SATELLITE COMMUNICATION (KEC062)
Section-wise Detailed Answers – B.Tech Semester VI
SECTION A
(Attempt all questions – brief but conceptually complete explanations)
Q1(a) Sun-synchronous orbits
A Sun-synchronous orbit is a near-polar orbit in which the satellite passes over any given point on the Earth’s surface at the same local solar time on every pass. This is achieved by carefully selecting the orbital inclination so that the orbital plane precesses at the same rate as the Earth revolves around the Sun. Sun-synchronous orbits are mainly used for Earth observation and remote sensing satellites because they provide consistent lighting conditions for imaging.
Q1(b) Difference between geosynchronous and geostationary orbit
A geosynchronous orbit is an orbit in which the satellite’s orbital period is equal to the Earth’s rotational period of 24 hours. However, the satellite may appear to move in a figure-eight pattern in the sky. A geostationary orbit is a special type of geosynchronous orbit in which the satellite is placed in the equatorial plane with zero inclination and circular orbit. In this case, the satellite appears stationary with respect to the Earth.
Q1(c) Justification for launch sites near the equator
Most satellite launching sites are located close to the equator because the Earth’s rotational velocity is maximum at the equator. Launching satellites eastward from the equator provides an additional velocity boost due to Earth’s rotation, reducing fuel consumption and increasing payload capacity. This makes launches more economical and efficient.
Q1(d) Sun Transit Outage
Sun Transit Outage occurs when the Sun comes directly behind a satellite relative to an Earth station antenna. During this alignment, the Sun’s strong microwave radiation interferes with the satellite signal, causing temporary signal degradation or complete loss. This phenomenon occurs around equinoxes and lasts for a few minutes each day over several days.
Q1(e) Factors responsible for orbital effects
Orbital effects arise due to Earth’s oblateness, gravitational perturbations from the Sun and Moon, atmospheric drag, and solar radiation pressure. These factors cause variations in orbital parameters such as inclination, eccentricity, and right ascension of ascending node, affecting satellite position and stability.
Q1(f) Significance of Telemetry, Tracking, and Command (TT&C) system
The TT&C system is essential for monitoring and controlling satellite health and operation. Telemetry provides data about satellite status, tracking determines satellite position and velocity, and command allows ground stations to control satellite functions. Together, they ensure safe and reliable satellite operation throughout its life.
Q1(g) Blocks of a GPS receiver
A GPS receiver consists of an antenna, RF front end, signal processor, correlators, microprocessor, clock, and display unit. These blocks work together to receive satellite signals, decode navigation messages, calculate position, velocity, and time information.
Q1(h) Need of P and C/A codes in GPS
The Coarse/Acquisition (C/A) code is used for civilian navigation and allows quick satellite acquisition. The Precision (P) code provides higher accuracy and is mainly used for military applications. These codes enable signal identification, synchronization, and resistance to interference.
Q1(i) Features of cryogenic engines in GSLV
Cryogenic engines use liquid hydrogen as fuel and liquid oxygen as oxidizer. They offer high specific impulse, improved efficiency, and higher payload capability. In GSLV, cryogenic engines enable launching heavier satellites into geosynchronous transfer orbit.
Q1(j) Launch vehicles developed by ISRO
ISRO has developed launch vehicles such as SLV, ASLV, PSLV, GSLV, and GSLV Mk-III. These vehicles support missions ranging from low Earth orbit satellites to heavy payloads for geostationary or interplanetary missions.
SECTION B
(Attempt any three – descriptive answers)
Q2(a) History and achievements of satellite communication systems
Satellite communication began with the launch of Sputnik in 1957, followed by early communication satellites like Echo and Telstar. Over time, geostationary satellites revolutionized global communication by enabling television broadcasting, telephony, and data services. Modern satellite systems support broadband internet, navigation, weather forecasting, and disaster management. These achievements have transformed global connectivity.
Q2(b) Orbital elements defining a satellite’s orbit
A satellite’s orbit is defined by six orbital elements: semi-major axis, eccentricity, inclination, right ascension of ascending node, argument of perigee, and true anomaly. These elements describe the size, shape, orientation, and position of the orbit in space and are essential for precise orbit determination.
Q2(c) Power received by receiver and related terms
The power received by a satellite receiver depends on transmitted power, antenna gains, wavelength, and distance. It can be expressed using the effective aperture area of the receiving antenna. Power flux density represents power per unit area at the receiver location, while Effective Isotropic Radiated Power (EIRP) is the product of transmitter power and antenna gain, indicating the effective radiated signal strength.
Q2(d) Implementation and significance of VSAT systems
Very Small Aperture Terminal (VSAT) systems use small earth stations for two-way communication via satellites. They are widely used in banking, education, remote offices, and rural connectivity. VSAT systems provide reliable communication where terrestrial infrastructure is unavailable.
Q2(e) Mechanism of satellite launching and west-to-east launch
Satellite launching involves placing the satellite into a transfer orbit using multi-stage rockets, followed by orbit injection. Launching from west to east utilizes Earth’s rotation to provide additional velocity, improving efficiency and reducing fuel requirements.
SECTION C
Q3(a) Types of satellite orbits and their advantages
Satellites operate in Low Earth Orbit, Medium Earth Orbit, Geosynchronous Orbit, and Highly Elliptical Orbit. Low Earth Orbits provide low latency and high resolution, Medium Earth Orbits are used for navigation systems like GPS, and Geostationary Orbits enable continuous coverage of specific regions. Highly elliptical orbits are useful for high-latitude coverage.
Q3(b) Types of satellites and applications
Satellites are classified as communication, navigation, weather, remote sensing, scientific, and military satellites. Communication satellites support broadcasting and data services, navigation satellites enable positioning, weather satellites assist forecasting, and remote sensing satellites support resource monitoring and environmental studies.
Q4(a) Kepler’s second law and orbital calculations
Kepler’s second law states that a satellite sweeps equal areas in equal times, implying variable velocity in elliptical orbits. The satellite moves faster at perigee and slower at apogee. Using given perigee and apogee distances, velocity at these points and time period can be calculated using orbital mechanics equations and Earth’s radius.
Q4(b) Significance of look angles
Look angles define the orientation of an earth station antenna toward a satellite. Elevation angle determines vertical pointing, while azimuth angle determines horizontal direction. Accurate calculation ensures proper antenna alignment and reliable communication with geostationary satellites.
Q5(a) Segments of a satellite system
A satellite system consists of space segment, ground segment, and control segment. The space segment includes satellites, the ground segment includes earth stations and user terminals, and the control segment manages satellite operations. Together, these segments enable end-to-end communication.
Q5(b) Significance of C/N and G/T ratios
Carrier-to-Noise ratio indicates signal quality, while G/T ratio represents receiver sensitivity. High C/N ensures reliable communication, and high G/T improves reception of weak signals. These parameters are crucial for satellite link design.
Q6(a) Position location principle of GPS
GPS determines position using trilateration based on distance measurements from multiple satellites. By measuring signal travel time and applying geometric equations, the receiver computes latitude, longitude, altitude, and time. GPS codes such as C/A and P codes enable accurate ranging and synchronization.
Q6(b) Working of Direct Broadcast Satellite (DBS) system
DBS systems transmit television signals directly from satellites to home receivers. Components include satellite transponders, uplink stations, dish antennas, low-noise block converters, and set-top boxes. DBS provides high-quality television services over wide areas.
Q7(a) History of Indian satellite systems and ISRO
India’s space program began with the establishment of ISRO in 1969. Satellites such as INSAT and GSAT revolutionized communication, broadcasting, and meteorology. ISRO’s achievements include interplanetary missions, navigation systems, and indigenous launch vehicles.
Q7(b) Generations of Indian launch vehicles and GSLV Mk-III
India developed SLV, ASLV, PSLV, and GSLV in successive generations. GSLV Mk-III is a heavy-lift vehicle capable of launching large satellites and crewed missions. It uses solid, liquid, and cryogenic stages, making it a key vehicle for future missions.
