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
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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
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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