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Ground BasedMeteorological Radars

Presented By:

David Franc

NOAA’s National Weather Service

September 2005

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Operational Functions Primary use is for operational

meteorology, research and navigation. Important functions include: Severe storm tracking Flash Flood Warnings Air Traffic Safety

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How do Meteorological Radars impact your life? Routine weather forecasts Severe weather and flash flood warnings Aviation and maritime safety

Personal travel safety Safe, timely transport of personal and commercial

goods Agriculture – your source of food Power management Highway management Water management

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Spectrum Allocations Meteorological radars operate under the

radiolocation and radionavigation services Three bands identified for ground-based

meteorological radars in the Radio Regulations 2 700-2 900 MHz: Relevant Footnote 5.423 5 600-5 650 MHz: Relevant Footnote 5.452 9 300-9 500 MHz: Relevant Footnote 5.475

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Frequency Band Selection Radar range inversely proportional to

frequency Increased propagation loss for higher

frequency bands Precipitation absorption Unambiguous range-velocity product

Other considerations Spatial resolution

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Propagation and Absorption Propagation losses increase as frequency

increases Radar propagation path is two way

Many meteorological radars used for precipitation estimation Cannot estimate rainfall in a storm if radar

returns are absorbed by the storm

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Unambiguous Range-Velocity Doppler radar performance limited by the unambiguous

range/velocity product

Where,

Ra = Unambiguous range

Va = Unambiguous velocity

As frequency increases, the maximum range or maximum observable velocity, or both must decrease.

Technology to overcome range/velocity limitations degrade radar performance in other ways

Ra Vac8

Ra Va

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Comparison of the BandsGeneral Trends:

System Cost: Highest- 2700-2900 MHz

Lowest- 9300-9500 MHz

System Complexity: Highest-2700-2900 MHz

Lowest- 9300-9500 MHz

Operating Range: ~450 km- 2700-2900 MHz

~200 km- 5600-5650 MHz

>75 km- 9300-9500 MHz

Severe Weather

Performance Rating: Highest - 2700-2900 MHz

Lowest - 9300-9500 MHz

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Meteorological Radar Products Radar generates three base data products:

Reflectivity Mean Radial Velocity Spectrum Width

Base products used to produce many high level products Examples: rainfall estimates, wind shear detection,

precipitation type, aircraft icing levels…

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Example of Meteorological Radar Volume Scan

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Sharing Study Considerations

Antenna movement

Antenna pattern

Protection criteria

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Antenna Movement Antenna moves to conduct a volume scan

Antenna will step through 2 to 10 elevations Full 360 rotation performed at each elevation

May take 10 to 15 minutes to complete entire volume scan

Dynamic simulations require much longer run times in comparison to radars rotating at a constant elevation

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Antenna Patterns Pencil beam antenna pattern Standard ITU-R parabolic antenna patterns

not applicable ITU-R patterns too broad for pencil beam Typically result in over estimation of

interference

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Antenna Pattern ComparisonComparison of Measured Antenna Pattern to F.1245 Calculated Pattern

(Radar G - M.1464-1)

-20

-10

0

10

20

30

40

50

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Offset Angle from Main Beam(degrees)

Ant

enna

Gai

n (d

Bi)

Measured

F.1245

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Antenna Pattern ComparisonComparison of Measured Antenna Pattern to F.1245 Calculated Pattern

(Radar G - M.1464-1)

0

5

10

15

20

25

30

35

40

45

50

-5 -4 -3 -2 -1 0 1 2 3 4 5

Offset Angle from Main Beam (degrees)

An

tenn

a G

ain

(dB

i)

Measured

F.1245

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Protection Criteria Protection criteria published in 3 ITU-R

recommendations 2700-2900 MHz: M.1464 5600-5650 MHz: M.1638 9300-9500 MHz: M.[8B.8-10GHZ]

Criteria in M.1464 based on testing Criteria in M.1638 and M.[8B.8-10GHZ]

currently based on basic radar theory

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Protection Criteria (continued) Testing to determine criteria (refer to Annex 3 of

M.1464) Inject interference signal at known level relative to

radar noise floor Alternate interference free and interference- injected

volume scans Collect the radar base data products - compare

interference and interference free base data of each resolution cell

Lowest interference level causing out of spec. base data results is protection criteria.

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Future Trends Sensitivity

Met radars currently process returns 3 to 6 dB below noise floor

Minimum signal to interference ratio (Smin/I) Lower memory and processing costs - radars

processing signals further below noise floor Lower usable S/N leads to lower required I/N

Phased array antennas Allow other volume scan strategies Can periodically return to an area of concern in

atmosphere during a volume scan

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Future Trends (continued) Radar networks integrating radars using two or

more frequency bands Low frequency- good storm performance at long

range Higher frequency- gap filler radars where short range

(mitigating cone of silence) or high resolution performance is needed

Increased automation Mode selection Severe weather signature detection

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Conclusion Meteorological radars operate differently and

produce different products than other radar types The differences need to be considered when

conducting sharing studies Limitations of physics dictate frequency band use Meteorological radars with higher sensitivity –

lead to greater interference sensitivity