Satellites are positioned in specific orbits based on their application, altitude, and orbital period.
Low Earth Orbit (LEO)
- Altitude: 160 km to 2,000 km above the Earth’s surface.
- Orbital Period: Approximately 90 to 120 minutes.
- Key Characteristics: High resolution due to proximity to Earth, rapid orbital speed, and short communication latency.
- Applications: Earth observation, remote sensing, military reconnaissance, and global communication constellations (e.g., Starlink, OneWeb).
Medium Earth Orbit (MEO)
- Altitude: 2,000 km to nearly 35,786 km.
- Orbital Period: 2 to 24 hours.
- Key Characteristics: Covers a larger geographical area than LEO. Satellites remain visible from a point on Earth for several hours.
- Applications: Navigation systems (e.g., GPS, GLONASS, Galileo) and semi-synchronous communication.
Geosynchronous and Geostationary Orbit (GEO)
- Altitude: Exactly 35,786 km above the Earth’s equator.
- Orbital Period: 24 hours, matching the Earth’s rotational period.
- Key Characteristics: A geostationary orbit is a specific type of geosynchronous orbit that is circular and lies exactly in the equatorial plane. From Earth, a geostationary satellite appears stationary in the sky.
- Applications: Satellite television, weather forecasting (e.g., INSAT series), and strategic military communications.
Polar and Sun-Synchronous Orbit (SSO)
- Altitude: 600 km to 800 km (a special type of LEO).
- Orbital Period: Approximately 100 minutes, passing from pole to pole.
- Key Characteristics: Passes over any given point of the planet’s surface at the same local mean solar time. This ensures consistent illumination angles for imaging.
- Applications: Environmental monitoring, weather tracking, and reconnaissance (e.g., India’s Cartosat and Resourcesat series).
| Orbit Type | Altitude Range | Orbital Period | Primary Applications | Notable Examples |
| Low Earth Orbit (LEO) | 160 – 2,000 km | 90 – 120 minutes | Remote Sensing, Broadband Internet | ISS, Starlink, EOS-04 |
| Medium Earth Orbit (MEO) | 2,000 – 35,500 km | 2 – 24 hours | Global Navigation, Positioning | GPS, GLONASS, Galileo |
| Geostationary Orbit (GEO) | 35,786 km | 24 hours | Telecommunications, Weather Forecasting | GSAT series, INSAT-3DR |
| Sun-Synchronous Orbit (SSO) | 600 – 800 km | ~100 minutes | Consistent Illumination Imaging | Cartosat-3, Oceansat-3 |
Core Subsystems of a Satellite
A satellite is divided into two primary functional components: the Bus (the structural framework) and the Payload (the functional instruments).
The Satellite Bus (Platform)
- Structure and Mechanisms: The physical framework made of lightweight composite materials (like carbon fiber or aluminum alloys) that withstand acoustic vibrations during launch.
- Power Subsystem (EPS): Generates electricity via solar panels and stores energy in Lithium-ion batteries for use when the satellite is in the Earth’s shadow.
- Attitude and Orbit Control System (AOCS): Manages the orientation (attitude) of the satellite using sensors (star trackers, sun sensors) and actuators (reaction wheels, thrusters).
- Telemetry, Tracking, and Command (TT&C): The communication bridge between the satellite and ground stations, transmitting health data (telemetry) and receiving operational commands.
- Thermal Control Subsystem: Maintains payload and electronics within safe temperature limits (-150°C to +150°C) using multi-layer insulation (MLI) blankets, heat pipes, and radiators.
The Payload
- Communication Payloads: Transponders (C-band, Ku-band, Ka-band) that receive, amplify, and retransmit signals.
- Remote Sensing Payloads: Optical cameras, multispectral scanners, and Synthetic Aperture Radar (SAR) sensors that function independent of daylight or cloud cover.
- Navigation Payloads: Atomic clocks (Rubidium or Caesium) that emit highly accurate time-stamped signals.
Indian Satellite Programs and Applications
The Indian Space Research Organisation (ISRO) categorizes its satellite deployment across three distinct operational pillars.
Communication Satellite Systems (INSAT and GSAT)
- Overview: India operates one of the largest domestic communication satellite systems in the Asia-Pacific region, primarily utilizing INSAT and GSAT series.
- Key Technological Milestones: Transition to High Throughput Satellites (HTS) like GSAT-11 and GSAT-20 (CMS-03) to provide high-speed internet to remote regions.
- Applications: Direct-To-Home (DTH) television, VSAT networks for banking, tele-education (EDUSAT), tele-medicine, and disaster management support.
Earth Observation and Remote Sensing (IRS and EOS)
- Overview: The Indian Remote Sensing (IRS) satellite constellation is one of the largest civilian remote sensing constellations globally. Recent naming conventions utilize the Earth Observation Satellite (EOS) nomenclature.
- Key Satellites and Payloads: * Cartosat Series: High-resolution mapping and urban planning.
- Risat / EOS-04 Series: Radar Imaging Satellites providing all-weather, day-and-night imaging capability.
- Oceansat Series: Oceanographic observations, wind vector studies, and chlorophyll monitoring for potential fishing zones.
- Applications: Crop acreage estimation, forest cover mapping, groundwater prospecting, flood zoning, and border surveillance.
Navigation Satellite Systems (NavIC and GAGAN)
- NavIC (Navigation with Indian Constellation): An independent regional navigation satellite system established by ISRO. It consists of 7 satellites (3 in geostationary orbit and 4 in geosynchronous orbit) covering India and a region up to 1,500 km beyond its borders. It utilizes both L5 and S bands.
- GAGAN (GPS Aided GEO Augmented Navigation): A Satellite-Based Augmentation System (SBAS) jointly developed by ISRO and the Airports Authority of India (AAI). It enhances the accuracy of GPS signals over the Indian airspace for civil aviation safety.
Emerging Frontiers in Satellite Technology
Innovations are driving down the cost of access to space while increasing deployment capabilities.
Small Satellites and Constellations
- CubeSats and Nano-Satellites: Satellites standardized into modular units of 10 × 10 × 10 cm (1U), weighing under 1.33 kg per unit.
- Mega-Constellations: Deploying thousands of small satellites in LEO to achieve low-latency global coverage.
- India’s Initiatives: ISRO developed the Small Satellite Launch Vehicle (SSLV) specifically to cater to the commercial launch market for small satellites and CubeSats.
Satellite-Based Internet of Things (IoT)
- Mechanism: Narrowband IoT (NB-IoT) enabled by LEO constellations allows sensors in remote locations (oil pipelines, maritime transport, agricultural fields) to transmit data directly to satellites without traditional cellular network infrastructure.
In-Orbit Servicing and Space Debris Management
- The Problem (Kessler Syndrome): A theoretical scenario where the density of objects in LEO is high enough that collisions cause a cascade effect, rendering space exploration perilous.
- Technological Solutions: Active Debris Removal (ADR) using robotic arms, nets, or lasers. ISRO has initiated project NETRA (Network for Space Object Tracking and Analysis) to detect space debris threats to Indian functional satellites. In-orbit refueling and servicing missions are also being developed to extend satellite lifespans.
Space-Based Quantum Communications
- Quantum Key Distribution (QKD): Utilizing satellites to transmit entangled photons, enabling unhackable cryptographic keys across long distances. ISRO has successfully demonstrated free-space quantum communication over tactical distances, paving the way for a secure satellite communication network.