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This paper evaluates the sensitivity of ITRF2008-based satellite laser ranging (SLR) station positions estimated weekly using LAGEOS-1/2 data from 1993 to 2012 to non- tidal time-vary ing gravity (TVG). Two primary methods for mod...
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This paper evaluates the sensitivity of ITRF2008-based satellite laser ranging (SLR) station positions estimated weekly using LAGEOS-1/2 data from 1993 to 2012 to non- tidal time-vary ing gravity (TVG). Two primary methods for modeling TVG from degree-2 are employed. The operational approach applies an annual GRACE-derived field, and IERS recommended linear rates for five coefficients. The experimental approach uses low-order/degree 4×4 coefficients estimated weekly from SLR and DORIS processing of up to 11 satellites (tvg4x4). This study shows that the LAGEOS-1/2 orbits and the weekly station solutions are sensitive to more detailed modeling of TVG than prescribed in the current IERS standards. Over 1993-2012 tvg4x4 improves SLR residuals by 18 % and shows 10 % RMS improvement in station stability. Tests suggest that the improved stability of the tvg4x4 POD solution frame may help clarify geophysical signals present in the estimated station position time series. The signals include linear and seasonal station motion, and motion of the TRF origin, particularly in Z. The effect on both POD and the station solutions becomes increasingly evident starting in 2006. Over 2008-2012, the tvg4x4 series improves SLR residuals by 29 %. Use of the GRGS RL02 50 × 50 series shows similar improvement in POD. Using tvg4x4, secular changes in the TRF origin Z component double over the last decade and although not conclusive, it is consistent with increased geocenter rate expected due to continental ice melt. The test results indicate that accurate modeling of TVG is necessary for improvement of station position estimation using SLR data.
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We examine the impact of using seasonal and long-period time-variable gravity field (TVG) models on GPS orbit determination, through simulations from 1994 to 2012. The models of time-variable gravity that we test include the GRGS ...
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We examine the impact of using seasonal and long-period time-variable gravity field (TVG) models on GPS orbit determination, through simulations from 1994 to 2012. The models of time-variable gravity that we test include the GRGS release RL02 GRACE-derived 10-day gravity field models up to degree and order 20 (grgs20x20), a 4 9 4 series of weekly coefficients using GGM03S as a base derived from SLR and DORIS tracking to 11 satellites (tvg4x4), and a harmonic fit to the above 4 9 4 SLR-DORIS time series (goco2s_fit2). These detailed models are compared to GPS orbit simulations using a reference model (stdtvg) based on the International Earth Rotation Service (IERS) and International GNSS Service (IGS) repro1 standards. We find that the new TVG modeling produces significant along, cross-track orbit differences as well as annual, semi-annual, draconitic and long-period effects in the Helmert translation parameters (Tx, Ty, Tz) of the GPS orbits with magnitudes of several mm. We show that the simplistic TVG modeling approach used by all of the IGS Analysis Centers, which is based on the models provided by the IERS standards, becomes progressively less adequate following 2006 when compared to the seasonal and long-period TVG models.
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We have investigated the quality of precise orbits for the SARAL altimeter satellite using Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) data from March 14, 2013 to Aug...
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We have investigated the quality of precise orbits for the SARAL altimeter satellite using Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) data from March 14, 2013 to August 10, 2014. We have identified a 4.31 ± 0.14 cm error in the Z (cross-track) direction that defines the center-of-mass of the SARAL satellite in the spacecraft coordinate system, and we have tuned the SLR and DORIS tracking point offsets. After these changes, we reduce the average RMS of the SLR residuals for seven-day arcs from 1.85 to 1.38 cm. We tuned the non-conservative force model for SARAL, reducing the amplitude of the daily adjusted empirical accelerations by eight percent. We find that the best dynamic orbits show altimeter crossover residuals of 5.524 cm over cycles 7-15. Our analysis offers a unique illustration that high-elevation SLR residuals will not necessarily provide an accurate estimate of radial error at the 1-cm level, and that other supporting orbit tests are necessary for a better estimate. Through the application of improved models for handling time-variable gravity, the use of reduced-dynamic orbits, and through an arc-by-arc estimation of the C_(22) and S_(22) coefficients, we find from analysis of independent SLR residuals and other tests that we achieve 1.1-1.2 cm radial orbit accuracies for SARAL. The limiting errors stem from the inadequacy of the DPOD2008 and SLRF2008 station complements, and inadequacies in radiation force modeling, especially with respect to spacecraft self-shadowing and modeling of thermal variations due to eclipses.
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The NASA GSFC DORIS analysis center has processed data from January 1993 to December 2014 and provided 1141 weekly solutions in the form of normal equations for incorporation into the DORIS solution for ITRF2014. The solution time...
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The NASA GSFC DORIS analysis center has processed data from January 1993 to December 2014 and provided 1141 weekly solutions in the form of normal equations for incorporation into the DORIS solution for ITRF2014. The solution time series, designated as gscwd26, were based on tracking data to eleven DORIS satellites divided generally into seven-day arcs. With respect to the ITRF2008 submission (Le Bail et al., 2010), the measurement model was updated to model the beacon frequency variations at certain DORIS sites, to apply the DORIS antenna phase law for the Starec and Alcatel antennae, and to apply the antenna offset corrections in the NASA GSFC orbit determination software rather than using the data-supplied corrections. We show that computing the antenna offset corrections in the orbit determination software is superior to using the offset corrections that are supplied with the DORIS data, and that this improves the RMS of fit for SPOT-2, Envisat, SPOT-4, and SPOT-5. The updates for the force model included: (1) the development of improved nonconservative force modeling for SPOT-2, SPOT-3, SPOT-5, Envisat, and HY-2A, and (2) the application of an updated static gravity model based on GRACE and GOCE data, and weekly models of the variations in the low degree gravity field deduced independently from tracking by Satellite Laser Ranging (SLR) and DORIS. The post-ITRF2008 DORIS coordinate WRMS after the launch of Envisat and SPOT-5 is improved from 11.20 to 12.45 mm with ITRF2008 (Le Bail et al., 2010), to between 8.50 and 9.99 mm with the gscwd26 SINEX solution. The application of the DORIS antenna phase laws shifts the DORIS scale wrt DPOD2008 by +6.0 mm from 1993/01/03 to 2002/06/06, and by +11.4 mm from 2002/06/13 to 2011/10/30. The application of more detailed models of time-variable gravity reduces the slopes in the Helmert transformation parameters Tx, and Ty (w.r.t. DPOD2008) after 2005. The annual amplitude in these parameters is reduced from 3.2 mm (for Tx), 4.1 mm (for Ty), to 1.7 mm (for Tx) and 2.8 mm (for Ty). Published by Elsevier Ltd. on behalf of COSPAR. This is an open access article under the CC BY-NC-ND license.
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This paper evaluates orbit accuracy and systematic error for altimeter satellite precise orbit determination on TOPEX, Jason-1, Jason-2 and Jason-3 by comparing the use of four SLR/DORIS station complements from the International ...
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This paper evaluates orbit accuracy and systematic error for altimeter satellite precise orbit determination on TOPEX, Jason-1, Jason-2 and Jason-3 by comparing the use of four SLR/DORIS station complements from the International Terrestrial Reference System (ITRS) 2014 realizations with those based on ITRF2008. The new Terrestrial Reference Frame 2014 (TRF2014) station complements include ITRS realizations from the Institut National de l'Information Geographique et Forestiere (IGN) ITRF2014, the Jet Propulsion Laboratory (JPL) JTRF2014, the Deutsche Geodaetisches Forschungsinstitut (DGFI) DTRF2014, and the DORIS extension to ITRF2014 for Precise Orbit Determination, DPOD2014. The largest source of error stems from ITRF2008 station position extrapolation past the 2009 solution end time. The TRF2014 SLR/DORIS complement impact on the ITRF2008 orbit is only 1-2 mm RMS radial difference between 1992-2009, and increases after 2009, up to 5 mm RMS radial difference in 2016. Residual analysis shows that station position extrapolation error past the solution span becomes evident even after two years, and will contribute to about 3-4 mm radial orbit error after seven years. Crossover data show the DTRF2014 orbits are the most accurate for the TOPEX and Jason-2 test periods, and the JTRF2014 orbits for the Jason-1 period. However for the 2016 Jason-3 test period only the DPOD2014-based orbits show a strong and statistically significant margin of improvement. The positive results with DTRF2014 suggest the new approach to correct station positions or normal equations for non-tidal loading before combination is beneficial. We did not find any compelling POD advantage in using non-linear over linear station velocity models in our SLR & DORIS orbit tests on the Jason satellites. The JTRF2014 proof-of-concept ITRS realization demonstrates the need for improved SLR+DORIS orbit centering when compared to the Ries (2013) CM annual model. Orbit centering error is seen as an annual radial signal of 0.4 mm amplitude with the CM model. The unmodeled CM signals show roughly a 1.8 mm peak-to-peak annual variation in the orbit radial component. We find the TRF network stability pertinent to POD can be defined only by examination of the orbit-specific tracking network time series. Drift stability between the ITRF2008 and the other TRF2014-based orbits is very high, the relative mean radial drift error over water is no larger than 0.04 mm/year over 1993-2015. Analyses also show TRF induced orbit error meets current altimeter rate accuracy goals for global and regional sea level estimation.
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DORIS is a globally distributed, all-weather satellite tracking system providing near-continuous precise Doppler coverage of the TOPEX/Poseidon (T/P), Jason-1, ENVISAT, and SPOT series of satellites. The DORIS system, which has be...
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DORIS is a globally distributed, all-weather satellite tracking system providing near-continuous precise Doppler coverage of the TOPEX/Poseidon (T/P), Jason-1, ENVISAT, and SPOT series of satellites. The DORIS system, which has been critical in establishing the high-precision orbit determination standards now enjoyed by these missions, continues to evolve and improve. There is a small, 5 to 10 μs, discrepancy between the DORIS time-tag computed by Centre National d'Etudes Spatiales (CNES) using the DORIS timing data, and a time-tag estimated using DORIS range-rate data with respect to orbits referred to Satellite Laser Ranging (SLR) or Global Positioning System (GPS) time. This discrepancy is evaluated using DORIS time biases estimated over T/P, Jason-1, and ENVISAT orbits, which are computed using SLR or GPS tracking and reference SLR or GPS time. Although DORIS is installed on other satellites, these are the only three where the DORIS time bias can be observed with the help of an alternate tracking system -SLR or GPS. For T/P, Jason-1 and ENVISAT, this DORIS time bias quantities to 5-10μs. Over the span of the T/P mission following cycle 92, this time bias has ranged from + 10 to —10μs. This paper addresses the precision and nature of the estimated time biases by evaluating such estimates over orbits computed with various gravity field models, computed using SLR+DORIS and GPS tracking, and computed by the Goddard Space Flight Center (GSFC), Jet Propulsion Laboratory (JPL), and CNES analysis centers. The paper includes descriptions of the DORIS measurement, time-tag processing, expected time-tag error, and time bias estimation. In describing the estimated DORIS time bias, the paper offers a new approach for evaluating the DORIS resolution capability.
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The direct recovery of surface mass anomalies using GRACE KBRR data processed in regional solutions provides mass variation estimates with 10-day temporal resolution. The approach undertaken herein uses a tailored orbit estimation...
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The direct recovery of surface mass anomalies using GRACE KBRR data processed in regional solutions provides mass variation estimates with 10-day temporal resolution. The approach undertaken herein uses a tailored orbit estimation strategy based solely on the KBRR data and directly estimates mass anomalies from the GRACE data. We introduce a set of temporal and spatial correlation constraints to enable high resolution mass flux estimates. The Mississippi Basin, with its well understood surface hydrological modelling available from the Global Land Data Assimilation System (GLDAS), which uses advanced land surface modeling and data assimilation techniques, and a wealth of groundwater data, provides an opportunity to quantitatively compare GRACE estimates of the mass flux in the entire hydrological column with those available from independent and reliable sources. Evaluating GRACE'S performance is dependent on the accuracy ascribed to the hydrological information, which in and of itself is a complex challenge (Rodell in Hydrogeol J, doi:10.1007/sl0040-006-0103-7, 2007). Nevertheless, the Mississippi Basin is one of the few regions having a large hydrological signal that can support a meaningful GRACE comparison on the spatial scale resolved by GRACE. The isolation of the hydrological signal is dependent on the adequacy of the forward mass flux modeling for tides and atmospheric pressure variations. While these
models have non-uniform global performance they are excellent in the Mississippi Basin. Through comparisons with the independent hydrology, we evaluate the effect on the solution of changing correlation times and distances in the constraints, altering the parameter recovery for areas external to the Mississippi Basin, and changing the relative strength of the constraints with respect to the KBRR data. The accuracy and stability of the mascon solutions are thereby assessed, especially with regard to the constraints used to stabilize the solution. We show that the mass anomalies, as represented by surface layer of water within regional cells have accuracy estimates of ±2-3 cm on par with the best hydrological estimates and consistent with our accuracy estimates for GRACE mass anomaly estimates. These solutions are shown to be very stable, especially for the recovery of semi-annual and longer period trends, where for example, the phase agreement for the dominant annual signal agrees at the 10-day level of resolution provided by GRACE. This validation confirms that mascons provide critical environmental data records for a wide range of applications including monitoring ground water mass changes.
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Changes in the directional reflectance properties of pressed and sintered polytetrafluoroethylene (PTFE) diffusers induced by exposure to vacuum ultraviolet (VUV) irradiation before and after controlled contamination with Pennzane...
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Changes in the directional reflectance properties of pressed and sintered polytetrafluoroethylene (PTFE) diffusers induced by exposure to vacuum ultraviolet (VUV) irradiation before and after controlled contamination with Pennzane are presented in this paper. A set of 99% reflective, white, optical grade diffuse calibration standards were irradiated with a VUV source positioned at 60 degrees to the diffuser normal. The bidirectional reflectance distribution functions before and after contamination and VUV irradiation were measured and compared at a number of scatter geometries and wavelengths in the UV, VIS and IR spectral ranges. The 8 degrees directional hemispherical reflectance's were also measured and compared from 200 nm to 2500 nm. Our results indicate a measureable impact of VUV irradiation on pressed and sintered PTFE diffusers as manifested by a directional dependent change in their reflectance. Such an effect needs to be considered in the on-orbit deployment of PTFE diffusers.
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We compute a series of Jason-2 GPS and SLR/DORIS-bused orbits using ITRF2005 and the std0905 standards (Lemoine et al., 2010). Our GPS and SLR/DORIS orbit data sets span a period of 2 years from cycle 3 (July 2008) to cycle 74 (Ju...
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We compute a series of Jason-2 GPS and SLR/DORIS-bused orbits using ITRF2005 and the std0905 standards (Lemoine et al., 2010). Our GPS and SLR/DORIS orbit data sets span a period of 2 years from cycle 3 (July 2008) to cycle 74 (July 2010). We extract the Jason-2 orbit frame translational parameters per cycle by the means of a Helmert transformation between a set of reference orbits and a set of test orbits. We compare the annual terms of these time-series to the annual terms of two different geocenter motion models where biases and trends have been removed. Subsequently, we include the annual terms of the modeled geocenter motion as a degree-1 loading displacement correction to the GPS and SLR/DORIS tracking network of the POD process. Although the annual geocenter motion correction would reflect a stationary signal in time, under ideal conditions, the whole geocenter motion is a non-stationary process that includes secular trends. Our results suggest that our GSFC Jason-2 GPS-based orbits are closely tied to the center of mass (CM) of the Earth consistent with our current force modeling, whereas GSFC's SLR/DORIS-based orbits are tied to the origin of ITRF2005, which is the center of figure (CF) for sub-secular scales. We quantify the GPS and SLR/DORIS orbit centering and how this impacts the orbit radial error over the globe, which is assimilated into mean sea level (MSL) error, from the omission of the annual term of the geocenter correction. We find that for the SLR/DORIS std0905 orbits, currently used by the oceanographic community, only the negligence of the annual term of the geocenter motion correction results in a - 4.67 ± 3.40 mm error in the Z-component of the orbit frame which creates 1.06 ± 2.66 mm of systematic error in the MSL estimates, mainly due to the uneven distribution of the oceans between the North and South hemisphere.
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A comprehensive analysis supported by experimental results is provided for RF and microwave power conversion efficiency (PCE) of photodiodes driven by optical signals with either shaped or pure sinusoidal intensity-envelopes where...
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A comprehensive analysis supported by experimental results is provided for RF and microwave power conversion efficiency (PCE) of photodiodes driven by optical signals with either shaped or pure sinusoidal intensity-envelopes where the shaping is performed with a Mach–Zehnder modulator (MZM). It is shown that optical envelope-shaping provides a significantly improved maximum theoretical PCE of 67% as compared to 50% for sinusoidal intensity-envelopes. Practical PCEs of 40.5%–53.5% have been achieved as compared to previously published efficiencies in the 32%–41% range. The results demonstrate that high-current photodiodes can be used as efficient wideband high-power output stages in microwave photonic distribution systems leading to less complex and more efficient antenna array backplanes.
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