HF Ground Wave Radar Waveform Design And Implementation Based On Multi-sweep Signal

By using vertical polarized electromagnetic wave High-frequency ground wave radar realizes over-the-horizon detection. It has been widely used in all-weather, large area, high precision, real-time detection of ocean surface dynamic parameters. HF radar can also achieve anti-stealth, over-the-horizon surface ship tracking and low/low altitude flight target penetration. At the same time, high frequency radar-based typhoon monitoring and tsunami warning and other meteorological disasters have received much attention. After nearly 50 years of development, high-frequency ground wave radar system has been a major improvement, such as the first 500m large antenna array has evolved into SeaSonde radar monopole/cross-loop antenna, the analogue superheterodyne receiver has been replaced by an all-digital radar system. With the development of science and technology, high-frequency ground wave radar has been put forward higher requirements on the marine environment detection and military target detection. So the shortcomings of existing radar signal system has become obvious as following:1) Different configurations of high-frequency ground wave radar can only complete a single task. A multi-functional hardware platform is required with the constraints of the field environment, for example, detecting short-range ship and monitoring remote current simultaneously.2) The strong high-frequency electromagnetic waves will bring interference to other electronic devices in the ocean application, so the peak power of radar signal is always limited by the International Telecommunication Union. On the other hand, the lower peak power of radar waveform is needed to meet the requirements of low probability of intercept in military applications.3) There are too many radio station to causes the serious radio frequency interference for HF radar. For the low utilization in the time-frequency plane of single sweep, the RFI will completely cover the sea echoes and point-like target echo and seriously reduces the radar performance, once it mixes with radar signal. Aiming at the above problems, this paper gives a new explanation of pulse compression based on time frequency distribition, and designs a time and frequency division multiplexed multi-sweep signal. In particular, the main research work includes the following contents:1) Based on the understanding of radar resolution theory, this paper proposes a new explanation of pulse compression on time-frequency distribution, which is divided into time compression and frequency compression. This provides a more comprehensive theoretical basis and a more concise and easy understanding of radar signal pulse compression.2) Based on the waveforms commonly used in high frequency radar, the characteristics and design methods of single sweep waveform are analyzed in detail, which pave the way for new waveform design. The FMICW signal is a commonly used signal in traditional high frequency radar. The ambiguity function of FMICW is deduced detailedly, which gives the waveform constraint and radar resolution in different processing modes. In this paper, we design and implement an FMICW signal based on nonuniform interrupt pulse to achieve the goal of near target detection and long-distance current detection by using different length interrupt pulse.3) A time and frequency division multiplexed sweep is proposed in this paper, which improves the utilization of time and frequency resources. The multiplexed sweeps can reduced the power density by spreading the energy on the time and frequency plane uniformly, so it improves anti-interference capabilities and decreases the Intercepted probability. This paper focuses on the characteristics of time division multiplexed sweep (TDMS) and frequency division multiplexed sweep (FDMS). The multi-sweep has a nail-plate ambiguity function whose range and Doppler resolution is same to the single sweep sigal. Considering the characteristics of mutli-sweep signal, we use the mixing and three FFT to implement the pulse compression. Compared with the stretch method in single sweep, the new method can compress all the energy fully. Our method is easier to implement than match filtering method.4) The characteristics of multi-sweep signals in practical applications are fully studied, including peak-to-average power ratio, Doppler tolerance and anti-jamming characteristics. In this paper, the peak-to-average power ratio(PAPR) is reduced by using phase modulation and window function modulation, at the same time a reasonable time-bandwidth product can also reduce PAPR. The Doppler tolerance of TDMS and FDMS can be obtained by computing the attenuation of main lobe in ambiguity function which are both 0.44/T (T denotes sweep time) under narrow-band condition.Compared to the single sweep, TDMS reduce the probability of radio frequency interference to 1/M (M denotes the multiplexed number of TDMS), FDMS improves the signal to interference ratio, which can be 20log10 N (N denotes the multiplexed number of FDMS).5) A set of all-digital high-frequency radar receiver for multi-sweep signal is developed, in order to verify the performance of the new waveform in real environment, which includes system software and hardware scheme. A new time-frequency conversion method based on time-division multiplexing (DFT) and frequency-domain convolutional windowing method is proposed, which greatly reduces the computational complexity of the system and decreases the resource consumption. A hardware circuit is designed based RF direct sampling and digital pulse compression, the digital board is used to control the system sequence and implement digital pulse compression. The system also uses a GPS module to realize the clock synchronization in the bistatic radar field test. Based on the system design, closed-loop test indicates that sensitivity of the system is-125 dBm, the analog front-end gain is 39dB, channel isolation is greater than 100dB. The digital board test shows phase stability of 0.05 degrees with single clock source, while phase stability is 12 degrees with two GPS clock source. The system indicators can meet the application requirements.6) The monostatic field test shows that the direct waves of different sweeps produce an intermittent range spectrum, so the bistatic separation system is considered to reduce the direct wave amplitude. In the bistatic test, the influence of baseline distance and windmill on radar echo is analyzed. When the same total energy is transmitted, TDMS can obtain the same detection performance with different multiplexed number M. On the other hand, TDMS signal can improve the signal-to-noise ratio by 10log10M under the condition of same power density.