Earth observation satellites: the world and India

Detail

Key words used in this section

• Earth observation (EO) satellite: a satellite that carries sensors (instruments that record energy reflected or given off by the earth) to collect images and data about land, water, ice and air.
• Spatial resolution: the size of the smallest ground area that one image pixel shows. "30 m" means one pixel covers a 30 m × 30 m patch. A smaller number means a sharper image.
• Multispectral sensor: a sensor that records the same scene in several separate wavebands, such as blue, green, red and near-infrared. Each feature reflects these bands differently, so we can tell crops, water, soil and forest apart.
• Pushbroom sensor: a sensor with a long row of detectors placed across the satellite's path. It records a whole line of ground at once, and the line moves forward as the satellite moves, like a broom sweeping a floor. It has no moving mirror, so its images are steadier and sharper than those of the older whiskbroom (scanning-mirror) sensors such as Landsat's MSS.
• PAN (panchromatic) sensor: a sensor that records one broad band, which gives a black-and-white image. It usually has finer resolution than the multispectral bands on the same satellite.
• Optical vs radar sensing:
• Optical (passive) sensors record sunlight reflected by the earth. They need daylight and clear skies.

• Radar (active) sensors send out their own microwave pulses and record the echo. Microwaves pass through cloud, so radar works at night and in the monsoon.

• SAR (Synthetic Aperture Radar): a side-looking radar on a moving satellite. It joins the echoes from many points along the orbit, so they act like one very long antenna. This gives a sharp image from a small antenna.

• Radar bands:
• C-band: about 5.6 cm wavelength. Good for crops, floods and sea ice.
• S-band: about 10 cm.
• L-band: about 24 cm. It goes through tree canopy and dry soil, so it is good for forests, ground movement and soil moisture.

• The longer the wavelength, the deeper the wave goes into the surface.

• Sun-synchronous polar orbit (SSPO): a near-polar orbit at about 500–900 km height. The satellite crosses each place at the same local time every time it passes, so the sunlight is the same in every image. Most EO satellites use this orbit.

• Geostationary orbit: an orbit about 35,786 km above the equator. The satellite circles once in 24 hours, so it seems to stay fixed over one point. It watches the same half of the globe all the time. Weather satellites use this orbit.

World: three landmark programmes

Programme	Country / agency	Started	Key sensors and resolution	Why it matters
Landsat	USA (NASA + USGS)	1972 (first called ERTS-1: Earth Resources Technology Satellite)	MSS (Multispectral Scanner) 80 m; TM (Thematic Mapper) 30 m	Longest continuous record of the earth's land surface
SPOT	France (CNES)	1986	HRV (High Resolution Visible) pushbroom sensor: 20 m multispectral, 10 m PAN	First major civilian pushbroom imager; could look sideways to take stereo images
Sentinel	European Union (Copernicus programme, run with ESA)	2014	Sentinel-1: C-band SAR radar; Sentinel-2: 10 m multispectral	Free and open data; used for floods, crops and forests

• Landsat (USA):
• It began as ERTS-1 in 1972 and was later renamed Landsat. It is the "grandfather" of civilian Earth observation.
• Its sensors improved over time: MSS at 80 m, then TM at 30 m. The 30 m pixel is still the world's standard for mapping land use.
• Landsat 9 was launched on 27 September 2021 from Vandenberg Space Force Base, California, on an Atlas V 401 rocket. NASA built and launched it. After about 10 months of testing, the USGS took over daily operation [4].
• Eight Landsat satellites have reached orbit since 1972. Landsat 6 failed at launch in 1993. Together they have collected more than 9 million scenes of land, ice and coasts [4].

• Why the long record matters: the same type of image of the same place over more than 50 years lets us measure change. Examples are deforestation in the Amazon, glacier retreat in the Himalaya and the growth of cities such as Delhi and Bengaluru.

• SPOT (France):

• SPOT stands for Satellite Pour l'Observation de la Terre.
• SPOT-1 (1986) carried the HRV pushbroom sensor at 20 m (multispectral) and 10 m (PAN). This was sharper than Landsat TM.

• NCERT uses a SPOT image as an example of a satellite picture.

• Copernicus Sentinels (EU):

• The series began in 2014. Its data is free for anyone, so it is widely used in India by researchers, states and start-ups.
• Sentinel-1 carries radar, so it maps floods even under monsoon cloud.
• Sentinel-2 gives 10 m multispectral images. These are used for crop type, crop health and forest cover.

India's path: from experiments to the IRS family

Stage 1: Bhaskara (experiments)

• Bhaskara-I (1979) and Bhaskara-II (1981) were India's first experimental Earth observation satellites.
• Both flew on Soviet rockets.
• Payloads:
• TV cameras for land, water and forest images.

• Microwave radiometers (passive sensors that measure the natural microwave energy given off by the surface) for the ocean and the atmosphere.

• Result: India learned how to build sensors, receive data and use it. This prepared the way for a full working series.

Stage 2: Indian Remote Sensing (IRS) satellites

• IRS is India's main series of Earth observation satellites. It is used to manage natural resources, farming, forests, water and disasters.
• IRS-1A, the first in the series, was launched on 17 March 1988 from Baikonur (in Kazakhstan, then part of the USSR).
• LISS (Linear Imaging Self-Scanning sensor): the multispectral pushbroom camera used on IRS satellites.

Satellite	Year	Sensors and resolution
IRS-1A	1988	LISS-I (72.5 m), LISS-II (36.25 m)
IRS-1C	1995	LISS-III (23 m), PAN (5.8 m)
Resourcesat-1 (IRS-P6)	2003	LISS-III, LISS-IV, AWiFS

Stage 3: satellites for special jobs

• Resourcesat (for land and crops):
• Resourcesat-1 (IRS-P6, 2003) carried three cameras on one satellite:
  • AWiFS (Advanced Wide Field Sensor): a wide, coarse view, good for state-level crop estimates.
  • LISS-III: a medium view.
  • LISS-IV: a sharp, narrow view, good for single fields and villages.
• It is used to monitor crops, land and water. NCERT Fig. 6.11 shows a Resourcesat image.

• Resourcesat-2 and Resourcesat-2A continue the same job.

• Cartosat (for maps):

• Cartosat-1 (2005) took stereo pairs: two images of the same place taken from two angles, like our two eyes.
  • This gives a sense of height.
  • From it we make DEMs (Digital Elevation Models: 3D maps of the land's height).
  • DEMs are used for contour maps, planning roads and dams, and mapping watersheds.

• Cartosat-3 (2019) reaches about 25 cm resolution. This is enough to see single vehicles and the outline of buildings, so it is useful for city planning, cadastral (land-record) maps and defence.

• Oceansat (for the ocean):

• It maps:
  • chlorophyll (green pigment in tiny ocean plants called phytoplankton), which shows where fish food is plentiful;
  • potential fishing zones (PFZ), which are advisories telling fishers where fish are likely to gather;
  • sea-surface winds, which help cyclone and monsoon forecasts.
• Oceansat-3 (EOS-06) was launched in 2022.

Radar and weather satellites

RISAT: radar imaging through cloud

• RISAT (Radar Imaging Satellite): India's C-band SAR satellites.
• Why radar matters for India:
• Most floods and kharif crops happen during the south-west monsoon (June–September).
• At that time thick cloud covers the country, so optical satellites see little.

• SAR sees through cloud and works at night, so it can still map floodwater and paddy fields.

• RISAT-1 was launched in 2012. EOS-04 (RISAT-1A) followed in 2022.

• Uses: floods, crops (especially kharif rice), and border surveillance (watching the borders).
• EOS-09 failure:
• EOS-09 was a 1,696 kg radar satellite meant for a 505 km sun-synchronous polar orbit. It was to add to India's all-weather imaging for farming, disaster management and surveillance [3].
• It flew on PSLV-C61, ISRO's 101st launch, on 18 May 2025.
• The rocket worked normally through the 2nd stage. A problem in the 3rd stage meant the mission could not be completed [3].

INSAT-3DS: weather watch from geostationary orbit

• INSAT-3DS is a geostationary meteorological (weather) satellite. It is a follow-on to INSAT-3D (2013) and INSAT-3DR (2016) in India's third-generation weather satellite series [5].
• Launch: 17 February 2024, on GSLV-F14, from SDSC-SHAR, Sriharikota (Andhra Pradesh). The rocket put it into a geosynchronous transfer orbit [5].
• Funding and build: the Ministry of Earth Sciences (MoES) funded it [5]. It is built on ISRO's I-2k bus and weighed 2,275 kg at lift-off [5].
• Payloads:
• Imager: a 6-channel radiometer that takes pictures of clouds and the earth's surface [5].
• Sounder: a 19-channel instrument that measures temperature and humidity at different heights in the air [5].
• Data Relay Transponder, which collects data from ground weather stations [5].

• Satellite-aided Search and Rescue transponder, which relays distress signals from ships, aircraft and people in danger [5].

• Output: more than 40 data products [5], for example:

• sea surface temperature, rainfall, fog and smoke or fire;
• winds and cloud properties;

• temperature profiles (how air temperature changes with height).

• It sent its first images on 7 March 2024 [5].

• Main use: tracking cyclones in the Bay of Bengal and Arabian Sea every few minutes, and giving data to the IMD for forecasts.

NISAR: the first dual-band radar satellite

• NISAR (NASA-ISRO Synthetic Aperture Radar) is the first satellite to carry both L-band and S-band radar.
• Launch: 30 July 2025 on GSLV-F16 from Sriharikota, Andhra Pradesh [2].

• This was the first GSLV mission to a sun-synchronous polar orbit [2].

• Who built what [2]:

NASA (USA)	ISRO (India)
L-band radar	S-band radar
12 m unfolding (deployable) antenna	Spacecraft bus (the satellite's main body)
GPS receiver, high-rate data link, solid-state recorder	GSLV-F16 rocket and launch services

• Technology: it uses SweepSAR, which gives sharp images over a very wide 242 km swath (the strip of ground seen in one pass). It images by day and by night, in all weather [2].
• Coverage: it images almost all of the earth's land and ice every 12 days.
• What it tracks:
• Earthquakes and landslides: it finds ground movement of a few centimetres by comparing two radar images of the same place taken at different times. This method is called interferometry.
• Glaciers and ice sheets: it tracks how fast they flow and melt, for example in the Himalaya, Greenland and Antarctica.

• Crops and forests: L-band passes through the tree canopy, so it can measure the amount of woody plant matter (biomass).

• Its main aims are to study how land and ice move or change shape, land ecosystems, and ocean areas [2].

The big picture: how coverage is divided

• Polar, sun-synchronous satellites (Landsat, Sentinel, IRS, Cartosat, RISAT, NISAR):
• give fine detail of the ground;

• but revisit the same place only every few days.

• Geostationary satellites (INSAT-3DS):

• give coarser detail;

• but watch the same area all the time, which is ideal for fast-changing weather such as cyclones.

• Optical satellites see colour and plant health but are blocked by cloud. Radar satellites see through cloud. Using both gives coverage in every season.

Prelims Hooks

• ERTS-1 (1972) was the first Landsat. Landsat holds the longest continuous land record. Landsat 9 was launched on 27 September 2021 from Vandenberg, California [4].
• Resolution pairs: MSS 80 m / TM 30 m (Landsat); HRV 20 m / 10 m (SPOT, 1986); Sentinel-2 10 m. Sentinel-1 is radar, not optical. This is a common trap.
• Bhaskara-I (1979) and II (1981) were launched on Soviet rockets. IRS-1A was launched on 17 March 1988 from Baikonur, not Sriharikota.
• LISS order by resolution: LISS-I 72.5 m → LISS-II 36.25 m → LISS-III 23 m. IRS-1C (1995) also carried PAN 5.8 m.
• Match each satellite to its job: Cartosat → mapping/DEM (Cartosat-3 ≈ 25 cm, 2019); Oceansat → PFZ/chlorophyll (Oceansat-3 = EOS-06, 2022); Resourcesat → crops (AWiFS + LISS-III + LISS-IV); RISAT → C-band SAR (EOS-04 = RISAT-1A).
• INSAT-3DS (GSLV-F14, 17 February 2024) is geostationary, has a 6-channel imager and 19-channel sounder, and is funded by MoES [5].
• NISAR (GSLV-F16, 30 July 2025): NASA gave the L-band radar and 12 m antenna; ISRO gave the S-band radar, the bus and the rocket. It has a 242 km swath and was the first GSLV launch to a sun-synchronous orbit [2].
• PSLV-C61/EOS-09 (18 May 2025) failed because of a 3rd-stage problem. It was ISRO's 101st launch [3].

Mains Points

• Radar fills the monsoon gap (GS-III, disasters and agriculture):
• Cloud covers India during the south-west monsoon, so optical images fail exactly when floods and kharif sowing happen.
• C-band RISAT/EOS-04 and L+S-band NISAR [2] provide flood maps, paddy area estimates and landslide or subsidence monitoring. Possible use areas include Assam and Bihar floods and the Joshimath-type sinking of land in the Himalaya.

• The EOS-09 loss (May 2025) [3] shows why India needs backup satellites to keep its radar coverage running.

• Change over time needs long archives (GS-I and III):

• Landsat's record since 1972 [4] and India's IRS record since 1988 make it possible to track deforestation, glacier retreat, city growth and wetland loss.

• These records support forest cover reports, watershed planning and climate adaptation.

• From experiment to self-reliance:

• India moved from Bhaskara on Soviet rockets to a full fleet built and launched at home: Resourcesat, Cartosat, Oceansat, RISAT and INSAT-3DS.
• These satellites serve fishing advisories, crop forecasts, cyclone warnings (INSAT-3DS [5]) and cadastral mapping.

• NISAR shows India is now an equal partner in the world's top Earth observation missions [2].

• Open data is changing how satellite data is used:

• Free Sentinel and Landsat data has lowered the cost of satellite analysis.
• This supports India's push to open up geospatial data, so that states, start-ups and researchers can build services on it.