Sun-synchronous orbit

Also called: Near-polar sun-synchronous orbit · Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"

Meaning

A sun-synchronous orbit is a near-polar orbit in which a satellite crosses each latitude at the same local solar time on every pass. For example, an IRS satellite crosses India at around mid-morning every time. Because the sun is at the same angle on every pass, images from different dates can be compared fairly. This makes it the main orbit for earth observation satellites such as IRS and Landsat.

Explanation

How it works: cause → process → result

• Near-polar path
• The orbit is tilted almost 90° to the equator.

• So the satellite travels roughly from pole to pole [5].

• Earth turns underneath

• 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 [5].

• Result: the whole globe gets covered

• Over many days, these strips join up and cover the earth [5].

• The satellite comes back over the same place after some days. This is its revisit (the gap before it images the same spot again).

• Same clock time on every pass

• The orbit is set so the satellite always reaches a given latitude at the same local solar time (time measured by the sun's position, not by the clock in your city).
• So the sun angle, lighting and shadows look the same in every image.

Key figures (NCERT Box 6.1)

Feature	Value
Height	700–900 km
Orbits per day	About 14, so one orbit takes roughly 100 minutes
Coverage	81°N to 81°S
Resolution	Fine, from 182 m down to 1 m
Orbit zone	Low-earth orbit (LEO: about 160 to 2,000 km above the earth) [5]

• Why the poles are partly missed
• The orbit is near-polar, not exactly polar.

• So the area right around the poles is left out. Coverage stops at 81°N and 81°S.

• Why the images are sharp

• The satellite flies low.
• The closer the sensor is to the ground, the finer the detail it records.

Why "same solar time" matters

• Fair comparison over time
• Same sun angle → same shadows and brightness.

• So a change seen in the image is a real change on the ground, not just different lighting.

• Uses that need this

• tracking crops, forests, floods and urban growth
• earth resources work: land use, crops, forests, minerals

Where it occurs

• India
• IRS series: the first, IRS-1A, was launched in 1988. Later came Resourcesat and Cartosat.
• IRS data from Cartosat-2 series, SCATSAT-1, Resourcesat-2/2A and Cartosat-1 is received at AGEOS (Antarctica Ground Station for Earth Observation Satellites). It sits at India's Bharati station, Larsemann Hills, Antarctica, and was commissioned in August 2013 [2].
• AGEOS passes the data on to Shadnagar, near Hyderabad (Telangana), which is India's main earth receiving station [3].
• At Shadnagar, IMGEOS (Integrated Multi-Mission Ground Segment for Earth Observation Satellites) receives, processes and sends out data from many satellites [2].

• It is run by NRSC (National Remote Sensing Centre), Hyderabad, an ISRO centre. NRSC shares the data through Bhuvan, ISRO's geoportal (an online map portal) [3][4].

• Why a polar station helps

• A sun-synchronous satellite passes near the poles on almost every orbit.
• So a station near a pole sees many more passes than Shadnagar can.

• Data recorded on board over other parts of the world can be downloaded there. This gives India global coverage [2].

• World

• USA: Landsat (first launched in 1972).
• NASA's Aqua flies at about 705 km [5].

Don't confuse with

• Geostationary orbit: a circular orbit right above the equator at 35,786 km (NCERT: about 36,000 km). It takes 24 hours per orbit and stays over one fixed spot [5]. A sun-synchronous orbit is low (700–900 km), near-polar and keeps moving.
• Resolution: sun-synchronous gives fine images (182 m to 1 m). Geostationary gives coarse images (about 1 km × 1 km) because it is around 40 times farther away.
• Coverage and revisit: sun-synchronous covers 81°N–81°S strip by strip and comes back after some days. Geostationary sees about one-third of the globe non-stop from one spot, but cannot image the poles well.
• Same local solar time vs same spot: "same local solar time" describes sun-synchronous. "Fixed over one point, same view all day" describes geostationary.

Prelims Hooks

• Sun-synchronous orbit: height 700–900 km, about 14 orbits a day, coverage 81°N–81°S (NCERT Box 6.1).
• "Crosses each latitude at the same local solar time" → sun-synchronous, not geostationary.
• Match the satellite to the orbit: IRS, Landsat, Cartosat, Aqua → sun-synchronous. INSAT, GOES → geostationary [5].
• Resolution trap: sun-synchronous = fine (182 m to 1 m). Geostationary = coarse (~1 km). The higher the orbit, the coarser the image.
• Most earth observation satellites fly in LEO (about 160 to 2,000 km) [5]. Aqua flies at about 705 km [5].
• AGEOS at Bharati station, Larsemann Hills, Antarctica (2013) downloads IRS data and sends it to Shadnagar, Telangana [2].

Mains Points

• Disaster management needs both orbits (GS-III, disasters)
• Geostationary INSAT gives non-stop, coarse watch, which is good for warning about cyclones and the monsoon.
• Sun-synchronous IRS gives detailed, repeatable images for damage assessment, such as flood-damage mapping.

• IRS gives detail. INSAT gives continuity.

• Same-time lighting helps planning (GS-I geography; GS-III environment)

• Images taken under the same sun angle can be compared across years.

• So planners can reliably measure land-use change, glacier retreat, urban sprawl, forest cover and crop estimates, and map wasteland.

• Ground segment as strategic infrastructure (GS-III, infrastructure)

• The near-polar path lets the Antarctic station AGEOS, working with Shadnagar, give India its own global data download [2].
• This cuts dependence on foreign stations and supports data sovereignty (a country's control over its own data).

Related concepts

• Geostationary orbit
• Earth receiving station
• National Remote Sensing Centre

Read more

• Orbits and the ground segment