Lidar observations of the meteoric deposition of mesospheric metals.

The mesospheric sodium and iron layers at an altitude between about 80 and 110 kilometers are routinely monitored by atmospheric physicists using resonance fluorescence lidar techniques because these constituents are excellent tracers of mesopause chemistry and dynamics. The mesospheric metals are the products of meteoric ablation. Existing ablation profiles are model calculations based in part on radar observations of the ionized background atmosphere left in the wake of high-speed (> 20 kilometers per second) meteoroids. Thin trails of neutral metal atoms, ablated from individual meteoroids, are occasionally observed with high-power lidars. The vertical distribution of 101 sodium and 5 iron meteor trails observed during the past 4 years at Urbana, Illinois; Arecibo, Puerto Rico; and near Hawaii is approximately Gaussian in shape with a centroid height of 89.0 (+/- 0.3) kilometers and a root-mean-square width of 3.3 (+/- 0.2) kilometers. This directly measured ablation profile is nearly the same as the mean iron layer profile but is considerably different from existing models and the distribution of ionized meteor trails observed by radars. A lower limit on the influx to the mesopause region from the lidar meteors is approximately 1.6 x 10(3) sodium and 2.7 x 10(4) iron atoms per second per square centimeter, which corresponds to an annual flux of meteoric debris into the mesosphere of about 2.0 (+/-0.6) gigagrams. Because the lidars can detect only the ablation trails left by the larger meteors, the observations suggest that the actual meteoric influx may be larger than the more recently reported values, which range between 16 and 78 gigagrams per year.

Primary vaginal cancer: role of MRI in diagnosis, staging and treatment.

Primary carcinoma of the vagina is rare, accounting for 1-3% of all gynaecological malignancies. MRI has an increasing role in diagnosis, staging, treatment and assessment of complications in gynaecologic malignancy. In this review, we illustrate the utility of MRI in patients with primary vaginal cancer and highlight key aspects of staging, treatment, recurrence and complications.

Correction of laser tracking data for the effects of horizontal refractivity gradients.

Pulsed laser ranging systems are being used to measure accurately the distance from the earth to retroreflector equipped satellites. At the lower elevation angles horizontal refractivity gradients can introduce centimeter level errors into the range measurements. A correction formula which compensates for the gradient effects is developed and evaluated using typical meteorological data obtained from weather stations located near Washington, D.C.

Effects of random path fluctuations on the accuracy of laser ranging systems.

The effects of turbulence induced pathlength fluctuations on the accuracy of single color and two color laser ranging systems are examined. Correlation and structure functions for the path deviations are derived using several proposed models for the variation of C(n)(2) with altitude. For single color systems, random pathlength fluctuations can limit the accuracy of a range measurement to a few centimeters when the turbulence is strong (C(n)(2) ~ 10(-13) m(-2/3)), and the effective propagation path is long (>10 km). Two color systems can partially correct for the random path fluctuations so that in most cases their accuracy is limited to a few millimeters. However, at low elevation angles for satellite ranging (<20 degrees ) and over long horizontal paths, two color systems can also have errors approaching a few centimeters.

Technique for remotely measuring surface pressure from a satellite using a multicolor laser ranging system.

The optical path length from a satellite to the earth's surface is strongly dependent on the atmospheric pressure along the propagation path. Surface pressure can be measured by using a multicolor laser ranging system to observe the change with wavelength in the optical path length from the satellite to a ground target. The equations which relate surface pressure to the differential path lengths are derived, and the accuracy of the pressure measurement is evaluated in terms of the ranging system parameters. The results indicate that pressure accuracies of a few millibars appear feasible.

Target signatures for laser altimeters: an analysis.

The statistical characteristics of the received signal for short pulse laser altimeters are investigated. Expressions are derived for the mean and temporal covariances of the received pulse for a direct detection receiver. The effects of laser speckle, shot noise, and surface profile of the ground target are considered. The results are used to compute the means and variances of the total received energy, propagation delay, and rms width of the received pulse. These parameters are shown to be directly related to the statistics of the surface profile.

Spatial and temporal filtering technique for processing lidar photocount data.

Shot noise places a practical limit on the spatial and temporal resolution of lidar photocount data. A 2-D signal-processing technique that utilizes spatial and temporal filtering to reduce shot noise and increase resolution is described. The technique is applied to sodium lidar data collected during the fall of 1979 over Urbana, Illinois. Temporal filtering is shown to enhance the spatial resolution of the sodium profiles significantly by reducing shot noise by more than 10 dB. The signal-processing technique is applicable to a wide variety of lidar data.

Interdisciplinary collaboration: psychiatric medical backup in the outpatient clinic.

The provision of psychiatric medical backup to nonphysician therapists is currently an important issue in inpatient and outpatient practice. Like all team approaches, backup offers the possibility of increased efficiency, professional growth, and access to treatment. At the same time, the psychiatrist providing medical backup is confronted with practical and ethical dilemmas in meeting his or her responsibilities. The authors define the role of psychiatrist medical backup and propose a collaborative model for outpatient practice. Issues to be considered in the planning of backup arrangements are discussed. The importance of clear role definition and communication between collaborators is emphasized.

Remote sensing of sea state using laser altimeters.

The reflection of short laser pulses from the ocean surface is analyzed based on the specular point theory of scattering. The expressions for the averaged received signal, shot noise, and speckle-induced noise are derived for a direct detection receiver system. It is found that the reflected laser pulses have an average shape which is closely related to the probability density function associated with the ocean surface profile. The result is used to estimate the mean sea level and significant wave height from temporal moments of the reflected laser pulse.

Bistatic imaging lidar technique for upper atmospheric studies.

The bistatic imaging lidar technique is fundamentally different from traditional monostatic lidar techniques. The vertical density of an atmospheric layer, such as the mesospheric sodium layer, is measured by imaging an illuminated spot within the layer. The spot is illuminated with a laser and imaged with a telescope in a bistatic configuration. Profiles through the image contain information about the vertical structure of the layer as well as the laser beam cross section. These profiles can be interpreted as the output of a linear filter having the density profile of the layer as input and an impulse response which is related to the laser beam cross section and imaging geometry. The theoretical vertical resolution can be quantified in terms of laser beamwidth and separation distance between the laser and telescope. Theoretical analysis of the technique and experimental data verifying the feasibility and basic performance of the technique are presented.

Nonlinear resonant absorption effects on the design of resonance fluorescence lidars and laser guide stars.

Saturation effects may significantly reduce the backscattered signal expected for resonance fluorescence lidar systems. Saturation arises when the laser energy density within the layer is large enough to significantly alter the population densities of the atomic states within the layer. These altered state populations lead to nonlinear absorption of the laser energy resulting in reduced rate of fluorescence and increased rate of stimulated emission. The level of saturation is determined by the laser pulse length, pulse energy, beam-width, and linewidth. Analysis reveals that the saturation effects can be characterized by two parameters: the saturation time tau(s) and the rms laser linewidth Deltaomega(rms).The saturation time is the characteristic time of stimulated emission and is an indication of the level of saturation. The saturation time depends on pulse energy, pulse length, and beamwidth. The laser linewidth is important since it determines the effective absorption cross section of the atoms and also the percentage of atoms illuminated for an inhomogeneously broadened absorption line. The impact of saturation on lidar design is also examined. Design examples, including lidar systems for atmospheric research and laser guide stars for adaptive imaging applications in astronomy are studied in detail.

Satellite laser ranging using pseudonoise code modulated laser diodes.

Techniques for satellite laser ranging have historically employed high-energy pulsed lasers as the optical transmitter. The output power currently available from laser diodes and laser diode arrays makes them a viable alternative to the pulsed laser for ranging applications. We examine the modulation of a laser diode by a pseudonoise code which produces a cw ranging signal with equivalent average power to that of current pulsed systems. A maximum-likelihood receiver for estimating signal arrival time using ideal rectangular pulses is shown to be a correlation receiver. A Uramer-Rao bound on the range error is derived for a ranging system with Gaussian code pulses and a correlation receiver. Ranging performance is evaluated for a single cubecorner retroreflector and a tilted flat diffuse target. It is shown that range accuracy of the order of 1 cm is feasible.

Optical techniques for remote probing of the atmosphere.

The Optical Society of America and the Air Force Office of Scientific Research sponsored the series of five topical meetings collectively designated Winter 83 held in Incline Village, Nev. during the weeks of 10 and 17 Jan. 1983. The 1 Sept. 1983 issue of Applied Optics contains 24 of the papers presented at the Topical Meeting on Optical Techniques for Remote Probing of the Atmosphere.

Effects of wavefront sampling speed on the performance of adaptive astronomical telescopes.

Adaptive imaging systems have been developed to compensate for distortions introduced by atmospheric turbulence. Their performance is limited by the wavefront sampling rate which is constrained by the wavefront sensor processing time. This paper explores the degradation of system performance caused by limitations in the wavefront sampling rate. Errors introduced by finite image observation times are also examined. For bright images, a very short observation time per sampling period is shown to yield high system resolution. For the case of very faint images, continuous observation results in the best signal-to-noise ratio. Adaptively correcting a 4-m telescope is shown to improve the detection threshold by nearly three visual magnitudes.

Compensation of coherence effects in an optical spectrum analyzer.

An optical technique is described which is capable of compensating the effects of imperfect spatial coherent illumination on the performance of a modified Mach-Zehnder spectrum analyzer. Methods for experimentally determining the complex mutual coherence function of the illumination using the analyzer are also described. The addition to the system of a calibrated phase shifter controlled by a minicomputer is proposed, which permits the spectral amplitude and phase to be determined automatically in real time.

Ray tracing evaluation of a technique for correcting the refraction errors in satellite tracking data.

Refraction errors in satellite laser tracking data are evaluated by ray tracing through 3-D refractivity profiles. The results indicate that a range resolution of 1 cm or less appears feasible for satellite elevation angles above 10 degrees if the tracking data are corrected using both spherical and gradient correction formulas.

Analysis of a potassium lidar system for upper-atmospheric wind-temperature measurements.

We report a detailed analysis of wind-temperature (W/T) lidar systems based on mesospheric potassium as the tracer. Currently, most narrow-band (W/T) systems use sodium (Na) as the tracer because of its relatively large natural abundance, large cross section, and the ability to use Doppler-free Na spectroscopy to generate accurate absolute frequency markers. We show that a potassium-based system with existing near-infrared solid-state laser technology operating at the potassium D lines has the potential to make W/T measurements that are more accurate than current Na narrow-band systems and can be far simpler technically.

Validation of the Lidar In-Space Technology Experiment: stratospheric temperature and aerosol measurements.

The Lidar In-Space Technology Experiment (LITE) was flown on STS-64 in September 1994. The LITE employed a Nd:YAG laser operating at 1064, 532, and 355 nm to study the Earth's lower atmosphere. In this paper we investigate the nighttime stratospheric aerosol and temperature measurements derived from the 532- and 355-nm channels. The observations are compared with lidar observations obtained at Arecibo Observatory, Puerto Rico, and Starfire Optical Range, New Mexico, and with balloonsondes launched from the San Juan and Albuquerque airports. The backscatter ratios derived from the LITE and Arecibo data between 15 and 30 km differ by less than 5%. The Angstrom coefficients of the stratospheric aerosols derived from the 532- and 355-nm LITE channels exhibited only slight variation in altitude. The mean value between 15 and 30 km derived from three different orbital segments at approximately 20 degrees N and 35 degrees N was 1.7. The mean standard deviation was approximately 0.3. Temperature profiles were derived from the LITE data by correcting the 355-nm channel for aerosol scattering with the 532-nm signal and an assumed Angstrom coefficient. The rms differences between the corrected profiles and the balloonsonde data were as low as 2 K in the 15-30-km height range. The results were not particularly sensitive to the choice of the Angstrom coefficient and suggest that accurate temperature profiles can be derived from the LITE data in the upper troposphere and lower stratosphere provided that the aerosol loading is light.