RADAR is an electromagnetic system used to detect the presence, location, velocity, and tracking profile of distant objects—such as aircraft, ships, spacecraft, and motor vehicles—by transmitting radio waves or microwaves into space and processing the reflected echo signals.
The Governing Physics of RADAR
The fundamental operation of a RADAR system rests on two primary principles of classical physics: electromagnetic wave reflection and the Doppler Effect.
Pulse Transmission and Echo Sensing
The radar system radiates short, high-frequency electromagnetic energy pulses into space. When these waves strike an obstacle with different electrical properties than air, a fraction of that energy scatters in all directions, with a portion reflected back toward the source as an echo. Because radio waves travel at the constant speed of light (c ≈ 3 × 108 m/s), the precise distance (range, R) to the target object is directly calculated by measuring the time delay (t) between the transmission of the pulse and the reception of its echo:
The division by 2 accounts for the two-way path traveled by the electromagnetic pulse (from transmitter to target and back to receiver).
Target Speed Measurement (The Doppler Shift)
When a radar pulse reflects off a moving target, the frequency of the returned echo shifts relative to the originally transmitted frequency (f0). This change in frequency is known as the Doppler Shift (Δ f).
- If the target is moving toward the radar, the received frequency increases (f > f0).
- If the target is moving away, the received frequency decreases (f < f0).
The relative radial velocity (vr) of the target is mathematically derived from this shift:
Structural Components of a RADAR System
Transmitter
Generates high-power radio frequency pulses using specialized microwave oscillators, such as magnetrons, klystrons, or solid-state power amplifiers.
Duplexer
A high-speed electronic switch that connects a single antenna to both the transmitter and the receiver. It isolates the sensitive receiver from the high-power transmitter pulse during transmission, and then immediately switches to route the weak returned echo to the receiver.
Antenna
Focuses the high-frequency energy into a narrow, highly directional beam. The antenna sweeps across the sky to scan the designated operational area.
Receiver
Amplifies, filters, and down-converts the weak, microvolt-level returned echo signals without introducing electronic noise.
Signal Processor and Display
Processes the raw electronic data to filter out unwanted ground or weather clutter and renders the clean output visually on a Plan Position Indicator (PPI) screen or a modern digital tracking matrix.
Classification of RADAR Systems
RADAR systems are categorized based on their functional architecture and wave transmission profiles.
Based on Antenna Architecture
- Monostatic RADAR: The transmitter and receiver share a single antenna via a duplexer, or utilize two separate antennas placed immediately next to each other. This is the standard configuration for commercial and military installations.
- Bistatic / Multistatic RADAR: The transmitter and receiver antennas are separated by a very large, geographically significant distance. Bistatic networks are highly useful in military applications because they can detect stealth aircraft designed to deflect radar signals away from standard monostatic positions.
Based on Waveform Profile
- Pulse RADAR: Transmits discrete, high-power bursts of radio energy and waits for the echo during the silent intervals. It is primarily used to measure target range and coordinates.
- Continuous Wave (CW) RADAR: Transmits a continuous, uninterrupted stream of radio frequency energy. It cannot measure range because it lacks a time-reference marker, but it measures target velocity instantly via the Doppler shift.
- Frequency Modulated Continuous Wave (FMCW) RADAR: Transmits a continuous wave whose frequency changes periodically over time. By comparing the instantaneous frequency of the echo with the current transmitted frequency, it calculates both range and velocity simultaneously.
Introduction to GPS (Global Positioning System)
The Global Positioning System (GPS), originally developed by the United States Department of Defense as NAVSTAR, is a satellite-based radionavigation network that provides users with precise three-dimensional positioning (latitude, longitude, and altitude), velocity, and time information anywhere on Earth.
The Governing Physics of GPS
The Principle of Trilateration
GPS does not use angle measurements or radar reflections to determine location. Instead, it relies entirely on trilateration, a geometric method of determining the intersection points of overlapping spheres.
- If a receiver knows its exact distance from a single satellite (S1), it must lie somewhere on the surface of an imaginary sphere of radius R1 centered on that satellite.
- Knowing the distance from a second satellite (S2) narrows down the location to the circular intersection line where the two spheres meet.
- A distance measurement from a third satellite (S3) narrows the possibilities down to just two distinct points in space (one point is usually discarded automatically because it sits deep within space or inside the solid Earth).
- Consequently, a minimum of three satellites is geometrically sufficient to determine a 2D position (latitude and longitude) on the Earth’s surface. A fourth satellite is required in practice to resolve three-dimensional positioning (including altitude) and to correct the clock error of the user’s receiver.
Time-of-Flight (ToF) and Atomic Clocks
The distance from a user to a satellite is called the pseudo-range. It is calculated by multiplying the speed of light by the exact time it takes for the satellite’s radio signal to reach the receiver:
Relativistic Corrections in GPS
GPS operations must account for Albert Einstein’s theories of relativity to prevent significant positioning drift.
- Special Relativity: Because the satellites move at high speeds (≈ 14,000 km/h) relative to observers on Earth, their onboard clocks tick slower by about 7 microseconds per day.
- General Relativity: Satellites operate in a weaker gravitational field far above the Earth (≈ 20,200 km altitude). According to gravitational time dilation, clocks under less gravity tick faster. This effect causes the satellite clocks to run faster by about 45 microseconds per day.
Combining these two opposing effects means satellite atomic clocks run faster by a net total of 38 microseconds per day relative to clocks on the ground. If left uncorrected, this relativistic drift would cause GPS positioning calculations to fail by more than 11 kilometers every single day. Engineers prevent this by programming the satellite clocks to tick slightly slower before launching them into orbit.
Architecture of the GPS Network
The GPS system is organized into three distinct operational segments.
Space Segment
Consists of a constellation of satellites orbiting the Earth in Medium Earth Orbit (MEO) at an altitude of approximately 20,200 km. The satellites are arranged across 6 distinct orbital planes, ensuring that at least 4 to 8 satellites are visible from any point on Earth at any time.
Control Segment
A global network of ground tracking stations, including a Master Control Station (located at Schriever Space Force Base in Colorado, USA). The control segment tracks the satellites’ precise positions, monitors their atomic clocks, and uploads updated orbital data (ephemeris) twice a day.
User Segment
Comprises all civilian and military GPS receiver devices. These units pick up the satellite signals, perform trilateration equations, and calculate the user’s real-time position.
Comparative Overview: Global Navigation Satellite Systems (GNSS)
While GPS is the most widely used system, several nations have deployed independent global or regional navigation satellite systems.
| System Name | Operating Nation / Agency | Coverage Type | Constellation Status |
| GPS (NAVSTAR) | United States | Global | Fully Operational |
| GLONASS | Russian Federation | Global | Fully Operational |
| Galileo | European Union | Global | Fully Operational |
| BeiDou (BDS) | People’s Republic of China | Global | Fully Operational |
| NavIC (IRNSS) | India (ISRO) | Regional (India & 1500 km beyond boundaries) | Fully Operational |
| QZSS (Michibiki) | Japan | Regional (Asia-Oceania) | Operational |
Critical Technical Terms and Trivia for Civil Services Examination
- Duplexer Isolation: A key challenge in radar design is preventing the transmitter’s high-power signal from leaking directly into the receiver, which would instantly destroy the sensitive receiver circuits.
- Radar Cross Section (RCS): A measure of how detectable an object is by radar. Stealth aircraft utilize unique geometry and radar-absorbing composite materials to reduce their RCS to the size of a small bird.
- Selective Availability (SA): An intentional degradation of public civilian GPS accuracy originally enforced by the US military. This policy was permanently deactivated in May 2000, allowing civilian units to achieve high tracking accuracy (<5 meters).
- Ionospheric Delay: The primary source of error in modern GPS calculations. As satellite radio waves pass through the Earth’s charged ionosphere and troposphere, their speed changes slightly. Modern dual-frequency receivers correct for this by comparing two different signal frequencies (L1 and L2).