Transcript Document 7488349

TEC Measurements with Dual-Frequency Space
Techniques and Comparisons with IRI
T. Hobiger, H. Schuh
Advanced Geodesy, Institute of Geodesy and Geophysics, Vienna University of Technology, Vienna, Austria
C.K. Shum, L. Potts, S. Ge
Laboratory for Space Geodesy and Remote Sensing, The Ohio State University, Columbus, Ohio, USA
D. Bilitza
Raytheon ITSS, GSFC, SPDF, Code 612.4, Greenbelt, MD 20771
- Dual frequency altimeters (TOPEX/POSEIDON, JASON-1, ENVISAT)
- Global VLBI measurements spanning over two solar cycles
- DORIS global tracking systems for various altimetric and SPOT-n satellites
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Satellite
GPS
T/P
Jason
Envisat
DORIS
VLBI
Band
L1/L2
Ku/C
Ku/C
Ku/S
X/S
Frequencies
1.5/1.2 GHz
13.6/5.3 GHz
13.6/5.3 GHz
13.6/3.2 GHZ
2.0/401.25 MHz
8.4/2.3 GHz
Orbit
20,200 km
1300 km
1300 km
800 km
800-1300 km
TEC
slant
vertical
vertical
vertical
slant
slant
DORIS = Doppler Orbitography and Radio positioning Integrated by Satellite
L1
Ku
1.545 GHZ
13.6 GHz
1 TECU  16.2 cm delay
1 TECU  2.2 mm delay
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VLBI
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VLBI DATA SOURCE AND ESTIMATION TECHNIQUE
• VLBI provides only baseline (=differential) measurements
• Adopted estimation method developed at Vienna University of
Technology
- station dependent VTEC values modelled
by piece-wise linear functions
- longitudinal rotation of datapoints
equivalent to sun-fixed reference system
- north-south variations described by linear gradients
- adjustment algortithm prohibits negative TEC values
• Data from International VLBI Service for Geodesy & Astrometry
• Period: 1984 – 2005, depending on antenna involvement
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Map of all IVS VLBI stations
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Average of 20 years of data
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Altimeters
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Dual-Frequency Altimeters
• The ionosphere delay measured by dual frequency radar altimeters is in nadir
direction over ocean and large lakes.
• TOPEX/Poseidon (T/P) and JASON-1 carry Ku band (13.6 GHz) and C band
(5.3 GHz) altimeter flying in a 1300 km orbit, while ENVISAT with Ku band
(13.575 GHz) and S band (3.2 GHz) altimeter in 800 km orbit.
• The global sea level change study requires altimeter to be well calibrated and
to have long term stability.
• Evaluate the error budget from ionosphere contributions is very important for
sea level research.
• Due to the higher frequency of the altimeter instrument, the ionosphere delay is
small comparing to GPS, e.g., 1 TECU causes 16.2 cm delay at L1, but only 2.2
mm delay at Ku band.
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Single-Frequency Altimeters
• Most historic altimeters (GEOSAT, etc) were single-frequency (Ku band)
systems and relied on models for the computation of the ionospheric delay.
• IRI-95 was adopted to reprocess the GEOSAT Geophysical Data Record
(GDR) (Lillibridge and Cheney, 1997)
• IRI-95 was used for the processing of the GEOSAT Follow On (GFO) Interim
GDRs (Navy IGDR Users Handbook, 2000; NOAA GFO IGDR Format, 1998;
Shum et al., 2001).
• ERS-1 and ERS-2 data products were computed using IRI-95 (CERSAT, 1996).
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Bilitza et al., 1996
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Shengjie Ge et al., AGU Fall, 2004
Jump due to CODE processing problem
Equinoctial maxima
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Zhao et al., 2005
(GIM=CODE)
A 10 cm range correction was added to the C-band measurement to keep the dual-frequency ionosphere
delays from being negatively biased and to agree with calibration and validations with ionosonde data
(Imel, 1994). Range corrections of –20 mm and 130 mm were added to the TOPEX Side B altimeter Kuband and C-band range measurements, respectively, based on comparisons with JPL GIM.
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