page_title: Doppler effect
section_title: Satellite communication
hierarchical_section_title: Doppler effect / Applications / Satellite communication
caption_attribution_description: English: Possible Doppler shifts in dependance of the elevation angle (orbit altitudde h = 750 km). Fixed ground station.
context_page_description: The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.
  A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches 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.
  The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wave fronts is reduced, so the waves "bunch together".
context_section_description: Fast moving satellites can have a Doppler shift of dozens of kilohertz relative to a ground station. The speed, thus magnitude of Doppler effect, changes due to earth curvature. Dynamic Doppler compensation, where the frequency of a signal is changed progressively during transmission, is used so the satellite receives a constant frequency signal. After realizing that the Dopper shift had not been considered before launch of the  Huygens probe of the 2005 Cassini–Huygens mission, the probe trajectory was altered to approach Titan in such a way that its transmissions traveled perpendicular to its direction of motion relative to Cassini, greatly reducing the Doppler shift.
  Doppler shift of the direct path can be estimated by the following formula:
  where  is the velocity of the mobile station,  is the wavelength of the carrier,  is the elevation angle of the satellite and  is the driving direction with respect to the satellite.
  The additional Doppler shift due to the satellite moving can be described as:
  where  is the relative speed of the satellite.
caption: Possible Doppler shifts in dependence of the elevation angle (LEO: orbit altitude 
    
      
        
          h
        
      
      {\displaystyle h}
    
   = 750 km). Fixed ground station.[17]
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