Design And Implementation Of Double Antenna GNSS-R Data Acquisition System

GNSS-Reflectometry(GNSS-R)is the newly developed remote sensing detection technology based on the Global Navigation Satellite System(GNSS).This method adopts the direct reflection signals of GNSS for passive reception to overcome the shortcomings of traditional remote sensing detection,e.g.smaller detection areas and poor spatiotemporal resolution.At present,the GNSS-R can be used for the inversion of sea surface height,ocean wind field,significant wave height,soil moisture,vegetation height etc,which plays an important role in marine environment detection,navigation safety,meteorological forecast,and production and living fields.GNSS-R receives and processes satellites signals with the receiver to invert the information from land and sea surfaces.The essential parameters during the inversioin is the time delay power curve and distance difference between direct signals and reflected signals,and it is therefore important to acquire the inversion observation with the GNSS-R receiver.According to these considerations,this thesis designed and developed a data acquisition system of GNSS-R.The main research contents are as follows:(1)A dual-antenna GNSS-R data acquisition system is designed based on the characteristic of GNSS satellite direct signals and reflected signals.The acquisition system is composed of three modules,i.e.antenna,RF front-end,and baseband signal processor.An FPGA is adopted to complete the baseband signal processing and obtain the basic observation quantities in order to improve the work efficiency.A baseband signal processing algorithm is designed for the acquisition system,and the system can realize the whole process of data acquisition from receiving the original signal to outputting basic observation quantities.(2)The dual-antenna GNSS-R data acquisition system is developed with the above design.Depending on the characteristics of the direct reflection signal,the function-matched antenna is first selected to receive the direct signal and reflected signal.Two MAX2771 chips are used to realize the signal down-conversion in the RF front-end receiving link.The FPGA chip XC7S75 is used as the core processor to realize the signal processing and communication.Then,the hardware platform is carried out based on the design scheme,and the design and fabrication of the 6-layer printed circuit board are completed with full consideration of electromagnetic compatibility,signal integrity,impedance matching,and other factors.Particularly,the signal lines of direct reflection channels are made equal in length to minimize the impact of hardware design on the time delay between channels.Finally,the hardware platform of the dual-antenna GNSS-R data acquisition system is built with the hardware circuit as its core.(3)A direct reflection channel baseband signal processing algorithm is proposed based on the hardware platform of the data acquisition system.Firstly,the RF chip register is configured.Secondly,the RF module downconvert the signal down to an IF signal of 4.092MHz with a sampling width of 26MHz and quantization of 2bits.Next,the carrier and pseudo-code of the reflected signal are stripped from the signal.In order to improve the capture efficiency of the reflected signal,the obtained frequency offset of the direct signal and the pseudo-code phase value are passed to the reflection channel as reference values to realize the rapid capture of the reflected signal.Finally,the correlated power values of the two channels are transmitted to the computer through RS422 for correlated power curve fitting between the channels to determine the pseudocode phase delay amount,and then the propagation distance difference of the reflected signal is obtained.In this system,a dual-channel GNSS-R data acquisition system is designed and developed to process the direct reflection signals separately.The operating power of this hardware platform is 0.7W,which has the obvious advantages of low power consumption,low cost,and compatibility with multi-system and multi-frequency signals.The correlation power curve of the direct reflection signal is obtained by the baseband signal processing algorithm,and the propagation distance difference between the two channels is obtained after peak fitting.This work provides data support for the subsequent surface parameter inversion and also provides a reference for other GNSS-R receiving systems.