# Airborne Radar Staggered PRF Coherent Processing Method for Down-Looking Target Detection
## 1 Introduction
Space-time adaptive processing (STAP) is an important method for improving the detection performance of airborne radars, which has received considerable attention from researchers in recent years ([PERSON] et al., 2019; [PERSON] et al., 2020). When an airborne radar operates in a down-looking mode, the total clutter echoes from various directions are superimposed and cause Doppler broadening, which makes the detection of low-Doppler targets difficult. Ground moving target indication (GMTI) aims to detect slowly moving ground targets in the presence of strong clutter echoes. Research on GMTI and clutter suppression for radar platforms began in the 1960s, and typical platform motion compensation methods are displaced phase center antenna (DPCA) and STAP. However, both DPCA and STAP require at least two receiving channels. The application of STAP to side-looking airborne radars with relatively stationary ground clutter has achieved good performance, but due to severe distance dependence, especially for non-side-looking airborne radars, the performance is poor. When the PRF is high enough to cause range ambiguity, the clutter Doppler at close ranges changes rapidly for non-side-looking airborne radars, while the clutter Doppler at far ranges changes slowly, and clutter at different ambiguous ranges does not overlap. [PERSON] et al. (2019) proposed a subarray synthesis algorithm with elevation pre-filtering. Through this technology, ambiguous short-range clutter is eliminated before STAP, and the range dependence of clutter is alleviated, greatly improving the performance of STAP. [PERSON] et al. (2020) proposed using STAP combined with multiple interleaved mid-pulse repetition frequencies for processing. The update of the radar system can also provide new capabilities for clutter suppression and target detection of airborne radars. [PERSON] et al. (2019) proposed a frequency diverse array (FDA) MIMO radar range ambiguity clutter suppression method based on transmit beam main lobe correction. This method first uses designed transmit pulse weighting to compensate for the initial phase accumulated by the pulse frequency increment, thereby solving the problem of inconsistent high-gain illumination angles of the main lobe of the synthesized MIMO equivalent transmit beam pattern. In this paper, we conduct a study on the detection of airborne radar down-looking targets based on orthogonal signals combined with PRF staggered transmission. We employ multiple sets of PRF signal groups with orthogonal waveforms for target detection and range-Doppler ambiguity resolution. Additionally, we apply coherent processing to the results of multiple PRF signal groups, using the transmission intervals, to enhance the SNR of the targets, thereby facilitating the detection of low SNR targets. The major contributions of this paper are as follows:  
1. By using orthogonal DFC signals, targets under different PRFs can be separated, avoiding false target interference in different PRF signal groups;2. Staggered transmitting multiple sets of PRF signals and utilizing the first pulse emission time of each set of PRF to perform coherent fusion on the RD results can significantly improve the target's SNR, increase the coherent processing time, and improve the Doppler resolution of the target. This paper is organized according to the following framework. The second part of this paper mainly constructs the clutter model, down-looking detection model, and PRF staggered transmission model of airborne radar and introduces the ambiguous situation of the RD map of target and clutter under different PRFs. In the third part of the paper, the signal processing flow and radar blind zone optimization are mainly discussed and designed. The fourth part performs performance analysis on the algorithm's target detection and parameter estimation processes to validate the effectiveness of the algorithm presented in this paper.