Satellite Systems and Applications
Major Global Navigation Satellite System (GNSS)
Let us talk about a technology that works for you every single day — right inside your mobile phone: GNSS.
GNSS is a constellation of satellites that provides positioning, navigation, and timing (PNT) services globally. It tells you your location (latitude, longitude, altitude), speed, and precise time — anywhere on Earth.
Applications: navigation (cars, aircraft, ships), mobile location-based services, disaster management, military targeting, banking and telecom synchronisation, and surveying and mapping.
GPS (Global Positioning System) — USA
- Developed and operated by the USA, managed by the US Space Force (earlier the U.S. Air Force). Became fully operational in 1995.
- Constellation: at least 24 operational satellites, though typically 30 or more are active.
- Orbit: ~20,200 km altitude in Medium Earth Orbit (MEO), arranged in six orbital planes.
- Period: each satellite completes one full orbit in ~12 hours.
- Two services:
- (1) SPS (Standard Positioning Service) — free civilian service, accuracy ~5-10 m.
- (2) PPS (Precise Positioning Service) — encrypted, for authorised military and government users, with strong resistance to jamming and spoofing.
- Transmits on multiple frequencies (L1, L2, L5), and operates 24×7 in all weather conditions.
Notice this: GPS is not just about maps — its timing function is extremely critical. Telecom towers (4G/5G) need precise timing; financial transactions require time-stamping; and power grids use Phasor Measurement Units (PMUs), which rely on GPS timing to monitor phase angles and prevent blackouts.
GLONASS — Russia
- Originally developed during the Soviet era, restored to full global coverage in 2011.
- Operated by Roscosmos State Corporation in coordination with the Russian Ministry of Defence.
- Constellation: 24 operational satellites plus spares; Orbit: ~19,100 km MEO, arranged in three orbital planes with eight satellites each.
- Period: each satellite completes one orbit in 11 hours 15 minutes.
- Two services: Open Service (civilian) and Restricted Service (military, encrypted).
Galileo — European Union
- Developed and operated by ESA and EUSPA (European Union Agency for the Space Programme). Initial services began in 2016.
- Constellation: 24 operational satellites plus spares; Orbit: ~23,222 km MEO, arranged in three orbital planes.
- Key distinction: Galileo is fully civilian-controlled — unlike GPS and GLONASS, which were originally developed for military purposes.
- Four services: (1) Open Service, (2) Public Regulated Service (encrypted, for government use), (3) Search and Rescue (SAR) — detects distress signals, and (4) High Accuracy Service (HAS) — free, decimetre-level accuracy.
BeiDou — China
- Developed by CNSA along with Chinese defence authorities. Named after the ‘Big Dipper’ constellation.
- BeiDou-1 (2000): Regional coverage, China only
- BeiDou-2 / Compass (2012): Asia-Pacific coverage
- BeiDou-3 (2020): Full global navigation capability
- Around 35 satellites, in a unique mixed-orbit configuration: GEO (geostationary) + IGSO (inclined geosynchronous, figure-eight pattern) + MEO satellites.
- Unique feature: a short message communication service — users can send brief text-like messages via the satellite system.
QZSS (Quasi-Zenith Satellite System) — Japan
- Japan’s regional navigation system, designed to enhance GPS performance in Japan and the Asia-Pacific region. Operational since 2018.
- ‘Quasi-Zenith’ means: satellites follow highly inclined elliptical orbits that keep at least one satellite nearly overhead Japan at all times — ensuring strong signal reception even in urban canyons and mountainous areas.
- Currently operates with 4 satellites, expanding to a 7-satellite constellation for fully autonomous, non-GPS-dependent regional navigation.
GNSS — A Comparative View
| System | Country | Since | Coverage | Orbit |
| GPS | USA | 1995 | Global | MEO (~20,200 km) |
| GLONASS | Russia | 1996/2011 | Global | MEO (~19,100 km) |
| Galileo | EU | 2016 | Global | MEO (~23,222 km) |
| BeiDou | China | 2000/2020 | Global | Mixed (GEO+IGSO+MEO) |
| NavIC | India | 2018 | Regional (India+~1500km) | GEO + IGSO |
| QZSS | Japan | 2018 | Regional (Japan+Asia-Pac.) | QZO + GEO |
FACT: GAGAN is India’s ‘Satellite-Based Augmentation System.’ You will read more about this later in this section
Communication Satellites of India
Now that we understand how India’s space programme evolved, let’s dive deep into what these satellites actually do. Communication satellites are the backbone of India’s digital and broadcasting infrastructure. India’s communication satellites fall into two main families: INSAT and GSAT.
Think of INSAT as a Swiss Army knife (it does everything) and GSAT as a specialised high-capacity tool (dedicated communication only).
INSAT — Indian National Satellite System
Before INSAT, India was dependent on foreign satellites for everything — weather data, broadcasting, telecommunication. INSAT changed that. It became one of the largest domestic communication satellite systems in the Asia-Pacific region, and it has been operational since 1983.
Why Was INSAT Created?
Five key needs drove the creation of INSAT:
1. Strengthen national communication networks — connecting cities, towns, and remote villages
2. Support rural and remote connectivity — especially border areas and islands
3. Improve weather forecasting and cyclone warning — India’s coastline is highly cyclone-prone
4. Enable nationwide broadcasting — Doordarshan reaching every corner of India
5. Provide emergency and disaster communication — critical during floods, cyclones, earthquakes
INSAT Architecture — How It Works
INSAT satellites orbit in Geostationary Orbit (GEO) at approximately 35,786 km above the equator. At this altitude, the satellite moves at the same speed as the Earth’s rotation, so it appears fixed relative to the ground — like a stationary relay tower in the sky. This enables continuous coverage over India.
Key Payloads / Transponders on INSAT Satellites
| Payload | Band | Primary Use |
| C-band transponders | C-band | TV broadcasting, long-distance telecom |
| Extended C-band | Extended C | High-quality, rain-resistant communication links |
| Ku-band | Ku-band | DTH (Direct-to-Home TV) and VSAT services |
| Meteorological Sensors | Visible/IR | Cloud imaging, weather monitoring, cyclone tracking |
| Data Relay Transponders (DRT) | UHF | Data from ~3,000 Automatic Weather Stations (AWS) |
| Search & Rescue (SAR) payloads | 406 MHz | COSPAS-SARSAT emergency distress signals |
| 💡 DTH vs. VSAT (Common Confusion): DTH (Direct-to-Home) — You watch TV at home. It is a ONE-WAY receiver. Signal only comes down. VSAT (Very Small Aperture Terminal) — Used for internet, data, and voice. It is a TWO-WAY transceiver. Signal goes both up and down. Banks, ATMs, and remote offices use VSAT. |
INSAT Series — A Quick Journey
| Series | Period | Key Feature | Main Uses |
| INSAT-1 | 1982–1990 | Built with foreign collaboration (Ford Aerospace, USA) | Telecom, broadcasting & meteorology |
| INSAT-2 | 1992–1999 | First FULLY INDIGENOUS INSAT series | Telecom, meteorology, search & rescue |
| INSAT-3 | 2000–2004 | Improved payload capacity | DTH, telecom, meteorology |
| INSAT-4 | 2005–2017 | High-power Ku-band satellites | DTH, broadband services |
Applications of INSAT
- Telecommunications: Supports long-distance telephone services, VSAT networks, mobile backhaul, and communication in remote, border, and island regions
- Broadcasting: Enables Doordarshan and private TV channels nationwide; supports DTH (DD Free Dish) and satellite radio
- Meteorology: Cloud imagery, cyclone tracking, monsoon monitoring, relay of data from 3,000+ Automatic Weather Stations (AWS) and Automatic Rain Gauges (ARG)
- Disaster Management: Cyclone and flood warning dissemination; Search & Rescue support under COSPAS-SARSAT; emergency communication during natural disasters
- Education & Health: Supports tele-education and tele-medicine networks connecting rural hospitals with super-speciality centres
GSAT — Geostationary Satellite Series
As India’s digital economy grew, the multi-purpose INSAT satellites could no longer handle the increasing demand for high-speed broadband, defence communication, and DTH services. India needed dedicated, high-power communication satellites. Enter GSAT — operational since 2001. GSAT satellites modernise and extend the communication role of the INSAT system.
Why Was GSAT Developed?
INSAT was a multi-purpose platform combining communication AND meteorology. With the explosion of digital services, India needed dedicated communication bandwidth. GSAT was designed to:
1. Provide high-bandwidth communication capacity — for India’s growing digital needs
2. Support DTH broadcasting and broadband services
3. Enable VSAT and internet connectivity across India
4. Strengthen Digital India and BharatNet — through satellite backhaul in remote areas
5. Provide secure communication for defence and strategic users — Navy, Air Force, Army
6. Reduce dependence on foreign communication satellites — strategic autonomy
Key GSAT Satellites — You Must Know These
| Satellite | Year | Key Feature | Primary Use |
| GSAT-3 (EDUSAT) | 2004 | World’s FIRST dedicated educational satellite | Distance education, tele-education, teacher training |
| GSAT-7 (Rukmini) | 2013 | Multi-band: UHF, C, Ku-band | Indian NAVY communication |
| GSAT-7A (Angry Bird) | 2018 | Defence satellite | Indian AIR FORCE — airbases, radars, UAVs |
| GSAT-7B | Planned | Dedicated Army satellite | Tactical battlefield communication, border surveillance |
| GSAT-7R (CMS-03) | 2025 | India’s heaviest indigenous naval satellite | Next-gen Indian NAVY communication |
| GSAT-9 (South Asia Satellite) | 2017 | Regional cooperation | Gifted to SAARC nations (except Pakistan) |
| GSAT-11 (Big Bird) | 2018 | India’s heaviest-built satellite; HTS | BharatNet — rural broadband backbone |
| GSAT-20 (GSAT-N2) | 2024 | 48 Gbps capacity; ISRO-SpaceX collaboration | Broadband, in-flight & maritime connectivity |
| 🧠 Memory Tip: Remember the Defence GSAT family: GSAT-7 = Navy (Rukmini), GSAT-7A = Air Force (Angry Bird), GSAT-7B = Army (planned), GSAT-7R = Navy replacement. And GSAT-20 was launched by SpaceX Falcon-9 — India’s first collaboration with SpaceX for satellite launch! |
INSAT vs. GSAT — The Key Comparison
| Feature | INSAT | GSAT |
| Nature | Multipurpose satellites | Dedicated communication satellites |
| Primary Purpose | Telecom + Broadcasting + Meteorology + Disaster Warning | High-capacity communication & broadband |
| Era | 1983 – progressively replaced by GSAT | 2001 – present |
| Orbit | Geostationary Orbit (GEO) at 35,786 km | Geostationary Orbit (GEO) at 35,786 km |
| Meteorological Role | YES — imagers, data relay, cyclone warning | NO — purely communication |
| Strategic Use | Limited | Significant (GSAT-7, 7A, 7B for armed forces) |
| Present Status | Largely phased out / merged | Active backbone of India’s satcom infrastructure |
Indian Remote Sensing (IRS) Programme
While communication satellites let us talk to each other, remote sensing satellites let us watch over our country — its land, water, forests, coasts, and oceans — from space. The IRS Programme is India’s flagship Earth Observation (EO) satellite programme, and it is one of the largest civilian remote sensing satellite constellations in the world.
Think of IRS satellites as India’s ‘eyes in the sky’ — monitoring everything from crop health in Punjab to floods in Bihar, from glacial melting in the Himalayas to illegal fishing in the Indian Ocean.
Why Was IRS Needed?
- Foreign satellite data was costly and came with licensing restrictions
- Low temporal resolution — foreign satellites could not revisit India as frequently as needed
- Access during emergencies was uncertain — during wars or geopolitical tensions
- Strategic autonomy — independent planning, disaster response, and national security
IRS-1 Series (1988–1997) — Building the Foundation
The IRS-1 series laid the operational base for all subsequent Earth observation by India. These satellites operated in Sun-Synchronous Low Earth Orbit (SSO), enabling repetitive and consistent observation.
- Sensors: LISS-I, LISS-II, LISS-III (multispectral sensors — visible and near-infrared bands)
- Applications: Agricultural monitoring, land-use mapping, forestry, water resource management
| Satellite | Year | Launcher | Key Significance |
| IRS-1A | 1988 | Vostok (USSR) | India’s FIRST operational remote sensing satellite |
| IRS-1B | 1991 | Vostok (USSR) | Improved revisit time and data continuity |
| IRS-1C | 1995 | Molniya (Russia) | Introduced high-resolution imaging |
| IRS-1D | 1997 | PSLV-C1 | FIRST IRS satellite launched by an indigenous Indian rocket (PSLV) |
Cartosat Series — Mapping & Cartography
Cartosat is India’s high-resolution Earth-observation satellite family, designed primarily for cartographic mapping and geospatial applications. If IRS-1 gave India a broad view of the land, Cartosat gives detailed, sharp images — like upgrading from a low-resolution screen to a 4K display.
Key Features
- Sensors: Panchromatic (high-resolution black & white) and multispectral cameras; stereo imaging capability
- Orbit: Sun-synchronous Low Earth Orbit (SSO) for consistent illumination and repeat coverage
- Integration: Seamlessly integrates with GIS platforms like Bhuvan and National GIS
| Satellite | Year | Key Feature | Application |
| Cartosat-1 | 2005 | Stereo imaging — accurate 3D terrain mapping | Digital Elevation Models (DEMs), terrain analysis |
| Cartosat-2 | 2007–2018 | Very high-resolution optical imaging | Urban mapping, infrastructure planning |
| Cartosat-3 | 2019 | India’s highest-resolution civilian EO satellite | Advanced mapping, strategic uses |
Applications of Cartosat
- Infrastructure & Governance: Urban planning, Smart Cities, mapping roads/railways, land records modernisation (Digital India Land Records Modernisation Programme)
- Disaster Management: Damage assessment during floods, earthquakes, landslides; baseline mapping for recovery
- Strategic Uses: Border and coastal mapping; defence and internal security planning
Resourcesat Series — Agriculture & Natural Resource Monitoring
While Cartosat maps the physical landscape with high resolution, Resourcesat monitors how India’s resources — crops, water, forests, land — are being used. It is the backbone of India’s agricultural planning and natural resource management.
Unique Sensor System — Three Complementary Cameras
Resourcesat carries THREE cameras that work together like a zoom lens from different distances:
1. LISS-IV — High spatial resolution, narrow swath — for detailed, zoomed-in studies
2. LISS-III — Moderate resolution, multispectral — the workhorse camera
3. AWiFS (Advanced Wide Field Sensor) — Coarser resolution, very wide swath — for national-scale coverage
| Satellite | Year | Launcher | Key Feature |
| Resourcesat-1 (IRS-P6) | 2003 | PSLV-C5 | First satellite dedicated to resource monitoring; replaced IRS-1C/1D |
| Resourcesat-2 | 2011 | PSLV-C16 | Improved data continuity; strengthened agricultural monitoring |
| Resourcesat-2A | 2016 | PSLV-C36 | Extended mission life — ensured long-term EO data availability |
Applications of Resourcesat
- Agriculture & Food Security: Crop acreage estimation, crop health monitoring, yield forecasting, drought assessment — the base of India’s food planning
- Water Resources: Surface water mapping, irrigation planning, watershed assessment
- Forestry: Forest cover monitoring, vegetation health analysis, environmental impact assessment
Oceansat Series — Watching the Oceans
India has a coastline of 7,500 km and an Exclusive Economic Zone (EEZ) of over 2 million sq km. The Indian Ocean is critical for India’s trade, weather, and national security. The Oceansat series is India’s dedicated ocean observation family — supporting India’s Blue Economy, fisheries, and climate science.
Key Sensors on Oceansat
- OCM (Ocean Colour Monitor): Measures chlorophyll concentration and ocean colour — tells us where fish are likely to be found
- Scatterometer: Measures ocean surface wind speed and direction — critical for cyclone tracking
- MSMR / SSTM: Measures Sea Surface Temperature (SST) — important for cyclone intensity and monsoon forecasting
| Satellite | Year | Key Sensors | Significance |
| Oceansat-1 (IRS-P4) | 1999 | OCM, MSMR | India’s FIRST dedicated ocean satellite |
| Oceansat-2 | 2009 | OCM-2, Scatterometer | Improved fisheries advisory and wind data |
| SCATSAT-1 | 2016 | Advanced Scatterometer | Successor to Oceansat-2 scatterometer |
| Oceansat-3 (EOS-06) | 2022 | OCM-3, Scatterometer, SSTM | Enhanced ocean colour & wind monitoring |
| 💡 Potential Fishing Zone (PFZ) Advisories: Oceansat’s OCM sensor measures chlorophyll concentration in ocean water. High chlorophyll = high phytoplankton = more fish. ISRO generates PFZ advisories and sends them to fishermen, telling them exactly where to fish. This saves fuel, time, and lives — a direct benefit of space technology to the common man. |
RISAT Series — All-Weather Radar Imaging
All the IRS satellites we’ve discussed so far use optical sensors — they need sunlight to take pictures, just like a camera. But what if there are clouds? What if it’s night? Enter RISAT — India’s Radar Imaging Satellite. RISAT uses microwave radar (SAR — Synthetic Aperture Radar) that can penetrate clouds, rain, and fog, and works 24×7 regardless of light conditions.
| 💡 SAR vs. Optical Sensors: Optical satellites are like cameras — they need light and cannot see through clouds. SAR (Synthetic Aperture Radar) satellites emit their own microwave energy. Like a bat using echolocation, they send a signal and listen to the echo. They can image through clouds, rain, and in complete darkness. RISAT = all-weather, day-night imaging. |
Key Features of RISAT
- SAR Sensor: Synthetic Aperture Radar — operates in C-band (agriculture, soil moisture) or X-band (high-resolution surveillance)
- All-weather: Microwave radar penetrates clouds, rain, and fog — unlike optical satellites
- Day-Night: Does not depend on sunlight — can image at night or during storm
| Satellite | Year | Sensor Band | Key Application |
| RISAT-2 | 2009 | X-band | High-resolution imaging, strategic surveillance |
| RISAT-1 | 2012 | C-band | All-weather EO, agriculture & disaster monitoring |
| RISAT-2B | 2019 | X-band | Improved resolution, quick revisit capability |
| RISAT-2BR1 | 2019 | X-band | High-resolution imaging; strategic use |
| RISAT-1A | 2022 | C-band | Continuity of RISAT-1 capabilities |
| RISAT-1B | 2025 | C-band | Successor to RISAT-1A; lost during PSLV-C61 launch failure |
Satellite Navigation Systems of India
Imagine you are a missile and you need to know your exact location. Or you are a deep-sea fisherman in the middle of the ocean with a cyclone approaching. Or you are a pilot trying to land at a small airport in the Himalayas in thick fog. All of these situations require precise, reliable navigation. India has developed two systems for this: NavIC (an independent navigation system) and GAGAN (an aviation augmentation system).
NavIC — Navigation with Indian Constellation
NavIC (earlier called IRNSS — Indian Regional Navigation Satellite System) is India’s own GPS. It is an independent, indigenous regional satellite navigation system developed and operated by ISRO. Since 2018, it has been providing Positioning, Navigation, and Timing (PNT) services.
Why Was NavIC Needed? — The Kargil Lesson
The 1999 Kargil War was a turning point. During the conflict, the US restricted GPS data over the conflict zone for India. India’s military and civilian agencies found themselves blind — unable to navigate with precision in their own territory. This was a wake-up call: strategic dependence on foreign navigation systems is a national security risk.
- Strategic Autonomy: GPS signals can be degraded or denied during conflicts. India needed sovereign control over its navigation
- National Security: Military operations require high-accuracy, encrypted navigation — only possible with an indigenous system
- Regional Design Benefits: A regional system reduces operational cost and provides better accuracy over the service area.
Key Technical Features of NavIC
| Feature | Detail |
| Coverage | India + approximately 1,500 km beyond its borders (covers Indian Ocean Region) |
| Constellation | 7 operational satellites |
| Orbit types | 3 satellites in Geostationary Orbit (GEO) + 4 satellites in Geosynchronous Orbit (GSO) inclined at 29° |
| Orbital altitude | ~36,000 km |
| Frequency bands | Dual-frequency: L5 band + S band (S-band is unique — very few GNSS use it) |
| Accuracy (SPS) | ~5 metres over primary service area |
| Services | SPS (Standard Positioning Service) — civilian; RS (Restricted Service) — military, encrypted |
| Clock type | Each satellite has THREE Rubidium atomic clocks |
| 🧠 Memory Aid: NavIC = 7 satellites at 36,000 km. Think: 7/11 (like the store!) — 7 satellites, and 3+4=7. 3 are GEO (fixed, stationary), 4 are GSO (moving slightly but staying in same region). Dual frequency: L5 (shared with others) + S-band (India’s special weapon — very few countries use this). |
Applications of NavIC
- Smartphones & Consumer Electronics: MeitY mandated all 5G phones to support NavIC. New NVS-series satellites added L1-band compatibility with standard Qualcomm/MediaTek chipsets
- Maritime Safety (GEMINI Device): GEMINI = Gagan Enabled Mariner’s Instrument for Navigation and Information. Provides cyclone alerts, high-wave warnings to deep-sea fishermen beyond mobile coverage. Recognised by IMO (International Maritime Organisation) for the World-Wide Radio Navigation System
- Road Transport (AIS-140): All public transport vehicles (buses, taxis, trucks — yellow number plates) must have NavIC/GPS-enabled Vehicle Tracking Systems (VTS) and panic buttons. Aims to create an Intelligent Transportation System (ITS)
- Defence & Military: Restricted Service (RS) provides encrypted, high-accuracy signals for missile guidance, drone operations, troop movements, battlefield awareness
- Critical Infrastructure: High-precision timing for power grid synchronisation, telecom networks (4G/5G timing), banking and stock-exchange timestamping
- Disaster Management: Cyclone tracking, flood modelling, search and rescue operations
GAGAN — GPS Aided GEO Augmented Navigation
If NavIC is India’s own GPS, GAGAN is India’s GPS upgrade kit for aviation. GAGAN (GPS Aided GEO Augmented Navigation) is India’s Satellite-Based Augmentation System (SBAS), developed jointly by ISRO and the Airports Authority of India (AAI). It has been operational since 2015.
Important distinction: GAGAN does NOT replace GPS. It CORRECTS GPS signals, making them accurate and safe enough for aviation. Think of GAGAN as a spell-checker for GPS signals.
Why Was GAGAN Needed?
- GPS alone does not meet the ‘Safety-of-Life’ requirements for civil aviation (as per ICAO)
- GPS has precision gaps: ~5–15 m horizontal error; ~15–25 m vertical error — insufficient for guiding aircraft during landing
- GPS lacks real-time integrity alerts — it won’t immediately warn pilots if a satellite is sending wrong data
- India lies in the Equatorial Ionisation Anomaly (EIA) region — India’s position near the equator causes extreme ionospheric disturbances that degrade GPS accuracy uniquely
- Ground-based Instrument Landing Systems (ILS) are expensive and runway-specific — can’t cover every Indian airport
How GAGAN Works — Step by Step
| Step 1: GPS Satellites Send Signals GPS satellites transmit position and time signals. But these signals contain errors due to clock inaccuracies, orbit errors, ionospheric delays, and atmospheric effects. |
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| Step 2: Ground Reference Stations Detect Errors GAGAN has a network of precisely surveyed ground stations across India. They know their exact location and compare GPS-calculated position with true position to identify GPS errors. |
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| Step 3: Master Control Centre Computes Corrections All error data is sent to GAGAN’s Master Control Centre. It calculates correction messages, monitors satellite health, and generates integrity information (Is the signal safe to use?). |
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| Step 4: Corrections Uplinked to GEO Satellites Correction messages are transmitted to GSAT-8, GSAT-10, and GSAT-15 (GAGAN uses hosted payloads on these GEO communication satellites — no dedicated satellites!). |
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| Step 5: Aircraft Receives Corrected Signal The aircraft receives both the original GPS signal and the GAGAN correction signal. The receiver applies corrections in real time, providing aviation-grade accuracy and integrity. |
Key Features of GAGAN
- Not independent — an augmentation system: GAGAN augments GPS; it doesn’t replace it
- Accuracy: Horizontal: ≤ 3 m; Vertical: ≤ 4–5 m — suitable for precision-like aircraft landings
- Integrity Alerts: Issues alert within ~6 seconds if a GPS satellite sends faulty data — critical for pilot safety
- Vertical Guidance (APV-I): Enables precision-like landings without requiring expensive ILS ground equipment
- Ionospheric Correction: Uses ISRO-developed region-specific model (IGM-MLDF) tailored to equatorial conditions
- GEO Satellites Used: GSAT-8, GSAT-10, and GSAT-15 (hosted payloads — not dedicated satellites)
- ICAO-compliant: Interoperable with WAAS (USA), EGNOS (Europe), MSAS (Japan), BDSBAS (China)
- Certified operations: RNP 0.1 (en-route navigation) and APV-I (precision-like landing) certified by DGCA
Landing Guidance Systems — APV, LPV, and ILS
A very important and tricky topic — the three types of landing guidance. The key question is: how precisely can a pilot be guided to land at an airport? This is measured by ‘decision height’ (how low the aircraft can go before the pilot must see the runway or abort landing).
Three Key Systems
- APV (Approach with Vertical Guidance): A satellite-based landing procedure providing BOTH horizontal AND vertical guidance. Before APV, many airports only gave horizontal guidance, forcing pilots to judge their descent visually — risky in fog. With APV, aircraft follow a stable descent path. APV-I is enabled by GAGAN in India.
- LPV (Localiser Performance with Vertical Guidance): A satellite-based approach providing precision-like horizontal and vertical guidance via SBAS. More accurate than APV-I — close to ILS CAT-I performance. Technically feasible with GAGAN; nationwide rollout still evolving.
- ILS (Instrument Landing System): A traditional GROUND-BASED radio navigation system providing precise horizontal and vertical guidance. World’s most widely used precision approach system. Very expensive to install, runway-specific. Operational at major Indian airports.
Comparison Table — APV-I, APV-II, LPV, and ILS
| Feature | APV-I | LPV | ILS (CAT-I) |
| System Type | Satellite (SBAS) | Satellite (SBAS) | Ground-based radio |
| Vertical Guidance | Yes | Yes (localiser-like) | Yes (very precise) |
| Typical Decision Height | ~250 ft | ~250 ft | ~200 ft |
| Ground Infrastructure | NOT required | NOT required | Required (costly) |
| Cost of Installation | Low | Low | Very High |
| Coverage | Wide-area (satellite) | Wide-area (satellite) | Runway-specific only |
| Suitability for small airports | Excellent | Very good | Limited (costly) |
| Status in India | Operational via GAGAN | Rollout ongoing | Operational at major airports |
| 💡 Why APV matters for India: India has hundreds of small airports (especially under UDAN scheme), airports in hilly northeast terrain, foggy northern plains, and island airstrips. These airports CANNOT afford ILS. GAGAN’s APV-I enables precision-like landing guidance at ALL of these airports via satellite — at minimal cost. This is revolutionary for India’s aviation safety. |
RNP and PBN — Performance-Based Navigation
Two more important concepts that often confuse students — RNP and PBN.
RNP (Required Navigation Performance)
- A specification that defines the level of accuracy an aircraft must maintain 95% of the time during a specific flight phase. It is the core of Performance-Based Navigation (PBN).
- RNP 0.1 is enabled by GAGAN in India — the highest-accuracy type for dense airspace and high-precision en-route navigation.
PBN (Performance-Based Navigation)
- A modern air navigation concept where aircraft fly based on how accurately they can navigate, NOT on where ground-based aids are located.
- Components: RNAV (Area Navigation) + RNP. ICAO promotes PBN globally.
| RNP Type | Accuracy | Typical Use |
| RNP 10 | 10 nautical miles | Oceanic / remote areas |
| RNP 4 | 4 nautical miles | Oceanic with higher traffic |
| RNP 2 | 2 nautical miles | En-route continental |
| RNP 1 | 1 nautical mile | Terminal area (SID/STAR procedures) |
| RNP 0.3 | 0.3 nautical miles | Approach at regional airports |
| RNP 0.1 | 0.1 nautical miles | High-precision en-route / dense airspace — enabled by GAGAN |
| RNP AR | Variable (0.3–0.1 NM) | Terrain-challenged, curved approaches |
Important Sensors Used by ISRO
Satellites are only as useful as the sensors they carry. Different scientific questions require different sensors. ISRO uses a rich variety of sensors across its Earth Observation, meteorological, oceanographic, and planetary missions, collecting data across the entire electromagnetic spectrum. Let’s understand these sensors and their applications.
Optical & Multispectral Sensors
These are the cameras of the satellite world — they detect reflected sunlight in various wavelength bands. They are passive sensors (they don’t emit energy — they just receive what the Sun provides).
LISS Series (Linear Imaging Self Scanner)
- Used on: IRS-1 series and Resourcesat series
- Bands: Visible and Near-Infrared (NIR); some versions include Short-Wave Infrared (SWIR)
- Key Characteristic: Passive optical sensor — ideal for land resource studies; NOT suitable for weather observation (needs clear sky)
- Applications: Crop acreage estimation, forest cover mapping, land-use/land-cover (LULC) mapping
Panchromatic Camera (PAN)
- Used on: IRS-1 series and Cartosat series
- Key Characteristic: Very high spatial resolution, but produces only black-and-white images (no spectral colour information)
- Applications: Detailed city mapping, mapping roads/railways, border mapping, strategic surveillance
Hyperspectral Sensors
HySIS Payload (Hyperspectral Imaging Sensor)
- Used on: HySIS satellite
- Bands: Hundreds of narrow, contiguous spectral bands across VNIR and SWIR
- Key Characteristic: Fine spectral discrimination — identifies materials based on unique spectral ‘fingerprints’. Think of it as an optical spectrometer from space.
- Applications: Mineral and ore identification (mining sector), crop stress and disease detection, coastal and ecosystem studies
Microwave / Radar Sensors (Active Sensors)
Synthetic Aperture Radar (SAR)
- Used on: RISAT series satellites
- Radar Bands: X-band (higher resolution, surveillance) or C-band (agriculture, soil moisture)
- Key Characteristic: ACTIVE sensor — emits its own microwave energy. Day-and-night, all-weather imaging, unaffected by cloud cover or rain
- Applications: Flood mapping, landslide detection, cyclone assessment (disaster management); soil moisture estimation; border surveillance and strategic reconnaissance
Ocean Observation Sensors
| Sensor | Used On | Measures | Key Application |
| OCM (Ocean Colour Monitor) | Oceansat series | Chlorophyll concentration, ocean colour | Potential Fishing Zone (PFZ) advisories for fishermen |
| Scatterometer | Oceansat series, SCATSAT-1 | Ocean surface wind speed and direction (microwave backscatter) | Cyclone tracking, monsoon onset prediction, weather forecasting |
| MSMR / SSTM | Oceansat-1 (IRS-P4) | Sea Surface Temperature (SST), wind speed, water vapour | Climate studies, cyclone intensity, monsoon modelling |
Planetary & Space Science Sensors
| Sensor | Mission | Function | Key Achievement |
| TMC (Terrain Mapping Camera) | Chandrayaan-1 & 2 | Stereo imaging of planetary surfaces; creates Digital Elevation Models | Detailed topographic mapping of the Moon |
| M³ (Moon Mineralogy Mapper) | Chandrayaan-1 | Hyperspectral sensor — identifies minerals by spectral signatures | Confirmed presence of WATER & HYDROXYL on the Moon |
| XSM (X-ray Spectrometer Monitor) | Chandrayaan-2 Orbiter | Measures solar X-ray radiation and flux | Supports lunar X-ray fluorescence studies and solar monitoring |
Meteorological Sensors
| Sensor | Used On | Key Characteristic | Application |
| Imager | INSAT-3D, INSAT-3DR | Provides HORIZONTAL cloud and Earth surface imagery across visible, IR, and thermal bands | Weather monitoring, cyclone tracking, nowcasting |
| Sounder | INSAT-3D, INSAT-3DR | Provides VERTICAL profiles of atmospheric temperature and humidity — a thermometer-sounding system from space | Numerical weather prediction, atmospheric stability |
| VHRR (Very High Resolution Radiometer) | Early INSAT satellites | Coarse-to-moderate resolution; wide field of view | Cloud cover detection, SST estimation, weather monitoring |
| 💡 Imager vs. Sounder: A meteorological IMAGER takes a horizontal photograph of cloud patterns — like a bird’s-eye view. A SOUNDER takes a vertical cross-section of the atmosphere — like a doctor’s ultrasound. One looks wide, the other looks deep. |
Atomic Clocks — The Heart of Navigation Satellites
Every navigation satellite — NavIC, GPS, Galileo — relies on atomic clocks. Why? Because position calculation is based on measuring the time it takes for a signal to travel from satellite to receiver.
Even a tiny timing error of 1 microsecond translates to 300 metres of positional error (signal travels at speed of light!). Atomic clocks are accurate to 1 second error in millions to billions of years.
Types of Atomic Clocks and Accuracy
| Clock Type | Accuracy | Used In |
| Rubidium clocks | ~10⁻¹¹ (1 second error in ~300 years) | NavIC satellites — smaller and less costly |
| Caesium beam clocks | ~10⁻¹⁴ | GPS satellites and ground standards |
| Hydrogen masers | ~10⁻¹⁵ | Most accurate space-qualified clocks |
| Optical lattice clocks | ~10⁻¹⁸ | Most precise — research / future use |
How Atomic Clocks Work — In Simple Terms
| Step 1: Atomic Reference An atom (usually Caesium-133) has electrons that jump between two fixed energy levels. The energy difference between these levels is a CONSTANT OF NATURE — identical everywhere in the universe. |
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| Step 2: Microwave Stimulation The atom is exposed to microwave radiation. When the frequency exactly matches the energy difference, the atom absorbs the energy. This is called atomic resonance. |
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| Step 3: Resonance Locking Electronics continuously adjust the microwave frequency until maximum absorption is reached. This locked frequency is the atomic resonance frequency — the most stable signal in nature. |
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| Step 4: Timekeeping The locked frequency counts time precisely. By definition: 1 second = 9,192,631,770 oscillations of the Caesium-133 atom. This is the SI definition of the second! |
| 🎯 Key Fact: NavIC satellites carry Rubidium atomic clocks — each satellite has THREE. Rubidium clocks are smaller, lighter, and less costly than Caesium clocks, but slightly less accurate. The redundancy (3 clocks per satellite) ensures reliability. If one fails, the other two take over. |