Evaluation of GPS-based ionospheric TEC map by comparing with VLBI data

Precise ionospheric total electron content (TEC) map of the Earth is useful, if it is available for calibration of ionospheric dispersive delay for space measurement techniques using microwave such as GPS, VLBI, and spacecraft navigation. Recent rapid development of GPS techniques is making it more realistic that Earth's ionosphere TEC map measured by GPS observation is practically applicable in those space measurements. For the purpose of evaluating the accuracy of the ionospheric TEC map produced from GPS measurements, two cases of TEC maps were compared with dual band VLBI TEC measurements. In one case, local TEC maps produced from observation data using TECMETERs, which are a kind of GPS receiver for TEC measurements, were compared with VLBI data. As the second case, Global Ionosphere Maps (GIMs) generated by the Center for Orbit determination in Europe (CODE) were compared with VLBI data from short baseline to intercontinental baseline. The ionospheric group delay derived from the local TEC map of the first case had about 80% correlation with VLBI data on 109 km short baseline. Also the group delay computed by using the GIM data of the CODE (GIM/CODE) had about 90% correlation with VLBI data on that baseline. In comparisons on intercontinental baselines, correlations between GIM/CODE data and TEC measured by VLBI indicated almost unity. Then it was found that more than 90% of ionospheric TEC could be predictable with that TEC map. Through further statistical analysis of TEC comparison data, the error spectrum of GIM/CODE data was computed. Comparing the obtained error spectrum with error of the spherical harmonics component of the GIM/CODE data, the latter was always smaller than the former, and the error of GIM/CODE data is suspected to be underestimated, especially at low spatial frequency. It was inferred from the spectrum that more than 0.8 TECU of ionosphere perturbations remain in the higher spatial frequency region, which is not covered by the GIM/CODE model. Total accuracy of GIM/CODE data was evaluated around 3.7-3.9 TECU. Also phase delay rates derived from the GIM/CODE were compared with VLBI data. It indicated correlation around 0.6-0.8 on intercontinental baseline, but it is not enough accuracy for practical use in phase delay rate correction in VLBI observation. The reason for low coincidence is understood by the lack of small scale and short timescale TEC variation information in that TEC map model.