Global Positioning System Reference
In-Depth Information
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Figure 2.10
Gravity on the equipotential surface.
The astronomic latitude, longitude, and azimuth refer to the plumb line at the
ob
serving station. Figure 2.11 shows an equipotential surface through a surface point
P
and the instantaneous rotation axis and equator. The astronomic normal at point
P
, also called the local vertical, is identical to the direction of the gravity force at
th
at point, which in turn is tangent to the plumb line. The astronomic latitude
[34
Lin
—
0.1
——
Nor
PgE
at
P
is the angle subtended on the instantaneous equator by the astronomic normal.
Th
e astronomic normal and the parallel line to the instantaneous rotation axis span
th
e astronomic meridian plane at point
P
. Note that the instantaneous rotation axis
an
d the astronomic normal may or may not intersect. The astronomic longitude
Φ
Λ
is
the angle subtended in the instantaneous equatorial plane between this astronomic
m
eridian and a reference meridian, nominally the Greenwich meridian.
The geopotential number
C
is simply the algebraic difference between the poten-
tia
ls at the geoid and point
P
,
[34
C
=
W
0
−
W
(2.61)
Figure 2.11
Astronomic latitude.
