FSO performance analysis of a metro city in different atmospheric conditions

In this article, a free-space-optics (FSO) model for a metro city based on real-time visibility data under different weather conditions is presented. Here, the relation between atmospheric attenuation and visibility value and its impact on the FSO performance is carried out. Also, the hybrid-SIM modulation technique is applied for the bit error rate (BER) improvement of the system. Here, Bhubaneswar (the capital of Odisha, India) is considered for the above analysis. The total attenuation, BER, and link margin are calculated for the FSO performance evaluation of Bhubaneswar city and these are simulated using Matlab. The simulation results show that the proposed hybrid-SIM modulation technique achieves about 2.9 dB of SNR improvement than the existing BPSK-SIM technique. Also, from the link margin analysis, it is observed that the sunny day is the most suitable for FSO link transmission. These results can be helpful to decide the FSO parameters and the FSO setup can be designed for any city.

Characterizing Rain Cells as Measured by a Ka-Band Nadir Radar Altimeter: First Results and Impact on Future Altimetry Missions

The impact of large atmospheric attenuation events on data quality and availability is a critical aspect for future altimetry missions based on Ka-band altimetry. The SARAL/AltiKa mission and its Ka-band nadir altimeter offer a unique opportunity to assess this impact. Previous publications (Tournadre et al., 2009, 2015) already analyzed the impact of rain on the waveforms at Ka-band and proposed a definition of an elaborate rain flag. This notion tends to give a simpler black and white view of the atmospheric attenuation when the effect on the altimeter measurement is intense. However, in practice, there is a continuum of measurements that may be partially distorted or corrupted by rain events. The present study proposes a wider point of view, directly using the timeseries of the Ka-band altimeter backscattering coefficient for the first time, when previous studies relied on microwave radiometer (MWR) observations or model analyses with coarser resolutions. As guidelines for future Ka-band missions concerning the impact of the atmosphere, the Attenuation CElls Characterization ALgorithm (ACECAL) approach not only provides more representative statistics on rain cells (occurrences, amplitude, size), but also describes the internal structure of the cells. The actual atmospheric attenuation retrieved with ACECAL is about four times larger than the attenuation retrieved from the MWR. At a global scale, 1% of the measurements are affected by an attenuation larger than 23 dB and 10% of the atmospheric attenuation events have a size larger than 40 km. At regional scale, some areas of particular interest for oceanography as Gulf Stream, North Pacific and Brazil currents are more systematically affected compared with global statistics, with atmospheric attenuation up to 8 dB and cell size larger than 25 km when rain occurs. This study also opens some perspectives on the benefits that the community could be drawn from the systematic distribution of the rain cells parameters as secondary products of altimetry missions.

Vulnerability of Satellite Quantum Key Distribution to Disruption from Ground-Based Lasers

Satellite-mediated quantum key distribution (QKD) is set to become a critical technology for quantum-secure communication over long distances. While satellite QKD cannot be effectively eavesdropped, we show it can be disrupted (or ‘jammed’) with relatively simple and readily available equipment. We developed an atmospheric attenuation and satellite optical scattering model to estimate the rate of excess noise photons that can be injected into a satellite QKD channel by an off-axis laser, and calculated the effect this added noise has on the quantum bit error rate. We show that a ground-based laser on the order of 1 kW can significantly disrupt modern satellite QKD systems due to photons scattering off the satellite being detected by the QKD receiver on the ground. This class of laser can be purchased commercially, meaning such a method of disruption could be a serious threat to effectively securing high-value communications via satellite QKD in the future. We also discuss these results in relation to likely future developments in satellite-mediated QKD systems, and countermeasures that can be taken against this, and related methods, of disruption.

Characterizing Rain Cells as Measured by a Ka-band Nadir Radar Altimeter: First Results and Impact on Future Altimetry Missions

The impact of large atmospheric attenuation events on data quality and availability is a critical aspect for future altimetry missions based on Ka-band altimetry. The SARAL/AltiKa mission and its Ka-band nadir altimeter offer a unique opportunity to assess this impact. Previous publications (Tournadre et al. 2009, 2015) already analyzed the impact of rain on the waveforms at Ka-band and proposed a definition of an elaborate rain flag. This notion tends to give a simpler black and white view of the atmospheric attenuation when the effect on the altimeter measurement is intense. But in practice, there is continuum of measurements that may be partially distorted or corrupted by rain events. The present study proposes a wider point of view , the ACECAL approach providing statistics on rain cells occurrences as well as their amplitude and their size, as guidelines for future Ka-band missions concerning the impact of the atmosphere. At global scale, 1 % of the measurements are affected by an attenuation larger than 23 dB and 10 % of the atmospheric attenuation events have a size larger than 40 km. This study demonstrates that the data quality and availability over some regions of particular interest for oceanography as Gulf Stream, North Pacific and Brazil currents could be affected compared to global statistics. It also opens some perspectives on the benefits that the community could be drawn from the systematic distribution of the rain cells parameters as secondary products of altimetry missions.

Enhanced Spectrum Slicing--Wavelength Division Multiplexing Approach for Mitigating Atmospheric Attenuation in Optical Communication

Over the last decades, free-space optics (FSO) emerged as a prominent way of communication over radio frequency communication and microwave communication. Working of FSO in comparison with optical fiber cable (OFC) network is the same. The only difference between FSO & OFC is the transmission of an optical beam. The optical beam is transmitted through the free space in the case of FSO. Whereas the transmission of optical beam takes place using OFC core, i.e. glass fiber in the case of OFC. Fog, haze, rain, and clouds in the atmosphere directly affect FSO performance and the power of signal propagation. Further added, the wavelength of propagating beam is based on the size of fog particles which leads to atmospheric attenuation. To mitigate the impact of atmospheric attenuation on signals, the proposed study is based on a spectrum slicing (SS) - wavelength division multiplexing (WDM) system with Pre and Post Amplification. The analysis of SS-WDM-Pre and SS-WDM-Post has been done over various values of attenuation. The comparison analysis proves that the SS-WDM-Post is more efficient than both SS-WDM-Pre and traditional SS-WDM in terms of Q-factor.

Improving atmospheric path attenuation estimates for radio propagation applications by microwave radiometric profiling

Ground-based microwave radiometer (MWR) observations of downwelling brightness temperature (TB) are commonly used to estimate atmospheric attenuation at relative transparent channels for radio propagation and telecommunication purposes. The atmospheric attenuation is derived from TB by inverting the radiative transfer equation with a priori knowledge of the mean radiating temperature (TMR). TMR is usually estimated by either time-variant site climatology (e.g., monthly average computed from atmospheric thermodynamical profiles) or condition-variant estimation from surface meteorological sensors. However, information on TMR may also be extracted directly from MWR measurements at channels other than those used to estimate atmospheric attenuation. This paper proposes a novel approach to estimate TMR in clear and cloudy sky from independent MWR profiler measurements. A linear regression algorithm is trained with a simulated dataset obtained by processing 1 year of radiosonde observations of atmospheric thermodynamic profiles. The algorithm is trained to estimate TMR at K- and V–W-band frequencies (22–31 and 72–82 GHz, respectively) from independent MWR observations at the V band (54–58 GHz). The retrieval coefficients are then applied to a 1-year dataset of real V-band observations, and the estimated TMR at the K and V–W band is compared with estimates from nearly colocated and simultaneous radiosondes. The proposed method provides TMR estimates in better agreement with radiosondes than a traditional method, with 32 %–38 % improvement depending on frequency. This maps into an expected improvement in atmospheric attenuation of 10 %–20 % for K-band channels and ∼30 % for V–W-band channels.

A multi-band map of the natural night sky brightness including Gaia and Hipparcos integrated starlight

The natural night sky brightness is a relevant input for monitoring the light pollution evolution at observatory sites, by subtracting it from the overall sky brightness determined by direct measurements. It is also instrumental for assessing the expected darkness of the pristine night skies. The natural brightness of the night sky is determined by the sum of the spectral radiances coming from astrophysical sources, including zodiacal light, and the atmospheric airglow. The resulting radiance is modified by absorption and scattering before it reaches the observer. Therefore, the natural night sky brightness is a function of the location, time and atmospheric conditions. We present in this work GAMBONS (GAia Map of the Brightness Of the Natural Sky), a model to map the natural night brightness of the sky in cloudless and moonless nights. Unlike previous maps, GAMBONS is based on the extra-atmospheric star radiance obtained from the Gaia catalogue. The Gaia-DR2 archive compiles astrometric and photometric information for more than 1.6 billion stars up to G =21 magnitude. For the brightest stars, not included in Gaia-DR2, we have used the Hipparcos catalogue instead. After adding up to the star radiance the contributions of the diffuse galactic and extragalactic light, zodiacal light and airglow, and taking into account the effects of atmospheric attenuation and scattering, the radiance detected by ground-based observers can be estimated. This methodology can be applied to any photometric band, if appropriate transformations from the Gaia bands are available. In particular, we present the expected sky brightness for V(Johnson), and visual photopic and scotopic passbands.