Use of GPS Radio Occultation in Space Weather Applications and Ionospheric Research Tim Fuller-Rowell and Ernie Hildner NOAA Space Environment Center and CIRES University of Colorado GPS RO Data Users Workshop, Aug 22-24, 2005 Customers Requirements Current capability What GPS radio occultation (RO) data can provide How RO could be used in Space Weather Operations Value for research

Space Environment Center Boulder, Colorado Forecast and Analysis Branch 24/7 Space Weather Operational Center The nations official provider of real-time alerts, warnings, and products

Customers for Ionospheric Information High Frequency (HF) Communication –ground-to-ground or air-to-ground communication –establish accurate maximum useable frequencies –support automatic link establishment systems e.g., civilian aviation, maritime, frequency managers Single Frequency Positioning and Navigation –single frequency potential sub-meter accuracy positioning e.g., civil aviation, advanced vehicle tracking, potential for E911 improvements Dual Frequency Positioning and Navigation –decimeter accuracy cm e.g., real-time kinematic (RTK), autonomous transportation, off-shore drilling and exploration –rapid centimeter accuracy positioning 1-2 cm e.g., surveyors, possible InSAR (land radar) applications

Customers for Ionospheric Information Satellite Communication –specification and forecast of scintillation activity e.g., satellite operators, drilling companies Situational Awareness –Depressed maximum useable frequencies –Steep horizontal gradients –Unusual propagation paths –Larger positioning errors –High probability of loss of radio signals

Customer Requirements Three-dimensional electron density distribution –determine most effective HF communication frequency –establish ray tracing propagation paths –electron content (group delay or phase advance) along arbitrary lines-of-sight Maps of ionospheric irregularities Rapid updates (~ 5 minutes) Therefore, short data latency, quick data processing and model runs, and rapid dissemination of model outputs are required

D-region absorption Storm-time empirical F-region correction

Current NOAA Capability: US-TEC Product Since 2004, an experimental product characterizes the ionospheric total electron content over the continental US every 15 min for GPS application –Ionospheric data assimilation model (Kalman filter) ingesting ground- based GPS data to produce 2-D maps of total electron content over the Continental US –Evolved from a collaboration between NOAA’s NGS (National Geodetic Survey) and SEC Real-time ionospheric maps of total electron content every 15 minutes

Real-Time Stations Driving US-TEC Data from ~500 CORS (Continuously Operating Reference Stations) GPS dual-frequency receivers are collected by NGS (National Geodetic Survey) Over 100 stations are available in real-time: including data from US Coast Guard (shown in red), WAAS, and FSL sites

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 Applications: 1. Ionospheric correction for single frequency GPS and NDGPS positioning 2. Dual-frequency integer ambiguity resolution for rapid centimeter accuracy positioning Sat. 1 Sat. 5 Sat. 14 Sat. 29 …. etc

Current Status Currently data from about 60 real-time CORS stations transferred to SEC in receiver format every 5 minutes, with a 90 second latency Validation indicates 2 to 3 TEC units uncertainty in slant path Post-processing version used in differential mode indicates ambiguity resolution in only 3 epochs Parallel data stream has been established for reliability in preparation for transition to full operations (approved by NWS) Test period ended March 22nd, 2005 Some changes to product in response to customer feedback Expected release date Fall 2005

Potential of Radio Occultations Provides orthogonal look direction complementing ground-based GPS for ideal tomography imaging Full vertical profile All weather Day and night No instrument drift Global coverage Courtesy: Chris Rocken UCAR

How will it be used at SEC? Future Capability Ingest radio occultation data into current and future assimilative models including Gauss-Markov, physics-based and Ensemble Kalman filters (EnKF) for specification and forecasting the space environment US-Total Electron Content (US-TEC) –Regional Kalman filter data assimilation model –Driven by ground-based dual-frequency GPS –Already implemented at SEC –Could be modified to include RO Global Assimilation of Ionospheric Measurements (Utah State Univ.) Global Assimilative Ionospheric Model (Univ. Southern California/JPL) –GAIM I - Gauss-Markov Kalman Filter –GAIM II - Physics-based Kalman Filter –GAIM - USC –GAIM models already have the capability of ingesting radio occultations and other additional datasets –COSMIC I data ideal to test and validate utility of RO constellation in operations –GAIM I and II will be implemented operationally at AFWA –SEC backup or COOP for AFWA

COSMIC (GPS occultations) USA Canada Europe (EUREF) JPL Expansion in global ground-based GPS networks South America combined with COSMIC to improve global specification

Ocean coverage in the future

Operational Value to NOAA (if data are available timely!) Demand for ionospheric services is growing Radio occultation provides a uniquely helpful dataset for space weather monitoring Significant improvement in characterizing vertical structure of ionosphere has been demonstrated when RO data are assimilated Significant impact on NOAA’s mission

 Storm Electron Density [log/m 3 ] Storm Quiet 4.4LT Basu et al Research: Understanding the massive restructuring of the ionosphere during geomagnetic storms “Imaging” 3D structure essential

CHAMP (400 km) OSEC Bust 2005

Global Ionospheric Redistribution during Severe Geomagnetic Storms John Foster MIT Haystack Observatory NOAA SEC Jan. 6, 2005

NOAA’s Space Weather Mission Providing the Nation with critical space weather services to protect life and property in space, in the air, at sea, and on land Space Environment Center (SEC) is the national and the lead international warning center for disturbances in the space environment that can affect people and equipment. Within the National Oceanic and Atmospheric Administration (NOAA), SEC is one of the National Weather Service's (NWS) National Centers for Environmental Prediction (NCEP). National Oceanic and Atmospheric Administration (NOAA)National Centers for Environmental Prediction SEC conducts research to understand the space environment, and performs critical space weather operations, jointly staffed by NOAA and the U.S. Air Force, to provide forecasts and warnings of solar and geomagnetic activity to users in government, industry, the private sector, and the public.

Product on Web page: 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: a)vertical TEC b)slant path TEC for each GPS satellite in view uncertaintyrecent trend

Minter et al USTEC Validation Time of arrival Frequency Relative/Differential“Absolute”: using FORTE

Plans for USTEC Increase number of stations over CONUS to ~110 (FSL and FAA) Include Canadian stations to improve TEC on poleward side and provide values over North America Increase cadence to 5 minutes Provide short-term forecast (10 to 30 minutes) to bring up to, or just beyond, real-time Need sites to the south (Mexico and Caribbean) Buoys over oceans

GPS RO: Capabilities and Limitations Differential TEC precise to < 0.01 TEC units (1 TEC unit at L1 frequency equivalent to about 15 cm phase delay) Stable and reproducible Receiver biases or integer ambiguities needs to be determined with a data assimilation scheme, e.g., Kalman filter Some horizontal smearing occurs Comparison of derived profile with incoherent scatter radar data at Millstone Hill, MA facility G. A. Hajj & L. J. Romans, Radio Science 33, 175 (1998) Figure courtesy: Paul Straus Aerospace Corporation

Extreme Space Weather: Oct 2003 Superstorm Severe Plasmasphere Erosion (EUV images courtesy J. Goldstein)

Sunward Poleward TEC Plume Mapped to Equatorial Plane

Equatorial Anomalies Spread Poleward TEC Hole Poleward SAPS Electric Field Strips Away Outer Layers of Plasmasphere Key West Guiana GPS Samples Ionosphere/Plasmasphere TEC

March 31, :08 UT 285 E Longitude Corotating Plasmaspheric Bulge