(SEM VIII) THEORY EXAMINATION 2017-18 SATELLITE AND RADAR SYSTEMS

SECTION A (Brief Explanations)

False Alarm
A false alarm occurs when a radar receiver indicates a target even though no actual target is present. It is usually caused by noise or interference exceeding the detection threshold.

Average Power Calculation
Average power = Peak power × Duty cycle.
Duty cycle = Pulse width / Pulse repetition time.
Here pulse width = 2 µs and rest time ≈ 2000 µs, so duty cycle ≈ 2/2000 = 0.001.
Average power = 200 kW × 0.001 = 200 W.

Blind Speed
Blind speed is the target speed at which the Doppler frequency shift becomes zero or undetectable in MTI radar, making the target invisible to the radar.

Limitations of MTI Performance
MTI radar performance is limited by blind speeds, clutter reflections, system instability, and phase noise. These reduce detection accuracy.

Low Angle Tracking
Low angle tracking refers to radar tracking of targets close to the ground. It is difficult due to ground reflections and multipath interference.

Downlink Frequency Lower than Uplink
Downlink frequency is kept lower than uplink frequency because lower frequencies experience less atmospheric attenuation and rain fading, improving signal reliability.

Telemetry
Telemetry is the process of collecting data from remote locations and transmitting it to ground stations for monitoring and analysis, commonly used in satellites.

Kepler’s Law
Kepler’s laws describe planetary motion. The first law states that planets move in elliptical orbits with the sun at one focus. These laws are used in satellite orbit calculations.

Timing Accuracy
Timing accuracy refers to precision in measuring time intervals. In GPS and radar systems, accurate timing ensures precise distance and position measurement.

GPS Signal Levels
GPS signals reaching Earth are very weak, typically around –130 dBm. Sensitive receivers are required to detect and process these signals.

SECTION B (Medium Length Answers)

Radar Range Equation
The radar range equation relates transmitted power, antenna gain, wavelength, radar cross-section, and received power to determine maximum detection range. It shows that range is proportional to the fourth root of transmitted power and antenna gain. Higher gain and transmitted power increase detection range.

MTI Radar and Comparison with Pulse Doppler
MTI (Moving Target Indication) radar detects moving targets by using Doppler shift to distinguish them from stationary objects. It uses delay line cancellers to suppress clutter. Pulse Doppler radar also uses Doppler frequency but provides better velocity measurement and avoids blind speeds. Pulse Doppler radar is more advanced than MTI.

Tracking with Radar and Monopulse Tracking
Tracking radar continuously determines position of a target. Monopulse tracking uses simultaneous comparison of signals from multiple antenna beams to determine target direction accurately in a single pulse. It provides better accuracy than conical scanning.

General Link Equation (Satellite)
The satellite link equation relates transmitted power, antenna gains, free-space loss, and noise to received carrier-to-noise ratio (C/N).
C/N = (Received Carrier Power) / (Noise Power).
G/T ratio represents antenna gain to system noise temperature ratio and indicates receiver performance.

GPS Working Principle
GPS determines position by measuring time delay of signals from satellites. At least four satellites are needed to calculate three-dimensional position and correct receiver clock error. It uses trilateration technique.

SECTION C (10-Mark Style Answers)

Duplexer in Radar
A duplexer allows a single antenna to be used for both transmission and reception. During transmission, it protects the receiver from high power pulses. During reception, it allows weak echo signals to reach the receiver. The main limitation is switching time and power handling capability.

Radar Transceiver System
A radar transceiver includes transmitter, modulator, duplexer, antenna, receiver, and display unit. The transmitter generates pulses, the antenna radiates them, echoes are received, amplified, processed, and displayed on a screen.

Delay Line Canceller and Staggered PRF
Delay line canceller removes stationary clutter by subtracting consecutive pulses. Staggered Pulse Repetition Frequency reduces blind speed problem by varying pulse repetition interval.

CW Radar with Zero IF
Continuous Wave radar transmits continuous signal and detects Doppler shift for velocity measurement. Zero intermediate frequency simplifies receiver design. Drawbacks like inability to measure range are overcome by frequency modulation.

Geostationary Orbit
A geostationary orbit is a circular orbit at 35,786 km above Earth where satellite rotates with Earth’s rotation. It appears stationary to ground observers. Geosynchronous orbit has same period but may not remain fixed over one point.

Satellite Subsystems
Satellite consists of power subsystem (solar panels), communication payload, attitude control system, telemetry and tracking system, and propulsion system. Each subsystem ensures proper functioning and control.

GPS Segments
GPS has three segments: space segment (satellites), control segment (ground stations), and user segment (receivers). Signal acquisition refers to detecting and locking onto satellite signals for position calculation.

DBS Television Network
Direct Broadcast Satellite system transmits TV signals directly to users via satellite. It includes uplink station, satellite transponder, and home dish antenna. It provides wide coverage and high-quality transmission.