Global Positioning System Reference
In-Depth Information
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[88
Lin
—
1.8
——
Sho
PgE
Figure 3.13
Power spectral densities.
quency, standard running at 5.0 MHz. In order to account for relativistic effects, this
value is adjusted to 4.99999999782 MHz. As is the case with GPS, there are C/A-
codes on L1 and P-codes on L1 and L2, although the code structures differ. The
satellite clocks are steered according to UTC(SU). The GLONASS satellite clocks,
therefore, are adjusted for leap seconds.
The GLONASS broadcast navigation message contains satellite positions and
velocities in the PZ90 ECEF geodetic system and accelerations due to luni-solar
attraction at epoch
t
0
. These data are updated every 30 minutes and serve as ini-
tial conditions for orbital integration. The satellite ephemeris at the epoch
t
b
with
|
[88
15 min is calculated by numerical integration of the differential equations
of motion (3.69). Because the integration time is short, it is sufficient to consider a
simplified force model for the acceleration of the gravity field of the earth. Since the
gravitational potential of the earth is in first approximation rotationally symmetric,
the contributions of the tesseral harmonics
m
t
b
−
t
0
| ≤
0 are neglected in (3.71). Since
C
20
C
n
0
for
n>
2, we neglect the higher-order zonal harmonics. With these
simplifications the disturbing potential (3.71) becomes
=
=
µ
a
e
r
3
=
µ
a
e
r
3
C
20
P
2
(
cos
R
θ
)
J
2
P
2
(
cos
θ
)
(3.94)
J
2
3
a
e
r
3
=
µ
1
2
2
cos
2
θ −
