Coherent detection is a fundamental concept used in radar systems to extract useful information from received radar signals. It involves using a reference signal, often generated from the transmitted radar signal, to maintain phase coherence between the transmitted and received signals. This coherence allows for precise measurement of the phase difference between the two signals, which is essential for accurately determining the target's range, velocity, and other properties.
Let's break down the concept of coherent detection in radar systems:
Radar Signal Transmission: In a radar system, a radar transmitter emits an electromagnetic signal, often in the form of short pulses. These pulses travel through space until they encounter a target (such as an aircraft, ship, or object).
Signal Reflection: When the radar signal reaches the target, a part of it gets reflected back towards the radar system. This reflected signal is known as the echo or return signal.
Signal Reception: The radar receiver picks up the echo signal, which is typically much weaker than the transmitted signal due to the spreading of energy in space and scattering losses.
Coherent Reception: Coherent detection involves using a reference signal that is in sync with the transmitted signal. The reference signal is generated from the transmitted signal, often by mixing it with a portion of the transmitted signal itself or a stable local oscillator signal. This reference signal serves as a stable phase reference against which the received signal is compared.
Mixing and Downconversion: The received signal and the reference signal are mixed or multiplied together. This process is known as downconversion, and it shifts the received signal's frequency to a lower intermediate frequency (IF) or baseband.
Phase Detection: The downconverted signal is then passed through a phase detector. This detector compares the phase of the received signal with the phase of the reference signal. It effectively measures the phase difference between the two signals.
Phase Shift Information: The phase difference information is used to calculate the range (distance) to the target. By knowing the time delay between transmission and reception and the speed of light, the distance to the target can be accurately determined.
Doppler Shift Information: Additionally, coherent detection enables the radar system to detect the Doppler shift in the received signal. The Doppler shift is caused by the relative motion between the radar system and the target. By analyzing the rate of change of the phase difference, the radar system can infer the target's radial velocity (velocity component along the line of sight).
By employing coherent detection, radar systems can achieve high accuracy in measuring target properties like range and velocity, making it a vital technique in modern radar technology.