Automated daily process for global ionospheric total electron content mapsand satellite ocean altimeter ionospheric calibration based on Global Positioning System data

The accuracy of single-frequency ocean altimeters benefits from calibration of the total electron content (TEC) of the ionosphere below the satellite. Data from a global network of Global Positioning System (GPS) receivers pr ovides timely, continuous, and globally well-distributed measurements of io nospheric electron content. For several months we have been running a daily automatic Global Ionospheric Map process which inputs global GPS data and climatological ionosphere data into a Kalman filter, and produces global io nospheric TEC maps and ocean altimeter calibration data within 24 h of the end-of-day. Other groups have successfully applied this output to altimeter data from the GFO satellite and in orbit, determination for the TOPEX/Pose idon satellite. Daily comparison of the global TEC maps with independent TE C data from the TOPEX altimeter is performed as a check on the calibration whenever the TOPEX data are available. Comparisons of the global TEC maps a gainst TOPEX data will be discussed. Accuracy is best at mid-to-high absolu te latitudes (\latitude\ > 30 degrees) due to the better geographic distrib ution of GPS receivers and the relative simplicity of the ionosphere, Our h ighly data-driven technique is relatively less accurate at low latitudes an d especially during ionospheric storm periods, due to the relative scarcity of GPS receivers and the structure and volatility of the ionosphere. Howev er, it is still significantly more accurate than climatological models. (C) 2000 Elsevier Science Ltd. All rights reserved.