Radar Measurement Types and Basics

This article explores the different types of measurements performed on radar systems, along with the fundamental concepts behind them. We’ll cover a typical radar measurement setup and the various parameters that are crucial for understanding radar performance.

Radar works by transmitting a series of pulses towards a target and then receiving the reflected pulses back after a specific time delay (Δt). This round-trip time is the key to determining the target’s distance. Let’s delve into the different aspects of radar measurements.

Radar Pulse Waveform

Figure 1 below shows a typical pulse waveform from a radar transmitter and the key parameters we measure from it.

Figure 1: Typical radar pulse waveform.

Types of Radar Measurements

Here’s a breakdown of the common radar measurements:

Radar Range

This is the distance between the radar antenna and the target. We calculate it using the formula:

R = c * Δt / 2

Where:

• R is the range in meters.
• Δt is the round-trip time in seconds.
• c is the speed of light (approximately 3 x 10^8^ m/s).

Pulse Repetition Interval (PRI)

Also known as Inter-Pulse Period (IPP), PRI is the time between the start of two consecutive pulses in the radar’s pulse train.

Pulse Repetition Frequency (PRF)

PRF is the inverse of the PRI. It represents how many pulses the radar transmits per second.

PRF = 1 / PRI

Duty Cycle

This is the ratio of the pulse width (τ) to the PRI (T). It indicates the fraction of time the radar is actively transmitting.

Duty Cycle = τ / T

Pulse Width (τ)

As shown in Figure 1, pulse width is the duration of a single radar pulse. It’s typically measured in microseconds (µsec). Other pulse parameters include rise time and fall time.

Peak Power

The peak power is the maximum power output of the radar transmitter. It’s usually around 1 Megawatt (MW).

Antenna Gain

This is the measure of how effectively the radar antenna focuses its transmitted power. It’s measured in dBi (decibels relative to an isotropic radiator) and is typically around 30 dBi.

Peak Effective Isotropic Radiated Power (EIRP)

This parameter is the product of the radar transmitter power and the antenna gain, representing the total power radiated by the radar. It’s often around 1 Gigawatt (GW).

Frequency Range

This refers to the operating frequency of the radar. Radars use a wide range of frequency bands, including HF, VHF, UHF, L, S, C, X, Ku, K, Ka, and MMW. Spectrum measurements include out-of-band emissions, spurious emissions, and emissions within the necessary bandwidth.

Antenna Pattern Measurement

This measures the radiation pattern of the radar antenna, showing how the antenna radiates energy in different directions.

Radar Measurement Setup

Figure 2 shows a typical setup for radar measurements.

Figure 2: Block diagram of a typical radar measurement setup.

The setup includes:

• Directional Coupler: This device samples a portion of the power traveling between the radar transmitter and antenna.
• Signal Generator with Radar Application Software: This allows generating ideal radar pulses for comparison and benchmarking.
• Oscilloscope: Used for time-domain measurements of radar pulses.
• Vector Signal Analyzer (VSA): Used for frequency-domain measurements.
• Variable Attenuator: This device adjusts the signal level to protect the measuring equipment.
• Antenna, Band-pass Filter (BPF), Low-Noise Amplifier (LNA), and Wideband Detector: These components are used when performing radiated emissions measurements from the radar antenna, before connecting to the oscilloscope or VSA.

In practice, a signal from the radar transmitter is coupled using the directional coupler. The attenuated signal is then fed to an oscilloscope for time-domain analysis or to a Vector Signal Analyzer (VSA) for frequency-domain analysis. The setup can also be used for radiated emission measurements by connecting the VSA to an appropriate antenna.