Geoscience Reference
In-Depth Information
4.3.4 Determining Ionospheric Parameters from VLBI Data
Although VLBI is a differential space geodetic technique it is possible to derive
absolute ionosphere parameters, i.e. VTEC for each station. As shown by Hobiger
et al. (
2006
) VTEC values can be determined similar as troposphere parameters
(see section on mapping functions and gradients by Nilsson et al. (
2013
)inthis
topic) by taking advantage of the fact that the slant ionosphere delays are elevation
dependent and can be described by an empirical mapping function (Eq.
50
). Thus
VTEC values can be estimated for each station and constant instrumental delays can
be separated from these parameters within the adjustment process. As one of the
drawbacks, the estimation of ionosphere parameters from VLBI needs a mathemat-
ical relation between VTEC above the site and the VTEC of each observation as
described in Hobiger (
2005
) or Hobiger et al. (
2006
). Moreover, as VLBI provides
only a single scan per epoch and station, it is important that mapping function errors
are reduced to a minimum in order to obtain unbiased VTEC estimates. Dettmering
et al. (
2011a
) carried out a thorough investigation of systematic differences between
VTEC obtained by different space-geodetic techniques including VLBI by applying
the estimation strategy proposed by Hobiger et al. (
2006
). Thereby it is concluded
that VLBI derived ionosphere parameters are comparable to other space geodetic
techniques, like GPS, DORIS, Jason and F3C concerning the accuracy of the esti-
mation. Moreover, the mean biases found in that study are similar to those given in
Hobiger et al. (
2006
) being in the range of a few TECU.
4.3.5 Acquiring Ionospheric Information from DORIS
The Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS)
was developed by the French CNES, Institut Géographique National (IGN) and
Groupe deRecherche enGéodésie Spatiale (GRGS) tomeet scientific and operational
user requirements in very precise orbit determination. Although the DORIS system
was primarily designed for the precise orbit computation required for observing
the oceans by altimetry missions, the unique network of ground stations and its
highly accurate positioning capability have also played a great role for geodesy
and geophysical applications. This includes measuring continental drift, fitting the
local geodetic network, monitoring the geophysical deformations, determining the
rotation and the gravity parameters of the Earth, and contributing to the realization of
an international terrestrial reference system. Due to the fact that the DORIS system
uses two different frequencies for its measurement, it is capable of monitoring the
ionosphere as well.
The basic principle of the DORIS system is based on the accurate measurement
on board the spacecraft of the Doppler shift of radio frequency signals emitted by
ground beacons. Measurements are made on two frequencies:
2GHz and 400MHz.
About 56 ground beacon stations transmit dual frequency signals from locations
distributed all over the world. The satellites carrying the DORIS receivers include
Jason, TOPEX, ENVISAT, SPOT 2, SPOT 4, and SPOT 5. These satellites are at the
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