A. G. 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.

Radio occultation data analysis by the radioholographic method

The radioholographic method is briefly described and tested by using data of 4 radio occultation events observed by the GPS/MET experiment on 9 February 1997. The central point of the radioholographic method (Pavelyev, 1998) is the generation of a radiohologram along the LEO satellite trajectory which allows the calculation of angular spectra of the received GPS radio wave field at the LEO satellite. These spectra are promising in view of detection, analysis and reduction of multipath/diffraction effects, study of atmospheric irregularities and estimation of bending angle error. Initial analysis of angular spectra calculated by the multiple signal classification (MUSIC) method gives evidence that considerable multibeam propagation occurs at ray perigee heights below 20 km and at heights around 80–120 km for the 4 GPS/MET occultation events. Temperature profiles obtained by our analysis (radioholographic method, Abel inversion) are compared with those of the traditional retrieval by the UCAR GPS/MET team (bending angle from slope of phase front, Abel inversion). In 3 of 4 cases we found good agreement (standard deviation σT∼1.5°K between both retrievals at heights 0–30 km).

Global sounding of sporadic E layers by the GPS=MET radio occultation experiment

Abstract The GPS radio occultation technique is sensitive for layered structures with horizontal scales of around 100 km and with vertical scales of a few 100 m or more at the Earths limb. These structures cause strong fluctuations of the GPS L1 and L2 phase paths which have been measured by a GPS receiver onboard of Microlab-1 satellite in 730 km orbit during the GPS/Meteorology experiment (GPS/MET of UCAR, Boulder). By means of GPS/MET radio occultation data, profiles of electron density fluctuations are derived for the mesosphere/lower thermosphere region with a height resolution of around 1 km . Data analysis of 1900 radio occultation events in June/July 1995, 1540 events in October 1995, and 2690 events in February 1997 confirms seasonal dependence of sporadic E layers. The meridian slices of average sporadic E activity show a dominance of plasma irregularities in the summer hemisphere. The irregularities mainly occur at heights 90–110 km . Auroral and equatorial sporadic E, electron density depletions, and multiple ionization layers are also present in the high resolution GPS/MET data. The multiple layers often have a distance of around 5–10 km in height, and appear up to a height of 140 km (upper height limit for 50 Hz sampling rate of GPS receiver). For February and June, the GPS/MET observations are compared to ground-based observations of the Asia/Australia ionosonde chain.

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.

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.

A Method for Determination of Internal Gravity Wave Parameters from a Vertical Temperature or Density Profile Measurement in the Earth's Atmosphere

A method for determination of internal gravity wave (IGW) parameters from a single vertical temperature or density profile measurement in the Earth’s atmosphere has been developed. This method may be used for the analysis of profiles measured by any techniques in which the accuracy is enough to measure small (∼1%) amplitudes of the temperature or density fluctuations in the atmosphere. The criterion for the IGW identification has been formulated and argued. In the case when this criterion is satisfied then analyzed fluctuations can be considered as wave-induced. The method is based upon the analysis of relative amplitude thresholds of the temperature or density wave field and upon linear IGW saturation theory in which amplitude thresholds are restricted by dynamical instability processes in the atmosphere. In order to approbate the method we have used data of simultaneous radiosonde measurements of the temperature and wind velocity in the Earth’s stratosphere where the saturated IGW propagation has been detected. It is shown that the application of the method to radio occultation temperature data gives the possibility to identify IGWs in the Earth’s lower stratosphere and to determine values of key wave parameters.

Detection of layering in the upper cloud layer of Venus northern polar atmosphere observed from radio occultation data

[1] Observations of radio wave scintillations represent an important tool for measuring of small-scale irregularities in the atmosphere of Venus. Prominent features of enhanced scintillation located in the 60-km region were observed in Mariners 5 and 10, Venera 9, and Pioneer Venus occultations. It is possible that the enhanced scintillations are due to the random turbulence in the upper region which is caused by trapped small-scale gravity waves. However, other interpretations are possible. Thin stable layers, which are commonly observed in the Earth stratosphere under cloud-free conditions, could also contribute to scattering in the Venus stratosphere. If scintillations observed in different occultations are correlated, then these scintillations may be attributed to the persistent layers. Cross correlations of 32-cm radio wave amplitude fluctuations have been determined for seven radio occultation measurements of Venus’s northern polar atmosphere using Venera 15 and 16. Significant cross correlations were found between 59.0 and 61.5 km in four different radio occultations. Layering is revealed in the upper layer of the Venus clouds at altitudes of 59.0–61.5 km, which is specified by enhanced turbulence of the atmosphere. It is found that the lifetime of the small-scale layered irregularities is 2 d or more and that their horizontal extension in the meridional direction can exceed 180 km. A possible cause of emergence of the layered structures inside the upper layer of polar clouds of Venus is discussed.

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.

Identification of Inclined Ionospheric Layers Using Analysis of GPS Occultation Data

The ionosphere and atmosphere may have significant impacts on the high-stable navigational signals of the Global Positioning System (GPS) in the communication link satellite to satellite. The classification of the different types of the ionospheric impact on the phase and amplitude of the GPS signals at altitudes of 40-90 km is introduced using the CHAllenging Minisatellite Payload (CHAMP) radio occultation (RO) data. An analytical model is elaborated for the description of the radio wave propagation in the stratified ionosphere and atmosphere. The propagation medium consists of sectors having the spherically symmetric distributions of refractivity. The newly developed model presents analytical expressions for the phase path and refractive attenuation of radio waves. The model explains significant amplitude and phase variations at altitudes of 40-90 km of the RO ray perigee associated with the influence of the inclined ionospheric layers. An innovative eikonal acceleration technique is described and applied to the identification and location of the inclined ionospheric layers using the comparative analysis of the amplitude and phase variations of the RO signals.

Characteristics of internal waves in the Martian atmosphere obtained on the basis of an analysis of vertical temperature profiles of the Mars Global Surveyor mission

An original method of determining the characteristics of an internal gravity wave (IGW) was developed using the data of an analysis of individual vertical temperature profile in the planet’s atmosphere. The method is based on an analysis of relative wave amplitude determined from the vertical temperature profile, as well as on the proposition of the IGW linear theory, according to which the wave amplitude is limited by the processes of dynamic (shear) instability in the atmosphere. It is supposed that, when the amplitude of the internal wave reaches the shear instability threshold as the wave propagates upward, a dissipation of wave energy occurs such that the IGW amplitude is maintained at the atmospheric instability threshold. The application of the developed method to vertical temperature profiles obtained from radio occultation measurements of the MGS (Mars Global Surveyor) mission made it possible to identify IGWs in the Martian atmosphere and determine the values of key wave parameters such as intrinsic frequency, amplitudes of the vertical and horizontal disturbances of wind velocity, vertical and horizontal wavelength, intrinsic vertical and horizontal phase (and group) velocities, kinetic, potential, and total energy of IGWs per unit mass, vertical fluxes of wave energy and horizontal momentum. Identified in the Martian atmosphere IGWs, with a vertical wavelength of 4.5–8.2 km, are waves with low intrinsic frequencies close to inertial frequency. Their kinetic energy, as a rule, is greater than potential energy by an order of magnitude. The propagation of these waves causes a significant modulation of the stability of atmospheric stratification that leads to shear instability and the occurrence of thin layers of intermittent turbulence in the Martian atmosphere.