), is fired. This provides the necessary thrust (a “boost”) to circularize the orbit at 35,786 km and adjust its inclination to zero degrees (for a geostationary orbit), matching the Earth’s rotational speed.
    • Historical Context & Examples: The concept of geostationary communication satellites was first proposed by the science fiction writer Arthur C. Clarke in a 1945 paper titled “Extra-Terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?“. India’s key launch vehicle, the Geosynchronous Satellite Launch Vehicle (GSLV), is specifically designed to place satellites into GTO. Missions like Chandrayaan-2 (2019) and Mangalyaan (2013) also initially used Earth-bound orbits and transfer maneuvers, demonstrating the fundamental utility of this orbital mechanic principle for interplanetary missions as well.

  • Sun-Synchronous Orbit (SSO)

    • Concept: A Sun-Synchronous Orbit is a specific type of near-polar Low Earth Orbit (LEO) where the satellite passes over any given point on the Earth’s surface at the same local solar time. This means the angle between the satellite’s orbital plane and the Sun remains relatively constant throughout the year.
    • Scientific Principle - Orbital Precession: This is achieved by utilizing the phenomenon of orbital precession. The Earth is not a perfect sphere; it is an oblate spheroid (slightly flattened at the poles and bulging at the equator). This gravitational anomaly exerts a torque on the satellite, causing its orbital plane to rotate or “precess” around the Earth’s axis. In an SSO, this precession rate is deliberately matched to the rate at which the Earth revolves around the Sun (approximately 360 degrees in 365.25 days, or about 0.986 degrees per day).
    • Orbital Characteristics:
      • Altitude: Typically between 500 km and 800 km.
      • Inclination: The angle between the orbital plane and the equatorial plane is usually between 94 to 98 degrees (a retrograde orbit). This high inclination is what makes it a “near-polar” orbit. The precise inclination is calculated to achieve the required precession rate at a given altitude.
    • Applications and Significance: The constant illumination angle is invaluable for Earth observation.
      • Comparative Analysis: As noted by earth scientists, this allows for consistent lighting conditions, making it possible to compare images of the same location taken on different days, months, or years without the data being skewed by changes in shadows or solar illumination. This is critical for monitoring deforestation, urbanization, agricultural crop health, and glacial retreat.
      • Examples: Most Earth observation and remote sensing satellites, such as India’s Cartosat series and Resourcesat series (now part of the EOS series), are placed in SSO. The joint NASA-ISRO NISAR mission will also operate from an SSO.

  • Types of Satellites

    • Communication Satellites:

      • Function: These act as relays in space, receiving signals from a ground station (uplink) and transmitting them to another location (downlink). They bridge vast geographical distances, overcoming the line-of-sight limitation of terrestrial communication.
      • Orbit: Primarily placed in Geostationary Orbit (GEO) to ensure they remain at a fixed point relative to the Earth’s surface, allowing ground antennas to be permanently pointed at them.
      • Indian Context: ISRO’s Indian National Satellite (INSAT) system, initiated in 1983, is one of the largest domestic communication satellite systems in the Asia-Pacific. The satellites were later named GSAT (Geosynchronous Satellite). In a recent move towards a more application-centric approach, ISRO has renamed this series to CMS (Communication Satellite), with CMS-01 (formerly GSAT-12R) being the first in 2020.
      • Applications:
        • Telecommunications & Broadcasting: Direct-to-Home (DTH) television, radio networking, and internet services.
        • Education: The EDUSAT (or GSAT-3), launched by ISRO in 2004, was India’s first thematic satellite dedicated exclusively to serving the educational sector. It provided satellite-based two-way communication to classrooms for delivering educational content.
        • Telemedicine: Connecting rural primary health centers with super-specialty hospitals in urban centers for expert consultation.
        • VSAT (Very Small Aperture Terminal): These are small ground stations (dish antennas of 1-4 meters) used for reliable data transmission for services like banking (connecting ATMs), e-governance, and enterprise networks.
        • Search and Rescue: The Cospas-Sarsat Programme is an international satellite-based system for distress alert detection and information distribution. Satellites detect and locate signals from emergency beacons on ships, aircraft, or individuals. India is a member and provider of ground segment equipment and services, with mission control centers in Bengaluru and Lucknow.

    • Earth Observation (EO) or Remote Sensing Satellites:

      • Function: These satellites are designed to observe Earth from orbit, collecting data about its physical, chemical, and biological systems using advanced sensors. This is known as remote sensing—gathering information about an object without being in direct physical contact with it.
      • Orbit: Typically placed in LEO, especially Sun-Synchronous Orbits, to get high-resolution imagery with consistent lighting. For continuous monitoring of a large area (like weather systems over the Indian subcontinent), they may also be placed in Geostationary Orbit (e.g., INSAT-3D, INSAT-3DR).
      • Technologies: They employ various sensing technologies:
        • Optical Imaging: Capturing images in the visible and infrared spectrum.
        • Synthetic Aperture Radar (SAR): An active sensing technique that transmits microwave signals and records the echoes. It can “see” through clouds, darkness, and rain, making it an all-weather, day-and-night observation tool. RISAT series from ISRO are examples.
        • LiDAR (Light Detection and Ranging): Uses laser pulses to measure distances and create precise 3D maps of the Earth’s surface.
        • Spectroscopy/Hyperspectral Imaging: Captures data across hundreds of narrow spectral bands, allowing for detailed identification of materials, minerals, vegetation types, and soil composition.
      • Indian Context: ISRO began its EO program with the Indian Remote Sensing (IRS) satellite series, starting with IRS-1A in 1988. This has now been consolidated and renamed the EOS (Earth Observation Satellite) series.
      • Applications: Agriculture (crop acreage and health monitoring), forestry (mapping forest cover), geology (mineral exploration), disaster management (flood mapping, cyclone tracking, landslide risk assessment), urban planning, and national security.

  • NISAR Mission

    • Collaboration and Full Form: A joint mission between the National Aeronautics and Space Administration (NASA) and the Indian Space Research Organisation (ISRO). The name NISAR stands for NASA-ISRO Synthetic Aperture Radar.
    • Objective: It is an advanced Earth observation mission designed to systematically map the Earth using two different radar frequencies (L-band and S-band). It will measure changes in our planet’s surface with unprecedented detail, typically on the order of centimeters.
    • Technical Specifications:
      • It will be the first satellite mission to use two different radar frequencies simultaneously. NASA is providing the L-band SAR payload, while ISRO is providing the S-band SAR payload, the spacecraft bus, and the launch vehicle (GSLV Mk-II).
      • The use of SAR allows it to operate day and night and in all weather conditions.
    • Scientific Goals and Applications:
      • Environmental Monitoring: Tracking changes in ecosystems, biomass, sea-level rise, and the dynamics of ice sheets and glaciers.
      • Disaster Monitoring: Providing critical data for managing natural disasters like earthquakes (by monitoring crustal deformation), tsunamis, volcanic eruptions, and landslides. Its rapid revisit time will enable quick damage assessment.
      • Geological Studies: Understanding the processes of Earth’s crust and monitoring groundwater resources.

Prelims Pointers

  • Geostationary Transfer Orbit (GTO):
    • An intermediate, highly elliptical orbit.
    • Perigee is at Low Earth Orbit (LEO) altitude (e.g., ~200 km).
    • Apogee is at Geostationary Orbit (GEO) altitude (~35,786 km).
    • Used to move satellites from LEO to GEO in a fuel-efficient manner.
  • Sun-Synchronous Orbit (SSO):
    • A type of near-polar, Low Earth Orbit (LEO).
    • Satellite passes over a point on Earth at the same local solar time.
    • Altitude: 500-800 km.
    • Inclination: 94-98 degrees (retrograde).
    • The orbital precession (due to Earth’s equatorial bulge) is matched with Earth’s revolution rate around the Sun.
    • Ideal for Earth observation and remote sensing satellites.
  • Communication Satellites:
    • Typically placed in Geostationary Orbit (GEO).
    • ISRO’s series: INSAT → GSAT → CMS (current nomenclature).
    • EDUSAT (GSAT-3): India’s first satellite exclusively for education, launched in 2004.
    • VSAT: Very Small Aperture Terminal; small ground station antennas.
    • Cospas-Sarsat: International satellite-aided search and rescue program. India is a member.
  • Earth Observation (EO) Satellites:
    • Also known as Remote Sensing satellites.
    • Mostly placed in Sun-Synchronous Orbits (SSO).
    • ISRO’s series: IRS → EOS (current nomenclature).
    • Technologies used: Synthetic Aperture Radar (SAR), LiDAR, Hyperspectral Imaging.
    • Examples: Cartosat series, Resourcesat series, RISAT series.
  • NISAR Mission:
    • A joint project between NASA and ISRO.
    • Full form: NASA-ISRO Synthetic Aperture Radar.
    • Objective: Earth observation, disaster and environmental monitoring.
    • World’s first dual-frequency (L-band and S-band) radar imaging satellite.
    • To be launched by ISRO’s GSLV Mk-II.

Mains Insights

  • Socio-Economic Significance of India’s Satellite Program (GS-III)

    • From Self-Reliance to Global Service Provider: India’s space program, envisioned by Vikram Sarabhai, was founded on the principle of using advanced technology for societal good. This has evolved from achieving self-reliance in launch capabilities (PSLV, GSLV) to becoming a commercially viable player through organizations like NSIL (NewSpace India Limited), providing launch services and satellite data globally.
    • Empowering Governance (GS-II): Space technology is a force multiplier in public service delivery.
      • Cause-Effect: The use of satellite imagery (from EOS satellites) for crop yield estimation directly impacts the implementation and settlement of claims under PM Fasal Bima Yojana. Geo-tagging of assets created under MGNREGA using the ‘Bhuvan’ platform enhances transparency and accountability.
    • Bridging the Digital and Development Divide:
      • Communication satellites (INSAT/GSAT/CMS series) have been pivotal in expanding tele-education to remote areas (EDUSAT) and tele-medicine for rural healthcare, addressing issues of accessibility and quality. VSAT networks have been the backbone for banking services in under-connected regions.

  • Space Technology in Disaster Management (GS-III)

    • A Holistic Framework: Satellites play a crucial role across the entire disaster management cycle, shifting the paradigm from a relief-centric approach to one of preparedness and mitigation.
    1. Pre-Disaster Phase:
      • Early Warning: Meteorological satellites (e.g., INSAT-3DR) provide timely and accurate tracking of cyclones, forecasting their intensity and path, enabling mass evacuations.
      • Vulnerability Mapping: High-resolution data from EOS satellites helps in creating maps of landslide-prone zones, floodplains, and areas vulnerable to seismic activity.
    2. During-Disaster Phase:
      • Real-time Monitoring: SAR satellites (like RISAT) can penetrate cloud cover to provide images of flooded areas, helping to direct rescue teams.
      • Communication Support: In the event of terrestrial communication breakdown, communication satellites provide emergency connectivity for response agencies.
    3. Post-Disaster Phase:
      • Damage Assessment: Comparing pre and post-disaster satellite imagery provides a quick and accurate assessment of damage to infrastructure, agriculture, and housing, which is crucial for planning relief and reconstruction.
      • Example: The National Remote Sensing Centre (NRSC) provides near real-time flood inundation maps to state and central agencies during monsoon seasons.

  • International Cooperation and Space Diplomacy (GS-II)

    • Collaborative Missions: Missions like NISAR (with NASA) demonstrate India’s growing stature as a credible partner in advanced scientific research. It signifies a move from being a recipient of technology to a co-developer.
    • SAARC Satellite: The launch of the South Asia Satellite (GSAT-9) in 2017, offered as a ‘gift’ to neighboring countries, is a prime example of space diplomacy, using space assets to foster regional cooperation and goodwill.
    • Global Programs: India’s active participation in programs like Cospas-Sarsat and the International Charter ‘Space and Major Disasters’ showcases its commitment to using space for global humanitarian good.

Graph View