Differential GPS - Understanding GPS: Principles and Applications - page 389

Satellite

d m

d u

φ ′

φ′ m

IPP Ionosphere

h

φ

U

Earth

φ m

M

M - Reference station position
U - User position
IPP - Ionospheric pierce point
R - Radius of the Earth
E

R E

ψ pp

Figure 8.7 Ionospheric delay difference.

p
d

p
d

403 .

Iono Iono

ε − ε = ⋅ − cos φ ′ ⋅ ⋅ TEC

u m m

f

2

m m

100 km

km

403 .

≈− ⋅ cos 45 °⋅ ⋅ 50 × 10

16

( )

4 2

210 ×

9

1575 . × 10

= 003 . m

The variation of the ionospheric delay difference due to differences in elevation
angle as a function of separation is shown in Figure 8.8 for three values of satellite
elevation angle and a TEC of 50

10 16 electrons/m 2 .
Spatial variations in TEC within the ionosphere typically lead to much greater
differences in ionospheric delay than those attributable to elevation angle. The dif-
ference in vertical ionospheric delays (i.e., delays observed for a satellite that is
directly overhead) due to TEC gradients is typically in the range of 0.2-0.5m over
100 km when the ionosphere is undisturbed, but it can be greater than 4m over 100
km when the ionosphere is disturbed [6, 7]. Slant range delays during daylight hours
were evaluated in [8] for a network of GPS receivers over a 1-year time frame. The
conclusions from [8] were that the difference in ionospheric delays seen by two
receivers separated by 400 km in a midlatitude region is expected to be less than 2m

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