Geoscience Reference
In-Depth Information
τ
if
When the ionospheric-free linear combination Eq.
81
is evaluated, a biased delay
is obtained
f
gx
f
gx
−
f
gs
f
gx
−
τ
if
=
f
gs
τ
gx
−
f
gs
τ
gs
f
gx
f
gx
−
f
gs
f
gx
−
=
τ
if
+
f
gs
τ
inst
,
x
−
f
gs
τ
inst
,
s
,
(84)
τ
where the notation
is used to express overall instrumental delay, caused by a
weighted difference between X- and S-band receiving system delays. Although one
might think that this would cause a problem in further processing steps, geodetic
analysis is not affected by these instrumental delays. As long as instrumental delays
do not change between the scans, there will be no impact on geodetic results. They
can be treated as a constant bias of the delay measurements, independent of azimuth
and zenith distance and are absorbed into the clock models (Ray and Corey
1991
).
Also the computed ionospheric correction for X-band group delay measurements
Eq.
82
has to be replaced now by the intrinsic one (
τ
τ
ig
,
x
), including the receiving
system biases
f
gs
f
gx
−
τ
igx
=−
f
gs
(τ
gx
−
τ
gs
)
f
gs
f
gx
−
f
gx
+
=
f
gs
(τ
inst
,
s
−
τ
inst
,
x
)
.
(85)
τ
inst
The scaled difference between S- and X-band instrumental delay, denoted by
τ
inst
,
is always contained in the X-band ionospheric correction.
VLBI2010
Since 2003 the International VLBI service for Geodesy and Astronomy (IVS) has
been developing the next generation VLBI system called VLBI2010 .The VLBI2010
system concept differs from the current geodetic VLBI mode in a variety of ways
which also affects the calculations of the ionosphere contribution. With the current
data, the geodetic analyst is expected to remove the ionospheric dispersive delay
by forming linear combinations and iteratively solving the ambiguity. VLBI2010
will lead to a paradigm change where the dispersive delays are removed during
band-width synthesis respectively fringe fitting, taking advantage of the broad-band
observables which should permit access to phase delay observables.
