Minkang Cheng

Anatomy of apparent seasonal variations from GPS‐derived site position time series

[1] Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the ‘‘peering’’ approach. Peering is a way to depict the contributions of the comparatively well-known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that � 40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales. INDEX TERMS: 1223 Geodesy and Gravity: Ocean/Earth/atmosphere interactions (3339); 1247 Geodesy and Gravity: Terrestrial reference systems; KEYWORDS: seasonal variation, GPS, time series

Determination of long-term changes in the Earth's gravity field from satellite laser ranging observations

Temporal changes in the Earths gravity field have been determined by analyzing satellite laser ranging (SLR) observations of eight geodetic satellites using data spanning an interval of over 20 years. The satellites used in the analysis include Starlette, LAGEOS 1 and 2, Ajisai, Etalon 1 and 2, Stella, and BE-C. Geophysical parameters, related to both secular and long-period variations in the Earths gravity field, including the geopotential zonal rates ( , , , , and ) and the 18.6-year tide parameter, were estimated. The estimated values for these parameters are ; ; ; ; ; (centimeters) and S18.6+20 = −0.1±0.2 (centimeters). The amplitude and phase for the 18.6-year tide are in general agreement with the effects predicted by the Earths mantle anelasticity. The solution accuracy was evaluated by considering the effects of errors in various non-estimated dynamical model parameters and by varying the data span and data sets used in the solution. Estimates for from individual LAGEOS 1 and Starlette SLR data sets are in good agreement. The lumped sum values for and are very different for LAGEOS 1 and Starlette. The zonal rate determination is limited to degree 6 with the current SLR data sets. Analysis of the sensitivity of the solution for the zonal rates to the satellite tracking data span suggests that the temporal extension of the current SLR data sets will enhance the solution of zonal rates beyond degree 6.

Seasonal variations in low degree zonal harmonics of the Earth's gravity field from satellite laser ranging observations

Seasonal variations in the Earths gravity field were determined using satellite laser ranging (SLR) observations from multisatellite. The time series for the variations of the even zonal harmonics, Jl(l = 2, 4, 6, and 8), were determined using the SLR data from the geodetic satellites, including Starlette, Ajisai, Stella, LAGEOS I and LAGEOS II, during the period from October 1993 to December 1996. Owing to uncertainties in the eccentricity excitation for LAGEOS I and II, the variations of J3 and J5 were determined using only the SLR data from Starlette, Ajisai, and Stella. The seasonal variations of J2, J4, J6 and J8 become separable using the existing multisatellite SLR data sets collected in 15-day time intervals. The amplitude (normalized and in units of 10−10) and phase (in a cosine conversion and in units of degrees) for the annual variation in Jl (l= 2, 3, 4, 5, 6, and 8) are estimated to be (1.25 ± 0.1, 140 ± 10), (2.16 ± 0.21, 341 ± 19), (1.07 ± 0.1, 338 ± 15), (1.12 ± 0.16, 152 ± 16), (0.26 ± 0.17, 337 ± 9), and (1.03 ± .016, 209 ± 10), respectively. The observed annual variations of J3 and J5 are essentially opposite in phase. This phenomenon results in a different lumped sum effect for various satellites. For example, the lumped sum of J3 and J5 annual variation from LAGEOS I is twice as large as that from Starlette. The excitation due to the mass redistribution in the atmosphere and ocean and the changes in continental water storage were considered in this study using the available global geophysical data, which included the European Centre for Medium-Rang Weather Forecasts atmospheric surface pressure, the TOPEX/Poseidon altimetry derived sea surface anomalies, and the World Monthly Surface Station Climatic Data. Overall, the variation in the even zonal coefficients due to the atmospheric mass redistribution is responsible for 30% to 60% of the observed annual variations in the node residual for Starlette and LAGEOS I. Comparison indicates that the oceanic mass movement, in particular, the continental water change produce comparable contributions to the seasonal zonal variations. The amplitude of the observed annual variation of J2 is found to fall between the values predicted from the models of the surface water, ocean, and atmosphere with and without the inverted barometer (IB) oceanic response, but the phase is in good agreement with the IB models. The nontidal mass redistributions in atmosphere, ocean, and continental water change can only account for ∼13% of the semiannual variation in J2 but are the primary excitation sources for semiannual variations in the higher-degree zonal terms.

Geocenter Variations from Analysis of SLR Data

The Earth’s center of mass (CM) is defined in the satellite orbit dynamics as the center of mass of the entire Earth system, including the solid earth, oceans, cryosphere and atmosphere. Satellite Laser Ranging (SLR) provides accurate and unambiguous range measurements to geodetic satellites to determine variations in the vector from the origin of the ITRF to the CM. Estimates of the Global mass redistribution induced geocenter variations at seasonal scales from SLR are in good agreement with the results from the global inversion from the displacements of the dense network of GPS sites and from ocean bottom pressure model and GRACE-derived geoid changes.

Neutral Density Measurements from the Gravity Recovery and Climate Experiment Accelerometers

Predicting the orbits of space objects in low-altitude orbits requires an accuratemodel for the atmospheric neutral density. The current accuracy of semi-empirical models limits the prediction accuracy and impacts a number of operational decisions. The currentmodels are based on sparsemeasurements of the neutral density, collected over an extended period. One of the problems is observing the thermosphere density changes in response to the solar and geomagnetic variability on short temporal scales, such as those characterized by geomagnetic storms. The stochastic behavior of the solar forcing represents one of the major challenges in predicting satellite orbits. In situ measurements of thedensity canplay a significant role in improving the structure of the neutral densitymodels and in providing a timelymeasurement for enhancing the accuracy of the satellite predictions.Measurements fromorbiting accelerometers carried by the twin Gravity Recovery and Climate Experiment satellites have the potential for providing accurate and timelymeasurements to improve the satellite prediction accuracy. The objective of this paper is to describe the procedure for using the Gravity Recovery and Climate Experiment accelerometer measurements for determining accurate density measurements.

Anisotropic reflection effect on satellite, Ajisai

SummaryA model for the anisotropic reflection force acting on Ajisai is presented which includes the variable reflectivity coefficient and the force in the direction perpendicular to the incident light. This model significantly reduces the along-track orbit errors of Ajisai and smoothes the spike-like variation in the estimated drag coefficients from analysis of Ajisai laser ranging data. The model produces 17% smaller range residual RMS values in a one-year arc analysis of 1993 data, and a smaller residual RMS for a short-arc analysis in mid-year, the period from May to August, 1993.

Observed Temporal Variations in the Earth’s Gravity Field from 16-year Starlette Orbit Analysis

Satellite laser ranging data to Starlette, collected during the period from 1975 to 1990, have been analyzed to determine yearly values of the second degree annual (S a ) and semiannual (S sa ) tides, simultaneously with average values of other low degree and order tide parameters. The yearly fluctuations in the values for S a and S sa are associated with changes in the Earth’s second degree zonal harmonic caused by meteorological excitation. The Starlette-determined mean values for the amplitude of the annual and semiannual variations in J 2 are 32.3 × lO−11 and 19.5 × lO−11, respectively; while the rms about the mean values are 4.1 × lO−11 and 6.3 × lO−11, respectively. The annual δJ 2 is in good agreement with the value obtained from the combined effects of air mass redistribution without the oceanic inverted-barometer effects (non-IB) and hydrological change. Approximately 90% of the observed annual variation from Starlette is attributed to the meteorological mass redistribution occurring on the Earth’s surface.

Correction to “Variations in the Earth's oblateness during the past 28 years”

[1] In the paper ‘‘Variations in the Earth’s oblateness during the past 28 years’’ by Minkang Cheng and Byron D. Tapley (Journal of Geophysical Research, 109, B09402, doi:10.1029/2004JB003028, 2004), there was a printing error for the sign of exponent of GM. The listed GM values in paragraph 4 should be read as 3.986004414 10 m s 2 and 3.986004415 10 m s . JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B03406, doi:10.1029/2005JB003700, 2005

Determination of CHAMP Accelerometer Calibration Parameters

One of the key science instruments for gravity science aboard CHAMP is a three-axis STAR accelerometer, which measures the non-gravitational accelerations acting on the spacecraft. These measurements are used in precise orbit determination and for gravity field estimation. Usually, the accelerometer output is not an absolute value of non-gravitational accelerations and has to be calibrated by applying a bias and a scale factor for each axis. Since the bias and scale factor parameters cannot be obtained from ground tests, they have to be determined using the flight data. This paper describes different approaches for determination of the CHAMP accelerometer calibration parameters: comparison of observed accelerations with accelerations computed using models, and determination of the parameters in orbit determination and gravity field recovery schemes. The impact of these calibration parameters on the gravity field recovery are also discussed.

Neutral Density Measurements from the GRACE Accelerometers

(Abstract) Predicting the orbits of space objects in low altitude orbits requires an accurate model for the atmospheric neutral density. The current accuracy of semi-empirical models limits the prediction accuracy and impacts a number of operational decisions. The current models are based on sparse measurements of the neutral density, collected over an extended period. One of the problems is observing the thermosphere density changes in response to the solar and geomagnetic variability on short temporal scales, such as those characterized by geomagnetic storms. The stochastic behavior of the solar forcing represents one of the major challenges in predicting satellite orbits. In situ measurements of the density can play a significant role in improving the structure of the neutral density models and in providing a timely measurement for enhancing the accuracy of the satellite predictions. Measurements from orbiting accelerometers carried by the twin GRACE satellites have the potential for providing accurate and timely measurements to improve the satellite prediction accuracy. The objective of this paper is to describe the procedure for using the GRACE accelerometer measurements for determining accurate density measurements and thermospheric wind fields. Finally, selected results from the analysis of four years of GRACE accelerometer measurements are described.