Measured Visibility Data Research Articles

RDCP: A Real Time Sea Fog Intensity and Visibility Estimation Algorithm

A number of accidents at sea are primarily caused by low visibility due to sea fog. Therefore, it is important to estimate sea fog intensity and visibility in real-time in the ocean environment. Visibility meters utilize optical sensors rather than visional information, so that the measured visibility data occasionally includes errors. Moreover, visibility meters have significant costs so that it is not viable to install them at various locations. Therefore, this paper proposes an algorithm called RDCP (Reduced Dark Channel Prior), which provides reliable estimation at a low cost by processing images captured from cameras, since they receive identical information as human eyes. For the estimation, the RDCP algorithm firstly acquires dark channels from an ocean image, then applies an optimized threshold value to the dark channels and crops out the sky region in the ocean image. For the estimation performance evaluation, 320 raw images captured from cameras at four different ports in Republic of Korea are used. Considering facilities in oceans, which are usually operated in power-limited environment, the processing time performance of PDCP is also examined. The experiments demonstrate that the RDCP algorithm provides reliable estimation performance in real-time.

Design and Performance Evaluation of Free Space Optical Communication Link in Zaria, Nigeria

Free Space Optical Communication (FSOC) is an optical technology with a great chance to complement traditional wireless communication technologies. It offers essential advantages compared to Radio Frequency (RF), such as low power consumption, no frequency restrictions, no electromagnetic interference, security, and a significant increase in bandwidth. Similarly, they have proven a worthy substitute for fiber optics with notable advantages. FSOC systems are usually installed above ground level and are therefore exposed to prevailing weather conditions such as haze, fog, rain, etc. which negatively affect the optical signal transmission. There is a shortage of research on using FSO systems in tropical regions. For this reason, accurate free space optical communication channel modeling helps telecommunication operators to engineer and appropriately manage their networks. Therefore, this research work developed Free Space Optical Communication Channel Model to mitigate the effects of atmospheric attenuations by estimating the specifically induced attenuation caused by both haze and rainfall rates on the FSOC link in the Zaria geographical area using two years of measured visibility data and rainfall rates data of the study location obtained from Nigerian Meteorological (NIMET) agency, Zaria station, locatedat the Nigeria College of Aviation Technology (NCAT) Zaria and Center for Energy Research and Training (CERT), Ahmadu Bello University (ABU), Zaria. The performance of the FSOC system is analyzed and evaluated through Link Margin (LM) analysis by using the design specifications of a commercial optical transceiver (TereScope 5000). The haze-induced attenuation obtained at 850nm, 950nm, and 1550nm is 5.934dB, 6.402dB, and 3.152dB respectively. Therefore, the result shows that the 1550 nm wavelength has minimum haze-induced attenuation compared to 850nm and 950nm at a propagation link distance of 6km. Furthermore, from the results of the performance evaluation of the LM analysis for the combined effects of Geometrical Attenuation (Gatt), Haze-Induced Attenuation (HIA), and Rain-Induced Attenuation (RIA), the result shows that operating a 1550nm wavelength in transmission power greatly improves optical transmission when compared with 850nm and 950nm wavelengths. Generally, the overall outcome of the research concludes that the free space optical communication system has the robustness to handle successful wireless communication during the worst weather conditions in Zaria throughout the year for a link range of up to 6km

Towards 21-cm intensity mapping at z = 2.28 with uGMRT using the tapered gridded estimator I: Foreground avoidance

ABSTRACT The post-reionization (z ≤ 6) neutral hydrogen (H i) 21-cm intensity mapping signal holds the potential to probe the large-scale structures, study the expansion history, and constrain various cosmological parameters. Here, we apply the Tapered Gridded Estimator (TGE) to estimate P(k⊥, k∥) the power spectrum of the $z = 2.28\, (432.8\, {\rm MHz})$ redshifted 21-cm signal using a $24.4\, {\rm MHz}$ sub-band drawn from uGMRT Band 3 observations of European Large-Area ISO Survey-North 1 (ELAIS-N1). The TGE allows us to taper the sky response, which suppresses the foreground contribution from sources in the periphery of the telescope’s field of view. We apply the TGE on the measured visibility data to estimate the multifrequency angular power spectrum (MAPS) Cℓ(Δν) from which we determine P(k⊥, k∥) using maximum likelihood that naturally overcomes the issue of missing frequency channels (55 per cent here). The entire methodology is validated using simulations. For the data, using the foreground avoidance technique, we obtain a $2\, \sigma$ upper limit of $\Delta ^2(k) \le (133.97)^2 \, {\rm mK}^{2}$ for the 21-cm brightness temperature fluctuation at $k = 0.347 \, \textrm {Mpc}^{-1}$. This corresponds to $[\Omega _{\rm H\, {\small I}~}b_{\rm H\, {\small I}~}] \le 0.23$, where $\Omega _{\rm H\, {\small I}~}$ and $b_{\rm H\, {\small I}~}$, respectively, denote the cosmic H i mass density and the H i bias parameter. A previous work has analysed $8 \, {\rm MHz}$ of the same data at z = 2.19, and reported $\Delta ^{2}(k) \le (61.49)^{2} \, {\rm mK}^{2}$ and $[\Omega _{\rm H\, {\small I}~} b_{\rm H\, {\small I}~}] \le 0.11$ at $k=1 \, {\rm Mpc}^{-1}$. The upper limits presented here are still orders of magnitude larger than the expected signal corresponding to $\Omega _{\rm H\, {\small I}~} \sim 10^{-3}$ and $b_{\rm H\, {\small I}~} \sim 2$.

High accuracy wide-field imaging method in radio interferometry

ABSTRACT With the development of modern radio interferometers, wide-field continuum surveys have been planned and undertaken, for which accurate wide-field imaging methods are essential. Based on the widely used W-stacking method, we propose a new wide-field imaging algorithm that can synthesize visibility data from a model of the sky brightness via degridding, able to construct dirty maps from measured visibility data via gridding. Results carry the smallest approximation error yet achieve relative to the exact calculation involving the direct Fourier transform. In contrast to the original W-stacking method, the new algorithm performs least-misfit optimal gridding (and degridding) in all three directions, and is capable of achieving much higher accuracy than is feasible with the original algorithm. In particular, accuracy at the level of single precision arithmetic is readily achieved by choosing a least-misfit convolution function of width W = 7 and an image cropping parameter of x0 = 0.25. If the accuracy required is only that attained by the original W-stacking method, the computational cost for both the gridding and Fast Fourier Transform steps can be substantially reduced using the proposed method by making an appropriate choice of the width and image cropping parameters.

Ångström exponent errors prevent accurate visibility measurement

Abstract. Visibility is an indicator of atmospheric transparency, and it is widely used in many research fields, including air pollution, climate change, ground transportation, and aviation. Although efforts have been made to improve the performance of visibility meters, a significant error exists in measured visibility data. This study conducts a well-designed simulation calibration of visibility meters, which proves that current methods of visibility measurement include a false assumption, leading to the long-term neglect of an important source of visibility error caused by erroneous values of Ångström exponents. This error has two characteristics, namely (1) independence, which means that the magnitude of the error is independent of the performance of the visibility meter. It is impossible to reduce this error by improving the performance of visibility meters. The second characteristic is (2) uncertainty, which means the magnitude of the error does not show a clear change pattern but can be substantially larger than the measurement error of visibility meters. It is impossible to accurately estimate the magnitude of this error or its influence on visibility measurements. Our simulations indicate that, as errors in visibility caused by erroneous values of Ångström exponents are inevitable using current methods of visibility measurement, reliable visibility data cannot be obtained without major adjustments to current measurement methods.

Link performance evaluation of terrestrial FSO model for predictive deployment in Bhubaneswar smart city under various weather conditions of tropical climate

This paper proposes the link performance analysis of the terrestrial free-space optic (FSO) model, intended for deployment in Bhubaneswar smart city. The challenges of atmospheric adversity and its impact on the performance of FSO model have been analyzed from the real-time measured visibility data. A comparative study of different statistical models addressing the relation between measured visibility data and atmospheric attenuation has been carried out. The atmospheric losses such as haze, scintillation, rain, fog are considered for the link analysis due to the tropical savanna climate of the Bhubaneswar city. Thereafter the link margin is calculated for summer, rainy, winter season for the feasible establishment of FSO link. Comparison between two low absorption optical wavelengths such as 850 nm and 1550 nm has been considered for performance analysis. This is because of the commercial availability of optical components. Performance metrics such as received signal power, bit error rate (BER), signal to noise ratio (SNR), link margin has been used for the link performance analysis for a best fit FSO model for deployment in the city. Finally, the maximum achievable data rate is calculated for FSO link in all the three seasons for ensuring high-quality network reach.

Analysis of Optical Attenuation from Measured Visibility Data in Islamabad, Pakistan

FSOL (Free-Space Optical Links) are becoming very popular due to the inherent advantages of high data rates, rapid deployment, portability, cost effective and immunity to electromagnetic interference. FSOL is a line-of-sight technology that uses a modulated beam of light to transmit and receive the data of multiple of Giga Bit per second. FSOL uses the free space or atmosphere as a communication channel. Optical signal launched at transmitter end, travels through the atmosphere and reaches to the receiver, is severely affected by the local atmospheric conditions. Atmosphere contains different atmospheric particulates like fog, rain, snow, smog, clouds and haze. These atmospheric particulates, particularly fog, effect the propagation of optical signal passing through the atmosphere and cause significant amount of optical attenuation. In order to deploy FSO system, a detailed analysis of local weather condition is much more important. Here, we present the measured visibility data for analysis of optical attenuation. Visibility data was measured at COMSATS Institute of Information Technology from 2009-2012. Percentage CDF (Cumulative Distribution Function) is used to find out the detailed insight about the optical attenuation of FSOL. Yearly based CDFs is compared with each other which clearly suggest that almost 99% of time optical attenuation remains within the range of 2 dB/km.

Analysis of Optical Attenuation from Measured Visibility Data in Islamabad, Pakistan

Analysis of Optical Attenuation from Measured Visibility Data in Islamabad, Pakistan

Availability analysis of terrestrial free space optical (FSO) link using visibility data measured in tropical region

Haze and rain are the most impairments factors to free space optical (FSO) links. These weather conditions limit the visibility, and thereby causes high attenuation of the optical signal. This high attenuation reduces the availability performance of the FSO link. This study evaluates the performance of a terrestrial FSO link under tropical climate conditions. The performance analysis is evaluate over 5 km link distance. The cumulative distribution function (CDF) of the received signal to noise ratio (SNR) is used to study the outage performance of the FSO link under haze conditions. The performance analysis is based on three years measured visibility data in Malaysia. Based on the statistical analysis, link availability of 99.99% can be achieved for 37.44 dB SNR over 1 km link distance. Carrier class availability “five nine” can be achieved for 20 dBm transmission power using 1550 nm wavelength.

Retrieving visibility values using satellite remote sensing data

The recent development of satellite meteorology has allowed us to estimate spatially and frequently number of basic meteorological parameters. This paper presents the proposed methodology for retrieving visibility values based on the application of the darkest pixel atmospheric correction algorithm on satellite image data. The method is based on the use of the radiative transfer calculations followed by some key assumptions. Landsat-5 TM band 1 images (0.45–0.52 μm) have been used to determine the visibility value for each image date. A direct comparison between the measured visibility data from the airport meteorological stations with the determined visibility data was performed showing high correlation values. Indeed, by relating the determined visibility data with those measured on the Heathrow Airport station in the West London (UK), a correlation coefficient of r 2 = 0.97 has been found with the observed significance for the regression model to be less than 0.05, for four multi-temporal images acquired on 1985 and 1986. The algorithm has been tested also to Landsat TM images of the Paphos Airport area in Cyprus with satisfactory agreement between the visibilities measured at the meteorological station and those found from the images. The algorithm presented may be useful for assessing the atmospheric conditions of satellite images and also can assist the improvement and effectiveness of the available atmospheric correction algorithms.

Constraints on the HL Tauri Protostellar Disk from Millimeter‐ and Submillimeter‐Wave Interferometry

Millimeter and submillimeter interferometry is used to probe the dusty accretion disks around young protostars. New 460 GHz (λ = 650 μm) data from the Caltech Submillimeter Observatory and the James Clerk Maxwell Telescope CSO-JCMT Interferometer are combined with previous 345 GHz (λ = 870 μm) data from CSO-JCMT, 220 GHz (λ = 1.4 mm) data from the Owens Valley Radio Observatory (OVRO) Millimeter Array, 110 GHz (λ = 2.7 mm) data from the Berkeley-Illinois-Maryland Association Array (BIMA), and 43 GHz (λ = 7 mm) data from the VLA, in order to constrain the nature of the protostellar disk around HL Tau on size scales of 50 AU and above. A power-law disk model is fitted directly to the measured visibility data, and probability distributions are derived for the parameters. The effects of instrumental uncertainties are included in a consistent way. The position angle of the major axis of the emission is determined to be 127° ± 5° and the inclination 42° ± 5° (where 0° is face-on), assuming the disk is thin, flat, and circular. A strongly flared disk that is close to edge-on cannot be ruled out, however. The CSO-JCMT and OVRO data favor centrally concentrated distributions of the surface density Σ, where Σ ∝ r-p, and p > 1. This is not compatible with the relatively large sizes measured at lower frequency by BIMA and the VLA. No simple power-law disk model can be found that reproduces all of the millimeter and submillimeter data well. Such a model, with radial power laws in the surface density and temperature, and a single dust component, is therefore unlikely to be a good representation of the actual disk structure. One possibility that would help to reconcile the model with the data is the existence of more than one dust component, i.e., a range of grain sizes or structures in the disk and no unique value of the emissivity index β. Future models should allow for this as well as for disk geometries that are not thin and flat.