The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave (or other periodic event) for an observer moving relative to its source. It is named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague. A common example of Doppler shift is the change of pitch heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.
Due to the high orbital speed of the amateur-satellites, the uplink and downlink frequencies will vary during the course of a satellite pass. This phenomenon is known as the Doppler effect. While the satellite is moving towards the ground station, the downlink frequency will appear to be higher than normal and therefore, the receiver frequency at the ground station must be adjusted higher in order to continue receiving the satellite. The satellite in turn, will be receiving the uplink signal at a higher frequency than normal so the ground station’s transmitted uplink frequency must be lower in order to be received by the satellite. After the satellite passes overhead and begins to move away, this process reverses itself. The downlink frequency will appear lower and the uplink frequency will need to be adjusted higher. The following mathematical formulas relate the doppler shift to the velocity of the satellite.
|=||doppler corrected downlink frequency
pembetulan doppler untuk frequency pancaran menurun
|=||doppler corrected uplink frequency
pembetulan doppler untuk frequency pancaran menaik
|=||velocity of the satellite relative to ground station in m/s.
Positive when moving towards, negative when moving away.
halaju satelit relatif kepada stesen dibumi (mengambil kira putaran bumi)dalam meter sesaat.
rujukan positif bergerak menghampiri, rujukan negatif jika bergerak menjauhi
|=||the speed of light in a vacuum ( m/s).
kelajuan cahaya dalam persekitaran hampagas 300,000,000 meter sesaat
|Change in frequency
Perubahan pada frekuensi
Pembetulan Pancaran Menurun
Pembetulan Pancaran Menaik
Due to the complexity of finding the relative velocity of the satellite and the speed with which these corrections must be made, these calculations are normally accomplished using satellite tracking software. Many modern transceivers include a computer interface that allows for automatic doppler effect correction. Manual frequency-shift correction is possible, but it is difficult to remain precisely near the frequency. Frequency modulation is more tolerant of doppler shifts than single-sideband, and therefore FM is much easier to tune manually.
Stationary sound source produces sound waves at a constant frequency f, and the wave-fronts propagate symmetrically away from the source at a constant speed c. The distance between wave-fronts is the wavelength. All observers will hear the same frequency, which will be equal to the actual frequency of the source where f = f0 .
The same sound source is radiating sound waves at a constant frequency in the same medium. However, now the sound source is moving with a speed υs = 0.7 c. Since the source is moving, the centre of each new wavefront is now slightly displaced to the right. As a result, the wave-fronts begin to bunch up on the right side (in front of) and spread further apart on the left side (behind) of the source. An observer in front of the source will hear a higher frequency
f = c + 0/c – 0.7c f0 = 3.33 f0 and an observer behind the source will hear a lower frequency
f = c – 0/c + 0.7c f0 = 0.59 f0 .
Now the source is moving at the speed of sound in the medium (υs = c). The wave fronts in front of the source are now all bunched up at the same point. As a result, an observer in front of the source will detect nothing until the source arrives where
f = c + 0/c – c f0 = ∞ and an observer behind the source will hear a lower frequency
f = c – 0/c + c f0 = 0.5 f0 .
The sound source has now surpassed the speed of sound in the medium, and is traveling at 1.4 c. Since the source is moving faster than the sound waves it creates, it actually leads the advancing wavefront. The sound source will pass by a stationary observer before the observer hears the sound. As a result, an observer in front of the source will detect
f = c + 0/c – 1.4c f0 = -2.5 f0 and an observer behind the source will hear a lower frequency
f = c – 0/c + 1.4c f0 = 0.42 f0 .
source of information taken from wikipedia
sumber informasi ini diambil dari wikipedia