Validation of Jason-1 Nadir Ionosphere TEC Using GEONET

Abstract

The Jason-1 dual-frequency nadir ionosphere Total Electron Content (TEC) for 10-day cycles 1–67 is validated using absolute TEC measured by Japan's GPS Earth Observation Network (GEONET), or the GEONET Regional Ionosphere Map (RIM). The bias estimates (Jason–RIM) are small and statistically insignificant: 1.62 ± 9 TECu (TEC unit or 10¹⁶ electrons/m², 1 TECu = 2.2 mm delay at Ku-band) and 0.73 ± 0.05 TECu, using the along-track difference and Gaussian distribution method, respectively. The bias estimates are –3.05 ± 10.44 TECu during daytime passes, and 0.02 ± 8.05 TECu during nighttime passes, respectively. When global Jason-1 TEC is compared with the Global Ionosphere Map (GIM) from the Center for Orbit Determination in Europe (or CODE) TEC, the bias (Jason–GIM) estimate is 0.68 ± 1.00 TECu, indicating Jason-1 ionosphere delay at Ku-band is longer than GIM by 3.1 mm, which is at present statistically insignificant. Significant zonal distributions of biases are found when the differences are projected into a sun-fixed geomagnetic reference frame. The observed biases range from –7 TECu (GIM larger by 15.4 mm) in the equatorial region, to +2 TECu in the Arctic region, and to +7 TECu in the Antarctica region, indicating significant geographical variations. This phenomena is primarily attributed to the uneven and poorly distributed global GPS stations particularly over ocean and near polar regions. Finally, when the Jason-1 and TOPEX/Poseidon (T/P) TECs were compared during Jason-1 cycles 1–67 (where cycles 1–21 represent the formation flight with T/P, cycles 22–67 represent the interleave orbits), the estimated bias is 1.42 ± 0.04 TECu. It is concluded that the offset between Jason/TOPEX and GPS (RIM or GIM) TECs is < 4 mm at Ku-band, which at present is negligible.

Keywords:

• Jason-1
• GPS
• TEC
• GONET
• TOPEX/Poseidon

Acknowledgments

The first two authors acknowledge the support by the Japanese RISE/SELENE project and a grant from JSPS program. We thank Japan's Geographical Survey Institute for providing the GEONET data. The data processing using the IBM supercomputer and other platforms were carried out at the National Astronomical Observatory in Mizusawa and the Earth Science School in Kyoto University. TOPEX/Poseidon and Jason-1 altimeter data have been provided by NASA/JPL's PO-DAAC. The Ohio State University researchers acknowledge supports from grants by NASA's Earth Science Enterprise Interdisciplinary Science program, NOAA's Laboratory for Satellite Altimetry, NSF's Aeronomy program and NSF's Aeronomy program (ATM 0418844).

Color figures of this article are available at: http://geodesy.eng.ohio-state.edu/publications