Static Soil Moisture Retrieval Measurements Based on GNSS-R

Transcription

1 Intenational Confeence on Computational Systems an Communications (ICCSC 2017) Static Soil Moistue etieval Measuements Base on GSS- TEGDA Pei1,a an YUEKU Pei1,b 1 Steet Xuefuajie, 10, Dalian Economic Technological Development Zone, Liaoning, China, a @qq.com, bpeiyuekun@126.com Keywos: GSS-, Soil moistue, Softwae eceive, Dielectic constant. Abstact. GSS eflectomety (GSS-) is a emote sensing tool fo extacting Eath suface infomation. This wok pesents a peliminay esult of a static measuement applying the softwae GPS eceive. The system configuation consists of both ight Han Ciculaly Polaization (HCP) an Left Han Ciculaly Polaization (LHCP) antennas fo eceiving eflecte signals. aw sample ata wee stoe into two PCs fo post pocessing in oe to get Signal to oise atio (S) fo both polaizations. Powe eflectivity behavio was theoetically analyze an compae with obtaine esults. A tial ielectic constant etieval of the meaow was attempte, an the esults wee coheent with the in-situ soil moistue state. 1 Intouction Global avigation Satellite System eflectomety (GSS-) has been wiely stuie fo emote sensing of the Eath sufaces such as sea win an oughness, snow an ice epth, vegetation coveage an soil moistue since it was fist bought out in 1993 [1]. GSS tansmits signals lying in L ban, theefoe the eceive eflecte signal is sensitive enough to the elative pemittivity of the teain (top 0-4cm). Moe ecently, soil ielectic constant an soil moistue etieval has state to pouce some esults. Thee iffeent obseving stategies wee implemente fo this kin of applications: (1) Multipath effect an it s elation to soil moistue [2]. It exploits the elationship between the total eceive signal amplitue behavio an the soil moistue fo the etieval. (2) Intefeence Patten Technique (IPT). Soil moistue etieval is base on fining a specific notch point fom the intefeence patten, vesus satellite elevation [3]. (3) Bi-static metho. This is base on the sepaate eception of iect an eflecte signals using iffeent antennas an on the sepaate measuement of signal powes. Depening on the antenna configuation, two possible obseving systems exploiting the bi-static geomety can be futhe ientifie: (a) A own-looking Left Han Ciculaly Polaization (LHCP) antenna an an up-looking HCP antenna. LHCP antenna eceives eflecte GPS signal fom the suface. Powe eflectivity coul be obtaine eithe by using a bi-static aa equation o fom the powe atio between the eflecte signal an the iect signal [4]. eflectivity is then a function of ielectic constant of the soil, the elevation angle an the suface Publishe by CSP 2017 the Authos 29

2 oughness. (b) One HCP up-looking antenna an two own-looking antennas with one HCP polaize an the othe LHCP polaize [5]. With this configuation, it s possible to measue both the co-pola component of the teain eflectivity (using LHCP signal) an the coss-pola component (using the HCP antenna). The atio of these two eflectivities was veifie to be in goo coelation with soil moistue an it was inepenent fom the suface oughness. In this wok, the eceiving system is as epicte by (b) above an is shown in Fig. 1. It s base on GPS softwae eceive. The HCP an LHCP antennas eicate fo eceiving eflecte signal mounte on an am ae fixe on a tipo. The elevation angle of the am is ajustable epening on the specula point position on the goun. aw signals wee own convete an sample into igital values by aio fequency font-ens (SiGE GPS fonten) connecte to the antennas. Digital binay ata wee futhe estoe into laptops sepaately though ata gabbe softwae Gab evelope by AVSAS Goup of Politecnico i Toino, Italy. A sampling fequency of MHz is use, giving about 8 samples pe C/A coe chip. Fo static measuements, no navigation infomation is equie fom iect signal, but iect signal can be use as efeence fo calculating powe eflectivity. Howeve, ou setup oesn t consist of HCP antenna pointing upwa to eceive iect signal since ou fist concen is to eive ielectic constant of soil though the atio between co- an coss-polaization eflectivities in which case the iect signal contibution can be cancele out. See etails in the following section. But it s easy to a the iect signal eceiving pat epening on the etieval pocess we ecie to use. Fig. 1 eceive set up an geomety, the white ellipse inicates the Fesnel Zone The total aw-sample eflecte GSS signals acquie uing the measuement wee pocesse into Delay Dopple Maps (DDMs) an Delay Wavefoms exploiting a fully open loop scheme, in oe to evaluate Signal to oise atios (Ss) time seies, without the necessity to wait fo stana GSS close loop acquisition an tacking. Fo each available satellite, an with a 0.5 secon inteval, a S value elate to the eceive signal is ecoe. As shown in Fig. 1, we i seveal expeiments on the meaow in font of the GM builing of Tuin. The time slot was caefully chosen by using ou specula point peiction softwae such that the specula points of two GPS satellites wee close enough fo compaison of the etieve esults. One of the ielectic constant etieval esult of the meaow is shown in the esult section an it coincies with the in-situ soil moistue. 2 etieval Metho Stating fom S of iect HCP signal an eflecte LHCP an HCP signals, the etieval pocess is shown in the following: 30

3 PL SL P, S P, S P. (1) whee P is signal powe, supescipts an states fo eflecte an iect path, subscipts L, an ae LHCP, HCP an oise, espectively. Powe eflectivity chaacteizing the suface eceive by LHCP an HCP antennas ae popotional to: PL L P, P. (2) If the suface is pefectly smooth, the powe eflectivity of specula eflection can be witten as the moulus squae of Fesnel eflection coefficient without ega to the suface oughness by the citeia escibe in [6],, 2 L L,, 2. (3) whee is Fesnel coefficient, is the incience angle of GPS signal (complementay angle of GPS satellite elevation), is elative ielectic constant of the soil. Futhemoe, is popotional to soil moistue fo a cetain soil type. Applying the Fesnel eflection coefficient equations [7], the elationships between L ( ) an elevation angle of satellite une 3 iffeent soil moistue conitions (fo san) ae shown in Fig. 2 elationships between L ( ) an elevation angle of satellite une 3 iffeent soil moistue conitions As shown in Fig. 2, the LHCP Fesnel coefficient L is much highe than when the satellite elevation angle is high. L inceases while eceases as the volumetic soil moistue inceases. As ou measuements wee planne to eceive signals of satellite with highe elevation angle, an it coul be seen fom Fig. 2 that L shoul be highe than. egaing Eq. 1, 2 an 3, we expect S L highe than S in a pefectly smooth suface conition, an combine the equations we coul eive ielectic constant fom S compute. 3 Measuements an esults Fist tial expeiment was one using the geomety an configuation as shown in Fig. 1, two ays of signal wee expecte to have signal eflections fom the meaow uing the ata acquisition, which ae P 26 an P 28. esults of S of both HCP an LHCP uing the one minute ata collection wee shown in Fig

4 Fig. 3 S measuements on both HCP an LHCP of P 26 an P 28 As shown in Fig. 3, fo both P 26 an P 28, S of LHCP is moe than 10B highe than that of HCP. Duing time, Ss with same polaization ae in the simila level, except that the LHCP S fom P 28 has about 3 B incease. This coul be esulte fom seveal easons incluing tansmitte powe attenuation, suface oughness o gass coveage. Applying ou etieval algoithm, the peliminay esults of ielectic constant obtaine fom S of P28 is shown in Fig. 4. The ten of the ielectic constant is simila with an mainly etemine by the LHCP S, compaing with the black line in Fig. 3. The etieve ielectic constant was elatively low an it coespone to the in-situ soil moistue state, which was vey y. Fig. 4 etieve ielectic constant fom the output S of P 28 shown in Fig. 3 4 Conclusions The configuation using both LHCP an HCP to eceive eflecte GPS signals wee applie an teste. Peliminay esults show that the S follows the pinciple of LHCP an HCP eflectivity. A futhe HCP antenna fo eceiving iect signal as efeence can be exploite in oe to eliminate the effect of iect signal powe change an to get powe eflectivity iectly. Dielectic constant etieval pocesses ae une stuying an estimating consieing the suface oughness state an othe influence. Futue wok is on getting a moe eliable ielectic constant etieving algoithm an its veification. Futhe measuement campaigns ae une planning, with the emote sense suface well 32

5 known an contolle (soil moistue content can be manually change an monitoe), an the etieve ielectic constant will be compae with the in-situ measuement. efeences [1] S. Jin, G. P. Feng, an S. Gleason, emote sensing using GSS signals: Cuent status an futue iections, Avances in Space eseach, vol. 47, pp (2011) [2] K. M. Lason, J. J. Baun, E. E. Small, V. U. Zavootny,E. D. Gutmann, an A. L. Bilich, GPS multipath an its elation to nea-suface soil moistue content, IEEE Jounal of Selecte Topics in Applie Eath Obsevations an emote Sensing, vol. 3, no. 1, pp (2010) [3]. oiguez-alvaez, X. Bosch-Lluis, A. Camps, A. Aguasca, M. Vall-Llossea, E. Valencia, I. amos-peez, an H. Pak, eview of cop gowth an soil moistue monitoing fom a goun-base instument implementing the Intefeence Patten GSS- technique, aio Science, vol. 46, S0C03, pp (2011) [4] S. Gleason, S. Hogat, Y. Sun, C. Gommenginge, S. Mackin, M. Aja, an M. Unwin, Detection an pocessing of bistatically eflecte GPS signals fom low eath obit fo the pupose of ocean emote sensing,, IEEE Tansactions on Geoscience an emote Sensing, vol. 43, no. 6, pp (2005) [5] A. Egio, M. Capaini, G. uffini, S. Paloscia, E. Santi,L. Gueieo,. Pieicca an. Flouy, Global navigation satellite systems eflectomety as a emote sensing tool fo agicultue, emote Sensing, no. 4, pp (2012) [6] F. T. Ulaby,. K. Mooe, an A. K. Fung, Micowave emote sensing: active an passive, Aison Wesley, vol. 3. (1986) [7] S. J. Katzbeg an J. L. Gaison J. Utilizing GPS to etemine ionospheic elay ove the ocean, ational Aeonautics an Space Aministation, Langley eseach Cente. (1996) 33