Refractivity Measurements Using Phased Array Radar

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Refractivity Measurements Using Phased Array Radar

• Boon Leng Cheong

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Phased Array Radar in National Weather Radar
Testbed

• Also being referred to as SPY-1A Phased Array
  Radar (PAR)
• Navy's phased array radar being adapted for
  weather use
• Improve the leadtime of weather warnings

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Signals are bin sorted for range
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Array Processing
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Standard Weather Scanning in PAR
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Validation with WSR-88D
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Refractivity Measurements from Ground Clutters
Using PAR

• Use refractivity measurement as a proxy to
  estimate humidity map within 25 km
• Adapt the method by Frédéric 2004, JTECH, 21,
  pp. 560-573

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Ground Clutters in Close Ranges
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Refractive Index

• Refractive Index of a medium is
• Radio waves propagate in a medium with
  non-uniform refractivity bends and changes speed
• Near the surface, Near the surface, n 1.0003,
  and ?n in O(10-5) O(10 PPM) within 60 km
• Radio wave bends less than 1 horizontally within
  60 km

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Refractivity Function

• More convenient quantity, Refractivity
• Refractivity equation
• where p pressure in millibar (mb)
• T absolute temperature in Kelvin (K)
• e vapor pressure in mb

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Refractivity Function

• N changes predominantly by water vapor
• At conditions
• T 28 C
• p 1000 mb
• q 7 g kg-1
• ?1C ?1 N-unit
• ?1 mb ?0.3 N-unit
• ?1 g kg-1 ?6 N-unit

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Phase and Refractivity

• Time for electromagnetic wave to travel the 2r
  path depends on refractive index
• The phase of the received signal changes if the
  integrated refractivity along the path is changed

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Refractivity Measurements

• Two measurements of phase
• Use measurement at t0 as the reference map
• Ideal reference map is measured at
• Homogeneous humidity profile
• nref is calculated for the reference condition

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Local Refractivity
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To Obtain Absolute Refractivity

• We have
• Gradient of phase change along the radius tells us

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Idealized-simplified Example

• Refractivity structure is manifested into phase
  difference

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Noisy Measurements
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Ground Clutters in Close Ranges
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Finding Good Ground Clutters
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To-do List

• For refractivity measurements
• Collect longer dataset for a reference map
• Alternate(better?) scheme for clutter quality
  index
• Etc

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• Something Else

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Numerical Simulation

• Point scatterers in a 3D enclosing volume
• Positions of scatterers are updated based on the
  mean velocity and turbulent field
• Scatterer exiting the volume is replaced with a
  new scatterer in the opposite location

Modified from Holdsworth and Reid, 1995
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Parameters Update
Parameters include reflectivity, wind field,
turbulence, etc.
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Example Parameters
The parameters can be assumed have simple
structures. OR Something generated with more
sophisticated algorithms and fed into the
simulation scheme
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• The End