Title: Refractivity Measurements Using Phased Array Radar
1Refractivity Measurements Using Phased Array Radar
2Phased 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
3Signals are bin sorted for range
4Array Processing
5Standard Weather Scanning in PAR
6Validation with WSR-88D
7Refractivity 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
8Ground Clutters in Close Ranges
9Refractive 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
10Refractivity Function
- More convenient quantity, Refractivity
- Refractivity equation
-
- where p pressure in millibar (mb)
- T absolute temperature in Kelvin (K)
- e vapor pressure in mb
11Refractivity 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
12Phase 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
13Refractivity 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
14Local Refractivity
15To Obtain Absolute Refractivity
- We have
- Gradient of phase change along the radius tells us
16Idealized-simplified Example
- Refractivity structure is manifested into phase
difference
17Noisy Measurements
18Ground Clutters in Close Ranges
19Finding Good Ground Clutters
20To-do List
- For refractivity measurements
- Collect longer dataset for a reference map
- Alternate(better?) scheme for clutter quality
index - Etc
21 22Numerical 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
23Parameters Update
Parameters include reflectivity, wind field,
turbulence, etc.
24Example Parameters
The parameters can be assumed have simple
structures. OR Something generated with more
sophisticated algorithms and fed into the
simulation scheme
25 26