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911.
Elvis A. Shukla Jagdish Prasad M. S. S. Nagaraju Rajeev Srivastava D. L. Kauraw 《Journal of the Indian Society of Remote Sensing》2009,37(1):129-137
Soil resource characterization of Dhamni micro-watershed in Chandrapur district of Maharashtra was carried out using IRS-1D
LISS-III data in conjunction with field survey and ancillary data. The study indicates that nearly 84.2 per cent of the total
geographical area of the watershed is under cultivation. Forest (mainly degraded) occupy only 4.5 per cent area Whereas wasteland
with scrub cover 9.4 per cent area of the watershed. Nine soil series were tentatively identified and mapped as soil series
association in to five mapping units. These soils belong to order Inceptisol, Vertisol and Mollisol. Except the soils of wasteland
with scrub, other soils are moderately suitable for pigeonpea and soybean and have average to good productivity. 相似文献
912.
Conventions on thermal expansion modelling of radio telescopes for geodetic and astrometric VLBI 总被引:7,自引:6,他引:1
Axel Nothnagel 《Journal of Geodesy》2009,83(8):787-792
Thermal expansion of radio telescopes has long been recognized as an effect which cannot be neglected in geodetic and astrometric
VLBI data analysis if millimeter accuracy is desired. In this article, the author documents the conventions which are being
set by the International VLBI Service for Geodesy and Astrometry (IVS) for a consistent modelling of this effect in its routine
product generation. For the largest telescopes, the annual cycle of thermal expansion may change the height of the VLBI reference
point by as much as 20 mm. However, for telescopes which are used in present-day IVS operations, the variations rather range
from 4 to 6 mm. 相似文献
913.
Phase center modeling for LEO GPS receiver antennas and its impact on precise orbit determination 总被引:12,自引:5,他引:7
Adrian Jäggi R. Dach O. Montenbruck U. Hugentobler H. Bock G. Beutler 《Journal of Geodesy》2009,83(12):1145-1162
Most satellites in a low-Earth orbit (LEO) with demanding requirements on precise orbit determination (POD) are equipped with
on-board receivers to collect the observations from Global Navigation Satellite systems (GNSS), such as the Global Positioning
System (GPS). Limiting factors for LEO POD are nowadays mainly encountered with the modeling of the carrier phase observations,
where a precise knowledge of the phase center location of the GNSS antennas is a prerequisite for high-precision orbit analyses.
Since 5 November 2006 (GPS week 1400), absolute instead of relative values for the phase center location of GNSS receiver
and transmitter antennas are adopted in the processing standards of the International GNSS Service (IGS). The absolute phase
center modeling is based on robot calibrations for a number of terrestrial receiver antennas, whereas compatible antenna models
were subsequently derived for the remaining terrestrial receiver antennas by conversion (from relative corrections), and for
the GNSS transmitter antennas by estimation. However, consistent receiver antenna models for space missions such as GRACE
and TerraSAR-X, which are equipped with non-geodetic receiver antennas, are only available since a short time from robot calibrations.
We use GPS data of the aforementioned LEOs of the year 2007 together with the absolute antenna modeling to assess the presently
achieved accuracy from state-of-the-art reduced-dynamic LEO POD strategies for absolute and relative navigation. Near-field
multipath and cross-talk with active GPS occultation antennas turn out to be important and significant sources for systematic
carrier phase measurement errors that are encountered in the actual spacecraft environments. We assess different methodologies
for the in-flight determination of empirical phase pattern corrections for LEO receiver antennas and discuss their impact
on POD. By means of independent K-band measurements, we show that zero-difference GRACE orbits can be significantly improved
from about 10 to 6 mm K-band standard deviation when taking empirical phase corrections into account, and assess the impact
of the corrections on precise baseline estimates and further applications such as gravity field recovery from kinematic LEO
positions. 相似文献
914.
Georges Balmino 《Journal of Geodesy》2009,83(10):989-995
We have applied efficient methods for computing variances and covariances of functions of a global gravity field model expanded
in spherical harmonics, using the full variance–covariance matrix of the coefficients. Examples are given with recent models
derived from GRACE (up to degree and order 150), and with simulated GOCE derived solutions (up to degree and order 200). 相似文献
915.
The determination of potential difference by the joint application of measured and synthetical gravity data: a case study in Hungary 总被引:1,自引:1,他引:0
In an elementary approach every geometrical height difference between the staff points of a levelling line should have a corresponding
average g value for the determination of potential difference in the Earth’s gravity field. In practice this condition requires as
many gravity data as the number of staff points if linear variation of g is assumed between them. Because of the expensive fieldwork, the necessary data should be supplied from different sources.
This study proposes an alternative solution, which is proved at a test bed located in the Mecsek Mountains, Southwest Hungary,
where a detailed gravity survey, as dense as the staff point density (~1 point/34 m), is available along a 4.3-km-long levelling
line. In the first part of the paper the effect of point density of gravity data on the accuracy of potential difference is
investigated. The average g value is simply derived from two neighbouring g measurements along the levelling line, which are incrementally decimated in the consecutive turns of processing. The results
show that the error of the potential difference between the endpoints of the line exceeds 0.1 mm in terms of length unit if
the sampling distance is greater than 2 km. Thereafter, a suitable method for the densification of the decimated g measurements is provided. It is based on forward gravity modelling utilising a high-resolution digital terrain model, the
normal gravity and the complete Bouguer anomalies. The test shows that the error is only in the order of 10−3mm even if the sampling distance of g measurements is 4 km. As a component of the error sources of levelling, the ambiguity of the levelled height difference which
is the Euclidean distance between the inclined equipotential surfaces is also investigated. Although its effect accumulated
along the test line is almost zero, it reaches 0.15 mm in a 1-km-long intermediate section of the line. 相似文献
916.
The diurnal cycle of the tropospheric zenith total delay (ZTD) is one of the most obvious signals for the various physical
processes relating to climate change on a short time scale. However, the observation of such ZTD oscillations on a global
scale with traditional techniques (e.g. radiosondes) is restricted due to limitations in spatial and temporal resolution.
Nowadays, the International GNSS Service (IGS) provides an important data source for investigating the diurnal and semidiurnal
cycles of ZTD and related climatic signals. In this paper, 10 years of ZTD data from 1997 to 2007 with a 2-hour temporal resolution
are derived from global positioning system (GPS) observations taken at 151 globally distributed IGS reference stations. These
time series are used to investigate diurnal and semidiurnal oscillations. Significant diurnal and semidiurnal oscillations
of ZTD are found for all GPS stations used in this study. The diurnal cycles (24 hours period) have amplitudes between 0.2
and 10.9 mm with an uncertainty of about 0.5 mm and the semidiurnal cycles (12 h period) have amplitudes between 0.1 and 4.3 mm
with an uncertainty of about 0.2 mm. The larger amplitudes of the diurnal and semidiurnal ZTD cycles are observed in the low-latitude
equatorial areas. The peak times of the diurnal cycles spread over the whole day, while the peak value of the semidiurnal
cycles occurs typically about local noon. These GPS-derived diurnal and semidiurnal ZTD signals are similar with the surface
pressure tides derived from surface synoptic pressure observations, indicating that atmospheric tides are the main driver
of the diurnal and semidiurnal ZTD variations. 相似文献
917.
Christopher Kotsakis 《Journal of Geodesy》2009,83(1):31-50
All gravity field functionals obtained from an Earth gravitational model (EGM) depend on the underlying terrestrial reference
frame (TRF), with respect to which the EGM’s spherical harmonic coefficients refer to. In order to maintain a coherent framework
for the comparison of current and future EGMs, it is thus important to investigate the consistency of their inherent TRFs,
especially when their use is intended for high precision studies. Following the methodology described in an earlier paper
by Kleusberg (1980), the similarity transformation parameters between the associated reference frames for several EGMs (including
the most recent CHAMP/GRACE models at the time of writing this paper) are estimated in the present study. Specifically, the
differences between the spherical harmonic coefficients for various pairs of EGMs are parameterized through a 3D-similarity
spatial transformation model that relates their underlying TRFs. From the least-squares adjustment of such a parametric model,
the origin, orientation and scale stability between the EGMs’ reference frames can be identified by estimating their corresponding
translation, rotation and scale factor parameters. Various aspects of the estimation procedure and its results are highlighted
in the paper, including data weighting schemes, the sensitivity of the results with respect to the selected harmonic spectral
band, the correlation structure and precision level of the estimated transformation parameters, the effect of the estimated
differences of the EGMs’ reference frames on their height anomaly signal, and the overall feasibility of Kleusberg’s formulae
for the assessment of TRF inconsistencies among global geopotential models. 相似文献
918.
Calibrating the GOCE accelerations with star sensor data and a global gravity field model 总被引:1,自引:0,他引:1
A reliable and accurate gradiometer calibration is essential for the scientific return of the gravity field and steady-state
ocean circulation explorer (GOCE) mission. This paper describes a new method for external calibration of the GOCE gradiometer
accelerations. A global gravity field model in combination with star sensor quaternions is used to compute reference differential
accelerations, which may be used to estimate various combinations of gradiometer scale factors, internal gradiometer misalignments
and misalignments between star sensor and gradiometer. In many aspects, the new method is complementary to the GOCE in-flight
calibration. In contrast to the in-flight calibration, which requires a satellite-shaking phase, the new method uses data
from the nominal measurement phases. The results of a simulation study show that gradiometer scale factors can be estimated
on a weekly basis with accuracies better than 2 × 10−3 for the ultrasensitive and 10−2 for the less sensitive axes, which is compatible with the requirements of the gravity gradient error. Based on a 58-day data
set, scale factors are found that can reduce the errors of the in-flight-calibrated measurements. The elements of the complete
inverse calibration matrix, representing both the internal gradiometer misalignments and scale factors, can be estimated with
accuracies in general better than 10−3. 相似文献
919.
Lars E. Sjöberg 《Journal of Geodesy》2009,83(10):967-972
The topographic bias is defined as the error/bias committed by continuing the external gravity field inside the topographic
masses by a harmonic function. We study the topographic bias given by a digital terrain model defined by a spherical template,
and we show that the topographic bias is given only by the potential of an inner-zone cap, and it equals the bias of the Bouguer
shell, independent of the size of the cap. Then we study the effect on the real Earth by decomposing its topography into a
template, and we show also in this case that the topographic bias is that of the Bouguer shell, independent of the shape of
the terrain. Finally, we show that the topographic potential of the terrain at the geoid can be determined to any precision
by a Taylor expansion outside the Earth’s surface. The last statement is demonstrated by a Taylor expansion to fourth order. 相似文献
920.
Many regions around the world require improved gravimetric data bases to support very accurate geoid modeling for the modernization
of height systems using GPS. We present a simple yet effective method to assess gravity data requirements, particularly the
necessary resolution, for a desired precision in geoid computation. The approach is based on simulating high-resolution gravimetry
using a topography-correlated model that is adjusted to be consistent with an existing network of gravity data. Analysis of
these adjusted, simulated data through Stokes’s integral indicates where existing gravity data must be supplemented by new
surveys in order to achieve an acceptable level of omission error in the geoid undulation. The simulated model can equally
be used to analyze commission error, as well as model error and data inconsistencies to a limited extent. The proposed method
is applied to South Korea and shows clearly where existing gravity data are too scarce for precise geoid computation. 相似文献