In equatorial orbit from west to east with period of 24 hours Easy to track

In equatorial orbit from west to east with period of

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In equatorial orbit, from west to east with period of 24 hours Easy to track down the satellite Large delay time Polar Satellite Geostationary Satellite Not always in the same position relative to earth so dishes must be moved Cannot be used for continuous comm. Used for remote sensing Remains in fixed position above point on equator so no need to move dish Can be used for continuous comm. Continuously monitor climatic change 16.9 Signal Attenuation Attenuation is the gradual decrease in power of a signal the further it travels Power ratios are expressed in decibels (dB) because the numbers involved are smaller and cover a wider range Attenuation/amplification between two positions can be expressed in dB by: no. of decibels = 10 lg ( ? ??? ? 𝑖? ) o If value is positive, there is an increase in power hence the signal has been amplified o If value is negative, there is a decrease in power hence the signal has been attenuated Attenuation of cables is given as attenuation per unit length and is found by: attenuation per unit length (dB km -1 ) = ????????𝑖?? (??) ?????ℎ ?? ????? (??) Signal must be distinguishable above the level of noise and this can be measured by the signal-to-noise ratio: signal-to-noise ratio = 10 lg ( ?𝑖???? ????? ??𝑖?? ????? ) Repeaters amplify both signal & noise so signal-to-noise ratio remains constant however regeneration of digital signal removes most noise high signal-to-noise ratio 17. M EDICAL I MAGING 17.1 X-Ray Production X-rays are a form of electromagnetic radiation Produced when high-speed electrons hit metal targets
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CIE A2-LEVEL PHYSICS//9702 P AGE 31 OF 34 Production of X-rays: o Heated filament undergoes thermionic emission releasing high-speed electrons o p.d between cathode & anode causes e - s to accelerate o Electrons bombard metal target emitting X-rays which leave through the window o Some kinetic energy of electrons transferred into metal target as thermal energy o Metal target is cooled by water or spun around to increase target area Tube Current: rate of arrival of electrons at metal target Intensity Hardness Depends on no. of e - s hitting anode per unit time Depends on acceleration of electrons Proportional to the heater current Proportional to p.d. between anode & cathode A more intense X-ray produces an image quicker A harder X-ray has more penetrating power Some ‘soft’ X -rays are always produced which cannot fully pass through the patient and contribute to the total radiation dose of the patient. To reduce the radiation dose hence cut off ‘soft’ X -rays, an aluminium filter used to absorb them 17.2 X-Ray Spectrum X-rays emerge from the tube with a range of energies as represented in the spectra below The spectra is made up of two components: o Braking radiation: X-ray photons released when e - decelerate as it strikes anode, attracted by the nucleus of an atom in the anode and loses energy o Characteristic radiation: rearrangement of e - s in anode when a high-speed e - strikes, excites orbital e - s which then de-excite, emitting photons & giving rise to spectrum lines; specific to material of anode 17.3 X-Ray Imaging
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