Yuei-An Liou

FORMOSAT-3/COSMIC GPS Radio Occultation Mission: Preliminary Results

The Formosa Satellite-3 and Constellation Observing System for the Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) radio occultation (RO) mission has been successfully launched on April 14, 2006. The FORMOSAT-3/COSMIC mission uses global positioning system (GPS) signals to study the atmosphere and the ionosphere with global coverage. Receivers that are installed onboard of the six small FORMOSAT-3/COSMIC satellites register the phase and the amplitude of radio waves at two GPS frequencies. We give a preliminary analysis of the first RO measurements that are provided by the FORMOSAT-3/COSMIC mission. The geographical distribution of the first FORMOSAT-3/COSMIC RO experiments is shown. We demonstrate that the performance of the first measurements allows obtaining the vertical profiles of the refractivity, temperature, and pressure for the considered FORMOSAT-3/COSMIC RO events with expected accuracy, which is quite similar to the accuracy of the previous Challenging Mini-Satellite Payload and Gravity Recovery and Climate Experiment RO missions. New elements in the RO technology are suggested for further improving the accuracy and broadening the application range of the RO method. We emphasize new directions in applying the RO method to measure the vertical gradients of the refractivity in the atmosphere, to determine the temperature regime in the upper stratosphere, and to investigate the internal wave activity in the atmosphere. We find a significant correlation between the phase acceleration and the intensity variations in the RO signals that are emitted by GPS satellites and registered by the FORMOSAT-3/COSMIC satellites. This correlation opens a way to locate the layered structures in the propagation medium based on simultaneous observations of the radio wave intensity and the phase variations in trans-ionospheric satellite-to-satellite links.

A land-surface process/radiobrightness model with coupled heat and moisture transport for freezing soils

Phase change of water is an important sink and source of energy and moisture within soils as well as a significant influence upon soil temperature and moisture profiles. These profiles play a crucial role in governing energy and moisture fluxes between bare soils and the atmosphere. They also codetermine radiobrightness, so that the difference between modeled and observed radiobrightness becomes a measure of error in a models estimate of temperature or moisture. The authors present a physically based, coupled-heat and moisture-transport, one-dimensional hydrology/radiobrightness (1 dH/R) model for bare, freezing and thawing, moist soils that are subject to insolation, radiant heating and cooling, and sensible and latent heat exchanges with the atmosphere. They use this model to examine thermal, hydrologic, and Special Sensor Microwave/Imager (SSMI) radiobrightness signatures for a three-month, dry-down simulation in the fall and winter of the northern United States Great Plains as part of an investigation of the effects of coupling heat and moisture transport. Given a typical initial moisture content of 38%, they find that coupled transport results in a reduction of ice in the surface soil by 21%. The range of diurnal variations in temperature are not significantly affected by coupled transport. Diurnal variations in the 19-GHz, H-polarized radiobrightness can be greater in the coupled transport case by 37 K. Total diurnal variation can exceed 57 K during periods of diurnal freezing and thawing.

Impact of surface meteorological measurements on GPS height determination

Although the topic of the positioning precision of the Global Positioning System (GPS) has been studied extensively, it focuses mostly on the error sources such as the ionospheric effect, antenna phase center variation and tropospheric influence. This investigation addresses the influence of the tropospheric effect on the results of the height determination. Used data are obtained from GPS receivers of a network and co-located surface meteorological instruments in 2003. Two approaches, parameter estimation and external correction, are utilized to correct the zenith tropospheric delay (ZTD) by applying the surface meteorological measurements (SMM) data. The GPS height can be affected by an incorrect pressure measurement up to a few meters, and the root-mean-square (RMS) of the daily solution can range from millimeters to a few centimeters, no matter which approach is adopted. The effect is less significant when using SMM for parameter estimation, but the trend of corrections on the GPS height is more consistent at either higher or lower altitudes. By external correction using SMM and Saastamoinen model, the GPS height reaches a few centimeters repeatability, while the RMS of the daily solution displays an improvement of about 2-3 mm.

Constellation Deployment for the FORMOSAT-3/COSMIC Mission

The FORMOSA Satellite Series No. 3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) spacecraft constellation consisting of six low-earth-orbiting satellites is the worlds first operational Global Positioning System (GPS) radio occultation mission. The mission has been jointly developed by the National Space Organization of Taiwan and the University Corporation for Atmospheric Research of the U.S. in collaboration with the Jet Propulsion Laboratory, NASA, and the Naval Research Laboratory for three onboard payloads, including a GPS Occultation Receiver, a triband beacon, and a tiny ionospheric photometer. The FORMOSAT-3/COSMIC mission was successfully launched from Vandenberg into the same orbit plane of the designated 516-km circular parking orbit altitude on April 15, 2006. After the six satellites completed the in-orbit checkout activities, the mission was started immediately at the parking orbit for in-orbit checkout, calibration, and experiment of three onboard payloads. Individual spacecraft thrust burns for orbit raising were performed to begin the constellation deployment of the satellites into six separate orbit planes. All six FORMOSAT-3/COSMIC satellites are maintained in a good state of health except spacecraft flight model no. 2, which has had power shortages. Five out of the six satellites had reached their final mission orbits of 800 km as of November 2007. This paper provides an overview of the constellation spacecraft design, constellation mission operations, constellation deployment timeline evolution, associated spacecraft mass property and moment of inertia results, orbit-raising challenges, and lessons learned during the orbit-raising operations.

Annual temperature and radiobrightness signatures for bare soils

The authors have developed physically based, diurnal, and annual models for freezing/thawing moist soils subject to annual insolation, radiant heating, and cooling, and sensible and latent heat exchanges with the atmosphere. Both models have the same weather forcing, numerical scheme, and soil constitutive properties. The authors find that surface temperature differences over a diurnal cycle between the annual and diurnal models are as much as -5 K in March, -7 K in June, -4 K in September, and 5 K in December for 38% (by volume fraction) moist soil. This difference occurs because the annual model includes the history of energy fluxes at the surface of the soil. The annual model is linked to microwave emission models for predictions of temporal radiobrightness signatures. The model predicts a relatively weak decrease in diurnal differences in soil temperature with increased moisture content, but a significant decrease in diurnal differences in radiobrightness. It also exhibits notable perturbations in radiobrightness when soils freeze and thaw. The moisture dependent, day-to-night radiobrightness difference is enhanced by as much as -42 K at 19.35 GHz horizontal polarization for frozen soil if daytime thawing occurs.

FORMOSAT-3/COSMIC Constellation Spacecraft System Performance: After One Year in Orbit

The FORMOSAT-3 mission, also known as constellation observing system for meteorology, ionosphere, and climate (COSMIC), is the third major project of the Formosa satellite (FORMOSAT) series implemented by the National Space Organization of Taiwan. FORMOSAT-3/COSMIC is a joint Taiwan/U.S. mission consisting of six identical low Earth orbit satellites. All six cluster satellites were successfully launched by a single Minotaur launch vehicle on April 15, 2006. The retrieved Global Positioning System (GPS) radio occultation (RO) data have been freely available online to the science community since shortly after the completion of satellite bus in-orbit checkout. Having completed the verification and validation, the worldwide science communities are highly satisfied with the RO data. Scientists have hailed the RO sensors as offering the most accurate, precise, and stable thermometers in space. After one year in orbit, all six FORMOSAT-3/COSMIC satellites were in good condition (except FM2, which had power shortage issues) and were on their way toward the final constellation of six separate orbit planes with 30 deg separation. Four out of six satellites had already reached their final mission orbit of 800 km by mid-May 2007. Together, the six satellites have generated a total of more than 2500 RO data per day. However, only 50%-70% of the RO data as received one year after launch could be retrieved into useful atmosphere profiles. The retrieved RO data, about 1800 per day on average, have been assimilated into numerical weather prediction models by many major weather forecast centers and research institutes. This paper provides an overview of the constellation mission, the spacecraft system performance after one year in orbit, the technical challenges we have encountered, and the performance enhancements we have accomplished.

Simultaneous observations of radio wave phase and intensity variations for locating the plasma layers in the ionosphere

7 [1] A new method is introduced to locate the layered 8 structures in the ionosphere based on simultaneous 9 observations of radio wave temporal intensity and phase 10 variations in trans-ionospheric satellite-to-satellite links. 11 The method determines location of the tangent point on the 12 trans-ionospheric ray trajectory where gradient of 13 refractivity is perpendicular to the ray trajectory and the 14 influence of a layered structure on radio wave parameters is 15 maximal. This new technique was applied to the 16 measurements provided during CHAMP radio occultation 17 (RO) mission. For the considered RO events, the locations 18 of the inclined plasma layers in the lower ionosphere are 19 found and the electron density distributions are retrieved. 20 The method is checked by measuring the location of the 21 tangent point on the ray trajectory in the neutral gas in the 22 atmosphere. The results showed a fairly good agreement. 23 Citation: Liou, Y. A., and A. G. Pavelyev (2006), Simultaneous 24 observations of radio wave phase and intensity variations for 25 locating the plasma layers in the ionosphere, Geophys. Res. Lett., 26 33, LXXXXX, doi:10.1029/2006GL027112.

Wave structures in the electron density profile in the ionospheric D- and E-layers observed by radio holography analysis of the GPS/MET radio occultation data

Abstract Vertical distribution of the electron density in the upper atmosphere can be studied using high-precision global positioning system (GPS). In this paper, we show that the radio holography method allows one to determine the vertical profile of the electron density and monitoring wave structures in the upper atmosphere. As an example of this approach, results of analysis of data corresponding to four GPS/Meteorology (GPS/MET) radio occultation events are presented. The radio holograms of the D-layer of the ionosphere reconstructed from radio occultation data revealed wave structures with vertical scales of about 1– 8 km and variations in the vertical gradient of the electron density from ±5×103 to ±8×10 3 electrons /( cm 3 km ) at altitudes of 72– 95 km . These structures may be caused by wind shear and atmospheric internal waves with vertical scales ranging from a few hundred meters to several kilometers, which produce vertical convergence of the plasma velocity and plasma advection. Theoretical consideration shows a possibility of qualitative determination of the vertical gradient of the horizontal wind velocity in the E-layer and estimation of the temperature variations in the neutral gas in the D-layer region from observed profiles of the electron density. Variations in the electron density are connected with the temperature changes. The connection coefficient depends on the vertical velocity of the neutral gas motion in the mesosphere. Maximums of the temperature deviation correspond to those in the electron density profile. The results indicate a possibility to estimate the form of the small-scale temperature vertical perturbations in the mesosphere using the radio occultation data.

A neural-network approach to radiometric sensing of land-surface parameters

A biophysically-based land-surface process/radiobrightness (LSP/R) model is integrated with a dynamic learning neural network (DLNN) to retrieve the land-surface parameters from its radiometric signatures. Predictions from the LSP/R model are used to train the DLNN and serve as the reference for evaluation of the DLNN retrievals. Both horizontally polarized and vertically polarized brightnesses at 1.4 GHz, 19 GHz, and 37 GHz for an incidence angle of 53/spl deg/ make up the input nodes of the DLNN. The corresponding output nodes are composed of land-surface parameters, canopy temperature and water content, and soil temperature and moisture (uppermost 5 mm). Under no-noise conditions, the maximum of the root mean-square (RMS) errors between the retrieved parameters of interest and their corresponding reference from the LSP/R model is smaller than 28 for a four-channel case with 19 GHz and 37 GHz brightnesses as the inputs of the DLNN. The maximum RMS error is reduced to within 0.5% if additional 1.4 GHz brightnesses are used (a six-channel case). This indicates that the DLNN produces negligible errors onto its retrievals. For the realization of the problem, two different levels of noises are added to the input nodes. The noises are assumed to be Gaussian distributed with standard deviations of 1 K and 2 K. The maximum RMS errors are increased to 9.3% and 10.3% for the 1 K-noise and 2 K-noise cases, respectively, for the four-channel ease. They are reduced to 6.0% and 9.1% for the 1 K-noise and 2 K-noise cases, respectively, for the six-channel case. This is an implication that 1.4 GHz is a better frequency in probing soil parameters than 19 GHz and 37 GHz.

Radiobrightness of prairie soil and grassland during dry-down simulations

We present 60-day summer dry-down simulations for prairie grassland based upon a one-dimensional hydrology and radiobrightness models for northern prairie. Our objective is to examine the effect of scaling upon the interpretability of mixed pixel radiobrightnesses. Even for relatively homogeneous regions like North American prairie, there are significant variations in land cover within the relatively large footprints of satellite microwave radiometers. In this paper we specifically address 1.4- and 19-GHz brightness in the presence of subpixel variability in canopy density. Bare soil and a densecanopy grassland can be viewed as extreme examples of prairie land cover. If land cover within any pixel is viewed as a mixture of these two extremes, then the terrain within that pixel can be modeled as some combination of the hydrology and radiobrightness models for bare soil and dense-canopy grassland. We examined two combination schemes: (1) a homogeneous combination where the dense-canopy grasses are simply spread uniformly over the pixel to achieve a desired vegetation column density between that of bare soil and dense-canopy grass, and (2) a tiled combination where the pixel is divided into a region of bare soil and a region of dense-canopy grassland. We examined H-polarized, 53° incidence angle, 19.35- and 1.4-GHz pixel brightnesses and found the 19-GHz brightness to be significantly greater for homogeneous pixels than for tiled pixels throughout the dry-down period. For example, a 19-GHz, 50% homogeneous pixel is 50 K brighter than a 50% tiled pixel at the beginning of the dry-down and 40 K brighter at 60 days. In contrast, the 1.4-GHz brightnesses are essentially identical for homogeneous and tiled pixels. Within the constraints of our simulation, subpixel variation in canopy density is a significant factor in the quantitative interpretation of the 19-GHz brightness of prairie grassland but is not a factor in the interpretation of the 1.4-GHz brightness.