Question 1

Why does the uplink in communication satellites normally operate at a higher frequency than the downlink?

Accepted Answer

Introduction / Context:In satellite communication, uplink and downlink frequencies are chosen carefully. Typically, uplinks use higher frequencies (e.g., 6 GHz for C-band uplink vs 4 GHz for downlink) because of differences in attenuation and power availability.Given Data / Assumptions:Uplink power transmitted from Earth station is much higher than satellite power capability.Downlink must reach many users with limited satellite power.Atmospheric attenuation increases with frequency.Concept / Approach:Since Earth stations can generate high-power signals, uplinks can be sent at higher frequencies (with higher attenuation). Satellites, with limited power, use lower frequencies for downlinks to ensure signals reach users reliably.Step-by-Step Solution:Earth station uplink frequency = 6 GHz (high power available).Satellite downlink frequency = 4 GHz (lower attenuation, weaker power source).Thus, uplink is set higher to manage system efficiency.Verification / Alternative check:Practical systems: C-band (uplink 6 GHz, downlink 4 GHz) and Ku-band (uplink 14 GHz, downlink 12 GHz).Why Other Options Are Wrong:Beam-shaping and polarization are secondary benefits, not the primary reason.Narrow beams are due to antenna design, not uplink frequency selection.Antenna size considerations apply, but attenuation is the decisive factor.Common Pitfalls:Confusing uplink frequency selection with antenna design or polarization efficiency.Final Answer:It results in lesser signal attenuation

Question 2

In a stop-and-wait Automatic Repeat reQuest (ARQ) system, how does the transmitting terminal operate after sending a data block?

Accepted Answer

Introduction / Context:Stop-and-Wait ARQ is one of the simplest error-control protocols in data communication. It ensures reliable delivery of data frames by requiring acknowledgements before sending new blocks.Given Data / Assumptions:Sender transmits one block at a time.Receiver responds with ACK (positive) or NAK (negative).Timeout is set if no response is received within a fixed duration.Concept / Approach:The sender must wait for confirmation before transmitting further data. This prevents data loss and ensures order but reduces throughput compared to more advanced ARQ protocols.Step-by-Step Solution:Step 1: Sender transmits one block of data.Step 2: Receiver checks and responds with ACK (if correct) or NAK (if error).Step 3: Sender either proceeds (ACK) or retransmits same block (NAK).Step 4: If no response within timeout, retransmission occurs.Verification / Alternative check:Networking textbooks confirm Stop-and-Wait ARQ as a fundamental example of reliable transmission.Why Other Options Are Wrong:Immediate sending without waiting breaks reliability.Only ACK and NAK responses govern transmission control.Continuous transmission is Go-Back-N or Sliding Window, not Stop-and-Wait.Common Pitfalls:Confusing Stop-and-Wait ARQ with Go-Back-N ARQ or Selective Repeat ARQ.Final Answer:It waits for positive or negative acknowledgement from the receiving terminal

Question 3

Satellite link budgeting: If the downlink frequency is constant and the satellite antenna diameter is reduced by half, by what factor must the RF output power be increased to keep the same EIRP over the larger coverage area?

Accepted Answer

Introduction / Context:In satellite communications, the Effective Isotropic Radiated Power (EIRP) sets the downlink signal strength perceived by Earth stations. EIRP equals the transmitter RF power multiplied by the antenna gain. If the dish size changes while frequency remains the same, antenna gain changes and the RF power must be adjusted to keep EIRP constant.Given Data / Assumptions:Downlink frequency is fixed (thus wavelength is constant).Antenna diameter is reduced by a factor of 2.Antenna efficiency remains approximately the same.We want the same EIRP at the edge of coverage.Concept / Approach:Parabolic dish gain G scales as G ∝ (D/λ)^2 * η where D is diameter, λ is wavelength, and η is efficiency. With constant λ and η, halving D reduces gain by a factor of (1/2)^2 = 1/4. To keep EIRP = P_tx * G constant, transmitter power must increase in the inverse ratio of the gain change.Step-by-Step Solution:Initial gain: G1 ∝ D^2.New diameter: D2 = D1/2 ⇒ G2 ∝ (D1/2)^2 = D1^2/4 = G1/4.To keep EIRP the same: P2 * G2 = P1 * G1 ⇒ P2 = P1 * (G1/G2) = P1 * 4.Verification / Alternative check:If the antenna beamwidth roughly doubles when diameter halves, coverage area increases and peak gain drops accordingly. Operators compensate with higher RF power or accept lower link margins; keeping EIRP constant requires a 4× power increase.Why Other Options Are Wrong:2: Underestimates the quadratic dependence on diameter.8 or 16: Overestimates; diameter-halving leads to 4×, not 8× or 16×.None: The precise factor is 4 under the given assumptions.Common Pitfalls:Assuming gain scales linearly with diameter; it scales with D^2 at constant frequency.Final Answer:4

Question 4

Sun outage for GEO links: When a geostationary satellite passes directly in front of the Sun (as seen from the Earth station), approximately how long and for how many days does the reception outage persist during each equinox season?

Accepted Answer

Introduction / Context:Sun outages (Sun transits) occur near the equinoxes when the Sun aligns with the satellite and Earth station antenna, flooding the receiver with solar noise and causing temporary link loss. Engineers need an order-of-magnitude estimate for planning service advisories.Given Data / Assumptions:Geostationary satellite and parabolic Earth station antenna.Outage occurs around vernal and autumnal equinoxes.Daily events recur for several days, with the worst day near the center of the window.Concept / Approach:The Sun’s apparent path causes a few minutes of alignment each day for roughly 1–2 weeks per season, depending on latitude and antenna beamwidth. Typical outages are several minutes per day, not hours, and persist about ten or so consecutive days per season.Step-by-Step Solution:Outage duration per day is small (a few minutes).Number of affected days per equinox season typically ≈ 7–12, commonly cited ≈ 10.Therefore a practical planning figure is about 5 minutes per day for roughly 10 consecutive days.Verification / Alternative check:Operational notices from satellite operators list Sun outage windows with growing and then shrinking daily durations, peaking near 5–10 minutes depending on station parameters.Why Other Options Are Wrong:10 minutes for 5 days or 30–60 minutes per day: duration too long or window too short.One hour: far exceeds typical solar noise alignment effects.Common Pitfalls:Confusing total equinox season length with the peak daily outage minutes.Final Answer:5 minutes on 10 consecutive days

Question 5

Satellite history: What was the name of the first geostationary communications satellite launched in 1965?

Accepted Answer

Introduction / Context:Recognizing milestone spacecraft helps contextualize the evolution of satellite communications. The first successful commercial geostationary satellite enabled intercontinental telephony and television relays.Given Data / Assumptions:Year: 1965.We are asked for the satellite name of the first geostationary communications satellite.Commercial/global service context (INTELSAT program).Concept / Approach:Early Bird, designated Intelsat I, was launched in 1965 and provided the first commercial GEO communications service across the Atlantic, ushering in the modern era of satellite telephony/TV. Other listed satellites entered service later or were for different regions or orbits.Step-by-Step Solution:Identify the correct program and year (INTELSAT, 1965).Match with the spacecraft name: Early Bird (Intelsat I).Eliminate regional or later programs (e.g., ANIK, WESTAR) and non-GEO orbits (e.g., Molniya highly-elliptical orbits).Verification / Alternative check:Historical records list Early Bird (Intelsat I) as the first commercial GEO satellite, establishing two-way transatlantic services.Why Other Options Are Wrong:ANIK/WESTAR: later national systems (Canada/US).MOLNIYA: Soviet HEO system, not geostationary.SYNCOM-1: an earlier GEO technology demonstrator (1963) but the first operational commercial GEO satellite was Early Bird in 1965; the option list centers on commercial naming.Common Pitfalls:Confusing technology demos with commercial firsts.Final Answer:EARLY BIRD (Intelsat-I)

Question 6

Companding in communication systems: Which statement about a compandor is FALSE?

Accepted Answer

Introduction / Context:Companding (compressing + expanding) is used in analog and digital telephony/audio to improve the perceived signal-to-noise and reduce quantization noise on low-level signals by reshaping dynamic range before and after transmission.Given Data / Assumptions:Compandor performs pre-emphasis (compression) at the transmitter and de-emphasis (expansion) at the receiver.Goal: improve SNR for weak portions of the signal.Quantization noise is most objectionable for low-level signals in uniform quantization.Concept / Approach:Companding law (e.g., μ-law, A-law) increases effective resolution for small amplitudes by mapping them to a larger code range, thus reducing the quantization step relative to the weak signal amplitude and improving the signal-to-quantizing-noise ratio (SQNR) for soft passages.Step-by-Step Solution:Identify compandor action: compress highs, boost lows pre-transmission.At receiver, expand to restore original dynamic range while background/quantization noise is de-emphasized on weak segments.Therefore, weaker signals gain better (not poorer) SQNR.Verification / Alternative check:Telephony standards (μ-law/A-law) show improved effective bits for small amplitudes compared to uniform PCM, confirming better SQNR for weak signals.Why Other Options Are Wrong:(a), (b), (d) correctly describe compression, preferential treatment for weak parts, and increased quantum steps traversed for low-level signals after companding.(c) is false because companding specifically improves—not worsens—the SQNR of weak portions.Common Pitfalls:Assuming companding benefits only strong signals; the key benefit is for low-level content.Final Answer:For weaker signals it gives a poor ratio of signal strength to quantizing error.

Question 7

INSAT-I series outcome: Of the four INSAT-I satellites planned by India, which one was (initially) the successful operational satellite?

Accepted Answer

Introduction / Context:India’s INSAT-I series was an early backbone for telecommunication, meteorology, and broadcasting. Understanding which missions succeeded helps track the program’s maturation.Given Data / Assumptions:Four satellites: INSAT-1A, 1B, 1C, 1D (launched across the 1980s–1990).Question style reflects the period when INSAT-1B emerged as the first successful operational satellite after 1A’s failure.We answer consistent with the classic exam framing.Concept / Approach:INSAT-1A experienced early failure and did not fulfill its operational life. INSAT-1B entered service successfully and became the program’s first sustained success. Later, INSAT-1D was also successful, but the question’s conventional answer points to 1B as the first proven success in the series.Step-by-Step Solution:Recall mission outcomes: 1A failed early; 1B succeeded; 1C partial problems; 1D later success.Identify the historically referenced initial success: INSAT-1B.Select INSAT-IB as the correct option.Verification / Alternative check:ISRO mission summaries document 1B’s long operational service supporting telephony, TV, and weather payloads.Why Other Options Are Wrong:INSAT-1A: did not achieve sustained service.INSAT-1C: issues limited operational effectiveness.INSAT-1D: successful but later; the classic “first successful” identification is 1B.Common Pitfalls:Answering 1D as “the only success”; multiple in the series ultimately worked, but 1B is the early, well-known success.Final Answer:INSAT-IB

Question 8

Attitude behavior terminology: In the context of geosynchronous spacecraft, the term “rotation” most closely refers to which real-world effect?

Accepted Answer

Introduction / Context:Geosynchronous spacecraft must maintain a precise attitude (orientation) for antenna pointing and power generation. Terms like rotation, spin, nutation, drift, and stabilization can be confused in exam questions. This item probes the qualitative meaning of “rotation” effects observed in GEO operations.Given Data / Assumptions:The satellite is geosynchronous (near-geostationary viewpoint).We consider common disturbances and control: station-keeping, attitude control, nutation damping.“Rotation” here is used qualitatively as unintended attitude motion rather than deliberate spin stabilization.Concept / Approach:In 3-axis stabilized GEO satellites, small oscillatory motions (nutation/wobble) can occur about the desired attitude due to environmental torques (solar pressure, gravity-gradient) and control activity. This “wobbling” is a rotational effect about the spacecraft body axes, distinct from orbital “drift” (longitudinal movement) or the control approach (“three-axis stabilization”).Step-by-Step Solution:Differentiate orbital motion (drift) from attitude motion (rotation/wobble).Three-axis stabilization is a control method, not the rotation effect.Thus, “rotation” in this qualitative context aligns best with “wobbling” (nutation/attitude oscillation).Verification / Alternative check:Attitude dynamics texts describe nutation as rotational oscillation, often damped by control algorithms and momentum wheels.Why Other Options Are Wrong:Drift: east–west longitude motion, not rotation about body axes.Three-axis/three-dimensional stabilization: control strategies, not the rotational effect itself.None: a clear rotational phenomenon (wobble) exists.Common Pitfalls:Conflating orbital slot drift with body attitude rotation.Final Answer:Wobbling (nutation/attitude oscillation) about its intended orientation

Question 9

Definition check: Which statement best defines a geostationary satellite from the ground observer’s perspective?

Accepted Answer

Introduction / Context:A geostationary satellite is a special case of a geosynchronous orbit aligned with the equator such that its angular velocity matches Earth’s rotation, making it seem fixed in the sky to ground observers—vital for fixed Earth station pointing.Given Data / Assumptions:Altitude ~35,786 km over the equator (mean).Orbital period matches Earth’s sidereal day (~23 h 56 min).Inclination ≈ 0°, eccentricity ≈ 0.Concept / Approach:The correct defining characteristic for users is the apparent stationarity relative to Earth’s surface. While the satellite is in constant orbital motion, its ground track is a fixed point when orbit is circular, equatorial, and geosynchronous.Step-by-Step Solution:Identify user-centric definition: “appears stationary.”Reject literal phrases implying no motion—satellite is moving fast (~3 km/s).Confirm 24-hour criterion alone is necessary but not sufficient; orbit must also be equatorial and circular.Verification / Alternative check:Any inclination/eccentricity causes apparent north–south or east–west drift (analemma), violating strict “stationary” appearance.Why Other Options Are Wrong:“Hangs motionless” / “stops moving”: physically incorrect—orbital velocity is required.“Travels around Earth in 24 hours”: necessary but not sufficient; must be equatorial and circular.“Remains stationary above the Earth”: ambiguous; better is “appears stationary to observers.”Common Pitfalls:Equating geosynchronous (any inclination) with geostationary (equatorial, circular).Final Answer:It appears stationary to everybody on Earth.

Question 10

Launch site trivia: India’s first domestic geostationary satellite INSAT-1A (10 April 1982) was launched from which country?

Accepted Answer

Introduction / Context:INSAT-1A inaugurated India’s domestic GEO program. Knowing the launch nation/vehicle is a common historical detail in satellite communications coursework and exams.Given Data / Assumptions:Date: 10 April 1982.Spacecraft: INSAT-1A, geostationary communications satellite.Question asks for the launch country.Concept / Approach:INSAT-1A was launched by a U.S. Delta launch vehicle from Cape Canaveral, USA. Although later Indian satellites flew on various foreign and domestic launchers, this first in the INSAT-I series went from the United States.Step-by-Step Solution:Match INSAT-1A with the historical launcher and site.Identify the country: USA.Select option “USA.”Verification / Alternative check:Public ISRO/INTELSAT archives list Delta launch from the U.S. for INSAT-1A.Why Other Options Are Wrong:USSR/UK: not the launch nation for INSAT-1A.UP: not a country; distractor.France: not applicable for this flight.Common Pitfalls:Confusing launch sites of later INSAT or GSAT missions with the first in the series.Final Answer:USA

Question 11

Multiplexing principle: Radio broadcasting with multiple stations in the same area is primarily an example of which multiplexing technique?

Accepted Answer

Introduction / Context:Broadcast radio assigns different stations to different carrier frequencies so that many programs can be transmitted simultaneously without mutual interference, provided receivers are tuned to one carrier at a time.Given Data / Assumptions:Conventional AM/FM analog broadcasting scenario.Single antenna may radiate one station per frequency; many stations share the spectrum.Receivers select one station by tuning to its frequency.Concept / Approach:This is classic Frequency Division Multiplexing (FDM): separate information channels occupy distinct, non-overlapping frequency bands simultaneously. Time or code division are not used in legacy analog broadcast systems.Step-by-Step Solution:Identify how multiple stations coexist: by frequency separation.Map to multiplexing type: frequency multiplexing (FDM).Select the corresponding option.Verification / Alternative check:Broadcast band plans assign channel center frequencies and bandwidths; receivers use selective IF filtering to extract the chosen station.Why Other Options Are Wrong:Time multiplexing: would interleave in time (not typical in analog broadcasting).Space multiplexing: relies on spatial separation/antennas; not the core method here.Code multiplexing: associated with CDMA-type systems, not analog broadcast radio.Common Pitfalls:Confusing modern digital broadcast multiplexes (e.g., OFDM subcarriers) with analog radio channelization; the principle remains frequency separation.Final Answer:Frequency multiplexing

Question 12

Cost metric (historical context): The approximate total cost per watt for generating power on a geostationary satellite is closest to which value (in ₹/W)?

Accepted Answer

Introduction / Context:The effective “cost per watt” in GEO accounts for the mass, design, launch, and qualification overhead to generate and deliver usable electrical power via solar arrays and batteries at geostationary orbit. It is a useful high-level metric in older exam literature to compare platform costs.Given Data / Assumptions:Historical textbook/competitive exam context (not a current market quote).Includes spacecraft power subsystem and launch-to-orbit cost basis.Typical multi-kilowatt GEO buses of the era referenced by the question.Concept / Approach:Older cost-per-watt rules of thumb placed GEO bus power around the order of ₹100 per watt (exam-era values), reflecting expensive, ruggedized solar arrays, batteries sized for eclipse seasons, and high launch costs per kilogram. Modern values vary widely by program and currency but the exam key aligns with the ₹100 figure.Step-by-Step Solution:Recognize the question as a standard historical benchmark, not a current procurement quote.Select ₹100 as the closest canonical value used in many exam references.Reject unrealistically low options for GEO-era costs.Verification / Alternative check:Traditional satellite engineering texts present order-of-magnitude costs consistent with the ₹100/W figure for GEO-era buses considered in exam syllabi.Why Other Options Are Wrong:₹5–₹50: far below typical GEO historical cost references.₹20: still too low given array/battery mass and launch costs.₹200: higher than the classic benchmark indicated by such questions.Common Pitfalls:Applying present-day LEO/CubeSat economics to legacy GEO benchmarks.Final Answer:₹100

Question 13

Why are very-low Earth orbit (LEO) satellites generally not used alone for fixed satellite communications to a single ground user?

Accepted Answer

Introduction / Context:Satellite communication architectures vary by orbit: GEO, MEO, and LEO. Coverage and contact time to a fixed user depend on orbital altitude and constellation size. This question targets why a single LEO cannot serve as a continuous link to one location on Earth.Given Data / Assumptions:LEO altitude typically 500–2000 km.Orbital period is about 90–120 minutes.Ground station remains fixed; satellite moves rapidly across the sky.Concept / Approach:Due to their short orbital periods and low altitude, LEO satellites provide only brief visibility windows (a few minutes) to any fixed point per pass. Continuous service requires a constellation and seamless satellite handovers. In contrast, a geostationary satellite near 35,786 km altitude remains fixed in the sky relative to the observer, enabling continuous coverage within its footprint.Step-by-Step Solution:Relate altitude to orbital period: lower altitude → faster orbital motion.Visibility window to fixed user is limited.Therefore, a single LEO cannot provide 24/7 contact to a single ground user.Verification / Alternative check:LEO constellations (e.g., Iridium, Starlink) employ many satellites to ensure continuous coverage through overlapping footprints and handovers, confirming the limitation of single-satellite LEO links.Why Other Options Are Wrong:Sonic booms and melting are irrelevant.Propagation through the atmosphere is generally reliable at designed frequencies.Ionospheric effects exist but do not categorically prohibit LEO use.Common Pitfalls:Confusing LEO advantages (low latency, lower path loss) with continuous-coverage capability from a single satellite.Final Answer:Because a single LEO does not provide 24-hour/day continuous contact to a fixed user

Question 14

Indian space history: INSAT-IB replacing INSAT-IA The communication satellite INSAT-IB had to take over the role of INSAT-IA because INSAT-IA failed within how many months of its launch?

Accepted Answer

Introduction / Context:India’s early INSAT programme faced teething problems. Understanding the timeline of INSAT-IA's failure and the subsequent operational takeover by INSAT-IB is a common general-awareness topic in satellite communications and Indian space history.Given Data / Assumptions:INSAT-IA was India’s first operational multipurpose communication satellite of the INSAT-1 series.INSAT-IB was the immediate successor in the same series and assumed services after INSAT-IA failed.The question asks the number of months after launch when INSAT-IA effectively collapsed in service.Concept / Approach:We recall the chronological facts from standard references on the INSAT programme. INSAT-IA was launched in 1982 and suffered power bus and attitude control problems, leading to loss of service a few months later. INSAT-IB was launched in 1983 to restore and expand services—telecommunications, TV broadcasting, and meteorology.Step-by-Step Solution:Identify INSAT-IA launch and early failure window in 1982.Determine approximate operational lifetime before failure.Corroborate that the failure occurred roughly five months after launch, necessitating INSAT-IB's takeover of services.Verification / Alternative check:Multiple historical summaries note that INSAT-IA became inoperative within about five months, prompting urgent deployment and utilization of INSAT-IB to maintain national services.Why Other Options Are Wrong:12 months (a) and 20 months (b) are too long; 36 months (d) is far beyond the known early failure; 9 months (e) also exceeds the documented time frame.Common Pitfalls:Learners may confuse INSAT-IA's short life with later satellites that performed nominally for several years, or they may mix up dates from different INSAT-1 satellites.Final Answer:5

Question 15

India’s first three-axis stabilised geostationary communication satellite Identify India’s first three-axis stabilised (3-axis controlled) geostationary communication satellite used for experimental telecom/TV relay.

Accepted Answer

Introduction / Context:Satellite attitude control is critical for pointing antennas and solar arrays. Early Indian spacecraft evolved from spin-stabilised to three-axis stabilised platforms. Knowing which satellite first achieved 3-axis stabilisation in geostationary orbit is important in India’s spaceflight history.Given Data / Assumptions:The candidates include early Indian satellites: Aryabhata, Rohini, Bhaskara and the experimental geostationary payload APPLE.Three-axis stabilisation is required for precise payload pointing and efficient power generation.The query focuses on the first Indian example at geostationary altitude.Concept / Approach:APPLE (Ariane Passenger Payload Experiment), launched in 1981, was India’s first experimental geostationary communication satellite. It used a 3-axis stabilisation scheme and enabled India’s initial experiments in TV/telecom relay from geostationary orbit, paving the way for the INSAT series.Step-by-Step Solution:Distinguish LEO experimental/science sats (Aryabhata, Rohini, Bhaskara) from GEO communication missions.Note APPLE’s mission profile: experimental telecom relay from GEO with 3-axis control.Conclude APPLE is the first three-axis stabilised Indian GEO communication satellite.Verification / Alternative check:Standard ISRO retrospectives describe APPLE as India’s first 3-axis stabilised GEO experimental communication satellite used for TV/telecom link trials.Why Other Options Are Wrong:Rohini/Aryabhata/Bhaskara were not 3-axis geostationary communication spacecraft; INSAT-1D is later and not the first.Common Pitfalls:Confusing first GEO satellite experiments (APPLE) with first operational INSAT satellites; mixing LEO science missions with GEO comsats.Final Answer:APPLE

Question 16

Mainstay of India’s satellite network after INSAT-1B After the end of service of INSAT-1B (INSAT-IB), which programme was expected to become the mainstay of India’s satellite communication network for the coming years?

Accepted Answer

Introduction / Context:India’s national INSAT programme evolved across generations. After INSAT-1A and INSAT-1B, the roadmap pointed to indigenous follow-on satellites with improved payloads, capacity, and reliability to support telecom, TV, and meteorology services nationwide.Given Data / Assumptions:INSAT-1B provided critical services for years.Next-generation plans aimed at self-reliance with higher capability platforms.International fleets (INTELSAT/ARABSAT) existed but were not intended to be India’s primary mainstay.Concept / Approach:The INSAT-2 series (e.g., INSAT-2A/2B/2C/2DT/2E) represented India’s indigenous second generation. It expanded transponder capacity and diversified services. Therefore, post-INSAT-1B, the strategic mainstay for continuity and growth was the INSAT-2 series rather than depending on external regional/global systems.Step-by-Step Solution:Identify programme succession: INSAT-1 → INSAT-2 series.Assess capacity and mission objectives aligned with national requirements.Conclude INSAT-2 series as the intended mainstay after INSAT-1B’s service.Verification / Alternative check:Historical mission summaries show the transition to INSAT-2 class as India scaled up services and autonomy.Why Other Options Are Wrong:INTELSAT-V and ARABSAT are international systems; INSAT-1D is within the first-generation class and not the long-term mainstay; GSAT-4 only is too specific and not representative of the broader planned backbone.Common Pitfalls:Confusing temporary leasing or cooperative arrangements with the strategic national mainstay.Final Answer:INSAT-2 series

Question 17

Earth station uplink capacity factors The traffic-handling capacity (on the uplink) of an Earth station depends primarily on which of the following technical factors?

Accepted Answer

Introduction / Context:Earth station uplink capacity (e.g., number of carriers, data throughput) is limited by the carrier-to-noise ratio at the satellite transponder input. This, in turn, depends on the station’s transmitted signal strength, the satellite receive antenna characteristics, and the overall noise performance of the space segment.Given Data / Assumptions:Fixed satellite service with bent-pipe transponders.Uplink budget includes Earth station EIRP, free-space loss, atmospheric loss, satellite receive G/T, and transponder constraints.Traffic capacity correlates with achievable C/N and intermodulation limits.Concept / Approach:Higher EIRP improves the received carrier level at the satellite. Greater satellite receive gain increases the effective signal at the low-noise amplifier input. Lower satellite noise temperature (higher G/T) improves the C/N for the same received carrier. All these factors influence how many carriers or how high a data rate can be supported within required link margins.Step-by-Step Solution:Model uplink: C/N at satellite input is proportional to received carrier power and inversely to system noise power.Received carrier ∝ Earth station EIRP * satellite receive gain / path loss.System noise set by satellite front-end and antenna temperature affects denominator.Verification / Alternative check:Link budget calculations show sensitivity of C/N to EIRP, satellite G/T, and path/noise parameters. Operators raise EIRP, use high-gain antennas, and choose satellites with better G/T to expand capacity.Why Other Options Are Wrong:Choosing only one factor ignores the multiplicative/ratio nature of link budgets; all listed factors jointly affect uplink capacity.Common Pitfalls:Overlooking intermodulation and back-off; ignoring weather and pointing losses which further affect capacity.Final Answer:All of the above

Question 18

Global coverage with geosynchronous communication satellites How many geosynchronous communication satellites are typically required to provide near-global coverage of all inhabited regions of Earth (excluding high polar latitudes)?

Accepted Answer

Introduction / Context:Geostationary satellites at approximately 35,786 km altitude provide wide Earth coverage from the equatorial plane. A classic communications engineering result is that only a small constellation is needed for near-global service (excluding extreme polar regions, which are poorly viewed from GEO).Given Data / Assumptions:Satellites are positioned near longitudes about 120° apart.Coverage excludes high-latitude polar areas where elevation angles are very low.Goal is continuous coverage of inhabited regions.Concept / Approach:The visible Earth disk from GEO spans roughly one-third of the globe per satellite, depending on minimum elevation constraints. With three satellites spaced about 120° in longitude, overlapping coverage footprints can encompass essentially all inhabited areas.Step-by-Step Solution:Divide 360° Earth longitude into three sectors of roughly 120° each.Place satellites over the equator at these longitudes.Check elevation limits for high latitudes; inhabited regions are largely served.Verification / Alternative check:Many global operators deploy triads of GEO satellites to support TV, telephony, and data services worldwide, adding more spacecraft for capacity rather than fundamental coverage.Why Other Options Are Wrong:Two satellites leave major gaps; five or ten exceed the minimum needed for coverage (though may be used for redundancy/capacity).Common Pitfalls:Assuming perfect polar coverage is possible from GEO; polar regions typically require other orbits (e.g., Molniya, polar LEOs).Final Answer:3

Question 19

Definition of a modem in communications systems In practical data communication, what is a modem best described as?

Accepted Answer

Introduction / Context:The word modem is a portmanteau of MOdulator-DEModulator. It is foundational in telecommunications, enabling digital data to be transported over channels originally designed for analog waveforms, such as telephone lines or radio carriers.Given Data / Assumptions:Digital data must traverse band-limited or analog channels.Carrier waveforms are used to convey information efficiently and robustly.Receivers must recover baseband data via demodulation.Concept / Approach:A modem converts digital symbols to an analog passband signal by modulation (e.g., ASK, FSK, PSK, QAM) and then reconstructs the symbols at the receiver via demodulation, often with synchronization, equalization, and error correction support.Step-by-Step Solution:Identify that modulation maps bits/symbols onto amplitude/phase/frequency of a carrier.Recognize demodulation inverts this mapping to retrieve data.Conclude that a modem is the unit that performs both roles.Verification / Alternative check:From voiceband dial-up to cable/DSL/optical and wireless air-interfaces, the modem block always encapsulates modulation/demodulation functions.Why Other Options Are Wrong:(a) pertains to electromechanical devices; (b) digitizing speech is part of vocoders/codecs, not modems per se; (c) interference suppression is a separate function; (e) echo cancellers are distinct subsystems.Common Pitfalls:Confusing codecs with modems; assuming modulation alone defines a modem without the demodulator counterpart.Final Answer:An electronic unit that performs modulation and demodulation of a carrier

Question 20

Orbital period of geosynchronous “system” satellites Geosynchronous communication satellites that appear fixed over one longitude complete one orbit around Earth in approximately how many hours?

Accepted Answer

Introduction / Context:Geosynchronous satellites match Earth’s sidereal rotation rate so that, viewed from the ground, they revisit the same position over the equator each day (geostationary if zero inclination and eccentricity). This principle is central to fixed satellite services and broadcast distribution.Given Data / Assumptions:Altitude ~35,786 km above Earth’s equator.Negligible eccentricity and inclination for geostationary case.Sidereal day period ~23 h 56 min (~24 h commonly stated).Concept / Approach:Kepler’s third law sets orbital period versus semi-major axis. At GEO altitude, the orbital period equals Earth’s rotational period, hence about 24 hours. This synchrony keeps the satellite’s sub-satellite point nearly constant in longitude.Step-by-Step Solution:Choose orbital radius that satisfies T ≈ 24 h.Satellite rotates once per sidereal day, staying over the same meridian.Ground antennas can be fixed to a single look angle.Verification / Alternative check:Any deviation from 24 h produces an apparent drift in the sky; station-keeping maintains the near-constant position.Why Other Options Are Wrong:12 h corresponds to MEO like GPS; 6 h/1 h are LEO/MEO-like times, not GEO.Common Pitfalls:Mixing solar day (24 h) with sidereal day (≈23 h 56 min); for general purposes, 24 h is acceptable.Final Answer:24 hours

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