US-TEC Target Users: Positioning and Navigation community

1. US-TECTim Fuller-Rowell, Eduardo Araujo-Pradere, Mike Husler, Mihail CodrescuNOAA Space Weather Prediction Center and CIRES University of Colorado NOAA GPS Workshop

2. US-TEC Target Users: • Positioning and Navigation community • Collaboration between SWPC, NGS, FSL, and NGDC • Kalman filter over CONUS + ground-based GPS data, IRI background model, solve for receiver biases • 15-minute cadence • 15 to 30 minute latency • 2 - 3 TEC unit accuracy (~34 - 48 cm delay at L1 frequencies) NOAA GPS Workshop

3. Product on Web page:http://www.swpc.noaa.gov/ustec/http://www.sec.noaa.gov/ustec • Vertical TEC map over CONUS updated every 15 minutes • Estimated uncertainty in TEC • Location of current data sites • Difference from 10-day average to show recent trend • Data files: • vertical TEC • slant path TEC for each GPS satellite in view recent trend uncertainty NOAA GPS Workshop

4. US-TEC Data Sources • NDGPS Coast Guard stations including inland sites via CORS/NGDC - dual data feed for reliability • GPS/MET sites ESRL Seth Gutman • IGS stations over Canada NOAA GPS Workshop

5. Situational awareness:TEC Gradients Use information on TEC gradients to estimate time interval needed to record dual-frequency GPS data to achieve cm level positioning accuracy quiet ionospheric conditions small gradients, shorter intervals disturbed ionospheric conditions - steeper gradients, longer intervals NOAA GPS Workshop

6. Slant-Path TEC Maps 2-D maps of of slant path TEC over the CONUS for each GPS satellite in view updated every 15 minutes A B Sat. 1 A B Sat. 14 C A B C A B C Sat. 29 Sat. 5 C ….etc Applications: 1. Ionospheric correction for single frequency GPS 2. Support dual-frequency integer ambiguity resolution for more rapid decimeter and centimeter accuracy positioning C NOAA GPS Workshop

7. Differential Code and Phase NOAA GPS Workshop

8. Kalman Filter Project state, x Project error covariance, P Compute Kalman gain, K, for observation matrix, H Update state estimate with observations, z Update error covariance NOAA GPS Workshop

9. h EOF2 EOF1 EOF3 Electron density Empirical ortho-normal mapping functions NOAA GPS Workshop

10. Inclusion of time evolution terms Significant (1TECU ) changes in the ionosphere can occur over 4 minutes Include terms in the state vector, x, for linear time changes Append terms to the observation matrix, H, Solving for time also allows extended data windows (1 hour) NOAA GPS Workshop

11. NOAA GPS Workshop

12. “Differential” Validation • Integrate through US-TEC model at two different times. • Compare directly to the phase difference in the original RINEX data file. • As time separation increases, errors in US-TEC map become uncorrelated and approach true uncertainty. NOAA GPS Workshop Araujo-Pradere et al. 2006

13. US-TEC “Differential” Validation Slant path RMSE IRI US-TEC • Validation stations not included in assimilation process • Build up statistics every 5th day over 6 months • Daily average RMSE for each site NOAA GPS Workshop

14. Validation Statistics: “differential” TEC 2.4 TEC units

15. Absolute validation: FORTE Fast Onboard Recording of Transient Events satellite (Los Alamos, Abe Jacobson) • Phase or arrival time as function of frequency • Separate O and X traces • Fit to k/f2 dependence provides TEC estimate • Broad-band RF receiver 30-300 MHz at 800 km altitude • Designed to monitor lightning • Pulse transmitted from Los Alamos (simulated lightning) • Possible to estimate line-of-sight TEC between transmitter and FORTE satellite • Broad-band signal/receiver eliminates phase ambiguity so produces an “absolute” TEC estimate (uncertainty estimate is about 1 to 2 TEC units) • Issues are • bending of the rays, • plasmaspheric content, and having to • sub-sample US-TEC vertical domain Frequency Time of arrival NOAA GPS Workshop

16. Comparison of FORTE and US-TEC NOAA GPS Workshop

17. US-TEC Validation Summary Differential TEC: Slant = 2.4 TEC units Vertical = 1.7 TEC units “Absolute” FORTE ray tracing: Slant = 2.7 TEC units Vertical = 1.9 TEC units • Estimated US-TEC slant path total electron content uncertainty < 3 TEC units (equivalent to about 45 cm of signal delay at L1 frequencies) • Estimate US-TEC vertical total electron content uncertainty < 2 TEC units (equivalent to about 30 cm of signal delay at L1 frequencies) NOAA GPS Workshop

18. US-TEC Future Plans…. • Add FAA and other stations over CONUS and Mexico • Short-term forecast (15 to 60 minutes) to bring up to, or just beyond, real-time • TEC gradients - situational awareness • Increase cadence to 5 minutes during storms • Develop multi-regional capability - partnerships • Global capability - ISES • Longer-term forecast - combining terrestrial and space weather NOAA GPS Workshop