Which of the following frequency bands is not used in INSAT/GSAT satellite communication?

Correct — option (3), MF.

ISRO's mission pages describe the communication payloads of INSAT/GSAT satellites, which work from geostationary orbit, in bands such as C, extended C, Ku, Ka and S, all measured in gigahertz.

GSAT-30 (launched 17 January 2020): C and Ku bands
GSAT-19 (2017) and GSAT-29 (2018): Ka/Ku-band high throughput transponders
GSAT-17 (2017): normal C, extended C and S bands

MF, medium frequency, is the ITU name for 300 kHz to 3 MHz, wavelengths of 1,000 m down to 100 m. It carries medium-wave AM radio broadcasting.

MF waves travel as ground waves along the Earth's surface, and as sky waves refracted back to Earth by the ionosphere. At microwave frequencies, radio travels by line of sight, the path between a ground dish and a satellite.

The other three options are microwave letter bands. In IEEE Std 521, C is 4–8 GHz, Ku 12–18 GHz and Ka 27–40 GHz, more than a thousand times the frequency of any MF signal.

The idea to remember: INSAT/GSAT payloads work in microwave letter bands such as C, Ku and Ka; MF is the band of medium-wave AM broadcasting.

Why the others are wrong

• (1)
  Ka — Ka is used. ISRO describes GSAT-19 and GSAT-29 as carrying Ka/Ku-band high throughput communication transponders, and GSAT-14 (launched 5 January 2014) carried two Ka-band beacons at 20.2 GHz and 30.5 GHz for attenuation studies.

  In IEEE's scheme Ka is 27–40 GHz, the highest of the four bands in this stem. ISRO's 20.2 GHz Ka-band beacon shows that satellite usage of the letter can reach below that edge.

• (2)
  C — C band is used. ISRO's page says GSAT-30 provides communication services in C and Ku bands, and that GSAT-17 carries payloads in normal C-band, extended C-band and S-band.

  C is 4–8 GHz in IEEE's scheme. INSAT-3A, launched on 10 April 2003, carried twelve normal C-band, six extended C-band and six Ku-band transponders.

• (4)
  Ku — Ku band is used. ISRO described GSAT-31, launched on 6 February 2019, as augmenting the Ku-band transponder capacity in geostationary orbit, and GSAT-15 carried 24 communication transponders in Ku-band.

  Ku is 12–18 GHz in IEEE's scheme. It sits just under the K band (18–27 GHz), and Ka sits just above it.

Radio waves are grouped into bands by frequency. The ITU scheme names decade bands: MF is 300 kHz–3 MHz, HF 3–30 MHz, VHF 30–300 MHz, UHF 300 MHz–3 GHz, SHF 3–30 GHz and EHF 30–300 GHz.

Microwave bands also carry letter names. In IEEE Std 521, L is 1–2 GHz, S 2–4 GHz, C 4–8 GHz, X 8–12 GHz, Ku 12–18 GHz, K 18–27 GHz and Ka 27–40 GHz.

Other naming systems draw some of these edges differently, and the range a letter covers can vary between fields of use.

How a wave travels depends on its frequency. MF waves follow the ground or are refracted back by the ionosphere. At microwave frequencies radio travels by line of sight, which is how a home dish receives a geostationary satellite.

Key facts

• MF (medium frequency) is the ITU band from 300 kHz to 3 MHz, wavelengths 1,000 m to 100 m; it carries medium-wave AM broadcasting.
• IEEE Std 521 letter bands: S 2–4 GHz, C 4–8 GHz, Ku 12–18 GHz and Ka 27–40 GHz.
• ISRO: GSAT-30, launched on 17 January 2020, provides communication services from geostationary orbit in C and Ku bands.
• ISRO: GSAT-19 (June 2017) and GSAT-29 (November 2018) carry Ka/Ku-band high throughput communication transponders.
• ISRO: GSAT-17, launched on 29 June 2017, carries payloads in normal C-band, extended C-band and S-band.

Common traps

• Reading MF as a microwave letter band. MF is an ITU decade band of 300 kHz–3 MHz; C, Ku and Ka are letter names for gigahertz bands.
• Mixing up Ku and Ka. In IEEE's scheme Ku (12–18 GHz) sits under the K band and Ka (27–40 GHz) above it.
• Treating a letter's range as fixed everywhere. IEEE puts Ka at 27–40 GHz, yet ISRO calls GSAT-14's 20.2 GHz beacon a Ka-band beacon.

UnlockIAS will link similar questions from RAS Pre 2018 and 2013 here once those papers are published on this site.

• practice — not a real PYQ

  Which one of the following is the frequency range of the Medium Frequency (MF) band in the ITU designation?

  • (a)30 kHz – 300 kHz
  • (b)300 kHz – 3 MHz
  • (c)3 MHz – 30 MHz
  • (d)4 GHz – 8 GHz
  Answer(2) — MF is 300 kHz to 3 MHz. Option (1) is the Low Frequency (LF) band, option (3) is the High Frequency (HF) band, and option (4) is the C band in IEEE's letter scheme.
• practice — not a real PYQ

  GSAT-30, launched in January 2020, provides communication services from geostationary orbit in which bands?

  • (a)C and Ku bands
  • (b)S and UHF bands
  • (c)Ka and Q/V bands
  • (d)L1 and L5 bands
  Answer(1) — ISRO's page says GSAT-30 provides communication services in C and Ku bands. Option (2) names bands ISRO lists for other satellites (S on GSAT-17, UHF on INSAT-3A's data relay transponder); option (3) names bands from GSAT-29's payloads, its Ka/Ku-band transponders and Q/V-band payload; option (4) is GSAT-15's GAGAN navigation payload.