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Signal propagation and the radio spectrum
Published in L. Tetley, D. Calcutt, Understanding GMDSS, 2012
By referring to the table of the radio frequency spectrum it is possible to gain some initial idea of the approximate range over which radiowaves may be received. The table indicates how the frequency spectrum has been divided into bands which have well-known titles. For instance Search and Rescue (SAR) operations would most likely be conducted using VHF equipment. This equipment when used on channel 16 has a carrier frequency of 156.8 MHz which assigns it to the very high frequency (VHF) band. If all other parameters remain constant, the anticipated radio range of signals propagated on the VHF band, or those higher, is effectively by ‘line-of-sight’. Consequently, SAR communications between a liferaft and a surface vessel could expect to have a range of about 10 to 12 nautical miles depending upon the system installation and the relative heights of the antennae. For radio ranges beyond the horizon to be achieved by earthbound stations it would be necessary to use repeater stations or utilize the LF, MF or HF bands, each of which has its own limiting characteristics. Mobile satellite communications tend to use the L band, whereas earth satellite stations use the C band for line-of-sight communications with a satellite.
Satellites
Published in Mohammad Razani, Commercial Space Technologies and Applications, 2018
C-Band (4–8 GHz): Primarily used for satellite communications, for full-time satellite TV networks or raw satellite feeds. Commonly used in areas that are subject to tropical rainfall, since it is less susceptible to rainfade than Ku band (the original Telstar satellite had a transponder operating in this band, used to relay the first live transatlantic TV signal in 1962). This is the most commonly used frequency band in domestic, regional, and global communication satellites. Even today with the advancement of technology that has made Ku band available and to some extent the Ka band, still C band is the most used frequency band in satellite communication.
Single Longitudinal Mode Laser Diodes
Published in Joachim Piprek, Handbook of Optoelectronic Device Modeling and Simulation, 2017
When used as the light source in fiber-optic communication systems, a semiconductor laser operated with multiple longitudinal modes suffers the mode partition noise (MPN) [1, 2], which stands as the dominant limiting factor to the transmission span of the optical signal in fiber, as the MPN jeopardizes the signal by introducing the intersymbol interference (ISI) among the pulses in the stream—an effect that cannot be simply suppressed through increasing the laser output power. If a multiple longitudinal mode laser is directly modulated, the power allocated to each of its mode fluctuates in a random manner due to the mixed homogeneous and inhomogeneous gain broadening nature of direct bandgapped semiconductors, with the randomness originated from the spontaneous emission noise. Since the signal components carried by different longitudinal modes propagate at different speed due to fiber dispersion, these components won't arrive at the destination at the same time, which results in the pulse spreading over and spilling out of its allocated time slot, and consequently causes ISI. Such ISI bears a random nature due to the random power fluctuation of the multiple longitudinal modes as the signal carriers. Hence, it cannot be eliminated through linear equalization or phase delay compensation. Also, because the random fluctuation is in proportion to the total power, increasing the laser output power won't solve the problem, if doesn't make it worse. Normally, the power penalty soars even starting from a moderate MPN level. For example, at ~1550 nm (the center of the C-band), the maximum transmission capacity–distance product is only 5 Gbps-km, which means by using a typical multiple longitudinal mode Fabry–Pérot (FP) laser, the 2.5 and 10 Gbps optical signal can only be transmitted for 2 km and 500 m, respectively.
Mining large-gradient subsidence monitoring using D-InSAR optimized by GNSS
Published in The Imaging Science Journal, 2021
Haodi Fan, Xugang Lian, Wenfu Yang, Linlin Ge, Haifeng Hu, Zheyuan Du
At present, the Sentinel satellite has a C-band wavelength of 5.6 cm. In D-InSAR technology, the maximum shape variable of adjacent pixels obtained by SAR satellite interferometry in adjacent revisit period is λ/4. Therefore, the maximum deformation of adjacent pixels can be obtained for C-band is 1.4 cm. According to the basic condition given by Zebker et al. in 1992 in order to carried out the interferometry smoothly, the displacement of the unit pixel corre-sponding to the ground object along the radar LOS during the acquisition of the images cannot exceed half of the wavelength [37]. The maximum vertical settlement monitored by InSAR is ΔRmax = W/cosθ, where ΔRmax is the maximum vertical settlement, W represents the line-of-sight subsidence and θ is the radar incidence angle. In summary, the subsidence range of traditional D-InSAR monitoring is very limited.
Ultra-High bit rate all-optical AND/OR logic gates based on photonic crystal with multi-wavelength simultaneous operation
Published in Journal of Modern Optics, 2019
Tamer S. Mostafa, Nazmi A. Mohammed, El- Sayed M. El-Rabaie
In this work, a novel design based on a square lattice of silicon rods is proposed to realize ultra-high bit rate AND/OR logic gate operation. It achieves successful logic operation at multi-wavelength in the commercial C-band. The simultaneous operation is provided without altering the design continually as related literature used to. Internal and external complexity issues are also addressed and minimized. Finite difference time domain (FDTD) method has been used for the simulation. To establish an accurate optimization process, an enhanced illustration tool is developed and used to extract the best wavelength that examines the highest possible (CR) and (BR) for each gate separately. Subsequently, it used to extract the optimum conditions for operating both gates at the same wavelength.
Modelling of a variable optical switch based on the parametric amplification in a photonic crystal fibre
Published in Journal of Modern Optics, 2018
Hassan Pakarzadeh, Mostafa Sharifian
We have modelled and proposed a variable optical switch based on the optical parametric amplification in the photonic crystal fibre. Coupled amplitude equations were numerically solved to simulate the switch gain as a function of the power, wavelength and the SOP of the control wave as well as the signal wavelength. The results show that the switch gain is increased by increasing the control power and the maximum gain of 15 dB is obtained when the SOP of the control wave is 450. The proposed switch which preserves the operating wavelength exhibits a very wide and flat gain exceeding the C-band. This unique feature together with an ultrahigh speed switching has potential applications for WDM systems.