排序方式： 共有7条查询结果，搜索用时 31 毫秒
1
1.
The Doppler orbitography and radiopositioning integrated by satellite (DORIS) system was originally developed for precise orbit determination of low Earth orbiting (LEO) satellites. Beyond that, it is highly qualified for modeling the distribution of electrons within the Earth’s ionosphere. It measures with two frequencies in L-band with a relative frequency ratio close to 5. Since the terrestrial ground beacons are distributed quite homogeneously and several LEOs are equipped with modern receivers, a good applicability for global vertical total electron content (VTEC) modeling can be expected. This paper investigates the capability of DORIS dual-frequency phase observations for deriving VTEC and the contribution of these data to global VTEC modeling. The DORIS preprocessing is performed similar to commonly used global navigation satellite systems (GNSS) preprocessing. However, the absolute DORIS VTEC level is taken from global ionospheric maps (GIM) provided by the International GNSS Service (IGS) as the DORIS data contain no absolute information. DORIS-derived VTEC values show good consistency with IGS GIMs with a RMS between 2 and 3 total electron content units (TECU) depending on solar activity which can be reduced to less than 2 TECU when using only observations with elevation angles higher than \(50^\circ \) . The combination of DORIS VTEC with data from other space-geodetic measurement techniques improves the accuracy of global VTEC models significantly. If DORIS VTEC data is used to update IGS GIMs, an improvement of up to 12 % can be achieved. The accuracy directly beneath the DORIS satellites’ ground-tracks ranges between 1.5 and 3.5 TECU assuming a precision of 2.5 TECU for altimeter-derived VTEC values which have been used for validation purposes. 相似文献
2.
Hart-Davis Michael G. Laan Stendert Schwatke Christian Backeberg Björn Dettmering Denise Zijl Firmijn Verlaan Martin Passaro Marcello Seitz Florian 《Ocean Dynamics》2023,73(8):475-491
Ocean Dynamics - With the continued rise in global mean sea level, operational predictions of tidal height and total water levels have become crucial for accurate estimations and understanding of... 相似文献
3.
Systematic differences between VTEC obtained by different space-geodetic techniques during CONT08 总被引：1，自引：1，他引：0
The ionosphere is a dispersive medium for microwaves, and most space-geodetic techniques using two or more signal frequencies
can be applied to extract information on ionospheric parameters, including terrestrial as well as satellite-based GNSS, DORIS,
altimetry, and VLBI. Because of their different sensitivity regarding ionization, their different spatial and temporal data
distribution, and their different signal paths, a joint analysis of all observation types seems reasonable and promises the
best results for ionosphere modeling. However, it has turned out that there exist offsets between ionospheric observations
of the diverse techniques mainly caused by calibration uncertainties or model errors. Direct comparisons of the information
from different data types are difficult because of the inhomogeneous measurement epochs and locations. In the approach presented
here, all measurements are combined into one ionosphere model of vertical total electron content (VTEC). A variance component
estimation is applied to take into account the different accuracy levels of the observations. In order to consider systematic
offsets, a constant bias term is allowed for each observation group. The investigations have been performed for the time interval
of the CONT08 campaign (2 weeks in August 2008) in a region around the Hawaiian Islands. Almost all analyzed observation techniques
show good data sensitivity and are suitable for VTEC modeling in case the systematic offsets which can reach up to 5 TECU
are taken into account. Only the Envisat DORIS data cannot provide reliable results. 相似文献
4.
Eva Boergens Sven Buhl Denise Dettmering Claudia Klüppelberg Florian Seitz 《Journal of Geodesy》2017,91(5):519-534
River water-level time series at fixed geographical locations, so-called virtual stations, have been computed from single altimeter crossings for many years. Their temporal resolution is limited by the repeat cycle of the individual altimetry missions. The combination of all altimetry measurements along a river enables computing a water-level time series with improved temporal and spatial resolutions. This study uses the geostatistical method of spatio-temporal ordinary kriging to link multi-mission altimetry data along the Mekong River. The required covariance models reflecting the water flow are estimated based on empirical covariance values between altimetry observations at various locations. In this study, two covariance models are developed and tested in the case of the Mekong River: a stationary and a non-stationary covariance model. The proposed approach predicts water-level time series at different locations along the Mekong River with a temporal resolution of 5 days. Validation is performed against in situ data from four gauging stations, yielding RMS differences between 0.82 and 1.29 m and squared correlation coefficients between 0.89 and 0.94. Both models produce comparable results when used for combining data from Envisat, Jason-1, and SARAL for the time period between 2002 and 2015. The quality of the predicted time series turns out to be robust against a possibly decreasing availability of altimetry mission data. This demonstrates that our method is able to close the data gap between the end of the Envisat and the launch of the SARAL mission with interpolated time series. 相似文献
5.
SARAL/AltiKa completed its first year in orbit in March 2014. The 1 Hz GDR-T data of the first 10 cycles of the mission are used to perform a comprehensive quality assessment by means of a global multi-mission crossover analysis. Within this approach, SARAL sea surface heights are compared with data from other current missions, mainly Jason-2 and Cryosat-2, to reveal its accuracy and consistency with the other altimeter systems. Alongside with global mean range bias and instrumental drifts, investigations on geographically correlated errors as well as on the realization of the systems origin are performed. The study proves the high quality and reliability of SARAL. The mission shows only a small range bias of about ?5 cm with respect to Jason-2 and neither significant time-tag bias nor instrumental drifts. With 1.3 cm the scatter of radial errors is in the same order of magnitude as for Cryosat-2 and Jason-1 GM and will probably further improve using an enhanced sea state bias (SSB) model. However, the wet tropospheric corrections from SARAL radiometer still show some systematic effects influencing the range bias as well as geographically correlated error patterns and the z-component of the origin. Improved inflight calibration will be necessary to overcome these effects. 相似文献
6.
SARAL uses the same orbit as ERS and Envisat and can be used to extend inland water height time series derived from these missions. This article investigates the potential of SARAL for this application over the Great Lakes and the Amazon basin. SARAL/AltiKa is the first altimeter using Ka-band that is rarely influenced by ionospheric effects but susceptible for atmospheric water. Our investigations show clear waveform disruptions for SARAL due to precipitation. It is demonstrated that the quality of water heights improved when using alternative retracker products, for example, the ice-1 product. The improvement depends on the weather and yields up to 3.8 cm for wet conditions. The advantage of the smaller footprint of SARAL is demonstrated for land-water transitions where SARAL provides better water level heights up to 6 km to the lakeshore whereas Envisat is limited to about 11 km. SARAL provides also more reliable water level heights for narrow Amazon rivers than Envisat. Furthermore, the hooking effect is decreased for SARAL. Comparing water level time series of SARAL-only, Envisat-only, and multi-mission with in-situ data demonstrates that SARAL has the potential to extend Envisat long-term time series and to decrease the RMS by about 10% for large lakes and 40% for selected rivers. 相似文献
7.
Combination of different space-geodetic observations for regional ionosphere modeling 总被引：2，自引：1，他引：1
Denise Dettmering Michael Schmidt Robert Heinkelmann Manuela Seitz 《Journal of Geodesy》2011,85(12):989-998
Most of the space-geodetic observation techniques can be used for modeling the distribution of free electrons in the Earth’s
ionosphere. By combining different techniques one can take advantage of their different spatial and temporal distributions
as well as their different observation characteristics and sensitivities concerning ionospheric parameter estimation. The
present publication introduces a procedure for multi-dimensional ionospheric modeling. The model consists of a given reference
part and an unknown correction part expanded in terms of B-spline functions. This approach is used to compute regional models
of Vertical Total Electron Content (VTEC) based on the International Reference Ionosphere (IRI 2007) and GPS observations
from terrestrial Global Navigation Satellite System (GNSS) reference stations, radio occultation data from Low Earth Orbiters
(LEOs), dual-frequency radar altimetry measurements, and data obtained by Very Long Baseline Interferometry (VLBI). The approach
overcomes deficiencies in the climatological IRI model and reaches the same level of accuracy than GNSS-based VTEC maps from
IGS. In areas without GNSS observations (e.g., over the oceans) radio occultations and altimetry provide valuable measurements
and further improve the VTEC maps. Moreover, the approach supplies information on the offsets between different observation
techniques as well as on their different sensitivity for ionosphere modeling. Altogether, the present procedure helps to derive
improved ionospheric corrections (e.g., for one-frequency radar altimeters) and at the same time it improves our knowledge
on the Earth’s ionosphere. 相似文献
1