Satellite communication lecture8
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This document discusses factors that can impair satellite communications, including the atmosphere. It covers several key points: 1) The ionosphere and troposphere can both introduce impairments on signals depending on frequency. The ionosphere affects signals below 3 GHz while the troposphere affects those above 3 GHz. 2) Effects in the ionosphere include scintillation, which causes rapid fluctuations, as well as absorption and dispersion. Scintillation is most severe near the equator, poles and during sunrises/sunsets. 3) The troposphere's refractive index fluctuations can also cause scintillation on signals from C-band through V-band frequencies used in satellite communications. Its effects vary seasonally and daily
Satellite communication lecture8
- 1. SATELLITE COMMUNICATION LECTURE-8 PROPAGATION FACTORS RAIN ATTENUATION MODELS, TROPOSPHERIC AND IONOSPHERIC EFFECT
- 2. TRANSMISSION IMPAIRMENTS • The effect of the earth’s atmosphere on radio waves propagating between earth and space is a constant concern in the design and performance of satellite communications systems. • These conditions, when present alone or in combination on the earth-space link, can cause uncontrolled variations in signal amplitude, phase, polarization, and angle of arrival, which result in a reduction in the quality of analog transmissions and an increase in the error rate of digital transmissions. • The frequency of the radio wave is a critical factor in determining whether impairments to space communications will be introduced by the earth’s atmosphere.
- 3. RADIO FREQUENCY AND SPACE COMMUNICATION • The frequency of the radio wave is a critical factor in determining whether impairments to space communications will be introduced by the earth’s atmosphere. • A radio wave will propagate from the earth’s surface to outer space provided its frequency is high enough to penetrate the ionosphere, which is the ionized region extending from about 15 km to roughly 400 km above the surface. The various regions (or layers) in the ionosphere, designated D, E, and F, in order of increasing altitude, act as reflectors or absorbers to radio waves at frequencies below about 30 MHz, and space communications is not possible. As the operating frequency is increased, the reflection properties of the E and F layers are reduced and the signal will propagate through.
- 4. PROPAGATION MECHANISMS The mechanisms are usually described in terms of variations in the signal characteristics of the wave, as compared to the natural or free space values found in the absence of the mechanism. • Absorption-A reduction in the amplitude (field strength) of a radiowave caused by an irreversible conversion of energy from the radiowave to matter in the propagation path. • Scattering- A process in which the energy of a radiowave is dispersed in direction due to interaction with inhomogeneities in the propagation medium. • Refraction-Achange in the direction of propagation of a radiowave resulting from the spatial variation of refractive index of the medium. • Diffraction-A change in the direction of propagation of a radiowave resulting from the presence of an obstacle, a restricted aperture, or other object in the medium. • Multipath-The propagation condition that results in a transmitted radiowave reaching the receiving antenna by two or more propagation paths. Multipath can result from refractive index irregularities in the troposphere or ionosphere; or from structural and terrain scattering on the earth’s surface.
- 5. PROPAGATION MECHANISMS • Scintillation- Rapid fluctuations of the amplitude and phase of a radio wave caused by small scale irregularities in the transmission path (or paths) with time. • Fading- The variation of the amplitude (field strength) of a radio wave caused by changes in the transmission path (or paths) with time. The terms fading and scintillation are often used interchangeably; however, fading is usually used to describe slower time variations, in the order of seconds or minutes, whereas scintillation refers to more rapid variations, in the order of fractions of a second in duration. • Frequency dispersion- A change in the frequency and phase components across the bandwidth of a radio wave, caused by a dispersive medium. A dispersive medium is one whose constitutive components (permittivity, permeability, and conductivity) depend on frequency (temporal dispersion) or wave direction (spatial dispersion).
- 7. PROPAGATION BELOW 3GHz • Satellite communications applications that operate in the frequency bands below about 3 GHz include: • user links for mobile satellite networks (land, aeronautical, and maritime); • satellite cellular mobile user links; • command and telemetry links supporting satellite operations; • deep space communications; • specialized services where wide directivity ground antennas are important. • The ionosphere is the primary source of transmission impairments on satellite communications links operating in the frequency bands up to about 3 GHz. Two main characteristics of the ionosphere contribute to the degradation of radio waves: • background ionization quantified by the total electron content (TEC) along the propagation path; and • ionospheric irregularities along the path. • The degradations related to TEC include Faraday rotation, group delay, dispersion, Doppler frequency shift, variation in direction of arrival, and absorption. The main effect attributed to ionospheric irregularities is scintillation.
- 8. IONOSPHERIC SCINTILLATING • Ionospheric scintillation consists of rapid fluctuations of the amplitude and phase of a radio wave, caused by electron density irregularities in the ionosphere. • Scintillation effects have been observed on links from 30 MHz to 7 GHz, with the bulk of observations of amplitude scintillation in the VHF (30 to 300 MHz) band. • Ionospheric scintillation is most severe for transmission through equatorial, auroral, and polar regions; and during sunrise and sunset periods of the day. The principal mechanisms of ionospheric scintillation are forward scattering and diffraction. • The fluctuation rate for ionospheric scintillation is fairly rapid, about 0.1 to 1 Hz. A typical scintillation event begins after local ionospheric sunset and can last from 30 minutes to several hours. Scintillation events occur almost every evening after sunset for equatorial locations in years of maximum solar activity. Peak-to-peak fluctuations of 10 dB and greater are not uncommon during these periods.
- 9. PROPAGATION ABOVE 3GHz Many satellite communications links operate in the frequency bands above 3 GHz, including C-band, Ku-band, Ka-band, and V-band. Satellite applications operating in these frequency bands include: • fixed satellite service (FSS) user links; • broadcast satellite service (BSS) downlink user links; • BSS and mobile satellite service (MSS) feeder links; • deep space communications; • military communications links. The troposphere is the primary source of transmission impairments on satellite communications operating in these bands.
- 10. TROPOSPHERIC SCINTILLATING • Tropospheric scintillation is produced by refractive index fluctuations in the first few kilometers of altitude and is caused by high humidity gradients and temperature inversion layers. • The effects are seasonally dependent, and vary day to day and with the local climate. • Tropospheric scintillation has been observed on line of site links up through 10 GHz and on earth-space paths at frequencies to above 50 GHz.