Airborne lidar forward sensing along the flight direction can serve for notification of clear air turbulence (CAT) and help to prevent injuries or fatal air accidents. The validation of this concept was presented in the framework of the DELICAT (DEmonstration of LIdar-based CAT detection) project. However, the strong variations in signal level, which were observed during the DELICAT measurements but not explained, sometimes indicated the need of a better understanding the observational errors due to geometrical factors. In this paper, we discuss possible error sources pertinent to this technique, related to fluctuations of the flight parameters, which may lead to strong signal variations caused by the random deviations of the sensing beam from the forward flight trajectory. We analyze the variations in backscattered lidar signal caused by fluctuations of the most important forward-sensing flight parameter, the pitch angle. The fluctuation values considered in the paper correspond to the error limits of the compensational gyro platform used in civil aviation. The part of the pitch angle fluctuations not compensated for by the beam-steering device in the presence of aerosol concentration variations can lead to noticeable signal variations that can be mistakenly attributed to wind shear, turbulence, or fast evolution of the aerosol layer. We formulate the criteria that allow the recognition of signal variations caused by pitch angle fluctuations. Influence of these fluctuations is shown to be stronger for aerosol variations on smaller vertical scales. An example of DELICAT observations indicating a noticeable pitch angle fluctuation impact is presented.

The operation of a lidar intended for clear air turbulence (CAT) positioning on the basis of the backscatter enhancement (BSE) effect is analyzed using a turbulence model with a power-law spectrum. Systematic distortions occurring due to a need to regularize the lidar positioning problem solution are estimated. It is shown that the effect of molecular viscosity of air on the positioning result can be neglected if the wave parameter, which characterizes the diffraction manifestation, is higher than 3. This corresponds to sounding ranges of more than 1 km for optical or UV lidars. The analysis results show that the BSE lidar positioning accuracy weakly depends on the exponent in the turbulence spectrum in regions of severe turbulence. The results can justify a physical experiment for the design of an aircraft system for the lidar detection of CAT regions ahead of the flight course.

Airborne lidar sounding of aerosol clusters by a laser beam sent forward in the aircraft flight direction is considered. Observability of the temporal evolution of short-lived (from a few to tens of seconds) aerosol clusters with horizontal scales from a few hundreds to a few thousands of meters are shown by numerical simulation. It is shown that, unlike stationary lidar observations, airborne lidars allow the evolution of aerosol clusters to be analyzed without involving additional hypotheses about their spatial and temporal structure.

The possibility of lidar positioning of regions with higher clear-air turbulence (CAT) is shown. The turbulence is indicated by air density fluctuations generated by it. A scheme with a lidar based on using the backscattering enhecement (BSE) effect in a turbulent medium is considered. A stable solution of the positioning problem is obtained using the statistical regularization method. As is shown on models, CAT regions that are dangerous for civil aviation flights can be detected using such a lidar.

Backscattering enhancement (BSE) effect is due to the fact that both the initial and back-scattered waves propagate through the same inhomogeneities of the refractive index. Mean value of the back-scattered intensity is higher than it would be with the same obstacle but no inhomogeneities. This effect is named backscattering enhancement (BSE) effect. Numerical modeling of lidar that based on BSE effect was carried out in Rutov-Obukhov approximation in our work. The integral equation was considered which bundles up the distribution of turbulence intensity throughout the space between the source and a scatterer. Coefficient BSE was determined as ratio of relation dispersions of radiation intensity fluctuation that scattered straight back and at an angle.BSE coefficient does not depend on the nature of scatterings in cases of aerosol or molecular scatterers. As example variants of turbulence intensity distribution C-n(2) between sources in form select layer or boundaries of half-space with enhanced turbulence intensity scatterers were considered. Possibility of detection the sort out the regions with enhanced turbulence intensity was showed in the case isotropic turbulence for molecular or aerosol scatterings. Inhomogeneous distribution of turbulence intensity is reliably picked out on dependence of BSE coefficient on distance between source and probing laser beam.The lidar scheme for BSE measurements with space modulation of probing beam is suggested. It allows suppressing systematic errors. Lidar allows measure BSE coefficient along with the routine lidar sensing. The dependence of BSE coefficient on the line along propagation path has considered for finite receiving aperture and finite diameter of probing laser beam. The results of modeling demonstrate that BSE measurements make it possible to remotely sort out the regions with enhanced turbulence intensity at distances determined by the maximum sensing range.

The numerical examination of 2D spectra of strong stellar scintillations observed through the Earth’s atmosphere from space is carried out. The atmosphere contains a combination of statistically independent, anisotropic large-scale and isotropic small-scale inhomogeneities of the refractive index. 2D spectra and vertical and horizontal 1D spectra calculated using them are presented. It is shown that the strong scintillation spectra are not equal to the sum of the spectra formed by separate, statistically independent components. The combination of large- and small-scale inhomogeneities results in a greater dispersion of scintillations in comparison with the absence of latter ones. However, the presence of this combination can lead both to an increase and decrease of dispersion in comparison with the sum of dispersions of the anisotropic and isotropic components depending on their intensity relation. The new effect in 1D horizontal spectra behavior is found in the region of small wave numbers; i.e., the presence of small-scale atmospheric inhomogeneities results in the suppression of spectral power of scintillation formed by only an anisotropic component.

A phenomenological model is proposed for the three-dimensional (3D) spectrum of temperature irregularities generated by internal waves in the atmosphere. This model develops a theory (Chunchuzov, 2002) based on the assumption that the field of the Lagrange displacements of the medium’s particles that are caused by a statistical ensemble of internal waves with randomly independent amplitudes and phases is stationary, homogeneous, axially symmetric in a horizontal plane, and Gaussian. To fit the model to measured spectra of fluctuations in the stratosphere and mesosphere, an additional assumption is introduced into the model that the degree of anisotropy of irregularities depends on their vertical size. An explicit expression is presented for the 3D spectrum. The model vertical spectrum follows a power law with an exponent of −3. The horizontal spectrum has three asymptotically power portions. Two of these are characterized by an exponent of −3, whereas an intermediate portion has an exponent of −1 to −3, depending on the rate at which the degree of anisotropy decreases as the vertical size of temperature irregularities increases. Simple asymptotic formulas are obtained for the horizontal spectrum. Within the range of a few decades, the model is in good agreement with the published results of measuring the spectra in the upper troposphere, stratosphere, and mesosphere.

The first experimental studies of the spatial oblique and vertical spectra of temperature fluctuations in a stably stratified troposphere at heights of 2 to 8 km were conducted. The measurements were taken over northern European Russia. The spectra cover the wave number range from 5 10−4 to 3 10−2 rad/m. The estimates obtained for the spectral density are analyzed on the basis of a model developed previously for the three-dimensional (3D) spectrum of temperature fluctuations generated by a statistical ensemble of internal waves. This model made it possible to consider both oblique and horizontal spectra from a unified point of view and to use a unified set of parameters on the basis of the 3D spectrum concept. The quantitative estimates obtained for the parameters of the 3D spectrum have shown that large-scale temperature inhomogeneities with a vertical size of more than a hundred meters are strongly extended along the land surface. They have approximately the same form; their horizontal sizes are at least 20 times greater than their vertical sizes. The anisotropy of temperature inhomogeneities decreases with a decrease in their vertical sizes and reaches 1.5–2 for vertical sizes of 10–20 m or smaller.

Using the approximation of an anisotropic statistically-homogeneous phase screen, we consider spectra of strong scintillations. Numerical calculations are made for the model of large-scale anisotropic inhomogeneities typical of the Earth’s stratosphere. The spectrum transformation is studied for the transition from weak scintillations to the asymptotic regime of strong scintillations. We show that with increasing level of the scintillations, their spectra rapidly broaden to the region of large wave numbers which exceed both the inverse internal scale of the irregularities and the inverse radius of the Fresnel zone by orders of magnitude. Notable deviations of the two-dimensional spectra from the predictions based on perturbation theory are shown to occur for scintillation variance exceeding 0.1. The obtained two-dimensional spectra of scintillations give a complete picture of the behavior of one-dimensional spectra which can be retrieved from satellite observations made for different angles between the orbit plane and the direction to the source. Vertical and horizontal one-dimensional spectra are studied in detail. Approximate algebraic formulas are derived and their validity is proved by applying them to the calculation of spectra of strong scintillations for a wide (several decades) range of the wave number values.

Observation of stars from a spacecraft through the Earth’s atmosphere is a constituent part of remote sensing of the atmosphere. Recorded scintillation signals contain data on the structure of air-density irregularities induced by turbulence and internal waves. Currently, parameters of the structure in the stratosphere are determined using the procedures based on the weak-scintillation theory. However, during stellar occultation by the Earth’s atmosphere, scintillation becomes stronger as the line of sight plunges into denser air layers. This paper considers the problem of remote sensing of stratospheric irregularities under strong-scintillation conditions. The scintillation spectra are calculated in the phase-screen approximation under the assumption that the spectrum of the phase added by the screen corresponds to observations through the stratosphere. It is assumed that stratospheric irregularities of air density are generated by an ensemble of saturated internal waves whose three-dimensional spectrum contains two characteristic wave numbers corresponding to the outer and inner scales. In the calculation, no restrictions are imposed on the observed scintillation amplitude. It is shown that the effect of the scintillation amplitude on the observed scintillation spectra appears most prominent for large wave numbers corresponding to irregularities whose sizes are smaller than the inner scale. For these wave numbers, deviations from the weak-scintillation theory become appreciable if the rms relative fluctuation of light intensity exceeds 0.3. In contrast, for small wave numbers corresponding to scales exceeding the outer scale, the weak-scintillation theory remains valid to rms values as large as 2. Analysis of calculated spectra has shown that the parameters of the three-dimensional spectra of stratospheric irregularities can be retrieved under the conditions of relatively strong scintillation characterized by an rms value below 1.5–1.6.

The differences between the refraction angles measured and calculated for the reanalyses of the European Centre for Medium-Range Weather Forecasts were statistically analyzed on the basis of 64 radio occultation events recorded by the Microlab-1 satellite. It is shown that, for minimum ray heights below 20 km, the main contribution to the differences is made by spatial refractive-index fluctuations neglected by the model. The power spectral density of these fluctuations is mainly concentrated within the vertical wave-number range 0.5–10 rad/km. For heights above 30 km, the deviations are mainly determined by ionospheric disturbances and may vary several times during changes of the site and time of observations. This suggests that the results of satellite radio-occultation sounding of the neutral atmosphere can be used as an indirect quantitative estimate of local discrepancies between the actual field of the refractive index and its values calculated on the basis of a hydrodynamic atmospheric general circulation model.

Spatial spectra and characteristic scales of stratospheric density fluctuations obtained from the space station Mir observations of stellar scintillations are analyzed in this paper. The remote sensing method described is based on a well‐known stellar scintillation phenomenon that arises when observing the stars through the Earth's atmosphere. To interpret scintillation spectra, a model of the three‐dimensional (3‐D) spectrum of atmospheric density fluctuations consisting of both turbulent and internal wave‐associated spectral components is presented here. With the model chosen, we explain scintillation spectra at low frequencies by suggesting the atmospheric density fluctuations to be caused by a random ensemble of internal waves with the −5 power law decay for their 3‐D energy spectrum. For the wave‐associated anisotropic part of the spectrum, so‐called outer and internal vertical scales are introduced to explain behavior of the observed scintillation spectra in their low‐frequency range. These scales being earlier proposed only theoretically have been simultaneously revealed in the scintillation spectra presented here. The estimates of the outer scale for the different orbits of the space station are obtained and compared with those found from lidar and rocket soundings of the stratosphere. A possible cause of the observed variation in the wave number bandwidth of the wave‐associated part of the spectrum of air density fluctuations is discussed.

The scintillations of stars observed through the Earth's atmosphere are generated by random irregularities of air density. We propose the qualitative theory for description of coherency and correlations of optical scintillations measured at two wavelengths. It is based on a two‐component model of air density irregularities: One of the components corresponds to anisotropic irregularities, while the second one is generated by locally isotropic turbulence. The main conclusion of the developed theory is that chromatic aberration results in low coherency of isotropic scintillations. The scintillations measured by GOMOS fast photometers (FP) on board the Envisat satellite have confirmed the theoretical conclusions. The coherency of scintillation measurements at wavelength 672 and 499 nm visualize the regions of high coherency where the anisotropic irregularities dominate. Observations have allowed also the detection of layers with low coherence. They are located generally between altitudes of 30 and 40 km. The thickness of the layers and their altitude distribution depend on observation location. It is expected that the locally isotropic turbulence is strongly developed within these layers. We show that the low values of the cross‐correlation coefficient of two‐wavelength scintillations can be used as a qualitative indicator for the presence of layers with prevailing isotropic turbulence. The obtained results showed that the analysis of two‐wavelength coherency and cross‐correlation functions is a sensitive approach which will allow visualizing IGW breakdown in the stratosphere.

One-dimensional spatial spectra of stratospheric phase fluctuations of radio signals are considered using the data of 194 events of radio occultation observations in the GPS-Microlab-1 satellite system. Investigations are carried out for heights ranging from 15-18 km to 25-28 km. Among the spectra of eikonal increments, three groups with the power-law decrease of the spectral density are revealed with the corresponding exponents approximately equal to -5/3, -3, and -4. Spectra with exponents smaller than -5 are attributed to the fourth group; they occur in approximately half the cases. It is shown that the form of the spectrum depends on the temperature profile and the wind-velocity vertical gradient in the region of sounding.

Spectral analysis of stellar scintillation observed aboard the Mir space station through the Earth's stratosphere demonstrates the presence of narrow peaks in the estimated spectra. The paper considers the results of observations performed under optimal conditions at two orbits. Statistical estimation of the reliability of the peaks shows that, with a probability of more than 0.99, they correspond to the presence of vertically quasi-periodic structures in the field of air-density fluctuations at heights of about 35 km. The estimates of the quasi-periods and vertical lengths of the observed structures are presented. The contribution of the structures to the mean square of density fluctuations in the layer in which these structures were observed is estimated, and the possible causes of their formation are discussed.

Measurements of stellar scintillations caused by the atmosphere, conducted on board the space station Mir, are analyzed here. From scintillation spectra we obtained the parameters of spatial spectra of atmospheric density fluctuations (the altitudes 20–70 km) with vertical scales from hundreds of meters to tens of centimeters. The observed characteristic scales of atmospheric inhomogeneities are interpreted with a nonlinear model of internal wave spectrum and a theory of locally homogeneous and isotropic turbulence.

We investigate the capabilities of the back-propagation and radio-holographic methods for the interpretation of data of radio occultation sounding of ionospheric perturbations in regions of sporadic E-layers. Several occultations of Microlab-1 satellite are analysed, where multipath propagation from E-layers is detected. It is shown that the back-propagation technique can be used for processing radio occultation data in multipath zone in order to derive vertical profiles of the refraction angle. The holographic technique allows for the detection of complicated horizontal structures of ionospheric inhomogeneities, where the standard Abel inversion technique cannot be used for the derivation of vertical profiles of electron concentration.

Signals caused by the simultaneous emission of stars and the natural emission of the upper atmosphere are analyzed on the basis of photometric measurements onboard the Mir research platform for two series of five sessions. During each series, the measurements were performed at approximately the same latitudes and local solar time. It has been determined that the atmospheric emission brightness changed by a factor of 4. The effective vertical profiles of the volume emission rate have been reconstructed. During half of the sessions, each of these profiles has two maximums in the luminous layer between altitudes of 80 and 100 km. During three sessions, luminous layers were recorded at 55- to 75-km altitudes.