Ground GNSS Ionosphere Sounding - GNSS Remote Sensing: Theory, Methods and Applications - page 85

Fig. 4.11 NmF2 differences
of monthly averaged median
of GPS reconstruction with
the IRI-2001 model

4

2

0

−2

Jan
Apr
Jul
Oct

−4

−6

0 5 10 15 20 25

Local time (hour)

the IRI-2001, and just GPS reconstruction with the IRI-2001. Figure 4.11 shows
the comparison of the monthly averaged diurnal NmF2 difference (DNmF2) from
GPS reconstruction with respect to the IRI-2001 model at the grid point (37.5 ı N,
127.0 ı E) in winter, spring, summer and autumn, 2003, where the standard URSI
foF2 peak model for the IRI-2001 was used to estimate NmF2. It can be seen
that the GPS-derived NmF2 is larger than the estimation of the IRI-2001 model in
nighttime for all seasons (00:00-07:00 LT and 21:00-24:00 LT), but much smaller
in daytime for all seasons from 08:00 to 20:00 LT, indicating that the IRI-2001
model underestimates NmF2 values in nighttime for all seasons, and significantly
overestimates NmF2 values in daytime for all seasons.
For a good ionospheric model, its electron density profiles N (',, t ) should give
a good F2-layer peak electron density NmF 2(',, t ). To assess the IRI-2001 model
with GPS reconstruction, a simple deviation percentage can be defined as following:

DN mF 2 .'; ; t/ D Œ.NmF2 GP S .'; ; t/ NmF2 IRI .'; ; t// =
NmF2 IRI .'; ; t/ 100 % (4.31)

where NmF 2 GPS (',, t ) and NmF 2 IRI (',, t ) are the electron density peak values of
GPS reconstruction and IRI-2001, respectively, and DNmF 2(',, t ) is the deviation
percentage value of maximum ionospheric electron density NmF 2 GPS (',, t ) with
respect to the NmF 2 IRI (',, t ). T hus, the mo nthly averaged diurnal percentage
variations of the monthly median DN mF 2.t / can be obtained. Figure 4.12 shows
a comparison of monthly averaged density peak deviation percentage as calculated
from GPS reconstruction with respect to the IRI-2001 at the grind point (37.5 ı N,
127.0 ı E) in winter, spring, summer and autumn, 2003. It can be seen that the IRI-
2001 model underestimates the NmF2 values by 30 % in nighttime for all seasons,
and significantly overestimates NmF2 values in daytime for all seasons, especially
in autumn and winter.