# Presentation: 740257

## TEC Measurements with Dual-Frequency Space Techniques and Comparisons with IRI 

**TEC Measurements with Dual-Frequency Space Techniques and Comparisons with IRI**** **
**T. Hobiger****, ****H. Schuh**
***Advanced Geodesy, Institute o******f Geodesy and Geophysics, Vienna University of Technology, Vienna, Au******stria***
**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**

## Satellite  Band    Frequencies	     Orbit	TEC

- GPS        L1/L2    1.5/1.2 GHz	20,200 km	slant
- T/P	   Ku/C	    13.6/5.3 GHz 	   1300 km	vertical
- Jason	   Ku/C	    13.6/5.3 GHz 	   1300 km	vertical
- Envisat	   Ku/S     13.6/3.2 GHZ	     800 km	vertical
- DORIS                 2.0/401.25 MHz	 800-1300 km  slant
- VLBI	   X/S        8.4/2.3 GHz			slant
- DORIS = Doppler Orbitography and Radio positioning Integrated by Satellite
- L1	1.545 GHZ 	1 TECU  16.2 cm delay
- Ku	13.6 GHz	1 TECU  2.2 mm delay

## VLBI

## 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

## Slide 5

## Map of all IVS VLBI stations

## Slide 7

## Slide 8

## Slide 9

## Average of 20 years of data

## Altimeters

## 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.

## 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).

## Bilitza et al., 1996

## Slide 15

## Shengjie Ge et al., AGU Fall, 2004

- Equinoctial maxima

- Jump due to CODE processing problem

## 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 Ku-band and C-band range measurements, respectively, based on comparisons with JPL GIM.  **

**Zhao et al., 2005**

## Slide 18