The use of ionosondes in GPS ionospheric tomography at low latitudes

A new technique is presented for the incorporation of ionosonde observations into GPS ionospheric tomography. This approach greatly improves the vertical resolution of the images when using independent incoherent scatter radar observations as ground truth, addressing a traditional weakness of tomographic techniques. Ionosonde observations are used to set vertical basis functions adaptively within the inversion as well as providing electron density information for direct assimilation. The technique also improves slant total electron content (TEC) accuracy in the vicinity of the ionosonde. An experiment was performed in the equatorial region of South America as a 6-day case study due to the availability of incoherent scatter and ionosonde data during this period. Preliminary results were validated with the Jicamarca incoherent scatter radar and independent GPS slant TEC observations. Using the incoherent scatter radar as ground truth for the vertical profile, the new technique reduces mean NmF2 error to -0.07 × 10 11 electrons/m 3 compared with 0.27 × 10 11 electrons/m 3 in a control run with no ionosonde data, while root-mean square error is now 0.5 × 10 11 electrons/m 3 compared with 1.0 × 10 11 electrons/m 3 in the control. The new technique also results in 0.1 km mean error in hmF2, compared with -3.9 km in the control, while root-mean square hmF2 error is around 40 km in both cases. Using independent slant TEC observations, the mean error is -0.36 TECU compared with 0.64 TECU in the control run, while root-mean square error is 3.55 TECU down from 4.02 TECU, suggesting the new technique also improves TEC values.