Space Weather Effects on GPS

Thomas J. Bogdan, Director

Space Weather Prediction Center

NCEP/NWS/NOAA

325 Broadway, DSRC Room 2C109

Boulder, CO 80305, USA

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For Official Use Only

How GPS works

• Triangulation from known satellite positions

• Distance calculated based on radio signal travel time

• Must correct for delays due to propagation through the atmosphere

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For Official Use Only

GPS Error Sources

• Typical Error in Meters Standard GPS Differential GPS

(per satellites)

• Satellite Clocks 1.5 0

• Orbit Errors 2.5 0

• Ionosphere 5.0 0.4

• Troposphere 0.5 0.2

• Receiver Noise 0.3 0.3

• Multipath 0.6 0.6

The ionosphere is the largest source of error for Standard GPS and second largest for Differential GPS

http://www.trimble.com/gps/howgps-error2.shtml#3

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For Official Use Only

Ionosphere Effects on GPS

The ionosphere is defined as the region of the upper atmosphere where radio signal propagation is affected by charged particles.

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Scintillation Effects

SATCOM MESSAGE ERRORS

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Ascension Island6 April 1997

Average Signal Level

Receiver Fade Margin

064. THE QUICK BROWN FOX JUMPS OVER THE LAZY DOGS BACK 01234567<9"TI 

?65.( VHE UUicK vROWN$GOX JUMPS OVER†THE!McZY(f~d¸z.9g?_f_}_ j›sSL 33 s %E QUICK$BROWN(FOX JUM‘S OVER THE0L—ZY ?OGS ‚—CK 0123456789

TIÌU¤M ?67. (THE QUKcK BROWN GOY"JUMPS _V˜Z ™‰.O ÁOx'??.9 :c™l???0SsQJ.ª:.$~$U ¶S?IcK0 SˆN …? MUM“Û!_VGS* THE LAZY DOGS BACK 01=3456789 TIMES

069. THE QUICK BROWN FOX JUMPS OVER THE LAZY DOGS BACK 0123456789 TIM

SATCOM Message

S. Basu, private communication

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For Official Use Only

October 29th, 2003“walls” of TEC challenge

provision of integrity with differential GPS

TEC “walls”: 130 TEC units over 50 km

20 m of GPS delay;walls move 100 to 500 m/s

SED?“wall”

Courtesy: Tom Dehel, FAA

Ionosphere Challenges

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Mannucci et al., 2005

Anomaly crests

1 TECU = 1016 electrons m-2

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For Official Use Only

Space Weather Effects on WAAS

July 1, 2003 – March 1, 2005

% CONUS at 95% Availability

Magnetic Storm Index

(Nov 8, 2004)

15 hour loss on 10/29; 11.3 hour loss on 10/30, shorter losses on 11/20/2003

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Ionosphere Corrections

• Need to model the ionosphere to compute corrections

• Empirical climatological models: Klobuchar Coefficients

• Physics based numerical models: CTIPe, TIE-GCM

• Data assimilation schemes: WAAS, US-TEC, GAIM

Models can provide specification and forecast

The ionosphere is highly variable in space and time

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For Official Use Only

US-TEC Product

• Current NOAA capability for characterizing the total number of free electrons (TEC) in the ionosphere, with parallel input data streams for reliability

• Since 2004, a product characterizing the ionospheric TEC over the continental US (CONUS) has been running in real-time at NOAA’s Space Environment Center (SEC)

• The ionospheric data assimilation model uses a Kalman filter and ingests ground-based GPS data to produce 2-D maps of total electron content over the CONUS

• Product evolved from a collaboration between SEC and NOAA’s National Geodetic Survey (NGS), National Geophysical Data Center (NGDC), and Forecast Systems Laboratory (FSL)

Primary Product:Real-time ionospheric maps of total electron content every 15 minutes. Currently uses about 100 real-time GPS stations from

the CORS network

QuickTime™ and aTIFF (PackBits) decompressorare needed to see this picture.

http://www.sec.noaa.gov/ustec

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For Official Use Only

Applications: Ionospheric correction for single frequency GPS; NDGPS positioning;dual-frequency integer ambiguity resolution for rapid centimeter accuracy positioning (OPUS). (OPUS)OPUS)

Sat. 1

Sat. 5

Sat. 14

Sat. 29

….etc

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Slant-Path TEC Maps

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US-TEC Validation Summary

Differential TEC:Slant = 2.4 TEC unitsVertical = 1.7 TEC units

“Absolute” FORTE ray tracing:Slant = 2.7 TEC unitsVertical = 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)

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SWPC Ionosphere Goals

Produce global real-time specification and forecast Web display of GAIM output from AFWA Assimilation schemes using numerical models: CTIPe, IDEA

Improve US-TEC

CONUS: Specification with 10 minute latency US-TEC slant path total electron content uncertainty < 2 TECU US-TEC vertical electron content uncertainty < 1 TECU

CONUS: Provide Forecast 1 hour forecast as good as specification 3 hour forecast: uncertainty < 3 TEC units 6-12 hour forecasts

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Conclusions

Solar Maximum is on the way (2012?)

Ionosphere Services is a fast growing area in Space Weather

SWPC is committed to offer improved products and tools

SWPC is ready to collaborate on: Data Models and model results Research Services