1 Ground Based Meteorological Radars Presented By: David Franc NOAAs National Weather Service September 2005

2 Operational Functions Primary use is for operational meteorology, research and navigation. Important functions include: Severe storm tracking Flash Flood Warnings Air Traffic Safety

3 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

4 Spectrum Allocations Meteorological radars operate under the radiolocation and radionavigation services Three bands identified for ground-based meteorological radars in the Radio Regulations MHz: Relevant Footnote MHz: Relevant Footnote MHz: Relevant Footnote 5.475

5 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

6 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

7 Unambiguous Range-Velocity Doppler radar performance limited by the unambiguous range/velocity product Where, R a = Unambiguous range V a = 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

8 Comparison of the Bands General Trends: System Cost: Highest MHz Lowest MHz System Complexity: Highest MHz Lowest MHz Operating Range:~450 km MHz ~200 km MHz >75 km MHz Severe Weather Performance Rating:Highest MHz Lowest MHz

9 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…

10 Example of Meteorological Radar Volume Scan

11 Sharing Study Considerations Antenna movement Antenna pattern Protection criteria

12 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

13 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

14 Antenna Pattern Comparison

15 Antenna Pattern Comparison

16 Protection Criteria Protection criteria published in 3 ITU-R recommendations MHz: M MHz: M 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

17 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.

18 Future Trends Sensitivity Met radars currently process returns 3 to 6 dB below noise floor Minimum signal to interference ratio (S min /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

19 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

20 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