Evaluation of IRI95 ionosphere model for radar altimeter applications Original Research Article

Using the TOPEX dual-frequency (Ku- and C-band) radar altimeter measured nadir total electron contents (TECs) from an orbital altitude of 1354 km, the fidelity of the IRI95 ionosphere model and GPS-derived Global Ionosphere Map (GIM) have been evaluated. In particular, the models have been evaluated over the ocean for the time period 1993–1999, representing low, medium and high solar activity periods. In addition, the evaluation is also focused on the model performance for TEC variations due to local solar time, geographical locations, and long-term trend, or drift. The major spatial and temporal features of ionosphere depicted by IRI95 model agrees reasonably well when comparing with TOPEX ground-truth, except during high solar activity period in 1999. The error in IRI95 would translate into an apparent sea level change of 0.3–0.6 mm/yr for the majority (70%) of the global ocean. During high solar activities in 1999, the error reaches 1 mm/yr. Results indicate that GIM provides more accurate ionosphere delays during high solar activities. For the use of single-frequency altimeters (e.g., GFO) for sea level studies during high solar activity period, ionosphere models need further improvement.