New Voyager radio spectrograms of Uranus

New, high-resolution spectrograms of the Voyager-2 radio observations at Uranus were produced from the original, six-second Planetary Radio Astronomy (PRA) data and these show a number of new features which were not obvious in previous versions. Among these new features are the detailed structure of the so-called broadband-bursty (b-bursty) emissions, unexpected sloping striations in the smooth high-frequency (SHF) component, and the overlap of these two components during the first rotation after closest approach. In addition, a slightly different planetary rotation rate from the b-bursty emissions, was found, and at the initial onset of the SHF component, what appears to be the shadow of a Uranian plasmasphere. These new spectrograms were prepared using a special dithering algorithm to show signal strengths as gray shadings, and the data were also manually cleaned to suppress noise and interference. This produced spectrograms of exceptional quality and certain details of their production on a stand-alone personal computer are also discussed

A Study of Saturn's E-Ring Particles Using the Voyager 1 Plasma Wave Instrument

The flyby of Voyager 1 at Saturn resulted in the detection of a large variety of plasma waves, e.g., chorus, hiss, and electron cyclotron harmonics. Just before the outbound equator crossing, at about 6.1 R(sub s), the Voyager 1 plasma wave instrument detected a strong, well-defined low-frequency enhancement. Initially it was suggested that plasma waves might be responsible for the spectral feature but more recently dust was suggested as at least a partial contributor to the enhancement. In this report we present evidence which supports the conclusion that dust contributes to the low-frequency enhancement. A new method has been used to derive the dust impact rate. The method relies mainly on the 16-channel spectrum analyzer data. The few wide band waveform observations available (which have been used to study dust impacts during the Voyager 2 ring plane crossing) were useful for calibrating the impact rate from the spectrum analyzer data. The mass and, hence, the size of the dust particles were also obtained by analyzing the response of the plasma wave spectrum analyzer. The results show that the region sampled by Voyager 1 is populated by dust particles that have rms masses of up to few times 10(exp -11) g and sizes of up to a few microns. The dust particle number density is on the order of 10(exp -3) m(exp 3). The optical depth of the region sampled by the spacecraft is 1.04 x 10(exp -6). The particle population is centered about 2500 km south of the equatorial plane and has a north-south thickness of about 4000 km. Possible sources of these particles are the moons Enceladus and Tethys whose orbits lie within the E-ring radial extent. These results are in reasonable agreement with photometric studies and numerical simulations

Micron-Sized Particles Detected in the Vicinity of Jupiter by the Voyager Plasma Wave Instruments

Wideband waveform data obtained by the plasma wave instruments onboard the Voyager 1 and 2 spacecraft have been used to study micron-sized dust particles in the vicinity of Jupiter. The technique used was developed during the flybys of Saturn, Uranus, and Neptune, and makes use of the fact that a particle striking the spacecraft at 10-20 km/s causes a voltage pulse in the plasma wave receiver. The waveform of the voltage pulse is much different than the waveform of plasma waves and provides a highly reliable method of detecting micron-sized dust particles. Although the dust impact rate observed in the vicinity of Jupiter is much lower than the rates at Saturn, Uranus, and Neptune, the particles are easily detectable. Approximately 1200 48-second frames of wideband waveform data were examined in the vicinity of Jupiter. Dust impact signatures were found in approximately 20% of these frames. The peak impact rates are about 1 impact per second, and the peak number densities are about 10(exp -5) m(exp -3). Most of the impacts occurred near the equatorial plane at radial distances less than about 35 R(sub j) from Jupiter. Analysis of the detection threshold indicates that the particles have masses greater than 10(exp -11) g, which corresponds to particles with diameters of a few micrometers or larger

ANAM vs. NAM: Is the difference significant?

The Navy Aerosol Model (NAM, available in MODTRAN) is widely used as a tool to assess the aerosol extinction in the marine atmospheric surface layer. NAM was built as a regression model in the 1980s to represent the aerosol extinction at deck height as a function of the meteorological conditions. The recently developed Advanced Navy Aerosol Model (ANAM) utilizes additional experimental evidence to supersede NAM by correcting the underestimation of the concentration of aerosols larger than a few microns. More importantly, ANAM provides the aerosol extinction as a function of height between the surface and several tens of meters. Present-day naval surveillance and threat scenarios require detection of targets at the horizon, such as sea-skimming missiles, or small targets such as rubber boats. In either case, the propagation path from sensor to target is likely to come very close to the wave surface and in order to estimate detection ranges, an assessment of the transmission losses along the path is necessary. To answer the question posed in the title, we assess the two models using two meteorological data sets (784 cases) representative of diverse maritime conditions in regions of interest around the world

Air-sea interaction processes observed from buoy and propagation measurements during the red experiment

In recent years researchers have spent much effort towards gaining an understanding of the complex physical mechanisms through which the atmosphere and ocean interact with each other. This is due to the fact that knowledge of air-sea exchanges is important for a wide range of applications, such as the diverse topics of global climate modeling and near-horizon electromagnetic (EM) wave propagation assessment and prediction. EM propagation through the atmosphere is highly dependent upon the vertical profiles of air temperature and humidity and the horizontal variations in these profiles. It is well known that under most conditions these near-surface scalar profiles depend upon the turbulent air-sea fluxes. Traditional Monin-Obukhov similarity (MOS) theory has been used to successfully predict near-surface profiles over the ocean for most, but not all, stability conditions. It is also becoming increasingly clear that ocean waves influence near-surface profiles, although an understanding of the exact mechanisms through which this occurs and parameterizations to describe these processes so far have remained elusive (e.g. Hare et al. 1997; Hirstov et al. 1998)

Effects of atmospheric refraction and turbulence on long-range IR imaging in the marine surface layer, Comparisons between experiment and simulation

EOSTAR, a PC based Windows application, integrates the required modules necessary to calculate the electro-optical sensor performance on the basis of standard meteorological data. The primary output of EOSTAR consists of the synthetic sensor image (“what does the sensor see?”) and a coverage diagram (“detection probability versus range”). As part of the EOSTAR validation effort, the refraction and turbulence modules are being evaluated against literature data, similar models and experimental results. It is shown that the EOSTAR model can predict with reasonable success the occurrence of optical turbulence and refraction phenomena such as mirages. The major cause for discrepancies between the various models is attributed to the underlying micrometeorological bulk modules, whereas the sensitivity of the predictions on the values of the meteorological input parameters is held responsible for the discrepancies between model predictions and measurements

EOSTAR: Electro-Optical Signal Transmission and Ranging

The integrated model EOSTAR (Electro-Optical Signal Transmission and Ranging) is being developed to predict the performance of electro-optical (EO) sensor systems in the marine atmospheric surface layer. The model allows the user to define camera systems, atmospheric conditions and target characteristics, and it uses standard (shipboard) meteorological data to calculate atmospheric effects such as refraction, turbulence, spectrally resolved transmission, path- and background radiation. Alternatively, the user may specify atmospheric conditions, either interactively or in data files with a flexible format. Atmospheric effects can be presented graphically as distorted images of synthetically generated targets with spatially distributed emission properties. EOSTAR is a completely mouse-driven PC Windows program with a user-friendly interface and extensive help files. Many calculations are performed in real-time, although transmission and radiance take from a few seconds up to the order of ten seconds to complete for each new meteorological condition. The program can be used in a wide range of applications, e.g., for operational planning and instructio

Prediction and exploitation: the use of the EOSTAR model in the marine infrared propagation environment

Modern surface Navy ships require dependable and predictable communications, surveillance, and tracking systems. An accurate model for the propagation of infrared and optical frequencies through the atmosphere is a requirement for these systems, which operate over long nearly-horizontal paths that are close to the land or sea surface. The determination of the propagation environment for surface ships can be a difficult problem. The most critical portion is the 50-meter-thick surface layer containing the ship and extending to the horizon. Extended horizontal propagation paths within this atmospheric surface layer encounter relatively dynamic refractivity conditions. We will describe the application of the EOSTAR (Electro-Optical Signal Transmission and Ranging) model suite to provide accurate sensor performance predictions. The EOSTAR model is built upon a geometrical optics approach to infrared propagation: a ray is traced through the propagation environment, and path-dependent perturbations to the signal can be determined. EOSTAR is a valuable tool for prediction and exploitation of several phenomena common to this environment, and we will discuss the design and use of three individual modules within the EOSTAR suite: 1. Exploitation of a sub-refractive mirage to provide a passive ranging capability; 2. A path-dependent calculation of a refractive propagation factor, or geometric gain; 3. Exploitation of scintillation effects to provide an early detection capability, and the prediction of a signature frequency and variance to enable detection enhancement

EOSTAR : an electro-optical sensor performance model for predicting atmospheric refraction, turbulence, and transmission in the marine surface layer

A first version of the integrated model EOSTAR (Electro-Optical Signal Transmission and Ranging) to predict the performance of electro-optical (EO) sensor systems in the marine atmospheric surface layer has been developed. The model allows the user to define camera systems, atmospheric conditions and target characteristics, and it uses standard (shipboard) meteorological data to calculate atmospheric effects such as refraction, turbulence, spectrally resolved transmission, path- and background radiation. Alternatively, the user may specify vertical profiles of meteorological parameters and/or profiles of atmospheric refraction, either interactively or in data files with a flexible format. Atmospheric effects can be presented both numerically and graphically as distorted images of synthetically generated targets with spatially distributed emission properties. EOSTAR is a completely mouse-driven PC Windows program with a user-friendly interface and extended help files. Most calculations are performed in real-time, although spectral transmission and background radiation calculations take up to a few seconds for each new meteorological condition. The program can be used in a wide range of applications, e.g., for operational planning and instruction