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Remote sensing satellites such as IRS, ERS-1 and IKONOS are usually placed in which type of orbit?
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Answer: Option A — Sun-synchronous orbit
Detailed Solution
A sun-synchronous orbit (SSO) is a near-polar orbit, usually at an altitude of about 600-800 kilometres, in which a satellite passes over any given point on the Earth's surface at the same local solar time every day. For example, a satellite may cross the equator every day at about 10:30 a.m. local time.
This happens because the plane of the orbit is slightly inclined (about 98 degrees, slightly retrograde) and it precesses, or rotates slowly, by about one degree per day, in step with the Earth's movement around the Sun. The Earth's slightly flattened shape (its equatorial bulge) causes this precession.
Why remote sensing satellites use sun-synchronous orbits:
Because the satellite passes over each place at the same local time, the angle of sunlight and the lighting conditions are similar every time. This makes it easy to compare images taken on different days, months or years, for example to study changes in crops, forests, glaciers or cities.
Being a polar orbit, it allows the satellite to cover almost the entire surface of the Earth as the Earth rotates beneath it.
The relatively low altitude gives high-resolution images.
Examples of sun-synchronous satellites:
- Indian Remote Sensing (IRS) satellites, such as Resourcesat, Cartosat, Oceansat and the EOS series (for example, EOS-04, a radar imaging satellite launched by PSLV-C52 in 2022).
- ERS-1 (European Remote Sensing satellite), NIMBUS-7 (NASA), IKONOS (a commercial high-resolution imaging satellite) and the Landsat series of the USA.
ISRO's Polar Satellite Launch Vehicle (PSLV) is especially suited for launching satellites into sun-synchronous polar orbits, which is why it is called the "workhorse of ISRO". In February 2017, PSLV-C37 launched a record 104 satellites in a single mission. India's first indigenous remote sensing satellite, IRS-1A, was launched in 1988. The National Remote Sensing Centre (NRSC), Hyderabad, receives and processes the data.
Why not the other options?
- ✗Geostationary orbit (about 36,000 km) is used for communication and weather satellites.
- ✗Medium Earth orbit is used for navigation satellites such as GPS.
- ✗Highly elliptical orbits are used for some communication satellites over high latitudes, such as Russia's Molniya.
Quick tip
Remember: Sun-synchronous orbit = near-polar, about 600-800 km, same local time each pass; used for remote sensing (IRS, Cartosat, Landsat); launched by PSLV (104 satellites in 2017).
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Space Technology & ISRO Missions — Preparation
This question tests your understanding of Space Technology & ISRO Missions, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers India's space programme: the history of ISRO, launch vehicles such as the PSLV and GSLV, satellites and their uses, India's missions to the Moon, Mars and the Sun, and major international space missions and agencies. Questions often ask when and from where a satellite or mission was launched, what it was meant to do, and which launch vehicle carried it. For example, Aryabhata was India's first satellite (1975), and Chandrayaan-3 landed near the Moon's south pole in 2023. New missions are launched every year, so pair these facts with recent current affairs. Practise the space technology and ISRO questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.