A new fully-digital HF radar system for oceanographical remote sensing

Transcription

1 LETTER IEICE Electronics Express, Vol.10, No.14, 1 6 A new fully-digital HF radar system for oceanographical remote sensing Yingwei Tian 1a), Biyang Wen 1b),JianTan 1,KeLi 1, Zhisheng Yan 2, and Jing Yang 1 1 School of Electronic Information, Wuhan University, Wuhan, , China 2 Nanjing Research Institue of Eletronics Technology, Nanjing, , China a) tianyw@whu.edu.cn b) rspl@whu.edu.cn Abstract: This paper presents a new fully-digital high-frequency (HF) backscatter radar system for oceanographical remote sensing. The fully-digital receiver is designed that employs directly radio freqency (RF) sampling, digital down-conversion, and digital pulse compression techniques. The difference between this system and ordinary analogue system is illustrated. Results of closed-loop test and field experiment conducted on the coast of the Eastern China Sea are given to prove good performance of the system. Keywords: HF backscatter radar, digital down-conversion, RF directly sampling, digital pulse compression Classification: Electron devices, circuits, and systems References [1] D. E. Barrick: 2008 IEEE/OES 9th Working Conference on Current Measurement Technology (2008) 131. [2] D. E. Barrick, B. J. Lipa, P. M. Lilleboe and M. Isaacson: U. S. Patent (1994). [3] M. Menelle, G. Auffray and F. Jangal: Radar Conference (2008) 224. [4] D. Emery and G. Dickel: Radar Conference (2008) [5] D. Trizna and L. Xu: IEEE J. Ocean. Eng. 31 (2006) 904. [6] R. H. Khan and D. K. Mitchell: IEE PROCEEDINGS-F (1991) 411. [7] Q. H. Shao, X. R. Wan, D. L. Zhang, Z. X. Zhao and H. Y. Ke: J. Radars 4 (2012) 370. [8] B. C. Henderson: Watkins Johnson Company, Technical Note 17 (1990) 1. [9] W. Shen: Ph.D thesis Wuhan University, Wuhan, China (2009). [10] W. Shen, B. Y. Wen, S. C. Wu, J. Yang, X. J. Huang and Y. Liu: Systems Engineering and Electronics 29 (2007) [11] Z. S. Yan, B. Y. Wen, C. J. Wang and C. Zhang: IEICE Electron. Express 6 (2009)

2 1 Introduction HF surface wave radar (HFSWR) has been well-known for its capability in oceanographical remote sensing. During the past three decades, over 90% of HFSWRs are with superheterodyne receivers, the representative product as SeaSonde by CODAR [1, 2]. With the development of digital processing devices, fully-digital receiver has being more and more appreciated for its simplicity and flexibility in structure. However, HFSWRs with fully-digital receivers reported in the last decade are characterized by large size and expensive cost, mainly for hard targets surveillance [3, 4, 5]. In this paper, a new kind of fully-digital HF radar for oceanographical remote sensing is presented. In comparison with ordinary superheterodyne receivers [2], this fully-digital system employs nearly no analogue devices, where RF signals were directly sampled before any frequency-conversion done. The subsequent processings, including frequency-conversion, demodulation, pusle-compression, were all taken in digital devices such as FPGA chip. In addition to the advantage of simplicity and low-cost, the fully-digital structure also performs better compatibility than ordinary analogue structure with other waveforms besides FMICW used in this paper [2, 6], such as OFDM [7]. The detailed decription of the fully-digital HFSWR is set out in section 2. In section 3 we give the simulation results of the closed-loop test. Field experiment results at the Eastern China Sea are presented in section 4. Section 5 is dedicated to the conclusion. 2 Description of the system 2.1 Block diagram Fig. 1 shows the block diagram of the fully-digital HFSWR system. This system consists of antennas, transmitter, receiver, and Computer. The transmitter has a maximum output power of 100 watts. Except for the antennas and transmitter, the receiver is packaged in a chassis (19 inches in width and 3.5U high) which is connected to computer via USB cable. Fig. 1. Block diagram of the fully-digital HFSWR system. The receiver is composed of an arbitrary-waveform generator (AWG) based on direct digital synthesizer (DDS), analog front-end (AFE) served for filtration and amplification, A/D converter (ADC), system controller and 2

3 signal processor which are implemented in FPGA circuit. The clocks driven for DDS, ADC, and system controller are derived from the synchronized outputs of the same phased-locked loop (PLL) circuit to obtain good coherence. 2.2 Comparison with analogue IF receiver The fully-digital receiver is different from the analogue intermediate frequency (IF) receiver on both aspects of circuit structure and digital processing procedure, as shown in Fig. 2. Fig. 2. Comparison between fully-digital receiver and analogue IF receiver. The dashed boxes 1 and 2 are included only in analogue IF receiver, while 3 only in fully-digital receiver. Windowings before FFT and DFT are omitted to ease the comparison. In analogue IF receiver, mixer and local oscillator (LO) are included to realize the frequency-conversion. To maintain the performance of mixer, LO with enough power is usually needed. It s generally known that mixer can influence performance of the analogue circuit in following ways, converison loss, noise figure, and intermodulation distortion [8]. Worse still, phase noise of LO may be transferred to the IF signal. Although optimizing the design may decrease these malign effects, the best solution is to disuse it as that in fully-digital receiver, which will also cut down the cost. As for digital processing procedure, in IF receiver, LO equals the transmitted signal, the IF signal being sampled with lower rate and no decimation needed before FFT operation [2, 9]. In fully-digital receiver, the RF signal was sampled directly, then frequency down-converted and de-chirped. To reduce computation complexity, decimation based on CIC filter is applied. This procedure seems to be more complex than that in IF receiver, but results in a more flexible approach, which performs better stability and compatibility with other waveforms besides FMICW, such as OFDM. The fully-digital structure is also more convenient for external noise spectrum monitoring [10]. 2.3 Signal processing procedure In Fig. 2, the coordinate rotational digital computer (CORDIC) algorithm is employed to implement orthogonal transformation to eliminate the carrier 3

4 and sweep components [11]. The digitized echo can be denoted as s r (n) =A cos ( 2πf c (nδt τ 0 ) πα(nδt τ 0 ) 2) (1) where τ 0 represents the delay of the received signal, A is the amplitude, f c is the carrier frequency, α is the sweep slope, n is the sampling number, and ΔT is the sampling interval. Assign the input phase of CORDIC θ(n) as θ(n) = nδt (2πf c παnδt ) (2) Then we obtain a complex signal consisted of Inphase (I ) component and Quadrature (Q) component as s o (n) =s r (n) ( cos θ(n)+j sin θ(n) ) (3) Ignore the high frequency component, 2f c,ins o (n), we get s o(n) = A 2 exp ( j( 2πf c τ 0 πατ πατ 0 ΔTn) ) (4) As shown in (4), the third term of s o(n), 2πατ 0 ΔTn, corresponds to the range information, which can be extracted by employing DFT algorithm. S(k) = N 1 n=0 s o(n)e j2πnk/n (5) The range peak located at k = ατ 0 ΔTN,andN is the length of the sampled sequence, k is the range bin number. The simplified location is k = Bτ 0, while k N. Only a few points of k needed to be calculated. To achieve negligible amplitude variation through the range-spectrum, the decimation factor wouldn t be too large. The subsequent Doppler coherent integration is performed on computer as that in IF receiver. 3 Closed-loop test The closed-loop test is performed via applying a delayed and attenuated RF signal generated by DDS to receiver. The orignal ampltitude is 40 dbm, and attenuation is 80 db. The pulse compression result is presented in Fig. 3b. Another result without anttenuation is shown in Fig. 3a. The stability test result of amplitude and phase is shown in Fig Field experiment at the Eastern China Sea An original sea echo spectrum measured at 03:22 LT on March 16, 2013 on the coast of the Eastern China Sea is represented in Fig. 5. The incoherently processing over 13 minutes (equals 3 coherently integration period) is operated to achieve a smoother Doppler spectrum. In Fig. 5a, the large first-order (Bragg) peaks that originate from waves half the radar wavelength moving toward and away from the radar, surrounded by higher order continuum, is clearly depicted, and the SNR is about 35 db. In Fig. 5b, the Bragg peaks spread over 110 km, are clearly illustrated. 4

5 Fig. 3. Result of pulse compression output with different input amplitude. Different range delay is assigned for easy identification. The variation between output and input is due to the error of attenuation. Fig. 4. Stability test result at input amplitude of 40 dbm. The test last for 512 sweep peroids, and each sweep period is 0.5 s. Fig. 5. Sea echo spectrum of this system. The operation frequency is 13.5 MHz, and incoherently processing over 13 minutes is employed. The measured radial surface current map of the Eastern China Sea on March 16, 2013 is shown in Fig. 6. From this figure, we find that the system can map ocean surface current over 110 km range and 120 azimuth. The 5

6 Fig. 6. Radial surface current map of the Eastern China Sea on 16/03/2013. The radial current velocity of position C over one day is shown at the bottom. radial current velocity curve over one day fits the known regularity well. 5 Conclusion In this paper, a fully-digital HF backscatter radar for oceanographical remote sensing is described. This radar employs FMICW waveform and RF directly sampling, digital down-conversion, digital pulse compression techniques. Comparison between this system and ordinary IF analogue system shows that this system has particular advantages of simplicity, low-cost, highperformance and flexibility. According to the illustrated closed-loop test and field experiment results, this radar works well that the amplitude and phase stability are respectively less than db and 0.01, and the radial current velocity of sea surface waves over 110 km range coverage can be measured at 13.5 MHz operation frequency upon 100 W peak power. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC) under grant and public science and technology research funds projects of ocean under grant The authors wish to express their gratitude to the editor and the anonymous reviewers. 6