page_title: Weather radar
section_title: Doppler dilemma
hierarchical_section_title: Weather radar / Data types / Velocity / Doppler dilemma
caption_attribution_description: English: Maximum range from reflectivity (red) and unambiguous Doppler velocity range (blue) with pulse repetition rate. In a Doppler radar, the pulse repetition rate has an inverse effect on maximum unambiguous range and velocity. This dilemma leads the user to make a choice of PRF to maximize the range and the velocity usefulness. Range in km, velocity resolution in m/s and PRF in Hz.
context_page_description: Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type. Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.
  During World War II, radar operators discovered that weather was causing echoes on their screen, masking potential enemy targets. Techniques were developed to filter them, but scientists began to study the phenomenon. Soon after the war, surplus radars were used to detect precipitation. Since then, weather radar has evolved on its own and is now used by national weather services, research departments in universities, and in television stations' weather departments. Raw images are routinely used and specialized software can take radar data to make short term forecasts of future positions and intensities of rain, snow, hail, and other weather phenomena.
context_section_description: The phase between pulse pairs can vary from - and +, so the unambiguous Doppler velocity range is
  Vₘₐₓ = 
  This is called the Nyquist velocity. This is inversely dependent on the time between successive pulses: the smaller the interval, the larger is the unambiguous velocity range. However, we know that the maximum range from reflectivity is directly proportional to
  :
  x = 
  The choice becomes increasing the range from reflectivity at the expense of velocity range, or increasing the latter at the expense of range from reflectivity. In general, the useful range compromise is 100–150 km for reflectivity. This means for a wavelength of 5 cm (as shown in the diagram), an unambiguous velocity range of 12.5 to 18.75 metre/second is produced (for 150 km and 100 km, respectively). For a 10 cm radar such as the NEXRAD, the unambiguous velocity range would be doubled.
  Some techniques using two alternating pulse repetition frequencies (PRF) allow a greater Doppler range. The velocities noted with the first pulse rate could be equal or different with the second. For instance, if the maximum velocity with a certain rate is 10 metre/second and the one with the other rate is 15 m/s. The data coming from both will be the same up to 10 m/s, and will differ thereafter. It is then possible to find a mathematical relation between the two returns and calculate the real velocity beyond the limitation of the two PRFs.
caption: Maximum range from reflectivity (red) and unambiguous Doppler velocity range (blue) with pulse repetition frequency
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