Pulse compression works by modulating the transmitted pulse with a certain code or waveform, such as a linear frequency modulation (chirp), a phase shift keying (PSK), or a Barker code. The receiver then applies a matched filter or a correlator to the received signal, which compresses the pulse in time and enhances its peak power. The matched filter or correlator is designed to match the code or waveform of the transmitted pulse, so that it maximizes the output when the received signal matches the expected signal.
FM chirping allows a radar to use all solid state components and not have to use a klystron power tube in it's final output, as in the Federal Aviation Administration's Digital ASR 11.
Pulse compression is a signal processing technique used in radar systems to improve resolution and detection capabilities. It involves modulating the transmitted pulse, typically in frequency or phase, to increase the time-bandwidth product. The radar transmits a long, coded pulse, and upon receiving the echo, it processes this echo using a matched filter. This filter correlates the received signal with the transmitted code, effectively compressing the long pulse duration into a much shorter one. The primary advantage of this technique is that it combines the benefits of long pulse durations, which provide high energy and thus improved detection range, with the high resolution typically associated with short pulses.
Pulse compression offers numerous advantages for radar systems, such as improved range resolution and signal-to-noise ratio (SNR). Compressing the pulse in time allows for a finer resolution than the pulse width would allow, and increasing the peak power of the received signal enables detection of weaker targets. Additionally, pulse compression facilitates the use of long pulses, which have greater energy and can travel farther than short pulses, resulting in a larger range and better detection performance. Finally, it reduces the peak power requirement of the transmitter, allowing for energy savings and protection of the transmitter and antenna.
One of the main benefits of pulse compression is the enhanced range resolution it provides. By compressing a long pulse into a short one, the radar can distinguish between two closely spaced targets that might otherwise appear as a single target. This improvement in resolution is critical in environments where targets are clustered together, such as in air traffic control or battlefield surveillance. Additionally, pulse compression increases the signal-to-noise ratio (SNR). A longer transmitted pulse means more energy is sent out, which improves the radar's ability to detect weak echoes from distant or low-reflectivity targets.
Pulse compression has some challenges for radar systems, such as introducing range sidelobes which can mask or confuse the targets close to the main peak in range. To reduce range sidelobes, one can use certain codes or waveforms with low sidelobe levels, like Hamming or Hanning windows, or apply weighting functions to the output. Pulse compression also requires a high bandwidth, which can be limited by using certain codes or waveforms with narrow bandwidths, such as Barker codes or polyphase codes, or by using frequency hopping or spread spectrum techniques. Additionally, pulse compression increases the complexity and cost of the radar system since it needs sophisticated hardware and software to generate, modulate, transmit, receive, filter, and process the pulses. However, complexity and cost can be reduced by using simple codes or waveforms that are easy to generate and match, such as chirps or PSKs, or by using digital signal processing (DSP) or field-programmable gate arrays (FPGAs) to implement the pulse compression functions.
Despite its benefits, pulse compression also presents several challenges. One significant issue is the complexity of the signal processing required. Implementing the necessary algorithms for modulation, matched filtering, and pulse compression demands sophisticated hardware and software. This complexity can increase the cost and development time of radar systems. Moreover, the processing needs to be performed rapidly and accurately to ensure real-time operation, adding to the technical demands. Another challenge is managing the potential for range sidelobes, which are artifacts of the pulse compression process. These sidelobes can create spurious signals that may be mistaken for real targets, potentially leading to false detections.
Pulse compression radars can be classified into two main types, depending on how they modulate and filter the pulses. Analog pulse compression radars utilize analog devices and circuits, such as a voltage-controlled oscillator (VCO) and a surface acoustic wave (SAW) device, to modulate and filter the pulses. These systems are simple and fast, but have limited flexibility and accuracy. Digital pulse compression radars, on the other hand, use digital devices and algorithms like a direct digital synthesizer (DDS) and a digital correlator to modulate and filter the pulses. These systems are more flexible and accurate, but have higher power consumption and latency.
There are several types of pulse compression radars, each utilizing different modulation techniques. One common type is the linear frequency modulated (LFM) radar, also known as a chirp radar. In this system, the frequency of the transmitted pulse increases or decreases linearly over time. The received echo is then processed to compress the pulse and extract high-resolution range information. Chirp radars are widely used due to their simplicity and effectiveness in various applications. Another type is phase-coded radar, which uses phase modulation to encode the transmitted pulse. Binary phase-shift keying (BPSK) is a typical example, where the phase of the pulse is shifted according to a predetermined code.
Pulse compression radars are widely used in many applications and domains, such as airborne radar for navigation, surveillance, weather, or imaging. For example, the synthetic aperture radar (SAR) uses pulse compression to create high-resolution images of the ground or sea surface. Ground-based radars are installed on land or sea and are used for air traffic control, missile defense, or coastal monitoring. An example is the phased array radar (PAR), which uses pulse compression to steer the beam electronically and scan multiple targets simultaneously. Automotive radars are integrated into vehicles for collision avoidance, adaptive cruise control, or parking assistance. The frequency-modulated continuous wave (FMCW) radar uses pulse compression to measure the range and velocity of obstacles or other vehicles.
Pulse compression radars are used in a variety of applications across different fields. One prominent example is the AN/APG-77 radar used in the F-22 Raptor fighter aircraft. This radar system employs pulse compression to achieve high-resolution imaging and long-range target detection, crucial for air superiority missions. The advanced signal processing capabilities of pulse compression enable the radar to operate effectively in complex and contested environments. Another example is the Sentinel radar system, which is used for ground-based air defense. The Sentinel radar utilizes pulse compression to detect, track, and identify airborne targets, including drones, helicopters, and fixed-wing aircraft.
Challenges 1. Implementation Complexity: Requires sophisticated signal processing and computing. 2. Range Sidelobes: Can mask true targets; mitigation requires additional processing. 3. Doppler Sensitivity: Some techniques are sensitive to moving targets, potentially affecting accuracy. 4. Hardware Demands: Needs advanced, costly hardware for complex computations. 5. Interference Management: Susceptibility to interference from wideband systems necessitates careful design. Pulse compression significantly boosts radar capabilities but demands attention to design and operational issues like sidelobe suppression and Doppler effect management.