Use of State and Local Government Differential GPS for Weather Forecasting or How to Build a Low Cost National GPS-Met Network for NOAA Presented by Seth I. Gutman Chief, GPS-MET Observing Systems Branch NOAA Forecast Systems Laboratory Boulder, Colorado

FSL is building a 200-station GPS-Met Demonstration Network using sites belonging to NOAA, other federal, state, and local government agencies, and universities. Dual-use of these other agency sites has allowed us to evaluate GPS-Met quickly and at low cost. Collaboration with state and local government agencies will help NOAA to expand the GPS-Met network and improve local-to-national scale Wx forecasts. Use of a DGPS site for Wx forecasting adds little to the cost of the site and has no adverse impact on it’s primary use: improved positioning and navigation. Use of DGPS for Weather Forecasting

NOAA GPS-Met Demonstration Network Current & Planned Configuration NOAA GPS-MET SITES

Examples of Non-NOAA Sites Used for GPS-Met Mesa County, CO DGPS Local Government Hartsville, TN NDGPS U.S. DOT La Jolla, CA Scripps Institution of Oceanography Montauk Pt., NY Maritime DGPS U.S.Coast Guard

Dual-Use IPWV Data Comparison Driver, VA: DOT NDGPS Site Blacksburg, VA: NOAA FSL/NWS GPS-Met Site RNK: NWS RAOBS

The first collaborative use of DGPS for NOAA weather forecasting was with U.S. Coast Guard Maritime DGPS Program. It involved engineering evaluations, and solved system compatibility and interoperability issues. This was followed with agreements to use the DOT Nationwide Differential GPS (NDGPS) sites as part of the GPS-Met Demonstration Network. In 2000, FSL worked with Mesa County Colorado to integrate their Cooperative CORS site into the network. Future collaborations with state, and local government agencies will help NOAA expand the GPS-IPWV network and improve local-to-national scale Wx forecasts. Here’s an example / proof-of-concept. Collaborative Dual-Use of DGPS

FDOT is establishing about 45 DGPS sites all over Florida in the next few years. These sites will support statewide surveying for mapping, planning, design, construction, and use of transportation facilities. FDOT needed places to put their GPS receivers, and the Florida SOO’s are interested in GPS-Met, so Ronnie Taylor of NGS brought FDOT, Pat Welsh and the rest of the Florida SOO’s, and FSL together to form a project. We promised to work with FDOT and the SOO’s, and the WFO’s volunteered to host six systems. The fact that their GPS observations may also help improve NOAA Wx forecasts seems pretty cool * to FDOT. Florida Department of Transportation * Cool in this context is defined as consistent with their overall mission to protect life and property, and encourage commerce in the state.

FDOT Differential GPS Network

FDOT will:  Install the GPS receivers and communication systems at 45 sites and continuously retrieve the GPS data.  Place the observations on a server in Tallahassee. FSL will:  FTP these observations to Boulder every 30 minutes.  Add these sites to the GPS-Met Demonstration Network, calculate and monitor their positions, and calculate tropospheric signal delays in near real-time. Note that no surface met packages, normally required to parse the ZTD’s into their wet and dry components, will be installed at these sites. The ancillary pressure and temperature measurements will have to be derived some other way. How This Will Work

Suppose we know the difference in height between the GPS antenna and a model’s grid elevation. Given the analyzed pressure (altimeter) from the model, and accounting for any residual biases in an empirical manner, is it possible to estimate P sfc at the location of the antenna with an error  1 hPa? If so, we can expect IPWV retrieval accuracy roughly comparable to that achieved using standard NWS barometers and temperature sensors. Tm is not a problem, since estimates from the 40-km RUC model are already better than those estimated using T sfc measurements and a climate model. We need P sfc and Tm, valid at 15 and 45 minutes after the hour for near real-time data processing. We Propose The Following Experiment

If we can do this successfully, then there is no technical reason why we could not extend this strategy over the entire U.S. Lots of states beside Florida are starting to deploy DGPS networks. A coordinated outreach effort to the states will show them that sharing GPS and other data (e.g. mesonet observations) with NOAA is really in the best interests of the states’ residents. The local forecast offices should participate since they will be primary beneficiaries of these data and observations. Our work is cut out for us, but the cost & time savings to NOAA could be enormous. Conclusion