The ionosphere is an atmospheric region ionized by solar radiation

cosmic rays

and various particle radiations ranging from 60 km to 1000 km above the earth. Numerous free electrons and ions in the ionosphere can interfere with radio signals. Ionospheric parameters

especially F2 layer critical frequency (f0F2) and electron density peak height (hmF2)

play an important role in the propagation of HF waves. Therefore

the study of f0F2 and hmF2

especially its global distribution

is important.The overall objective of this paper is to study the annual

diurnal

and seasonal variations of f0F2 and hmF2 in the world on the basis of GNSS occulation datasets and to compare the values f0F2 obtained by GNSS occultation and digisondes from all stations in Japan.COSMIC

GRACE

and CHAMP electron density profile datasets and digisonde datasets in Wakkanai

Kokubunji

Yamagawa

and Okinawa for 2002—2016 are selected as the data sources. The maximum electron density NmF2 and hmF2 is obtained by using the electron density profile. Compared with ionosonde f0F2 datasets

the latitude and longitude of the selected NmF2 are within 5 degrees of ionosondes

and the time of the selected NmF2 is within 15 minutes of ionosondes.Results show that the annual variation of f0F2 and hmF2 has a significant positive correlation with solar activity and geomagnetic activity and significant spatial distribution characteristics. The maximum value of f0F2 distribution on both sides of the magnetic equator is approximately 15 degrees

and the maximum value of hmF2 distribution in the equatorial region and the northern and southern hemisphere peak heights is not symmetrical. In addition

the peak value of f0F2 and hmF2 appears during LT 14:00—16:00

and hmF2 exhibits significant equatorial ionization anomaly recovery after sunset. The f0F2 has obvious seasonal characteristics; the peak value of f0F2 appears in spring and autumn at middle and low latitudes

and is evident in abnormal winter phenomenon. However

the seasonal distribution of hmF2 evidently change. The values f0F2

which were obtained by occultation and digisondes from all stations in Japan

were compared

and results indicate that the ionospheric parameters obtained by the two detection methods have acceptable correlation. The correlation in the Okinawa station is the best

with a correlation coefficient of 0.88. The correlation in the Wakkanai station is poor

with a correlation coefficient of 0.75. The correlation between Yamagawa and Kokubunji stations is close at 0.84 and 0.85

respectively.The preliminary study shows that the global f0F2 and hmF2 distribution is affected by many factors

including solar activity

geomagnetic activity

electric field

and wind and high-energy particle sedimentation.The values f0F2 obtained by occultation and digisondes from all stations in Japan were compared

and results show that the ionospheric parameters obtained by the two detection methods have acceptable correlation. However

some evident differences were observed; these differences may be due to the electron density gradient and satellite retrieval technology.