A METHOD TO ESTABLISH GPS GRID IONOSPHERIC CORRECT MODEL

Transcription

1 A METHOD TO ESTABLISH GPS GRID IONOSPHERIC CORRECT MODEL Shuanggen Jn Shangha Astronocal Observatory, Chnese Acadey of Scences, Shangha 00030, Chna. E-al: or Abstract: The onospherc nfluence s one of the largest error sources n GPS postonng and navgaton after closng the Selectve Avalablty (SA. Therefore, t s avalable to establsh a real te onospherc correcton odel to elnate or tgate the onospherc nfluence. In ths paper, a new ethod, Hardy Functon Interpolaton ethod, s presented to establsh a hgh precson grd onospherc odel (GIM over the Yangtze Rver Delta usng contnuous GPS data of SIGAN network, and the nternal and external accuracy of TEC fro the GIM are evaluated. It has shown that the real te and hgh precson GIM over the Yangtze Rver delta s well establshed usng the Hardy Functon Interpolaton. The nternal and external accuracy of TEC fro the GIM are all saller than 0.3 and better than the ethods of dstance weght functon of WAAS and sphercal haronc functon. In addton, these ethods are further used to ntally nvestgate and analyze the seasonal varatons of TEC over Yangtze Rver delta. Keywords: GPS, Grd onospherc odel (GIM, TEC, Hardy Functon Interpolaton INTRODUCTION The onospherc affects the transsson of electroagnetc waves, whch can result n dsturbance or ntersson of sgnals (Jn and Zhu, 00. The agntude of ths effect s deterned by the aount of total electron content (TEC and the frequency of electroagnetc waves. The Global Postonng Syste (GPS s a satellte crowave technque whose sgnals are transtted on crowave (L-band carrers through the Earth s atosphere, and t nevtably suffers the onospherc effect. The agntude of nfluence on GPS sgnals s usually n the range fro a few eters to tens of eters but t could reach ore than 00 eters durng severe onosphere stors (e.g. Lu et al, 999. Therefore, the onospherc effect ust be estated so that a correcton can be ade to elnate or tgate for hgh precson GPS postonng applcatons. Addtonally, precse estates of onospherc effect are also portant for space weather researches and predctons of the onospherc events. Now GPS recevers of double-frequency can ontor the varaton of onospherc electron content (e.g. Yuan and Ou, 00; Zhao X F, 003. Establshng a hgh precson GIM s one of the crtcal steps for nvestgatng the onosphere usng GPS data. IGS Ionosphere Workng Group also uses the GPS-based GIM to research and nvestgate the varaton and acton of global onosphere. The onosphere s related to the te, season, locaton, and actvty of the Sun, and therefore, the dfference of onospherc effects on the propagaton of rado n dfferent local areas or te s very large (e.g. Yuan Y and Ou J, 00. For soe (grd onospherc odels establshed by GPS, the paraeters of the onospherc odels are usually deterned by fttng all GPS data n the large area, whch lts the precson of the odel due to neglectng the local character of onosphere and also s not sutable for analyzng the local effects of onospherc

2 paraeters (e.g. Yuan and Ou, 00; Lu et al, 999; Zhao X F, 003. For a sall and local regon, any ethods are used to nvestgate grd onospherc odel usng GPS data fro ground GPS networks n the past several years, such as the ethods of dstance weght functon of WAAS (e.g. Skone, S., 998, sphercal haronc functon and the polynoal functon (e.g. Schaer, 999; Kojathy, 997. In ths paper, a new ethod, Hardy Functon Interpolaton ethod, s presented to establsh a hgh precson grd onospherc correcton odel over the Yangtze Rver delta regon usng GPS data of the Shangha Integraton GPS Applcaton Network (SIGAN. The nternal and external accuracy of TEC fro the GIM are evaluated to nvestgate the possblty that the GIM provdes hgh precson onospherc correcton. HARDY FUNCTION INTERPOLATION The current eprcal onospherc correcton odels, such as the Klobuchar odel, could only correct about 50% of the total onosphere effects (e.g. Klobuchar, 987. Therefore, the ore precse onosphere odel s requred. As an approxaton, the onosphere ay be consdered to be a thn layer,.e. an onospherc sphercal shell, to a heght of 350 k above the earth s surface (e.g. Otsuka Y, 00. The grd onospherc odel values are the vertcal onospherc delays or vertcal total onospherc content (VTEC at the specfed Ionospherc Grd Ponts (IGPs..e. the ntersecton ponts of the selected longtude and lattude lnes coverng the area. In 977, Hardy developed the Hardy Functon Interpolaton (HFI ethod and appled t to analyze crustal vertcal deforaton (e.g. Lu et al, 00. We here apply the HFI ethod to establsh GPS grd onospherc odel over a sall local regon. For the GPS ste coordnate (B, L j, the VTEC of fttng odel can be wrtten as (e.g. Lu et al, 00: n VTEC (B = Q ( B, B a ( where VTEC(B,L s the VTEC of the easureent ponts n a sngle plane, (B and L s the coordnate of grd node, and a s the vector of the VTEC value at the grd pont. The correspondng core functon can be expressed as follows: Q ( B, B [( B B ( L L ] ( In Eq. (, ε and β are the sooth factor, whose eprcal values are 0.0 and 0.5 respectvely (Lu et al, 00. If there are easureent ponts(b j,l j and n grd nodes (B, L, and let n grd nodes (B,L as central stes of core functon Q.Thus, the VTEC of each GPS epoch n the Eq. ( can be wrtten as follows: Naely: v VTEC Qa (3 ( v ( B v ( B v ( B VTEC VTEC VTEC Q ( B Q ( B, B, B a Q ( B, B n n a Q ( B, B n n a n (4

3 The error equaton s as follows: v Qa (5 v V T E C Through the Eq. (4 we can obtan the vector a by a weghted least squares adjustent to all GPS observatons n each epoch, naely a T T ( Q P Q Q P v (6 v v where P s the weght atrx of the VTEC. And that, we can obtan the VTEC of rando grd v stes, naely: VTEC h = Q t t a, where Q Q Q Q h h. ( h h h SIGAN network In order to accurately ontor and predct the atospherc state paraeters over the Yangtze Rver delta regon, Chna, startng fro 999 SHAO (Shangha Astronocal Observatory, Chnese Acadey of Scences, Shangha eteorology bureau, and Shangha nsttute of surveyng and appng jontly establshed the Shangha Integrated GPS Applcaton Network (SIGAN, together wth the nternatonal GPS servce (IGS staton Shangha (SHAO. The SIGAN network s a 4-hour contnuous operatng geodetc GPS arrays consstng 4 GPS statons n whch the dual frequency Ashtech TM GPS recevers were nstalled, coverng the Yangtze Rver delta regon (Fgure.. Snce July 00, the SIGAN network has been operatng norally. All the stes transt GPS data of 30 sec and surface eteorologcal data of 6 n to the center of data process (SHAO every 30 n. Fgure. GPS ste dstrbuton of Shangha Integraton GPS applcaton Network RESULTS AND APPLICATIONS To easure the TEC over the Yangtze Rver delta regon, we have developed a Grd Ionospherc odel of TEC over n the Yangtze Rver Delta usng the Hardy Functon Interpolaton ethod wth a spatal resoluton of 0. o x0. o. The TEC of each grd ste can be apped for every 30 seconds fro the GPS dual frequency phase and code observable. For exaple, at 6:00(UT on Septeber 7, 003, the dstrbuton of VTEC of each grd stes wth a spatal resoluton of 0. o x0. o and contour of VTEC over the Yangtze Rver delta regon are 3

4 shown clearly n Fgure ( TECU 0 6 e /, and the ean standard devaton of the TEC estated fro ths GIM s less than TECU. The VTEC varatons every sx hours over the Yangtze Rver regon are shown n Fgure 3. TECU Longtude (deg Lattude (deg 3 Fgure. The VTEC and contour over the Yangtze Rver delta regon (UT: 6:00 Fgure 3. VTEC varatons over the Yangtze Rver delta regon on Septeber 7, 003 4

5 The nternal and external accuracy of VTEC reflect the qualty of VTEC estated fro the Grd Ionospherc Model, where the nternal accuracy denotes the root ean square (RMS of the dscrepancy between the observng and fttng VTEC value, and the external accuracy represents the root ean square (RMS of VTEC. In order to test the relablty of VTEC estated fro the Grd Ionospherc Model of HFI (Hardy Functon Interpolaton, we develop another two knds of Grd Ionospherc Models usng the dstance weght functon of Wde Area Augent Syste (WAAS (e.g. Chao and Tsa 996; Skone S, 998; Lu et al, 999 and sphercal haronc functon ethod (Schaer, 999, respectvely. The correspondng nternal and external accuracy of VTEC are derved respectvely. Fgure 4 shows coparsons of the nternal and external accuracy of VTEC fro three Grd Ionospherc Models. It has seen that the accuraces fro the three ethods are alost consstent, and the ean nternal accuracy s 0., and the ean external accuracy s 0.3. However, the nternal and external accuracy fro Hardy Functon Interpolaton are slghtly better than the ones fro other two ethods. Therefore, t s avalable to establsh the GIM wth the HFI ethod. Fgure 4. Coparsons of the nternal and external accuracy of TEC fro GIMs (Note: the vertcal axs s the accuracy of vertcal delay n L frequency (unt: eter;, 3 and 5 represent the nternal accuracy fro Hardy Functon Interpolaton, dstance weght functon of WAAS and sphercal haronc functon ethods respectvely; and, 4 and 6 represent the external accuracy fro Hardy Functon Interpolaton, dstance weght functon of WAAS and sphercal haronc functon ethods respectvely In addton, the grdded TECs over the Yangtze Rver Delta n January (wnter, Aprl (sprng, July (suer and October (autun 003 are further derved fro the SIGAN network usng the Hardy Functon Interpolaton, dstance weght functon of WAAS and sphercal haronc functon ethods. These results are alost close to each other wth less than 5 TECU. The seasonal varatons of TEC over the Yangtze Rver delta are nvestgated and analyzed. The greater TECs are found n the equnoxes,.e. the so-called seannual anoaly. Generally speakng, the TEC s hgher n wnter than n suer fro n dayte, but opposte n nghtte. Ths s due anly to the fact that the loss rate of electron densty depends anly on the olecular ntrogen concentraton [N] wth soe contrbuton fro olecular oxygen concentraton [O] n F regon, whle the producton rate depends on the atoc oxygen concentraton [O]. The coposton changes can be a result of the equnox anoaly effect of TEC due to the convecton of atoc oxygen fro the suer to the wnter 5

6 hesphere (Torr and Torr, 973. In addton, Torr and Torr (973 also suggested that the seannual TEC anoaly s due to the seannual varatons n neutral denstes assocated wth geoagnetc and auroral actvty. Mllward et al. (996, usng the coupled therosphere onosphere plasasphere odel (CTIP, showed that the offset of the geoagnetc axs fro Earth s spn axs s the cause of the seannual anoaly of noonte NF n the South Aercan sector. Concluson and dscusson The Hardy Functon Interpolaton (HFI, dstance weght functon of WAAS and sphercal haronc functon ethods are used to establsh the grd onospherc odels (GIM over the Yangtze Rver Delta usng contnuous GPS data of SIGAN network. The nternal and external accuracy of VTEC fro three ethods are all better than 0.3, but the nternal and external accuracy fro the HFI are slghtly better than ones fro other two ethods. Therefore t s relable to establsh a real te and hgh precson GIM over the Yangtze Rver delta regon wth the Hardy Functon Interpolaton ethod. Usng the regonal GIM odel fro the Hardy Functon Interpolaton t can produce ore detaled nstantaneous aps of the regonal onosphere, as copared to other global odels, e.g., ontorng and forecastng the local onospherc events over the Yangtze Rver delta regon. Furtherore, the seasonal varatons of grdded TEC over the Yangtze Rver Delta n 003 are ntally nvestgated and analyzed fro the SIGAN network usng the Hardy Functon Interpolaton, dstance weght functon of WAAS and sphercal haronc functon ethods. It has shown that the greater TECs are found n the equnoxes, whle the TEC s hgher n wnter than n suer fro n dayte, but opposte n nghtte. Ths s due anly to the coposton changes n the olecular ntrogen concentraton [N], olecular oxygen concentraton [O] and atoc oxygen concentraton [O] n F regon. In the future, ore detaled analyss n spato-teporal varaton characterstcs of TEC over the Yangtze Rver Delta wll be analyzed usng ult-satellte data (e.g. GPS and TOPEX data. APPENDIX.REFERENCES Chao Y. C and Tsa, Y. J. (996 Generaton of onospherc correcton and confdence estates for WAAS. Insttute of Navgaton, Proceedng of the ION Annual Techncal Meetng, Massachusetts: Cabrdge, Gao Y, X. Lao and Z. Lu. (00 Ionosphere Modelng Usng Carrer Soothed Ionosphere Observatons fro a Regonal GPS Network. Geoatca, 56(: Lu J N, J. Y. Chen, and Y. P. Zhang, et al.(999 Prncple and Methods on Establshng WADGPS. Bejng: Press of Surveyng and Mappng, pp Lu J N, C. Sh, and Y. B. Yao (00 Hardy Functon Interpolaton and Its Applcatons to the Establshent of Crustal oveent speed fled. Geoatcs and Inforaton Scence of Wuhan Unversty, 6(6: Mllward, G.H., Rshbeth, H., Fuller-Rowell, T.J., Aylward,A.D., Quegan, S., and Moffett, R.J. (996 Ionospherc F-layer seasonal and seannual varatons. Journal of Geophyscal Research 0,

7 Jn Shuanggen and Wenyao Zhu (00 Advances of GPS eteorology and analyss of ontorng exaple. Journal of Nature, 4(: Klobuchar, J.A. (987 Ionospherc Te-Delay Algorth for Sngle-Frequency GPS Users. IEEE Transactons on Aerospace and Electronc Systes, Vol. AES-3, No.3: Kojathy, A. (997 Global Ionospherc Total Electron Content Mappng Usng the Global Postonng Syste, Ph.D. dssertaton, Departent of Geodesy and Geoatcs Engneerng Techncal Report NO. 88, Unversty of New Brunswck, Fredercton, New Brunswck, Canada. Mannucc A.J., Wlson B.D. and Edwards C.D. (993 A New Method for Montorng the Earth Ionosphere Total Electron Content Usng the GPS Global Network, Proceedngs of ION GPS 93, pp Otsuka Y., T. Ogawa, A. Sato, T.Tsugawa, et al. (00 A new technque for appng of total electron content usng GPS network n Japan. Earth Planets Space 54: Sardon, E., Rus, A., and Zarraoa, N.(994 Estaton of the transtter and recever dfferental bases and the onospherc total electron content fro Global Postonng Syste observatons. Rado Scence, 9(3: Skone, S. (998 Wde Area Ionosphere Grd Modelng n the Auroral Regon. Ph.D thess, The Unversty of Calgary, Calgary, Alberta, Canada. Schaer, S.(999 Mappng and Predctng the Earth s Ionosphere Usng the Global Postonng Syste. Ph.D dssertaton, Astronocal Insttute, Unversty of Berne, Swtzerland. Torr, M.R., and D.G. Torr (973 The seasonal behavour of the F-layer of the onosphere. Journal of Atospherc and Terrestral Physcs 35, Zhao X. F (003. Study on establshent of the Ionospherc Grd Model. Master dssertaton, Wuhan Unversty. Yuan Y. B and J. K. Ou (00 Usng GPS to nvestgate the onospherc delay and the change law of the electron densty. Chnese Global Postonng Syste, 6(: 9-. 7