African Workshop on GNSS and Space Weather: Introduction to Total Electron Content (TEC)

Anthea Coster , MIT Haystack Observatory

OutlineWhat is TEC and why is it important?How is TEC Measured?Where to obtain TEC? The Madrigal Database.Examples of how TEC measures Space Weather.Summary

DEFINITION OF TEC

From Attila Komjathy, JPL

TEC = Total Electron Content (1016 x el/m2)

Structure of the Ionosphere

D-region

F-region

Topside

E-region

The Ionosphere:Its regions and day/night transitions

Map of GPS receivers

Distributed vs. single point measurements

Wide Area Distribution of 'Raw' Information

Distributed networks of

sensors yield global

physics unattainable

with single-point

measurements

[Coster et al, 2003]

Example :

Global GPS-derived

ionospheric mapping

during geomagnetic

disturbances

What is TEC and why is it important?How is TEC Measured?Where to obtain TEC? The Madrigal DatabaseExamples of TEC StudiesSummary

OUTLINE

GPS Background

• At most 32 satellites

• 6 orbital planes

• 4~6 satellites per plane

• 55o inclination angle

• near circular orbit

• ~ 20000 km altitude

• ~12 hours round trip

(11 hour 58 min 2.05 sec)

GPS Background

Each GPS spacecraft:

• Carries highly accurate clock

• Transmits its clock and position

• Signals are transmitted on 2 (or 3)

frequencies

GPS Satellite Signal Structure

9

Carrier

• Doppler

• Range

Code modulation:

• Identifies SV

• Spread power

• Range

Navigation data

• SV orbit

• Error correction

• SV health

X

GPS

signal

Range Measurements

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Transmitted at SV:

tts tr

Received at RX:

)( sr ttc −=

Pseudorange:

Nsr +−= )(

2

Carrier phase:

sr

GPS Positioning

GPS Signal Multipath

GPS ANTENNA

Illustration of Atmospheric Effects

Elevation Refraction Range Delay

n = 2 11

11

2

1

412 4 2 2

12

-X X

X Y Y X YT T L

( )

( ) ( )

−

− − + −

Where:

XN

=

2

2 YH

=

N

e

Ne

m=

2

0

12

H

e

e B

m=

= the angular frequency of the radar wave,

Y = Y cos , Y = Ysin ,

= angle between the wave vector k and B

k = wave vector of propagating radiation,

B = geomagnetic field, N = electron density

e = electronic charge, m = electron mass,

and permittivity constant.

L T

e

0

,

=

Index of Refraction in the Ionosphere

Ionospheric Range Correction

22

2

2

1

2

2

12

1) - 1( f

ANn eNN −−

GPS Positioning

TEC from GPS is measured from the difference of the GPS pseudo-range measurement at two frequencies

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Where P1 and P2 are the pseudo-ranges measured by GPS at the two different frequencies, f1 and f2.

Total Electron Content (TEC) EstimationDual-Frequency Measurements

Ionospheric ParametersGPS can be used to measure

Ground-Based Receivers

• Total Electron Content

• Scintillation Parameters: S4 and σΦ

Space-Based Receivers

• Electron Density Profiles

• Scintillation Parameters: S4 and σΦ

Problem 1: Mapping Function

• Mapping function used to map TEC (line of sight) to Vertical TEC

• Assigns V-TEC to pierce point

• Function of Elevation Range: 1 < Z < 3

Problem 2: GPS Biases

• GPS delay difference between two frequencies provides TEC

• Delay differences are also introduced by the satellite and receiver

• Satellite biases are determined by IGS community and are fairly stable

• Receiver biases are determined by individual user and most users estimate one bias over a 24 - hour period.

What is TEC and why is it important?How is TEC Measured?Where to obtain TEC? The Madrigal Database.Examples of TEC StudiesSummary

OUTLINE

What is now available through CEDAR Madrigal Webhttp://cedar.openmadrigal.org/

Standard TEC Data in Madrigal available since 2000

1. Provided in 1 degree by 1 degree bins2. Provided every 5 minutes3. Vertical TEC data estimates and Errors on these estimates4. Geographic Lat and Long5. Only provides data where observations are available. Does not attempt to model TEC where data is not available. Uses all GNSS data available.6. New TEC products are on the horizon …i.e. GLONASS observations

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New Line of Site TEC Data in Madrigalavailable now for ~ 3 years

1. Provided for every receiver2. Provided every 20 second 3. Satellite and Receiver ID4. Geographic Lat and Long of Receiver5. Pierce Point: Altitude, Lat and Long6. Azimuth and Elevation to Satellite7. Files are LARGE8. HDF5 format

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Instructions for using Madrigal provided at school website.

What is TEC and why is it important?How is it MeasuredOne Place to obtain it: Madrigal DatabaseExamples of TEC StudiesSummary

OUTLINE

Astronomy Picture of the Day 17 September 2012

At the end of last month, a long standing solar filament suddenly erupted into space

producing an energetic Coronal Mass Ejection (CME)

Coronal Mass Ejection

Day 90, 2001 Day 101, 2001

Venetie, Alaska 1 March 2017

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10:03 UT

Example of Differential TEC showing response to Solar Flare X9.3 on 6 Sep 2017

16 YR Median Diff of Daytime GPS and ISR TEC at Millstone from 2006-2017. Median Diff. = 8.97 STD = 4.6

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SUMMARY

• Global TEC maps can be derived from multiple GNSS (GPS) receivers

• This data is available through Madrigal database

• TEC is a powerful tool to monitor space physics events: geomagnetic storms, solar flares, auroras