Satellite Communication¶

Exam-ready answer

(a) Satellite communication [2 marks]¶

Satellite communication uses an artificial earth satellite as a microwave relay between distant earth stations. The transmitting earth station sends an uplink; the satellite transponder receives, filters, amplifies and frequency-converts it; and the receiving station obtains the retransmitted downlink.

(b) Kepler's laws of orbital motion [3 marks]¶

1. Law of orbits: a satellite follows an elliptical orbit with the earth at one focus.
2. Law of areas: the earth-satellite radius vector sweeps equal areas in equal times; therefore the satellite moves faster near perigee and slower near apogee.
3. Law of periods: the square of orbital period is proportional to the cube of semi-major axis:

where \(T\) is orbital period, \(a\) is semi-major axis, and \(\mu=GM_E\) is earth's gravitational parameter.

(c) Most important atmospheric loss above 10 GHz [5 marks]¶

For an ordinary rainy earth-space path above about 10 GHz, rain attenuation is usually the most important variable atmospheric impairment. Raindrops absorb part of the wave energy and scatter part away from the receiving antenna. Because drop dimensions become significant relative to wavelength, attenuation generally rises with frequency, rain rate, effective rain-path length and polarization mismatch.

The specific attenuation is commonly represented by

where \(R\) is rain rate and \(k,\alpha\) depend on frequency and polarization. Rain fade reduces received \(C/N\), increases BER and may cause outage, particularly in Ku- and Ka-band links.

Mitigation methods are:

• adequate fade margin;
• uplink power control;
• adaptive coding and modulation;
• site diversity or a lower-frequency fallback; and
• greater receive antenna gain where practical.

Qualification: oxygen and water-vapour absorption must also be included in a link budget and become especially important near resonance bands such as about 22 GHz and 60 GHz. Thus rain is the expected dominant weather-dependent loss, not an exceptionless statement for every frequency and climate.

Practice target: 16–18 minutes; divide answer space approximately in the ratio 2:3:5.

Exam-ready answer

A satellite orbit is its gravitational free-fall path around earth. Altitude and orbital shape set period, velocity, footprint, propagation delay, free-space loss and ground-terminal tracking. By altitude/geometry, communication orbits are low earth orbit (LEO), medium earth orbit (MEO), geostationary earth orbit (GEO) and highly elliptical orbit (HEO).

HEO has low perigee and high apogee. A satellite dwells near apogee, so inclined HEO constellations can serve high latitudes that GEO covers poorly, at the cost of changing range, Doppler and tracking.

Kepler's laws govern every class: an orbit is an ellipse with earth at one focus, equal areas are swept in equal times, and

where \(T\) is in s, semi-major axis \(a\) in m and earth parameter \(\mu\approx3.986\times10^{14}\,\text{m}^3/\text{s}^2\).

A satellite is geostationary only if its orbit is circular, in the equatorial plane, prograde (same direction as earth rotation), and has \(T=86164\,\text{s}\), one sidereal day. Hence

For an overhead relay, minimum ground-to-ground one-way propagation time is roughly \(2h/c\): about \(6.7\,\text{ms}\) through a \(1000\,\text{km}\) LEO versus \(239\,\text{ms}\) through GEO, before routing/processing and with real slant paths often longer. Free-space loss also grows as \(20\log_{10}d\). Thus LEO gives low latency and small terminals but demands a large managed constellation, Doppler tracking and inter-satellite/user handovers. GEO gives fixed pointing and continuous broad coverage but high delay/loss, poor polar elevation, expensive launch/station keeping and strong Ku/Ka rain-fade exposure. MEO is a compromise, especially for navigation.

Practice target: 18 minutes; draw both orbit figures, derive GEO radius from the sidereal period and reproduce the six-row comparison.

Exam-ready answer

A GEO link has two physical sections: earth-station modem/upconverter/HPA/dish → uplink → satellite receive antenna, LNA, filter/frequency converter and HPA → downlink → receiving dish/LNA/downconverter/modem. Each section must deliver the required carrier-to-noise density ratio with fade and implementation margin.

The main design factors are:

1. transmitter power, feeder loss and antenna gain, combined as \(EIRP=P_t+G_t-L_t\) in dBW;
2. slant range and frequency, giving \(L_{fs}=32.44+20\log_{10}d_{km}+20\log_{10}f_{MHz}\) dB;
3. atmospheric gas, cloud and especially rain attenuation above about \(10\,\text{GHz}\), plus polarization and pointing losses;
4. receive antenna gain and system noise temperature, summarized by \(G/T\) in dB/K;
5. allocated bandwidth, modulation/FEC, bit rate and required \(E_b/N_0\) or BER;
6. satellite transponder bandwidth, gain, EIRP, intermodulation and input/output back-off; and
7. availability target, interference coordination, hardware loss, aging and link margin.

For either section,

then \(C/N=C/N_0-10\log_{10}B\) for receiver bandwidth \(B\) in Hz. At a representative \(38000\,\text{km}\) slant range, FSPL is about \(207.0\,\text{dB}\) at \(14\,\text{GHz}\) and \(205.6\,\text{dB}\) at \(12\,\text{GHz}\), before atmospheric loss.

Many systems place the higher frequency on the uplink because a ground station can use a larger dish and HPA to overcome the extra loss, while the power-limited satellite uses the lower-loss downlink; frequency separation also permits simultaneous relay. This is a common allocation practice, not a universal physical law. Ku/Ka designs need rain statistics, fade margin, uplink power control or adaptive coding, while C-band is less rain-sensitive but uses larger antennas. The final design must satisfy both uplink and downlink availability without saturating the transponder.

Practice target: 9 minutes; draw the two-section chain, write EIRP and C/N0 equations and state at least six design factors including qualified rain loss.

Exam-ready answer

Stabilization maintains spacecraft attitude so communication antennas point toward the footprint and solar arrays/sensors retain their required orientation. Station keeping is different: it corrects orbital longitude, inclination and eccentricity against earth oblateness, lunar/solar gravity and solar-radiation pressure. Tracking estimates satellite direction and drives a ground antenna, while an earth station originates/terminates traffic and controls the RF link.

Spin stabilization rotates the spacecraft body, using angular momentum for passive attitude stiffness; a bearing-driven despun platform keeps the communications antenna earth-pointing. It is mechanically simple but constrains payload/large-array arrangement. Three-axis stabilization uses sun/earth/star sensors and gyros, an attitude controller, and reaction wheels for roll, pitch and yaw. Thrusters provide acquisition and unload wheel momentum. It gives accurate flexible pointing but needs more sensors, actuators and control logic. North-south and east-west thruster burns keep a GEO satellite inside its assigned orbital box; propellant commonly limits service life.

Ground tracking may be program track from predicted ephemeris, manual/step track from measured beacon level, or closed-loop automatic/monopulse tracking from angular error. The servo converts beacon error into azimuth and elevation motor commands and uses shaft encoders for feedback. A fixed GEO terminal needs little continuous motion after alignment, but narrow-beam large dishes still need correction; LEO/MEO stations must follow rapidly changing azimuth/elevation and compensate Doppler.

The earth-station transmit chain is user data/codec → FEC and modulator at IF → upconverter → high-power amplifier (TWTA/SSPA) → feeder/duplexer → parabolic dish. The satellite transponder receives, filters, amplifies and frequency-translates it. The receive chain is dish/feed → low-noise amplifier or LNB placed near the feed → downconverter/IF filtering → demodulator, FEC decoder and user output. Frequency/time references, power supply, redundancy, alarms and monitor/control support both chains; a VSAT packages these functions into an indoor modem and small outdoor BUC/LNB/dish.

Dish gain and receiver figure of merit are

For \(D=3\,\text{m}\), \(f=12\,\text{GHz}\) so \(\lambda=0.025\,\text{m}\), and \(\eta=0.6\), \(G\approx8.53\times10^4=49.3\,\text{dBi}\). If system noise temperature is \(150\,\text{K}\), \(G/T\approx49.3-21.8=27.5\,\text{dB/K}\). Increasing diameter raises gain but narrows beamwidth, making pointing error more critical; placing the LNA before lossy cable protects low noise temperature.

These systems support broadcast, gateway, telemetry/control, remote enterprise and disaster links. Limitations are GEO delay, rain/gaseous loss, blockage, HPA nonlinearity, expensive precision antennas and failure from poor tracking or exhausted station-keeping fuel. Stable spacecraft attitude, accurate ground pointing and a high-EIRP/high-G/T earth station must therefore be designed as one link.

Practice target: 18–20 minutes; divide time among stabilization/station keeping, tracking loop and complete earth-station chains, then finish with the gain and G/T calculation.