Global Positioning System Reference
In-Depth Information
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Because the rotation axis moves in space, the coordinates of stars or extragalac-
tic radio sources change with time due to the motion of the coordinate system. A
conventional celestial reference frame (CCRF) has been defined for the fundamental
epoch
J2000
.
0
≡
January 1
,
2000
,
12h TT
(2.7)
The letter “J” in J2000.0 indicates “Julian.” In a separate section below, we treat the
subject of time in greater detail. Let it suffice here to simply state that TT represents
terrestrial time (McCarthy, 1996, p. 83), which is realized by the international atomic
time (TAI) as
32
s
.
184
TT
TA I
+
(2.8)
[21
We denote the respective coordinate system, called the mean celestial coordinate
system at epoch J2000.0, by ( X). The
Z
axis coincides with the mean pole. This
is the direction of a fictitious rotation axis that has been corrected for nutation, i.e.,
the fictitious rotation axis that is “driven” by precession only. The mean celestial
equatorial plane is the plane perpendicular to the direction of
Z
. The
X
axis lies
in the equatorial plane and points toward the vernal equinox (intersection of mean
celestial equatorial plane and ecliptic). In reality, the precise definition of the first
axis takes earlier definitions into consideration that were based on fundamental star
catalogues in order to maintain consistency.
Because the CCRF is defined for the epoch J2000.0, the directions of the axis
are stable in space per definition. The practical realization of the celestial frame, and
therefore the directions of the coordinate axes, is based on a set of celestial radio
source coordinates. The IERS selects the celestial radio sources and specifies the ob-
servation techniques and analysis procedures. The outcome of this coordinated effort
is the ICRF. Extragalactic radio sources, such as quasars, whose signals can be ob-
served with VLBI, play a key role in the establishment and maintenance of the ICRF.
Consider two widely separated VLBI antennas on the surface of the earth observing
the signals from a quasar. Because of the large distance to quasars, their direction is
the same to observers regardless of where the observer is on the earth's surface, as
well as where the earth is on its orbit around the sun. The VLBI observations allow
one to relate the orientation of the baseline, and therefore the orientation of the earth,
to the inertial directions to the quasars.
Any variation in the earth's daily rotation around the instantaneous rotation axis, in
polar motion, or any deficiencies in the adopted mathematical model of nutations, can
be detected. Today, many quasars and a global network of VLBI antennas are used to
measure and monitor these variations. The current ICRF solution includes more than
600 extragalactic radio sources. The details of VLBI are not discussed here, but they
are available in the specialized literature. VLBI techniques are very similar to those
used in GPS. In fact, the early developments in accurate GPS baseline determination
very much benefited from existing knowledge of and experience with VLBI.
Lin
—
0.0
——
Sho
PgE
[21
