CDS (I) 2022 General Knowledge · Question 105 of 120

Correct — A, It is the world's most powerful radio antenna. LOFAR, the Low-Frequency Array, is a radio telescope whose antenna network lies mainly in the Netherlands, with stations in several other European countries. It was designed and built by ASTRON, the Netherlands Institute for Radio Astronomy, and was opened on 12 June 2010. Instead of one steerable dish it uses about twenty thousand small fixed antennas grouped into stations — fifty-two of them since 2019 — whose signals are combined by computer, so the instrument is pointed in software rather than by moving metal. It observes between 10 and 240 megahertz, the lowest frequencies that reach the ground through the ionosphere, and it is the most sensitive radio observatory there is at those frequencies until the Square Kilometre Array comes on line. The option's wording is loose journalistic shorthand for that, but each of the other three describes something LOFAR simply is not.

• (b)It is a space technology launched by NASA to discover exoplanets. — LOFAR is not a spacecraft and NASA did not launch it. It is ground-based, European and operated by ASTRON. NASA's dedicated exoplanet hunters are Kepler and TESS, and neither is a radio instrument.
• (c)It is an arrangement of satellites in space for the purpose of creating Satellite Navigation System. — Satellite navigation constellations are things like GPS, GLONASS, Galileo, BeiDou and India's NavIC. LOFAR has no satellites at all — its antennas sit on the ground in fields across northern Europe.
• (d)It is a wireless communication technology for 5G services. — The connection with 5G is only that both live in the radio spectrum, and they live in different parts of it. LOFAR listens between 10 and 240 megahertz; it is a scientific receiver, not a communication technology, and it transmits nothing.

A radio telescope collects radio waves from space instead of light. At long wavelengths a single dish would have to be impossibly large to give useful resolution, so low-frequency astronomy is done by interferometry — many small antennas spread over a wide area, their signals brought together and combined so that the array behaves like one instrument as wide as the separation between its most distant elements. LOFAR is built entirely on that principle, with no moving parts, and it works in the band where astronomers look for the faint signals of the early universe, for pulsars, for solar and Jovian radio bursts, and for cosmic-ray air showers.

Terminology questions on scientific instruments reward one habit: read the acronym. Low Frequency Array says both that it is an array, meaning many elements rather than one, and that it works at low frequency, which is the radio end of the spectrum. That already rules out an exoplanet spacecraft, a navigation constellation and a 5G technology, all of which are about transmitting rather than receiving. The remaining option describes a radio instrument, and it is the answer. It is worth being straight about the wording, though: professional descriptions call LOFAR the largest and most sensitive radio telescope operating at its frequencies, not the most powerful antenna in the world, since a radio telescope has no power output to speak of — it only listens. As of the 2022 exam its successor, the Square Kilometre Array being built in South Africa and Australia, was still under construction.

• LOFAR is the Low-Frequency Array, a radio telescope with its antenna network mainly in the Netherlands and its core about 3 km north of Exloo.
• It was designed and built by ASTRON, the Netherlands Institute for Radio Astronomy, and was officially opened on 12 June 2010.
• It uses about 20,000 small antennas grouped into 52 stations, combined as an interferometer rather than a single steerable dish.
• Its observing range is 10 to 240 megahertz — among the lowest radio frequencies that penetrate the Earth's ionosphere.
• It is the most sensitive radio observatory at those frequencies until the Square Kilometre Array becomes operational.

• Assuming any instrument in the news with an acronym must be a spacecraft. LOFAR is entirely ground-based.
• Confusing radio astronomy with radio communication. A radio telescope receives; it does not transmit.
• Reading 'most powerful' literally. LOFAR is the most sensitive instrument at its frequencies, which is what the phrase is standing in for.

As a what-is-this-instrument item, or as a match between observatories and what they observe.

No directly related past PYQ was found.

• practice — not a real PYQ

  The Giant Metrewave Radio Telescope, one of the world's largest low-frequency radio telescopes, is located near

  • (a)Hyderabad
  • (b)Pune
  • (c)Bengaluru
  • (d)Nainital

  Answer(b) Pune — the array of thirty large dishes stands at Khodad near Narayangaon, north of Pune, and is run by the National Centre for Radio Astrophysics.

• practice — not a real PYQ

  A radio telescope array such as LOFAR achieves high angular resolution mainly by

  • (a)cooling its receivers to near absolute zero
  • (b)combining signals from antennas separated by long distances
  • (c)orbiting above the Earth's atmosphere
  • (d)using a single dish of very large diameter

  Answer(b) combining signals from antennas separated by long distances — in interferometry the resolving power depends on the separation between elements, not on the size of any one of them.