Yan Songhua

Research on Soil Moisture Sensor Nodes and Their Placement in Distributed Sensor Networks

In this paper two key factors of distributed sensor network for soil moisture monitoring are studied. One factor is the sensor design based on global navigation satellite system reflection (GNSS-R) signal, the other is the sensor placement. At first this paper introduces the construction and the advantage of non-contact soil moisture sensor, then the paper gives the optimization method based on greedy algorithm for sensor placement to conserving energy. Simulation results for multiple nodes placement confirm the validity of the proposed method.

Soil moisture estimation based on BeiDou B1 interference signal analysis

BeiDou Navigation Satellite System reflectometry (BDS-R) is an emerging area of BD (BeiDou) applications that uses multipath reflected signals in microwave remote sensing. Soil moisture estimation is one of the potential important applications of BDS-R. This study aims to investigate the feasibility of soil moisture estimation based on BD B1 band interference signals, which are composed of direct and reflected BD signals and can be readily captured using low-cost commercial BDS receivers. In this paper, a theoretical background for soil estimation from interference signals was introduced, and an analysis of field experimental data was conducted. First, a cosine model for the interference signal power was built, and a non-uniform power spectrum analysis was performed on the received interference signals to determine its main frequency. Then, a least squares curve fitting was applied on the interference signal power to extract its phase. The relationship between the soil moisture and the phase was then determined. Two months of experimental data were collected at BaoXie, Wuhan and analyzed for further inversion. Experimental results show that the phase of the interference signals increases with the increase of soil moisture. The correlation coefficient between the phase and the in-situ soil moisture value is approximately 0.8. Finally, the relationship between the phase and soil moisture is employed to estimate soil moisture. Results show that using BDS-R to measure soil moisture is feasible, which demonstrates a great potential of new application of the BD system.

A waveform generate unit suitable for software defined HFSWR

A new radio frequency waveform generate unit is presented for software-defined high frequency surface wave radar (HFSWR). Since HFSWR operates in congested band, to suppress interference and clutters, together obtain the optimal combined sequence of waveform parameters, the radar system should be designed based on software radio principle. In this paper we discuss a software defined HF radar system-OSMAR2003, and focus on design and implementation of waveform generate unit for it. The proposed waveform generation unit integrates a frequency synthesizer with low phase noise and high signal to noise rate and a re-configurable synchronization control unit. The design is started with examination of requirements of the waveform and then optimize architecture of the combination of the synthesizer technology to satisfy the system requirements. Followed is the FPGA (field programmable logic array) based synchro unit controlled by VXI-bus. The measurement shows that the transmitted signal can be synthesized quickly with phase noise less than -109 dBc/Hz at 1-kHz off carrier and spur suppression greater than 65 dBc, While the waveform parameters can be set by VXI-bus. The signal generate unit becomes an important part of the software defined HFSWR and field experiments provide the validity.

The effect of phase noise on the remote sensing of ocean surface currents

This paper studied the phase noise effect on the range spectrum and Doppler spectrum measured by high frequency (HF) radar which was used for remote sensing of sea surface state parameters. The presence of phase noise in the master oscillator caused a widening of the range spectrum and clutter velocity spectrum, degrading the range and velocity resolution. In this paper, a practical phase noise model is presented, and based on it, the relationship between the phase noise power spectrum density (PSD) and cumulative phase noise is investigated. Then, the basic minimum requirements for the PSD are calculated to maintain radar performance. The results show that a commercial off the shelf oscillator can meet the phase noise requirements.

A study on skywave-surfacewave hybrid networking technique for HF oceanography radar

HF over the horizon radar can be divided into sky wave radar and the surface wave radar according to the radio propagation mode. As a new mode, HF hybrid sky-surface wave radar is composed of the ionosphere reflection propagation mode and the surface wave receiving mode. The technique of HF sky wave and surface wave hybrid networking system for HF oceanography radar is presented and analyzed in this paper, which includes two modes: the hybrid sky-surface wave system consisting of a sky wave transmit path and a surface wave receive path, and the surface wave system consisting of a surface transmit path and a surface wave receive path. Based on this hybrid networking technique, the system combines the merits of surface wave HF radar and sky-surface wave radar, which will improve the ability of target detection and enlarge the detection range. Compared with the single mode radar system, this system has advantages of better detection performance at the far ranges and stronger anti-interference ability. The structure of the hybrid networking system is given. Besides receiving the sea echo from the surface wave radar, HF receiving array on the shore also receives the sea echo from the sky wave transmitting station that is reflected from the ionosphere. The sea echo signals from different transmitting stations is obtained and separated by frequency offset or timing delay. Finally, the experimental system and experimental data analysis are provided. The experimental results under different scenarios including hybrid sky-surface propagation mode and hybrid networking mode are presented. Preliminary results from field experiments show the technical feasibility and the good detection performance of the new radar networking system.

Sea state sensing with single frequency GPS receiver

Bi-static remote sensing with GPS signal (direct and reflect signals) has supported some researches in soil moisture measuring, sea state estimating and sea altimetry monitoring. In these researches the special designed receivers are generally required, such as the “delay-mapping” GPS receiver or open loop differential real-time receiver, because they can find the characteristics of the reflected electromagnetic wave affected by the object. Now the commercial single frequency GPS receiver becomes more lighter, smaller and lower power, it is potentially capable of sea remote sensing. However it is ignored. In this paper, a ground based GPS-R technology with single frequency receiver is introduced. The technology is based on both the scatter theory from the rough surface and the GPS double difference theory of carrier phase. This paper describes the GPS-R instrument, introduces the basic method and shows the result of GPS-R experiments on bridge. These results provide good initial conditions for widespread sea state monitoring if using GPS-R receiver network. this paper also discusses the results under the influence of the satellite elevation and single difference.

Low velocity target detection based on time-frequency image for high frequency ground wave radar

The Doppler spectral broadening resulted from non-stationary movement of target and radio-frequency interference will decrease the veracity of target detection by high frequency ground wave (HFGW) radar. By displaying the change of signal energy on two dimensional time-frequency images based on time-frequency analysis, a new mathematical morphology method to distinguish target from nonlinear time-frequency curves is presented. The analyzed results from the measured data verify that with this new method the target can be detected correctly from wide Doppler spectrum.

The design of frequency synthesizer for the ocean state measuring and analyzing radar

The design and realization of a synthesizer for high frequency radar, OSMAR2003 (ocean state measuring and analyzing radar), is proposed. To provide a 47.5 MHz local signal and 7 MHz transmitting signal, two kinds of frequency synthesizer architectures are presented, and their advantages and disadvantages are compared. The preferred synthesizer is based on an architecture supporting a flexible combination of direct synthesizer and DDS (direct digital synthesizer) techniques with a low frequency reference crystal oscillator. The benefit of this architecture is the generation of signals with low phase noise and low spurious level. The measured output spectrum and field experiments undertaken in Zhujiajian Island demonstrate successful frequency synthesis operation.