A. A. Pavelyev

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.

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.

Application of GPS radio occultation method for observation of the internal waves in the atmosphere

Received 1 February 2005; revised 10 October 2005; accepted 20 December 2005; published 24 March 2006. [1] In this study, we show that the amplitude radio occultation (RO) method, which employs high-precision global positioning system (GPS) signals, allows one to determine the vertical gradients of refractivity and monitor wave structures in the atmosphere on a global scale at altitudes ranging from 10 to 40 km. We show that the sensitivity of the RO amplitude data to the wave structures in the atmosphere with vertical periods from 0.8 to 4 km exceeds one of the RO phase data by a factor of order 10. As an example of this approach, analytical results of the Challenging Minisatellite Payload’s (CHAMP) RO events are presented. Wave clusters were found in the amplitude variations of the RO signals with interior vertical periods from 0.8 to 4 km in the tropopause and lower stratosphere within the heights of 15–24 km (low latitudes) to 10–15 km (moderate latitudes). We demonstrate that the amplitude variations can be considered as a radioholographic image of the wave structures in the atmosphere. For internal gravity waves (GW), we show that the GW portrait, which consists of the altitude dependence of the GW phase, amplitude and vertical spatial frequency, can be retrieved from the amplitude variations of the RO signal. The GW dispersion and polarization relationships allow one to estimate the vertical profile of the horizontal wind perturbations, its gradient and the GW intrinsic phase speed. In general, when the origin and type of internal waves are not known, the height dependence of the vertical gradient of refractivity can be applied for monitoring the seasonal and geographical distributions of wave activities at different levels in the atmosphere.

Application of radio holographic method for observation of altitude variations of the electron density in the mesosphere/lower thermosphere using GPS/MET radio occultation data

A radio holographic method for observation of wave phenomena in the upper atmosphere (height interval 60 –120 km) is described. The radio holography uses coherent properties of the rays propagating through the atmosphere. These properties arise due to the high stability of the GPS signal and its high sensitivity to layered structures in the atmosphere and ionosphere. High angular and vertical resolution of the radio holographic method has been revealed with the use GPS=MET radio occultation data. Multibeam propagation in the troposphere and weak re3ections from the ocean surface have been observed with vertical resolution better than 70 m. Radio holographic method gives the possibility to remove the contributions of the upper layers and to identify detailed structures in the electron density altitude pro6les in the D- and E-layer of the ionosphere. The main e7ects connected with the in3uence of the D- and E-layer on the phase path are correspondingly of the order of 0.5 –2 and 10 –30 cm. The instrumental accuracy of the GPS phase path measurements was of the order of 1–2 mm. The 6ne structures in the D-layer and the sporadic E-layer have been inferred. They have a vertical spatial period of 1–2 km; changes in the electron density vertical gradient from ±0:5 × 10 3 to ±10 × 10 3 electrons=(cm 3 km) and the electron density variations from 100 el=cm 3 to 1000 el=cm 3 . Electron density altitude distribution and its gradient with maximum values ∼45 × 10 3 el=cm 3 and ∼20 × 10 3 electrons=(cm 3 km); respectively, have been observed in an E-layer of the ionosphere at the height interval 92–100 km. The possibility of a qualitative measurement of the vertical distribution of the horizontal wind velocity in the E-layer region using GPS=MET radio occultation data is considered. c

Principle of Locality and Analysis of Radio Occultation Data

A fundamental principle of local interaction of radio waves with a refractive spherical medium is formulated and illustrated using the radio occultation (RO) method of remote sensing of the atmosphere and the ionosphere of the Earth and the planets. In accordance with this principle, the main contribution to variations of the amplitude and the phase of radio waves propagating through a medium makes a neighborhood of a tangential point, where the gradient of the refractive index is perpendicular to the radio wave trajectory. A necessary and sufficient condition (a criterion) is established to detect the displacement of the tangential point from the radio ray perigee using analysis of the RO experimental data. This criterion is applied to the identification and the location of layers in the atmosphere and the ionosphere by the use of Global Positioning System RO data. RO data from the CHAllenge Minisatellite Payload (CHAMP) are used to validate the criterion introduced when significant variations of the amplitude and the phase of the RO signals are observed at the RO ray perigee altitudes below 80 km. The detected criterion opens a new avenue in terms of measuring the altitude and the slope of the atmospheric and ionospheric layers. This is important for the location determination of the wind shear and the direction of internal wave propagation in the lower ionosphere and possibly in the atmosphere. The new criterion provides an improved estimation of the altitude and the location of the ionospheric plasma layers compared with the backpropagation radio-holographic method previously used.

New Applications and Advances of the GPS Radio Occultation Technology as Recovered by Analysis of the FORMOSAT-3/COSMIC and CHAMP Data-Base

Comparative analysis of phase and amplitude variations of GPS radio-holograms allows one to separate the influence of the layered and irregular structures. A possibility exists to measure important parameters of internal waves: the intrinsic phase speed, the horizontal wind perturbations, and, under some assumptions, the intrinsic frequency as function of height in the atmosphere. A new technique was applied to measurements provided during CHAllenging Minisatellite Payload (CHAMP) and the Formosa Satellite-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) radio occultation (RO) missions. As an example of this approach, we establish the atmospheric origin of amplitude and phase variations in the RO signal at altitudes 10–26 km. We observed for the first time in the RO practice examples of internal wave breaking at altitudes between 38 km and 45 km. We obtained geographical distributions and seasonal dependence of atmospheric wave activity with global coverage within the years 2001–2003.

Radio Wave Propagation Phenomena from GPS Occultation Data Analysis.

The RO remote sensing can be performed with any two cooperating satellites located on opposite sides with respect to the Earth’s limb and moving to radio shadow. Several RO missions are working now aboard the Low Earth Orbit satellites. These missions provide global monitoring of the atmosphere and ionosphere of the Earth at different altitudes with high spatial resolution and accuracy. Their data are very important for meteorology, weather prediction. The RO data can be used to detect the climate changes, connections between the ionospheric, atmospheric processes, and solar activity, and to estimate conditions for radio navigation and radio location.