Orbits and the ground segment

Detail

1. Why the orbit matters

• A satellite's orbit is the fixed path it follows around the earth. The orbit decides three things:
• how high the satellite flies, which sets how much detail it can see
• how often it returns over the same place

• how much of the earth it can see at one time

• Remote sensing satellites use two main orbit types: sun-synchronous and geostationary (NCERT Box 6.1).

• Most earth observation satellites fly in low-earth orbit (LEO: about 160 to 2,000 km above the earth) [5].

2. Sun-synchronous orbit

• Sun-synchronous orbit: a near-polar orbit in which the satellite crosses each latitude at the same local solar time on every pass.

• Example: an IRS satellite crosses India at around mid-morning every time.

• Near-polar orbit: the orbit is tilted almost 90° to the equator, so the satellite travels roughly from pole to pole [5].

• How it covers the whole earth
• The satellite moves north–south.
• Below it, the earth turns from west to east.

• So each new pass falls over a fresh strip of land further west, and over many days the strips cover the globe [5].

• Key figures (NCERT Box 6.1)

• Height: 700–900 km
• About 14 orbits a day, so one orbit takes roughly 100 minutes
• Coverage: 81°N to 81°S. The orbit is not exactly polar, so the area right around the poles is left out.

• Resolution: fine, from 182 m down to 1 m

• Why the same solar time matters

• The sun is at the same angle on every pass, so lighting and shadows look the same.

• Images from different dates or seasons can then be compared fairly. This is needed for tracking crops, forests, floods and urban growth.

• Examples

• India: IRS series (IRS-1A, 1988), Resourcesat, Cartosat
• USA: Landsat (1972)
• NASA's Aqua flies at about 705 km [5].

3. Geostationary orbit

• Geostationary orbit: a circular orbit right above the equator at 35,786 km (NCERT: "about 36,000 km"). At this height one orbit takes 24 hours, the same time the earth takes to spin once [5].
• Why it seems to "hang" in the sky
• It moves in the same direction as the earth's rotation (west to east) and at the same angular speed.
• So, seen from the ground, it stays over one fixed spot on the equator [5].

• It sends back the same view again and again, giving almost non-stop watch over one region [5].

• Key figures (NCERT Box 6.1)

• Coverage: about one-third of the globe from one position

• Resolution: coarse, about 1 km × 1 km, because it is around 40 times farther away than an IRS satellite

• Uses: telecommunication and weather watching, such as following cyclones and monsoon cloud bands hour by hour.

• India: INSAT (first INSAT launched in 1983)
• USA: GOES (Geostationary Operational Environmental Satellite) series [5]

4. Comparison table (NCERT Box 6.1)

Feature	Sun-synchronous	Geostationary
Height	700–900 km, near-polar	35,786 km above the equator [5] (NCERT: ~36,000 km)
Period	~14 orbits a day	1 orbit in 24 hours, moving with the earth's spin
Coverage	81°N–81°S, strip by strip	~⅓ of the globe, from one fixed spot
Revisit	Same place after some days	Continuous view
Resolution	Fine (182 m to 1 m)	Coarse (~1 km × 1 km)
Uses	Earth resources: land use, crops, forests, minerals	Telecom, weather, cyclone tracking
Examples	IRS, Landsat, Aqua [5]	INSAT, GOES [5]

• Exam pair: IRS vs INSAT (NCERT exercise 2(ii)). IRS gives detail. INSAT gives continuity.

5. The ground segment: from satellite to map

• Ground segment: all the facilities on earth that receive, correct, store and send out satellite data.
• Step by step (NCERT stages e to h)
• The sensor on the satellite records reflected or emitted energy.
• It transmits the image data electronically to an earth receiving station.
• At the station, the data are corrected for errors, such as distortion from the earth's curvature or rotation and sensor faults.
• Information is then extracted through image interpretation and classification.
• The result is turned into maps and tables for users.

6. Earth receiving stations

• Earth receiving station: a ground station with large dish antennas that receives image data sent down by the satellite.
• Shadnagar, near Hyderabad (Telangana)
• India's main receiving station [3].
• It receives data from both Indian and foreign earth observation satellites [3].

• It houses IMGEOS (Integrated Multi-Mission Ground Segment for Earth Observation Satellites). This facility receives, processes and sends out data from many satellites [2].

• AGEOS (Antarctica Ground Station for Earth Observation Satellites)

• Location: India's Bharati station, Larsemann Hills, Antarctica [2].
• Commissioned in August 2013 [2].

• Receives IRS data from satellites such as Cartosat-2 series, SCATSAT-1, Resourcesat-2/2A and Cartosat-1, and passes it on to Shadnagar [2].

• Why a polar station helps

• A sun-synchronous satellite passes near the poles on almost every orbit.
• A station near a pole can therefore "see" many more passes than Shadnagar can.
• So data recorded on board over other parts of the world can be played back and downloaded there. This gives India global coverage [2].

7. National Remote Sensing Centre (NRSC), Hyderabad

• NRSC: one of the main centres of ISRO, Department of Space [4].
• Set up as the National Remote Sensing Agency (NRSA) in 1974.

• Became an ISRO centre in 2008.

• Its job [3][4]

• set up ground stations to receive satellite data
• make data products and send them to users
• do aerial remote sensing
• give decision support for disaster management
• provide geospatial services for governance through Bhuvan, ISRO's geoportal

• build capacity (training)

• Campuses [4]

Campus	Role
Balanagar, Hyderabad	Main campus: administration, applications, aerial services
Shadnagar	Data reception, processing, sending out data; climate studies; disaster management
Jeedimetla, Hyderabad	Training and outreach
Begumpet Airport, Hyderabad	Aircraft operations for aerial surveys

• Regional Remote Sensing Centres (5) [4]
• Jodhpur (West)
• New Delhi (North)
• Kolkata (East)
• Nagpur (Central)
• Bengaluru (South)

• They run regional projects, write custom software and train people in GIS and image processing [3].

• Aerial services: aerial photography, digital mapping, aeromagnetic surveys, and topographic and cadastral (land-ownership) mapping [3].

• Wasteland mapping: NRSC prepared India's wasteland classification and maps (NCERT Class 12).
• Indian Institute of Remote Sensing (IIRS), Dehradun (Uttarakhand): an ISRO unit that trains users in remote sensing and GIS.

Prelims Hooks

• Geostationary height = 35,786 km above the equator, with a 24-hour period [5]. NCERT rounds it to ~36,000 km.
• Sun-synchronous height = 700–900 km. About 14 orbits a day. Covers 81°N–81°S (NCERT).
• Resolution trap: sun-synchronous = fine (182 m to 1 m). Geostationary = coarse (~1 km). The higher the orbit, the coarser the image.
• Match the satellite to the orbit: IRS, Landsat, Cartosat → sun-synchronous. INSAT, GOES → geostationary.
• "Same local solar time" describes a sun-synchronous orbit, not a geostationary one.
• A geostationary satellite sees ~⅓ of the globe and cannot image the poles well.
• Shadnagar (near Hyderabad, Telangana) = India's earth receiving station. Its IMGEOS facility belongs to NRSC [2].
• AGEOS at Bharati station (Larsemann Hills, Antarctica), commissioned 2013, receives IRS data [2].
• NRSA 1974 → NRSC (ISRO centre) 2008. It runs Bhuvan. IIRS is at Dehradun, not Hyderabad.
• NRSC regional centres: Jodhpur, New Delhi, Kolkata, Nagpur, Bengaluru [4].

Mains Points

• The two orbits work together:
• Geostationary INSAT gives non-stop, coarse watch, which is good for cyclone and monsoon tracking and early warning.
• Sun-synchronous IRS gives detailed, repeatable images for crop estimates, forest cover, wasteland and flood-damage mapping.

• Disaster management needs both: geostationary for warning, sun-synchronous for damage assessment (GS-III, disasters).

• Same-time lighting is useful for planning: images taken under the same sun angle can be compared across years. This lets planners measure land-use change, glacier retreat and urban sprawl reliably (GS-I, geography; GS-III, environment).

• The ground segment is strategic infrastructure:
• Shadnagar plus the Antarctic station AGEOS (2013) gives India its own global data download [2].

• This cuts dependence on foreign stations and supports data sovereignty.

• NRSC's model of central reception with regional delivery:

• one main reception and processing hub, plus five regional centres, Bhuvan and IIRS training [4]
• Together these turn raw data into governance tools: wasteland reclamation, watershed planning and disaster support (GS-III, infrastructure).