Tropospheric Effect on Electromagnetically Measured Range: Prediction from Surface Weather Data
TL;DR: In this paper, it was shown that at any point in a dry atmosphere, the atmospheric refractivity depends on both pressure and temperature (the ratio P/T), not of temperature.
Abstract: Knowledge of the height integral of atmospheric refractivity (n — 1), where n is the refractive index, is essential for prediction of atmospheric range effect at any elevation angle. Observed values of the height integral for the lower, nonionized atmosphere can be obtained from weather balloon ascent data. Year-long collections of data from widely separated locations were used to relate this integral to surface data. Although (n — 1) at any point in a dry atmosphere depends on both pressure and temperature (the ratio P/T), the height integral of the observed dry part of (n — 1) is a linear function of surface pressure only, not of temperature. This is theoretically correct since P/T is equivalent to density, and the integral of density with height yields surface pressure. By application of this finding, the equivalent height for a (theoretically justified) quartic (n — 1) model (dry part) should be found to vary directly as surface temperature; the value obtained (least-squares fit to observed data) is 40.1 km for surface T = 0°C with a height expansion coefficient of 0.149 km per surface degree C. This would reduce the equivalent height to zero near 0° Kelvin. This theoretical model matches observed height integrals with an rms error of a few millimeters out of 2.3 meters (far less than 1%). Agreement between stations is excellent. A study of the more variable but much smaller wet part is in progress. The wet part is significant at radio but not at optical frequencies.
Citations
More filters
TL;DR: In this article, the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the GPS stations relative to Paisha, Penghu, situated at the Chinese continental margin.
783 citations
TL;DR: In this article, a generic interferometric synthetic aperture radar atmospheric correction model was developed to assess the correction performance and feasibility, which includes global coverage, all weather, all-time useability, correction maps available in near real-time, and indicators.
Abstract: For mapping Earth surface movements at larger scale and smaller amplitudes, many new synthetic aperture radar instruments (Sentinel-1A/B, Gaofen-3, ALOS-2) have been developed and launched from 2014–2017, and this trend is set to continue with Sentinel-1C/D, Gaofen-3B/C, RADARSAT Constellation planned for launch during 2018–2025. This posesmore challenges for correcting interferograms for atmospheric effects since the spatial-temporal variations of tropospheric delay may dominate over large scales and completely mask the actual displacements due to tectonic or volcanic deformation. To overcome this, we have developed a generic interferometric synthetic aperture radar atmospheric correction model whose notable features comprise (i) global coverage, (ii) all-weather, all-time useability, (iii) correction maps available in near real time, and (iv) indicators to assess the correction performance and feasibility. The model integrates operational high-resolution European Centre for Medium-Range Weather Forecasts (ECMWF) data (0.125° grid, 137 vertical levels, and 6-hr interval) and continuous GPS tropospheric delay estimates (every 5 min) using an iterative tropospheric decomposition model. The model’s performance was tested using eight globally distributed Sentinel-1 interferograms, encompassing both flat and mountainous topographies, midlatitude and near polar regions, and monsoon and oceanic climate systems, achieving a phase standard deviation and displacement root-mean-square (RMS) of ~1 cm against GPS over wide regions (250 by 250 km). Indicators describing the model’s performance including (i) GPS network and ECMWF cross RMS, (ii) phase versus estimated atmospheric delay correlations, (iii) ECMWF time differences, and (iv) topography variations were developed to provide quality control for subsequent automatic processing and provide insights of the confidence levelwithwhich the generated atmospheric correctionmapsmaybe applied.
289 citations
TL;DR: The TOPEX/POSEIDON mission objective requires that the radial position of the spacecraft be determined with an accuracy better than 13 cm RMS (root mean square). This stringent requirement is an order of magnitude below the accuracy achieved for any altimeter mission prior to the definition of the TOPEX mission as mentioned in this paper.
Abstract: The TOPEX/POSEIDON mission objective requires that the radial position of the spacecraft be determined with an accuracy better than 13 cm RMS (root mean square). This stringent requirement is an order of magnitude below the accuracy achieved for any altimeter mission prior to the definition of the TOPEX/POSEIDON mission. To satislfy this objective, the TOPEX Precision Orbit determination (POD) Team was established as a joint effort between the NASA Goddard Space Flight Center and the University of Texas at Austin, with collaboration from the University of Colorado and the Jet Propulsion Laboratory. During the prelaunch development and the post launch verification phases, the POD team improved, calibrated, and validated the precision orbit determination computer software systems. The accomplishments include (1) increased accuracy of the gravity and surface force models and (2) improved peformance of both laser ranging and Doppler tracking systems. The result of these efforts led to orbit accuracies for TOPEX/POSEIDON which are significantly better than the original mission requirement. Tests based on data fits, covariance analysis, and orbit comparisons indicate that the radial component of the TOPEX/POSEIDON spacecraft is determined, relative to the Earth's mass center, with an root mean square (RMS) error in the range of 3 to 4 cm RMS. This orbit accuracy, together with the near continuous dual-frequency altimetry from this mission, provides the means to determine the ocean's dynamic topography with an unprecedented accuracy.
288 citations
TL;DR: In this paper, the authors present a state-of-the-art dual-frequency altimeter onboard the TOPEX/POSEIDON (T/P) satellite, which is significantly more accurate than any of the other altimeters that have been launched to date.
Abstract: Publisher Summary The basic concept of satellite altimetry is to measure the range from the satellite to the sea surface. The altimeter transmits a short pulse of microwave radiation with known power toward the sea surface. The pulse interacts with the rough sea surface and a part of the incident radiation reflects back to the altimeter. The chapter emphasizes on the correction algorithms applied to the dual-frequency altimeter onboard the TOPEX/POSEIDON (T/P) satellite. This state-of-the-art altimeter sets the standard for future altimeter missions as it is significantly more accurate than any of the other altimeters that have been launched to date. To provide assurance that the performance requirements for altimeter measurement accuracy are met or exceeded, extensive calibration and validation (cal/val) are important elements of altimeter missions. Cal/val embraces a wide variety of activities, ranging from the interpretation of information from internal-calibration modes of the sensors to the validation of the fully corrected sea-level estimates using in situ data. The chapter concludes with a summary of the T/P mission design and an assessment of the performance of the T/P dual-frequency altimeter in addition, as well as an overview of future altimeter missions.
272 citations
TL;DR: In this article, a water vapor radiometer (WVR) was used to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations, and the results showed that WVR data yielded a 13% smaller weighted root-mean-square scatter of the baseline length estimates compared to the use of a Kalman filter.
Abstract: An important source of error in very-long-baseline interferometry (VLBI) estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. We present and discuss the method of using data from a water vapor radiometer (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data or Kalman filtering to correct for atmospheric propagation delay at the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The lengths of the baselines range from 919 to 7941 km. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. For the most frequently measured baseline in this study, the use of WVR data yielded a 13% smaller weighted-root-mean-square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the “best” minimum elevation angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass. For use of WVR data along with accurate determinations of total surface pressure, the best minimum is about 20 degrees; for use of a model for the wet delay based on the humidity and temperature at the ground, the best minimum is about 35 degrees.
268 citations
References
More filters
01 Aug 1953
TL;DR: In this paper, a relation 77.6 e N = ~ p + 4,810-T T where p = total pressure in millibars e=partial pressure of water vapor in millibrars T=absolute temperature=°C+273
Abstract: Recent improvements in microwave techniques have resulted in precise measurements which indicate that the conventional constants K1 = 79°K/mb and K22?=4,800°K in the expression for the refractivity of air, N=(n-1) 106=[K1/T](p+ K2'e/T) should be revised. Various laboratories appear to have arrived at this conclusion independently. In much of radio propagation work the absolute value of the refractive index of the atmosphere is of small moment. However, in some work it is important and it seems highly desirable to decide upon a particular set of constants. Through consideration of the various recent experiments this paper arrives at a relation 77.6 e N = ~ p + 4,810-T T where p=total pressure in millibars e=partial pressure of water vapor in millibars T=absolute temperature=°C+273 This expression is considered to be good to 0.5 per cent in N for frequencies up to 30,000 mc and normally encountered ranges of temperatures, pressure and humidity.
650 citations
TL;DR: In this paper, a model for the height profile of tropospheric refractivity N and expressions derived from it for computing corrections for satellite Doppler or range data were presented.
Abstract: This paper presents a new model for the height profile of tropospheric refractivity N and expressions derived from it for computing corrections for satellite Doppler or range data. (N ≡ 106 (n - 1), where n is the index of refraction.) The model is theoretically based on an atmosphere with constant lapse rate of temperature, as will be shown. It treats the ‘dry’ and ‘wet’ components of N separately and represents each as a fourth-degree function of height above the geoid; each component profile starts with its locally observed surface value and decreases to zero at an effective height that is different for the two components. The height parameters were obtained by a least-squares fit to observed data. A latitude dependence has been found for the ‘dry’ height. The model has been found capable of closely matching any local average N profile observed in a world-wide sample of locations throughout the height range of meteorological balloon data (up to 24 km); samples are shown. The corrections based on it are readily evaluated and are finite and usable at all elevation angles. Their effectiveness is evidenced by figures showing two different kinds of observed data: first, Doppler residuals for several satellite passes without and with the use of the correction; and the ‘navigation’ error in station-to-orbit slant range from Doppler data, again without and with the correction. The use of the correction removed obvious systematic errors. The fact that satellite Doppler data display identifiable tropospheric effects is of interest with regard to future study of the troposphere.
524 citations
TL;DR: In this paper, a two-quartic tropospheric refractivity profile for correcting satellite data has been modeled by Hopfield [1969, 197O] based on a troposphere with constant lapse rate of temperature.
Abstract: A two-quartic tropospheric refractivity profile for correcting satellite data has been modeled by Hopfield [1969, 197O]. Her model is theoretically based on a troposphere with constant lapse rate of temperature. It treats the “dry” and “wet” components of the tropospheric refractivity N separately and represents each as a fourth-degree function of height above the geoid. Expressions are given to compute the contributions to both range and range rate data.
20 citations