A new technique for mapping of total electron content using GPS network in Japan

Abstract

The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15• × 0.15• in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path.

Regional ionosphere map over Japanese Islands

“…In the array of medium scale structure, relatively weaker enhancement has been found at each night with longer periods. This phenomenon has been studied during the magnetic storm period (Otsuka et al, 2002). However, December 2001 is a quiet month of geomagnetic activity.…”

“…After that, the instrumental biases is estimated and removed from the relative TEC so as to obtain the absolute TEC for each satellite-receiver pair along the satellite track. This procedure has been described in detail by Otsuka et al (2002), and will not be mentioned more at here. Till this step, the TEC has been obtained in geographical reference with time resolution of 30 seconds, with a potential spatial resolution of 0.15 • × 0.15 • .…”

“…GEONET is not only the powerful mission to monitor the earthquake, but also the powerful system for regional meteorological and space meteorological observe (Iwabuchi et al, 2000;Otsuka et al, 2002). A useful technique has been developed by the group of Otsuka et al (2002) for mapping regional TEC with a spatial resolution of 0.15 • × 0.15 • for every 30 seconds, from the undifferenced GPS dual frequency phase and code observable, which has been obtained by GEONET. The standard deviation of the TEC estimated from this technique is less than 1 TECU (3 TECU) at the northern (southern) part of Japan.…”

Determination of GPS receiver differential biases by neural network parameter estimation method

“…Otsuka et al [2002] developed a technique to remove instrumental biases with a least squares fitting procedure and construct two-dimensional maps of the absolute TEC over Japan by using GPS data of GEONET. Although this method can be derived the TEC maps over Japan with considerably high spatial and temporal resolutions, the error becomes more noticeable with lowering latitude of the observation areas because of the assumption that the hourly average of vertical TEC is uniform within an area covered by a receiver.…”

Transition region of TEC enhancement phenomena during geomagnetically disturbed periods at mid-latitudes

“…The high resolution TEC maps over Japan have been derived with GEONET data since 1997 (Saito et al, 1998(Saito et al, , 2001. Otsuka et al (2002) developed a new technique to derive the absolute values of vertical TEC, in which a weighted least-squares fitting is used to determine unknown instrumental biases, assuming that the hourly TEC average is uniform within an area covered by a GPS receiver. This technique used in previous studies (Tsugawa et al, 2003(Tsugawa et al, , 2004 can produce absolute values of TEC in the accuracy of 3×10 16 electrons/m 2 .…”