Geostationary orbit

Geography glossary

Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"

Meaning

A geostationary orbit is a circular orbit right above the equator, at a height of 35,786 km (NCERT rounds it to "about 36,000 km"). A satellite there goes round the earth once in 24 hours, the same time the earth takes to spin once [2]. Because it moves with the earth's spin, the satellite seems to stay still over one spot. It can watch about one-third of the globe without a break, which is why India's INSAT uses it for weather watching and telecommunication.

Explanation

How it works: why the satellite seems to "hang" in the sky

  • The height sets the speed of the orbit.
  • The higher a satellite flies, the longer it takes to go round the earth once.

  • At 35,786 km, one orbit takes exactly 24 hours [2].

  • It moves with the earth.

  • The satellite travels west to east, the same way the earth spins.
  • Its angular speed (how many degrees it turns in an hour) matches the earth's.

  • So, seen from the ground, it stays over one fixed spot on the equator [2].

  • The result is a non-stop view.

  • It sends back pictures of the same area again and again.
  • This gives almost non-stop watch over one region [2].

Three conditions it must meet

  • Above the equator: the orbit must lie over the equator. If it is tilted, the satellite drifts north and south during the day and does not stay still.
  • Circular path: the orbit must be circular, so the satellite's speed and height stay the same.
  • Same direction and period as the earth: it must move west to east and take 24 hours for one orbit.
  • If any one of these is missing, the satellite will not stay over one spot.

What it can and cannot see (NCERT Box 6.1)

  • Coverage: about one-third of the globe from one position.
  • Resolution (the smallest object a picture can show): coarse, about 1 km × 1 km.

  • Why it is coarse: the satellite is about 40 times farther away than an IRS satellite, and the farther the camera, the less detail it can see.

  • The poles are a weak spot.

  • From above the equator, the earth's curve hides the high latitudes.

  • So a geostationary satellite cannot image the poles well.

  • Trade-off: it gives continuity (non-stop watch) but not detail.

Uses

  • Telecommunication: a dish on the ground can point at one fixed spot in the sky. It never has to turn and follow the satellite.
  • Weather watching: forecasters can follow cyclones and monsoon cloud bands hour by hour.

Where it occurs

  • Location in space: a single ring of orbit above the equator, at 35,786 km [2]. Every geostationary satellite sits somewhere on this ring.
  • India: INSAT series
  • India's geostationary satellites for telecommunication and weather.
  • The first INSAT was launched in 1983.

  • It watches cyclones over the Bay of Bengal and the Arabian Sea, and the monsoon cloud bands moving over India.

  • USA: GOES (Geostationary Operational Environmental Satellite) series, used for weather watching [2].

  • Contrast in India's satellite fleet:
  • INSAT (geostationary) is used for weather and communication.
  • IRS, Resourcesat and Cartosat (sun-synchronous, 700–900 km) are used for detailed mapping of land, crops and forests.

Don't confuse with

  • Sun-synchronous orbit: a near-polar orbit at 700–900 km, with about 14 orbits a day. It crosses each place at the same local solar time, giving fine detail (182 m to 1 m). A geostationary orbit is far higher, gives a continuous but coarse (~1 km) view, and does not pass at the "same solar time".
  • Low-earth orbit (LEO): about 160 to 2,000 km above the earth [2]. Most earth observation satellites fly here. A geostationary orbit is far above LEO, at 35,786 km.
  • Geosynchronous orbit: this orbit also takes 24 hours, but it may be tilted to the equator. Seen from the ground, the satellite then drifts north and south each day. A geostationary orbit is the special case that lies exactly over the equator and is circular, so the satellite looks fixed. Every geostationary orbit is geosynchronous, but not every geosynchronous orbit is geostationary.
  • Near-polar orbit: this orbit is tilted almost 90° to the equator, so the satellite runs roughly from pole to pole [2]. A geostationary orbit has 0° tilt and stays over the equator.

Prelims Hooks

  • Geostationary height = 35,786 km above the equator, with a 24-hour period [2]. NCERT rounds this to ~36,000 km.
  • Match the satellite to the orbit: INSAT and GOES use geostationary orbits [2]. IRS, Landsat and Cartosat use sun-synchronous orbits.
  • Resolution trap: geostationary = coarse (~1 km × 1 km), while sun-synchronous = fine (182 m to 1 m). The higher the orbit, the coarser the image.
  • Coverage: one geostationary satellite sees about one-third of the globe, but it cannot image the poles well.
  • "Same local solar time" describes a sun-synchronous orbit, not a geostationary one. This is a common "which of the following" trap.
  • Exam pair (NCERT exercise): IRS gives detail; INSAT gives continuity. The first INSAT was launched in 1983.

Mains Points

  • Disaster management needs both orbits (GS-III, disasters):
  • Geostationary INSAT gives a non-stop view, so cyclones and monsoon bands can be tracked hour by hour. This supports early warning and the evacuation of coastal people.
  • Sun-synchronous IRS gives detailed pictures after the event, used for flood-damage and crop-loss assessment.

  • So geostationary satellites are for warning, and sun-synchronous satellites are for damage assessment.

  • A trade-off between seeing often and seeing in detail (GS-I, geography):

  • The high orbit gives a continuous watch over one-third of the globe, but only coarse (~1 km) pictures, and the poles are poorly covered.

  • That is why India runs two kinds of fleet: INSAT for weather and telecom, and IRS, Resourcesat and Cartosat for land use, forests and wasteland mapping.

  • Strategic infrastructure (GS-III, infrastructure): fixed geostationary satellites give steady links for telecommunication and weather data. They need no tracking dishes, so the service reaches across the whole country.

Related concepts

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Sources

  1. 1Class 11, Ch 6 "Introduction to Remote Sensing" (primary)
  2. 2Orbits, NASA Earthdataearthdata.nasa.gov · tier 2