Global Positioning System Reference
In-Depth Information
Atmospheric Research (NCAR) (DS464.0; (http://dss.ucar.edu/datasets/ds464.0). Notably,
these results are adjusted by a scale factor 2.28 mm/hPa, which are accounted for in the
comparison. These pressure data origin from more than 8000 land and ocean weather
stations including the Global Telecomuncation System (GTS) and marine reports from the
Comprehensive Ocean-Atmosphere Data Set (COADS) (Dai and Wang 1999). The plots of
diurnal and semidiurnal ZTD cycles show general similarities, indicating that the diurnal
and semidiurnal atmospheric tides are probably the main driver of the diurnal and
semidiurnal ZTD variations derived from GPS (Jin et al., 2009).
3. Ionospheric sounding
The GPS consists of a constellation of 24 operating satellites in six circular orbits 20,200 km
above the Earth at an inclination angle of 55º with a 12-h period. The satellite transmits two
frequencies of signals (f1 = 1575.42 MHz and f2 = 1227.60 MHz). The equations of carrier
phase (L) and code observations (pseudorange P) of double frequency GPS can be expressed
as follows:
i
i
i
i
i
i
i
(7)
L
=
λφ
= −
ρ
d
+
d
+
c
(
τ τ λ
)
(
b
+
N
)
k j
,
kkj
,
ion k j
,
,
tropj
,
j
kkj
,
k j
,
i
i
i
i
i
+
i
(8)
P
= +
ρ
d
+
d
+
c
(
ττ
)
+
d
+
d
ε
k j
,
ion k j
,
,
tropj
,
j
qk
,
q k j
,
,
j
where superscript i and subscript j represent the satellite and ground-based GPS receiver,
respectively, ρ is the distance between satellite i and GPS receiver j,
d d are the
ionospheric and tropospheric delays, respectively, c is the speed of light in vacuum space, τ
is the satellite or receiver clock offset, b is the phase delay of satellite and receiver instrument
bias, d is the code delay of satellite and receiver instrumental bias, λ is the carrier
wavelength, φ is the total carrier phase between the satellite and receiver, N is the
ambiguity of carrier phase, and ε is the other residuals. From Eq.(7) and (8), the ionospheric
delay can be determined, which is useful for ionospheric delay correction and space weather.
and
ion
trop
3.1 2-D ionospheric imaging
The ionospheric delay can be determined from the double frequency GPS phase and code
(pseudorange) observations as
11
i
i
(9)
L
=
φφ
−=−
40.3
F z VTEC
( )
(
β
, )
s
+
B
4
1
j
2
j
4
2
2
f
f
1
2
11
i
i
(10)
PP P
=
−=
40.3
FzV
( )
C sb
(
β
, )
+
4
1
j
2
j
4
2
2
f
f
1
2
is (
)
i
i
i
i
where
Fz is the mapping function,
, and
b is
()
B
B
=−
λ
(
b
+
N
)
+
λ
(
b
+
N
)
4
1
1
j
1
j
2
2
j
2
j
i
i
(
dq
dq
)
+
(
dq
dq
)
. The Differential Code Biases ( b 4 ) can be obtained through GPS
1
j
2
j
1
2
N
carrier phase observations, and
B
can be obtained through the formula,
,
(
p
+−
Lb N
) /
4
4
4
i
=
1
where N is the epoch of GPS observation (Jin et al., 2008). For the TEC representation, a
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