Satellite communication

In short

Satellite communication uses a repeater in orbit to relay radio signals between earth stations that may be thousands of kilometres apart, giving coverage that is independent of terrestrial cable or tower infrastructure. A geostationary satellite at 35,786 km appears fixed in the sky and covers about a third of the globe, which is why DTH television, VSAT networks, disaster communication, maritime links and broadcast feeds all ride on satellites. The subject combines orbital mechanics, antennas, noise theory and link budgeting, so it is heavily examined in GATE EC/IN, ISRO and DRDO entry tests and university electives. In India the knowledge is applied at ISRO/URSC/SAC, NSIL, Tata Play and Dish TV, Hughes Communications India, Nelco, BSNL, DRDO labs and in the navigation systems NavIC and GAGAN.

Explained

Orbits and orbital mechanics

• A satellite orbit is an ellipse with the Earth's centre at one focus (Kepler's first law); it sweeps equal areas in equal times (second law), so it moves fastest at perigee and slowest at apogee; third law gives T² ∝ a³.
• Six orbital elements: semi-major axis a, eccentricity e, inclination i, right ascension of the ascending node Ω, argument of perigee ω, true anomaly (or mean anomaly) - these fix the orbit in space and the satellite in the orbit.
• LEO (160-2,000 km): period 90-120 min, low delay (~5-20 ms one way), small path loss, but each satellite is visible only 5-15 min, so a constellation with inter-satellite links and handover is needed (Iridium, Starlink, Eutelsat OneWeb).
• MEO (~8,000-20,000 km): period ≈5-12 h; used by GPS/Galileo/GLONASS and by O3b/mPOWER data satellites.
• GEO (35,786 km, circular, i = 0°): period equals one sidereal day, 23 h 56 min 4 s (86,164 s), so the satellite appears stationary; three GEO satellites 120° apart give near-global coverage except the poles (|lat| > ~81°).
• HEO / Molniya: high eccentricity, i ≈ 63.4°, 12 h period - long dwell over high latitudes; GTO is the transfer ellipse from which a GEO satellite is circularised by an apogee motor burn.
• Perturbations: Earth's oblateness (J2) regresses the node of inclined orbits, while the equatorial ellipticity (tesseral J22 term) drives east-west drift of GEO satellites toward two stable longitudes (≈75°E and 105°W); Sun-Moon attraction tilts the orbit by about 0.85°/year; solar radiation pressure acts on eccentricity.
• Station keeping: thrusters hold the satellite in a box of typically ±0.05° to ±0.1° in longitude and latitude; the usable life of a GEO satellite (12-15 years) is set largely by station-keeping propellant - electric (ion/Hall) propulsion extends it.

Geometry: look angles, coverage and delay

• Sub-satellite point: the point on the equator directly below a GEO satellite, at the satellite's longitude.
• Look angles: azimuth (from true north, clockwise) and elevation (above the local horizon) to which the earth-station antenna is pointed; both follow from the station's latitude/longitude and the satellite longitude.
• Elevation below about 5-10° is avoided: longer path through the atmosphere, more rain and gaseous loss, more ground noise pick-up and multipath.
• Slant range is 35,786 km only for a station at the sub-satellite point, rising to about 41,700 km at the edge of coverage; one-way delay is therefore 120-140 ms and round trip 240-280 ms - a "double hop" exceeds 500 ms, which hurts voice and TCP throughput.
• Coverage/beams: global beam, hemispheric, zonal and spot beams; narrow spot beams give high gain and allow frequency and polarisation reuse, the basis of HTS (high-throughput satellites) with tens of Gbit/s capacity.
• Eclipse: near the equinoxes the satellite enters Earth's shadow for up to ~72 min around local midnight and runs on batteries. Sun outage: for a few days near each equinox the Sun lines up behind the satellite, floods the receiving antenna with noise and interrupts reception for a few minutes a day.

Space segment: subsystems and transponder

• Payload (communication subsystem): receive antenna → LNA → frequency translating down-converter → input multiplexer (channelisation) → TWTA or SSPA per channel → output multiplexer → transmit antenna.
• Transponder: one such channel, typically 36 MHz in C-band, 54/72 MHz in Ku, up to several hundred MHz in Ka.
• Bent-pipe (transparent) transponder: only amplifies and shifts frequency - modulation-agnostic, simple. Regenerative (on-board processing): demodulates, decodes, switches and re-modulates - better noise performance and routing, but fixed to one waveform.
• TWTA nonlinearity: near saturation it generates intermodulation products when several carriers share a transponder, so operators apply input/output backoff (several dB); a single large carrier per transponder can run close to saturation.
• Bus subsystems: AOCS (attitude and orbit control - spin or three-axis stabilised, sensors plus reaction wheels and thrusters); TT&C (telemetry, tracking and command, on a separate narrowband link); electrical power (solar arrays + Li-ion/NiH₂ batteries, kW to 15 kW); thermal control (heat pipes, OSR, louvres, heaters); propulsion; antennas (horn, parabolic reflector with shaped/multi-feed beams, phased arrays).
• Frequency translation is essential so that the high-power downlink does not swamp the satellite's own receiver (e.g. 6 GHz up / 4 GHz down).

Frequency bands, propagation and impairments

• Uplink uses the higher frequency of a pair because extra path loss and rain loss are easier to overcome with ground power and large dishes than on board the satellite.
• Free-space loss rises as 20 log f, so Ka links need much tighter beams; gaseous absorption peaks at 22 GHz (water vapour) and 60 GHz (oxygen).
• Rain attenuation is the dominant impairment above ~10 GHz: predicted by ITU-R P.618/P.838 using rain rate exceeded for 0.01% of the time; India's tropical rain rates (60-120 mm/h in many zones) make Ku/Ka links demanding during the monsoon.
• Rain also raises sky noise temperature, degrading C/N beyond the attenuation itself; counter-measures are uplink power control, adaptive coding and modulation (ACM), site diversity and link margin.
• Other effects: tropospheric and ionospheric scintillation (strong near the magnetic equator, relevant in India), Faraday rotation of linear polarisation below ~3 GHz (hence circular polarisation at L/S band), depolarisation (XPD) from rain in dual-polarised Ku links, Doppler shift for LEO (tens of kHz at 2 GHz) needing frequency tracking.

Earth segment and link budget

• Transmit chain: baseband → FEC encoder → modulator (IF 70/140 MHz) → up-converter → HPA (SSPA, TWTA or klystron) → feed/OMT → reflector antenna.
• Receive chain: antenna → LNA/LNB mounted at the feed (to keep noise temperature low) → down-converter → demodulator/FEC decoder.
• Antenna types: prime-focus, offset-fed (no feed blockage - all DTH dishes), Cassegrain (large gateways). Tracking may be fixed, step-track, monopulse or program track.
• Figure of merit G/T (dB/K) is the single number describing a receiver: antenna gain minus system noise temperature in dB. Typical: 0.6 m DTH dish ≈ 14 dB/K; 3.8 m Ku VSAT hub ≈ 32 dB/K.
• System noise temperature Ts = antenna noise (sky + ground, 20-50 K clear sky) + LNA noise (50-100 K) + contributions of later stages divided by preceding gain (Friis).
• Link budget steps: EIRP → free-space loss → other losses (atmosphere, rain, pointing, polarisation mismatch, feeder) → add receive G/T → add 228.6 to get C/N₀ → subtract 10 log B for C/N, or 10 log Rb for Eb/N₀ → compare with the threshold Eb/N₀ of the modulation/FEC → the difference is the link margin.
• The overall C/N combines uplink, downlink and intermodulation contributions reciprocally, so the worst link dominates; saturation flux density (SFD) fixes the uplink EIRP needed to drive the transponder to saturation.

Access techniques, modulation and standards

• FDMA: carriers separated in frequency with guard bands; simple but needs backoff against intermodulation. SCPC (one carrier per channel) and MCPC (multiplexed carriers) are FDMA variants.
• TDMA: stations transmit bursts in a frame (reference burst, preamble, guard time, traffic burst) using the whole transponder bandwidth - no intermodulation, but needs network timing and synchronisation.
• CDMA: all users share time and band with distinct codes - interference-tolerant, used in mobile satellite and navigation (covered in its own topic).
• DAMA (demand-assigned) versus PAMA/pre-assigned: DAMA allocates a channel only when a call or burst is requested, which multiplies the capacity of thin-route VSAT and telephony networks. Random access (slotted ALOHA) is used on VSAT return links for bursty data.
• Topologies: star (remote-hub-remote, two hops between remotes) and mesh (remote to remote in one hop, needing bigger remotes); iDirect/Hughes TDM forward + MF-TDMA return is the common VSAT architecture.
• Modulation and coding: QPSK is the workhorse; 8PSK/16-32APSK with LDPC+BCH in DVB-S2/S2X give 2-4.5 bit/s/Hz with ACM; return links use DVB-RCS2. Older DVB-S used QPSK with convolutional + Reed-Solomon coding.
• Regulation (India): spectrum and orbital slots are coordinated through ITU Radio Regulations filings; domestically the Telecommunications Act, 2023 (which replaced the Indian Telegraph Act, 1885) governs licensing, with DoT issuing VSAT CUG, GMPCS and NLD authorisations, the WPC wing assigning frequencies, SACFA giving siting clearance and ISRO's NOCC monitoring INSAT-network earth stations; TRAI recommends tariffs and policy. The Indian Space Policy 2023 opened the sector to private operators through IN-SPACe authorisation, with NSIL as the commercial arm. Licensing conditions for foreign LEO constellations and satellite spectrum assignment rules are still evolving, so verify current DoT/TRAI notifications before quoting specifics.

Indian systems and navigation

• INSAT/GSAT series (built by URSC, payloads by SAC) carry C, extended-C, Ku and Ka transponders for DTH, telephony, VSAT, tele-education, telemedicine and disaster warning; INSAT-3D/3DR/3DS carry meteorological imagers and sounders for IMD. Indian orbital slots lie roughly between 48°E and 100°E.
• HTS examples: GSAT-11, GSAT-19 and GSAT-29 use multiple Ka/Ku spot beams with frequency reuse to deliver multi-Gbit/s for rural broadband and BharatNet backhaul.
• Control: the Master Control Facility (MCF), Hassan and its Bhopal station perform orbit raising, station keeping and payload operations; launches use PSLV, GSLV-Mk II and LVM3 from SDSC SHAR.
• NavIC (IRNSS): regional navigation with satellites in GEO and inclined geosynchronous orbits, broadcasting on L5 (1176.45 MHz) and S band (2492.028 MHz), serving India and about 1,500 km around it; newer satellites add L1. GAGAN is the ISRO-AAI GPS satellite-based augmentation system, certified for aviation approaches in Indian airspace.

Key points

• Geostationary condition - circular, equatorial, sidereal-day period: radius 42,164 km, altitude 35,786 km, velocity ~3.07 km/s.
• Sidereal day - 86,164 s, not 86,400 s; using the solar day gives a wrong orbit radius.
• Frequency translation - the satellite always retransmits on a different frequency to avoid self-interference.
• Uplink is the higher frequency of the band pair, because ground stations can afford power and aperture.
• EIRP - transmit power plus antenna gain, in dBW; it is what the far end "sees".
• G/T - receive antenna gain minus system noise temperature in dB/K; the figure of merit of any receiving terminal.
• Boltzmann's constant - 1.38 × 10⁻²³ J/K, i.e. −228.6 dBW/K/Hz, the constant that appears in every link budget.
• Free-space path loss - 92.44 + 20 log d(km) + 20 log f(GHz) dB; about 196 dB for a 4 GHz GEO downlink.
• Overall C/N - reciprocals add, so the weakest of uplink, downlink and intermodulation dominates.
• Rain fade - negligible at C band, 3-10 dB at Ku and far worse at Ka; it also raises sky noise temperature.
• Transponder - a 36/54/72 MHz repeater channel; backoff is needed when it carries multiple carriers.
• Bent-pipe vs regenerative - amplify-and-forward versus demodulate-decode-remodulate on board.
• Multiple access - FDMA (frequency), TDMA (time), CDMA (code); DAMA assigns capacity on demand.
• Delay - one hop GEO ≈ 250 ms round trip; LEO ≈ 20-50 ms, which is why constellations suit interactive traffic.
• Station keeping - ±0.05-0.1° box; propellant decides the satellite's life.
• Eclipse and sun outage - both cluster around the equinoxes; one drains batteries, the other floods the receiver with noise.
• VSAT - small-aperture terminal (0.9-3.8 m) in a star or mesh network, licensed in India as a CUG service under DoT.
• NavIC - India's regional navigation system on L5 and S band, with GAGAN as the GPS augmentation layer.

Formulas and examples

1. Deriving the geostationary radius. T = 86,164 s, so a³ = μT²/4π² = (3.986 × 10¹⁴ × 7.4242 × 10⁹)/39.478 = 7.496 × 10²² m³ → a = 4.2164 × 10⁷ m = 42,164 km. Altitude = 42,164 − 6,378 = 35,786 km. Velocity v = √(3.986 × 10¹⁴/4.2164 × 10⁷) = 3.07 km/s. *(Same method for a 600 km LEO: a = 6,978 km, a³ = 3.398 × 10²⁰, T = 2π√(3.398 × 10²⁰/3.986 × 10¹⁴) = 5,802 s = 96.7 min.)*

2. Look angles and slant range for a Bengaluru station (13.0°N, 77.5°E) to a satellite at 83.0°E.

• cos γ = cos 13.0° × cos 5.5° = 0.97437 × 0.99539 = 0.96988 → γ = 14.1°.
• d = √(6,378² + 42,164² − 2 × 6,378 × 42,164 × 0.96988) = √(1.2968 × 10⁹) = 36,012 km.
• tan El = (0.96988 − 0.15127)/0.24358 = 3.361 → El = 73.4°; azimuth is just east of south (~157°).
• One-way delay = 36,012/299,792 km/s = 120 ms → round trip ≈ 240 ms.

3. Downlink budget, C-band. Satellite EIRP 50 dBW, f = 4 GHz, d = 38,000 km, earth station D = 3 m, η = 0.6, Ts = 150 K, B = 36 MHz.

• λ = 0.075 m → G = 0.6 × (π × 3/0.075)² = 0.6 × 15,791 = 9,475 → 39.8 dB; G/T = 39.8 − 10 log 150 = 39.8 − 21.8 = 18.0 dB/K.
• Lfs = 92.44 + 20 log 38,000 + 20 log 4 = 92.44 + 91.60 + 12.04 = 196.1 dB.
• C/N₀ = 50 − 196.1 + 18.0 + 228.6 = 100.5 dBHz → C/N = 100.5 − 10 log(36 × 10⁶) = 100.5 − 75.6 = 24.9 dB.
• Carrying 45 Mbit/s (QPSK, rate 3/4, 30 Mbaud), Eb/N₀ = 100.5 − 10 log(45 × 10⁶) = 100.5 − 76.5 = 24.0 dB - far above the ~2.3 dB Eb/N₀ (≈4 dB Es/N₀) DVB-S2 QPSK 3/4 threshold, so this link is bandwidth-limited, not power-limited.

4. Uplink, saturation flux density and overall C/N. Earth station 5 m dish, η = 0.6, 100 W HPA, f = 6 GHz, d = 38,000 km; satellite G/T = 0 dB/K.

• G = 0.6 × (π × 5/0.05)² = 0.6 × 98,696 = 59,218 → 47.7 dB; EIRP = 20 + 47.7 = 67.7 dBW.
• Ψ = 67.7 − 10 log(4π × (3.8 × 10⁷)²) = 67.7 − 162.6 = −94.9 dBW/m² - just enough if the transponder SFD is −95 dBW/m².
• Lfs = 92.44 + 91.60 + 15.56 = 199.6 dB → C/N₀ = 67.7 − 199.6 + 0 + 228.6 = 96.7 dBHz → C/N(up) = 21.1 dB.
• With C/N(down) = 25.0 dB and C/N(IM) = 24.0 dB: sum of reciprocals = 0.007763 + 0.003162 + 0.003981 = 0.014906 → ratio 67.1 → C/N(total) = 18.3 dB. Note it is below the worst individual link.

5. Ku-band rain fade. Clear-sky downlink C/N = 16 dB with Ts = 150 K; rain attenuation for 99.9% availability A = 6 dB.

• Signal drops by 6 dB. Added sky noise ΔT = 275 (1 − 10^−0.6) = 275 × 0.749 = 206 K → Ts ≈ 356 K (assuming it couples fully into the antenna).
• Noise rise = 10 log(356/150) = 3.8 dB → total degradation 6 + 3.8 = 9.8 dB → C/N = 6.2 dB. A fixed-rate QPSK 3/4 carrier needing ~5 dB (≈4 dB plus implementation margin) just survives; ACM would instead drop to QPSK 1/2 and keep the link up.

Where it is used

• DTH and broadcasting: Tata Play, Dish TV, Sun Direct and DD Free Dish on Ku-band GSAT/foreign satellites; SNG uplink trucks and teleports for news feeds.
• Enterprise and rural broadband: VSAT networks by Hughes Communications India and Nelco for ATMs, bank branches, petrol pumps, lottery terminals, BharatNet backhaul and e-governance.
• Government, defence and disaster management: NDMA/NDRF portable terminals, GSAT-7/7A military satellites, Coast Guard and railway communication, cyclone warning dissemination through INSAT.
• Mobile and maritime: Inmarsat/Thuraya satellite phones (licensed in India under DoT/GMPCS rules), fishing-vessel transponders, aeronautical connectivity.
• Navigation and timing: NavIC receivers in vehicle trackers and fishermen's devices; GAGAN for aviation approaches; satellite time for power-grid and telecom synchronisation.
• Job roles: RF/link-budget engineer, payload and earth-station engineer, NOC/teleport operations, AOCS and TT&C engineer at ISRO/URSC/SAC, systems roles at NSIL and IN-SPACe-authorised private operators.

Interview questions

• Q: Why must a geostationary satellite be in the equatorial plane? A: Any inclination makes the satellite trace a figure-of-eight in latitude over a day, so it would no longer appear fixed and fixed antennas could not track it.
• Q: Why is the sidereal day used instead of 24 hours? A: The satellite must match the Earth's rotation with respect to the stars, which takes 23 h 56 min 4 s; using 86,400 s gives a radius error of about 80 km.
• Q: Why is the uplink frequency higher than the downlink? A: Higher frequency means more path and rain loss, which is cheaper to overcome with high-power ground amplifiers and large dishes than with limited satellite power.
• Q: What is G/T and why is the LNA mounted at the feed? A: G/T is receive gain minus system noise temperature in dB/K; placing the LNA at the feed avoids lossy cable ahead of it, which would add noise and dominate Ts.
• Q: Why does the overall C/N come out lower than both the uplink and downlink C/N? A: The noise contributions add, so the reciprocals of the ratios add; the total is always worse than the worst individual link.
• Q: What is transponder backoff and why is it needed? A: Operating the TWTA below saturation to keep it in its linear region; without it, multiple carriers generate intermodulation products that fall in-band.
• Q: How is rain fade handled at Ku and Ka band? A: Design margin from ITU-R P.618 statistics, uplink power control, adaptive coding and modulation, site diversity, and accepting a lower availability figure such as 99.5-99.9%.
• Q: Difference between bent-pipe and regenerative payloads? A: A bent pipe only amplifies and translates frequency, so uplink noise is retransmitted; a regenerative payload demodulates and decodes, decoupling the two links and allowing on-board switching.
• Q: Why do LEO constellations need many satellites and inter-satellite links? A: Each satellite is visible for only minutes and covers a small footprint, so continuous service needs overlapping coverage, fast handover and relaying between satellites or many gateways.
• Q: What causes sun outage and when does it occur? A: Near the equinoxes the Sun passes directly behind the GEO satellite as seen by the earth station; the Sun's noise temperature swamps the carrier for a few minutes a day over several days.
• Q: Why does TCP perform poorly over a GEO link, and what is done about it? A: The ~250 ms round trip limits the window-based throughput and slows recovery; TCP acceleration/spoofing proxies, large windows and selective acknowledgements are used.
• Q: What frequencies and coverage does NavIC use? A: L5 at 1176.45 MHz and S band at 2492.028 MHz (L1 on newer satellites), covering India and about 1,500 km beyond its borders from GEO and inclined geosynchronous orbits.

Common mistakes

• Using the 24-hour solar day, or forgetting to add the Earth's radius, when computing the geostationary altitude.
• Mixing dB and ratios - adding reciprocals of C/N values that are still in dB, or forgetting that the 228.6 term is already in dB.
• Taking the slant range as 35,786 km for every station instead of computing it from the look-angle geometry.
• Omitting the +228.6 (or equivalently k) term, or substituting B in MHz instead of Hz in 10 log B.
• Confusing C/N with Eb/N₀: the first uses noise bandwidth, the second uses bit rate, and they differ by the spectral efficiency.
• Ignoring the noise-temperature rise during rain and quoting only the attenuation - the real C/N loss is several dB larger.
• Assuming a satellite "amplifies" at the same frequency, or that a bent-pipe transponder cleans up uplink noise.

Related topics

Noise in communication systems · Sampling and digital modulation · Spread spectrum and CDMA · OFDM and MIMO · Radar systems · Telecommunication networks and switching