Basics of Chirp Radar – Radar Operation – Part 3

Chirp Radar is a type of radar that uses frequency modulation and pulse compression (which may confuse many as chirp has also been used as a synonym for “pulse compression”), which combines the capabilities of frequency modulation and pulse radar. Some of the benefits of a Chirp radar over a simple pulse radar are range enhancement, range resolution enhancement, the use of lower peak input power for comparable performance, a greater level of noise immunity, reduced power supply voltage, and a slightly more jamming resistance radar architecture.

Key Takeaways 

• Chirp radar, also known as linear frequency-modulated pulse radar (FMOP), uses a frequency-swept transmitted pulse and matched filtering to achieve pulse compression, improving range resolution and sensitivity.  

• Compared with simple pulsed radar, chirp radar offers benefits such as better range resolution, greater noise immunity, lower peak transmitter power, and enhanced dynamic range.  

• The basic mechanism: transmit a sweep (chirp), receive the echo, then apply a time-delay or matched filter so that the long pulse is compressed into a short, high-amplitude return signal, thereby distinguishing closely spaced targets. 

• Key trade-offs: while chirp radar enhances resolution and sensitivity, the system must handle bandwidth, matched-filter implementation, side-lobe control, and, in modern defence applications, vulnerability to sweeping jammers. 

• Applications for chirp radar include weather radar, marine radar, long-range detection systems, and other platforms where improved resolution and lower peak power are valuable.  

A limitation of pulse radar without compression is that two objects within the same spatial extent of the pulse will appear as one target. Using pulse compression, such as a chirp, otherwise known as linear frequency modulation, a pulsed radar can then use frequency comparison to discern between multiple objects. Frequency modulation on Pulse (FMOP) or chirp radar is only one form of pulse compression, of which there are many linear, nonlinear, time-dependent, and various other types of modulation.

The benefit of linear frequency modulation for pulse compression is that the circuitry is relatively simple compared to other more complex pulse compression techniques, and the result of the compression is a much higher amplitude signal without the need for a single high power supply of the same level. Chirp radar works by adding a time delay to the receive signal and the swept frequency transmitted signal in such a way that the transmitted signal is “compressed” in time to yield a shorter pulse and the receive signal as the reverse operation is performed. The transmitted signal is typically called the chirp, where the received signal that has gone through pulse compression is generally known as the collapsed or compressed signal.

A lineary delay versus frequency network is used to create the compressed signal, but can also be used reciprocally to decompress the return signal for analysis/display. A weighted envelope is sometimes applied to the output collapsed chirp pulse to reduce the sidelope power levels, which results in an increase the the dynamic range of the radar. Non-linear chirps may also be used to reduce side-lobe levels and increase the dynamic range without applying amplitude weighting and reduced the overall signal strength.

Recommendations 

1. Select components rated for wide bandwidth operation.
   Chirp radars require components (amplifiers, mixers, filters) that can support high instantaneous bandwidths without introducing distortion.
    

1. Use precision-timing and synchronization modules.
   Matched filtering and pulse compression depend on precise timing alignment between transmit and receive paths.
    

1. Optimize antenna and cabling for low phase distortion.
   Nonlinearity phase can reduce pulse compression efficiency and degrade the range of accuracy.
    

1. Incorporate adaptive filtering and signal processing.
   Advanced DSP or FPGA-based solutions can dynamically adjust chirp parameters for better interference mitigation.
    

1. Regularly calibrate your radar system.
   Calibration ensures accuracy in a range of measurements and compensates for temperature or component drift. 

Chirp radars are still used today in a variety of applications, Most notably, chirp radars are used in weather observation and marine radar. There are still legacy radars that use linear frequency modulated chirp radar. However, sweeping radar jammers may be used to mitigate the effectiveness of this radar type. Hence, militaries generally use more advanced pulse compression techniques with modern radar.