# Airborne Radar Staggered PRF Coherent Processing Method for Down-Looking Target Detection
## 1 Introduction  
Radar (Radio Detection and Ranging) actively emits electromagnetic waves and utilizes their reflections to acquire information about the target, including its distance, azimuth, and velocity. Modern radar performs various essential functions, such as detection, positioning, recognition, tracking, and imaging, playing significant roles in both military and civilian applications. In aviation, airborne radar is widely used, providing reliable situational awareness, navigation, and monitoring capabilities during adverse weather conditions and nighttime flights. Advancements in technology have led to airborne radar systems that offer higher resolution, increased data accuracy, and expanded monitoring ranges. When operating in the down-looking mode, airborne radar must consider the presence of ground clutter. The Doppler frequency of ground clutter varies based on the angle between the radar and the ground, causing significant range and Doppler ambiguities. Pulse Doppler radar is commonly employed for ground detection to distinguish targets from strong clutter echoes. The medium-pulse repetition frequency (MPRF) waveform is widely used in pulse Doppler radar to accurately measure target range and velocity in the presence of strong clutter. The adoption of multiple pulse repetition frequencies (Multi-PRF)in MPRF radar is necessary to resolve range and Doppler frequency ambiguities effectively. For optimal PRF selection, minimizing blind zones in terms of distance and velocity while ensuring full resolution of real distance and Doppler frequency ambiguities is crucial. [PERSON]. [1] was the first to conduct an analysis of the MPRF, and perform a distance and Doppler ambiguity analysis of MPRF waveforms. The clutter distribution in the down-looking mode of airborne radar was also explored, and the clutter's ambiguous characteristics on the Range Doppler (RD) map were analyzed. [PERSON] et al. [2] introduced a ground clutter model for the down-looking mode of radar, and to reduce the computational complexity of traditional clutter calculation methods, [PERSON] et al. [3] derived the expression of the boundary of range-Doppler cells in spherical coordinates, describing the process of calculating ground clutter range-Doppler spectra only in spherical coordinates. [PERSON] et al. [4] presented a ground clutter echo model under high PRF, and discussed the clutter-induced distance ambiguity in the RD results. [PERSON] et al. [5] introduced a blind zone model for distance-velocity measurement caused by ground clutter in airborne MPRF radar. Since a set of fixed PRFs has fixed ambiguous characteristics, multiple sets of PRF signals can be used in conjunction with the Chinese remainder theorem to perform de-ambiguity processing of targets after ambiguity [6; 7; 8]. This method of combining multiple sets of PRFs is also widely used in practical applications today. [PERSON] et al. [9] proposed a target parameter extraction method under MPRF target distance and velocity ambiguity, using the Chinese remainder theorem combined with clustering methods to solve the problem of target cross-distance units in different PRF groups. [PERSON] et al. [10] used interleaved transmission of different PRF signals and can perform coherent processing, effectively expanding the Doppler unambiguous measurement capability. In recent years, due to its superior anti-jamming capability and the ability to optimize the range-Doppler two-dimensional ambiguity, the use of random PRF has been increasingly researched and applied in addition to the traditional fixed PRF approach [10; 11; 12; 13; 14; 15]. [PERSON] et al. [16] proposed a radar operating mode that uses pulse trains with time intervals to eliminate range and Doppler ambiguity by appropriately setting the pulse interval difference. [PERSON] et al. [17] pointed out the beneficial electronic countermeasures brought by the use of random frequency and pulse repetition interval agile signals to radar systems.