Remote Sensing — Principles and Resolutions
Remote sensing acquires information about Earth without physical contact, using reflected/emitted electromagnetic radiation. Active sensors (RADAR, LiDAR) emit their own energy; passive sensors (most optical cameras) rely on sunlight. Four resolutions: (1) Spatial — smallest object detectable (pixel size); (2) Spectral — number/width of wavelength bands; (3) Radiometric — sensitivity to brightness differences (bit depth); (4) Temporal — revisit frequency over the same area. Memory aid: 'SSRT — Space, Spectrum, Radiance, Time.' Synthetic Aperture Radar (SAR) works day-night and through clouds (microwave penetrates cloud) — used in RISAT and the upcoming NISAR. India's main remote-sensing satellites: CARTOSAT (cartography/high spatial resolution), RESOURCESAT (natural resources), OCEANSAT (ocean colour), RISAT (radar).
Communication and Weather Satellites
Communication satellites sit in geostationary orbit so ground antennas can point at a fixed spot. They use transponders that receive an uplink signal, amplify and shift frequency, and retransmit (downlink). Common bands: C-band (lower frequency, rain-resistant), Ku-band, Ka-band (higher frequency, more bandwidth but rain fade). India's INSAT/GSAT series handle broadcasting, telecom, tele-education, telemedicine and disaster warning. Weather satellites are of two types: geostationary (INSAT, continuous view of one hemisphere) and polar-orbiting (global coverage, higher resolution). INSAT-3D carries an imager and sounder for atmospheric profiling. Memory aid: 'GEO for comms & continuous weather watch; Polar for global, sharper weather data.'
Example — Disaster Management and Agriculture Use
During floods and cyclones, SAR satellites (RISAT) map inundation even through cloud cover, while INSAT-3D tracks cyclone tracks and intensity — feeding IMD's early warnings (helping evacuate ahead of cyclones like Fani/Amphan). In agriculture, the FASAL and CHAMAN programmes use RESOURCESAT/multispectral data to estimate crop acreage and yield via vegetation indices (NDVI = (NIR−Red)/(NIR+Red); healthy vegetation reflects strongly in near-infrared). Forest cover assessment by FSI, groundwater prospecting, and the SVAMITVA scheme (drone + satellite property mapping) are other uses. Memory aid: 'Red = stress, NIR-high = healthy crop.' Bhuvan portal disseminates this geospatial data for governance.
Satellite Applications and Remote Sensing — Flashcards
Cover the answer, recall, then check. 12 cards on must-know facts for UPSC Prelims.
Q1. Difference between active and passive remote sensing?
A1. Passive sensors detect natural radiation (reflected sunlight/emitted heat); active sensors (radar/SAR, LiDAR) emit their own energy and record the reflection.
Q2. Which Indian satellite series uses radar for all-weather imaging?
A2. RISAT (Radar Imaging Satellite) — its Synthetic Aperture Radar (SAR) penetrates clouds and works day or night.
Q3. What is GAGAN?
A3. GPS Aided GEO Augmented Navigation — an ISRO-AAI satellite-based augmentation system that improves GPS accuracy for civil aviation over India.
Q4. What is Bhuvan?
A4. ISRO's web geoportal offering satellite imagery and thematic maps of India.
Q5. What are Cartosat satellites used for?
A5. High-resolution earth imaging for cartography, urban and rural planning, and terrain mapping.
Q6. What is spatial resolution?
A6. The smallest ground feature a sensor can distinguish; higher spatial resolution means smaller detectable objects.
Q7. Which agency manages remote-sensing data in India?
A7. The National Remote Sensing Centre (NRSC), Hyderabad, under ISRO.
Q8. What was EDUSAT used for?
A8. It was India's first satellite dedicated exclusively to education / tele-education (2004).
Q9. Name key applications of remote sensing.
A9. Crop and agriculture monitoring, disaster management, forestry, water resources, urban planning, and mineral prospecting.
Q10. What is a transponder on a satellite?
A10. A device that receives, amplifies and re-transmits signals; transponder capacity is leased for TV, telecom and internet.
Q11. Which orbit suits high-resolution earth observation, and which suits continuous weather watch?
A11. Polar/Sun-synchronous orbits for detailed earth observation; geostationary orbit (INSAT/Kalpana) for continuous weather monitoring.
Q12. What is a VSAT?
A12. Very Small Aperture Terminal — a compact satellite ground station enabling two-way voice/data links via satellite.