While the fundamental principle of transmitting and receiving electromagnetic waves remains constant, radar systems have evolved into various types, each optimized for specific applications and operational environments. These variations primarily stem from differences in the waveform transmitted and the signal processing applied to the received echoes. Here, we briefly introduce the main categories; a deeper dive into each will follow in subsequent modules.

1.3.1 Continuous Wave (CW) Radar

• Concept: CW radar continuously transmits an unmodulated electromagnetic wave (constant frequency and amplitude). It does not send out pulses; instead, it maintains a continuous transmission.
• Principle of Operation: Its primary function is to detect the Doppler shift in the frequency of the returned signal. When a target moves relative to the radar, the frequency of the reflected wave changes. If the target is moving towards the radar, the frequency increases (positive Doppler shift); if it's moving away, the frequency decreases (negative Doppler shift).
  The Doppler frequency (fd ) is directly proportional to the relative velocity (vr ) between the radar and the target and is given by:
  fd =λ2vr
  Where λ is the radar wavelength. The factor of 2 accounts for the round-trip travel of the wave.
• System Architecture: CW radars typically require separate transmitting and receiving antennas to prevent the strong transmitted signal from directly saturating the sensitive receiver. The received signal is mixed with a portion of the transmitted signal (often called the local oscillator) in a mixer. The output of the mixer is the beat frequency, which is the Doppler frequency.
• Key Advantage: Unmatched precision in measuring radial velocity (velocity component directly towards or away from the radar). It's also relatively simple and inexpensive to implement for short-range applications.
• Key Limitation: Cannot determine target range. Since the transmission is continuous, there is no discrete time reference (like a pulse edge) to measure the time delay of the echo. It only tells you if something is moving and how fast, but not how far away it is.
• Applications: Police speed guns, automatic door openers, motion sensors, industrial flow measurement, baseball speed measurement.

1.3.2 Frequency Modulated Continuous Wave (FMCW) Radar

• Concept: FMCW radar transmits a continuous wave, but its frequency is deliberately varied (modulated) over time, typically in a linear ramp (a "chirp"). This modulation provides the necessary time reference to measure range.
• Principle of Operation:
• The radar transmits a frequency-modulated signal, usually a linear sweep from a lower frequency (fmin ) to an upper frequency (fmax ) over a defined time period (Tsweep ).
• When this signal reflects off a target at range R and returns to the radar, it arrives after a time delay (τ=2R/c).
• Because the transmitted signal's frequency is constantly changing, the received signal's frequency will be different from the instantaneous frequency of the signal currently being transmitted.
• By mixing the received signal with a portion of the currently transmitted signal, a "beat frequency" (fb ) is generated. This beat frequency is directly proportional to the time delay (τ) and thus to the target's range.
• The relationship is:
  fb =Tsweep Bsweep ×τ=Tsweep Bsweep ×c2R
  Where Bsweep is the total frequency deviation (fmax −fmin ) during the sweep.
  From this, range can be determined:
  R=2×Bsweep fb ×Tsweep ×c
  If the target is also moving, a Doppler shift will be superimposed on the beat frequency. More advanced FMCW techniques (e.g., using up- and down-sweeps) allow for simultaneous measurement of both range and velocity.
• Key Advantages:
• Can measure both range and velocity simultaneously.
• Operates at relatively low average transmitted power, which reduces power consumption and makes it less detectable by passive receivers.
• Offers excellent range resolution for a given bandwidth.
• Key Limitations: Signal processing can become more complex for multiple targets, especially when separating range and velocity.
• Applications: Automotive collision avoidance systems, radar altimeters (for aircraft height measurement), industrial level sensing, short-range surveillance, ground penetrating radar (GPR) for close sensing.

1.3.3 Pulsed Radar

• Concept: Pulsed radar transmits short bursts (pulses) of high-power electromagnetic energy and then "listens" for echoes during the quiet periods between transmitted pulses.
• Principle of Operation:
• A high-power transmitter generates short pulses of radio frequency (RF) energy.
• These pulses are directed towards the target by an antenna. Often, the same antenna is used for both transmitting and receiving, with a device called a duplexer switching the antenna between the transmitter and receiver.
• After transmitting a pulse, the radar system switches to a receive mode and waits for echoes.
• The time taken for a pulse to travel to the target and return (round-trip time, Δt) is measured.
• The range (R) to the target is then calculated using the formula:
  R=2c×Δt
  Where c is the speed of light.
• Key parameters for pulsed radar include:
• Pulse Width (τp ): The duration of each transmitted pulse. Shorter pulses generally lead to better range resolution.
• Pulse Repetition Frequency (PRF): The number of pulses transmitted per second. PRF affects the maximum unambiguous range.
• Pulse Repetition Interval (PRI): The time between the start of one pulse and the start of the next (PRI=1/PRF).
• Key Advantages:
• Directly measures range, which is its primary advantage.
• Can also determine the angular position (azimuth and elevation) of targets.
• Capable of achieving very long detection ranges due to high peak transmitted power.
• Key Limitations:
• Minimum Detectable Range: Limited by the pulse width and the time required for the duplexer to switch.
• Maximum Unambiguous Range: Limited by the PRI. If an echo from a distant target arrives after the next pulse has been transmitted, it can be mistakenly interpreted as a closer target (range ambiguity).
• Requires high peak power, which can lead to larger, more complex, and more expensive components.
• Applications: Air traffic control (ATC), military surveillance, weather forecasting, maritime navigation, search and rescue, ground-based weapon systems.