Tianmu-1 is China’s first occultation meteorological constellation capable of on-orbit compatibility among BeiDou, GPS, Galileo and GLONASS. Using the ionospheric electron-density profiles retrieved by Tianmu-1 occultations in March 2025, we analyzed the daily number of occultations and their global distribution. The quality of the peak electron density (NmF2), F2-layer peak height (HmF2) and total electron content (TEC) derived from Tianmu-1 was assessed against F2-layer critical frequency (foF2) and peak height (hmF2) measured by ionosondes, and against the post-processed Global Ionospheric Map (GIM) TEC provided by the International GNSS Service (IGS). The results show that Tianmu-1-derived NmF2 correlates with ionosonde data at 0.93, with zero mean bias, 17.9 % mean relative bias and an RMSE of 0.27 × 10⁶ el cm⁻³. For HmF2, the correlation is 0.75, the mean bias is also zero, the mean relative bias is 6.83 %, and the RMSE is 29.6 km. After mapping Tianmu-1 TEC to the ground-to-GNSS-satellite altitude range with the NeQuick model, the TEC accuracies for GPS, BDS, GLONASS and Galileo are comparable when referenced to IGS GIM TEC, yielding RMSEs of 9.64, 9.52, 9.75 and 9.56 TECu, respectively. Compared with the IRI 2020 model, the NeQuick-normalized results exhibit better agreement with IGS GIM TEC: the mean bias is only −1.87 TECu and the RMSE is reduced to 9.62 TECu, whereas the IRI model produces a mean bias of −5.59 TECu and an RMSE of 10.69 TECu.
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