# A High Signal-Noise Ratio UWB Radar for Buried Pipe Location Using Golay Complementary Sequences
## 1 Introduction  
Detection and location of underground pipes before excavation work is extremely important. It can ensure faultless working processes and minimize the impacts on the existing pipes in the intervention area. Once the buried pipes are damaged, the undesirable fluid losses due to leaks can cause enormous environmental and economic repercussions. Ultra-wideband (UWB) ground penetrating radar (GPR) [1; 2; 3] as one of the non-destructive techniques [4; 5; 6; 7; 8] to locate the underground pipes attracts increasing attention because it is capable of accurately locating both metallic and non-metallic pipes without prior knowledge. However, conventional UWB GPR utilizes short pulse as the transmit signals, which leads to a tradeoff between the resolution and the detection distance. In order to expand the detectable range without degrading the resolution, large time-bandwidth signals are used as the transmit signals in recent years, such signals include frequency modulated continuous wave (FMCW) signals [9; 10; 11], stepped frequency signals [12; 13; 14], code signals [15; 16; 17], and chaotic signals [18; 19; 20]. UWB GPR based on FMCW signals or stepped frequency signals suffer from having strong sidelobes [21], thereby resulting in low signal-noise ratio (SNR). To suppress these undesired sidelobes, different mismatched filters have been reported in the literature [22; 23; 24]. Unfortunately, these sidelobes suppression techniques come at the cost of signal-noise ratio (SNR) loss and resolution reduction [25]. Pseudorandom binary sequences (PRBS or m-sequences) have low sidelobes and exhibit good correlation properties. Additionally, their sidelobes can be further reduced by extending the length of the PRBS [26]. Thus, the UWB GPR based PRBS can enhancethe SNR [27]. Extensive studies show chaotic signals possess many properties that make them an attracting candidate for radar system design [28; 29; 30]. The random nature of chaotic signals shows good correlation properties, which can improve the SNR of radar [31]. Although, both PRBS and chaotic signals are considered good options in terms of their low sidelobes, these two signals are not ideal choices if we take sidelobe level into account. Golay complementary sequences are a pair of equal length sequences that have the following property: when their correlation functions are algebraically added, their sidelobes are canceled while the correlation peak is double of that individual one [32]. Therefore, the Golay complementary sequences are very suitable for GPR. [PERSON] et al. used Golay complementary sequences to encode Costa frequency hopping continuous waveforms. Their results indicated the improvement in sidelobe compared to the same frequency hopping waveform encoded with the Barker code [33]. Utilizing Golay complementary sequences as the probe signal and combining with single sideband transmission, [PERSON] et al. improved the existing noise radar system. They compared the influence of noise on the PRBS-based radar and Golay-based radar in terms of peak sidelobe, secondary sidelobe, and integrated sidelobe values. Their theoretical and experimental results in free space demonstrated the Golay codes offered better performances even when the sequence length ratio of PRBS code and Golay codes is 2:1 [34; 35; 36; 37; 38]. Using Golay codes with a carrier of 12 MHz to degrade the sidelobe and a time-gain amplifier in the receiver to obtain the weak returned signal from the deep target, two experimental results of [PERSON] et al. demonstrated the detection performance of the proposed low frequency coded GPR [39]. In this paper, Golay complementary sequences are introduced for underground pipe location. A Golay complementary sequences generator with tunable code length and frequency is achieved and utilized as probe signals. The buried depth of the plastic pipe or metallic pipe can be obtained by the correlation function.