GPS-based ionospheric corrections for single frequency radar altimetry Attila Komjathy*, George H. Born Colorado Center for Astrodynamics Research, University of Colorado at Boulder, CB 431, Boulder, CO 80309-0431, USA Received 5 February 1999; received in revised form 12 May 1999; accepted 24 May 1999 Abstract The global ionospheric total electron content maps (GIMs) provide integrated electron densities between the ground and the GPS satellite altitude (20,200 km). Satellite altimeter ionospheric delay corrections require integrated electron densities between the ground and altimeter satellite altitude. In the case of the Geosat Follow-On (GFO) spacecraft, ¯ying at 800 km, we estimated that using GIM TEC data alone, up to a 2 cm path delay can be introduced into the GFO measurements for high solar activity period by not taking into account the electron content above this altitude. Furthermore, the GIMs can have errors of 20±30 TECU in low latitudes for high solar activity in areas where there is little GPS data (such as over the oceans). In this paper, we describe the results of ingesting GIM TEC data into the International Reference Ionosphere model (IRI-95) to mitigate these two eects. # 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Current satellite missions such as the European Remote Sensing satellites (ERS-1, ERS-2) and the Geosat Follow-On (GFO) mission are equipped with single frequency radar altimeters to obtain ocean height measurements for the study of ocean circula- tion and its variability. Unlike with dual frequency altimeters the path delay due to the ionosphere can- not directly be removed from the altimeter range measurements. For a radar altimeter operating at a frequency of 13.6 GHz, such as the one carried by GFO, this path delay can be greater than 20 cm at solar maximum or during ionospheric storms due to disturbed solar and geomagnetic conditions (1 cm path delay at 13.6 GHz corresponds to 4.6 TECU; 1 Total Electron Content Unit, TECU, corresponds to 10 16 electrons/m 2 ). Therefore, to maximize the accu- racy of ocean height measurements obtained with single frequency radar altimeters, it is necessary to use alternative means to remove the propagation delay imposed by the ionosphere. A workshop was convened by the Jet Propulsion Laboratory 1±3 December 1998 to discuss the ``GPS Applications to the Structure and Dynamics of the Earth's Oceans and the Ionosphere: Measurements, Analysis, Instrument Calibration, and Related Technologies''. The research discussed here was pre- pared and presented in the workshop session ``Ionospheric Calibration: Accuracy for Ocean Altimetry''. Participants were asked to demonstrate the ability of their proposed model/technique to reproduce the total electron content (TEC) data de- rived from the TOPEX/Poseidon (T/P) dual-fre- quency altimeter data. Journal of Atmospheric and Solar-Terrestrial Physics 61 (1999) 1197±1203 1364-6826/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S1364-6826(99)00051-6 * Corresponding author. Tel.: +1-303-492-4829; fax: +1- 303-492-2825. E-mail address: komjathy@gps.colorado.edu (A. Komjathy).