1. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Improved Method for Measuring the Satellite-to-Satellite TEC in the Ionosphere by S. Syndergaard H.-H. Benzon G. B. Larsen M. B. Sørensen P. Høeg Atmosphere Ionosphere Remote Sensing Division Danish Meteorological Institute

2. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Setting the scene: Ionosphere occultation principle

3. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Ionosphere tomography combining ground and space measurements Convenient to assume straight lines

4. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 GPS observables and traditional TEC measure Disregarding troposphere, clocks & ambiguity bias: On occultation links the traditional measure is not exactly the straight line TEC Discrepancy becomes appreciable for large electron density gradients

5. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Occultation geometry

6. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 S-S TEC with first order dispersion correction Mainly proportional to the square of vertical TEC gradients Accuracy depends on validity of spherical symmetry assumption (“major” term) (“minor” term)

7. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Simulated TEC residuals, double Chapman layers E-layer: Max. dens.: 2·10 11 m -3 Scale height: 5 km F-layer: Max. dens.: 3·10 12 m -3 Scale height: 60 km

8. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 GPS/MET ionosphere profile, occ. 641, Feb 20 1997 Local time: 12:30 Latitude: 36 N Longitude: 144 E Solar flux: 73

9. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 GPS/MET TEC profile, occ. 641, Feb 20 1997 Local time: 12:30 Latitude: 36 N Longitude: 144 E Solar flux: 73

10. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 GPS/MET estimated TEC residuals, occ. 641, Feb 20 1997

11. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Ørsted ionosphere profile, April 18 1999 Local time: 14:45 Latitude: 33 S Longitude: 13 W Solar flux: 113

12. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Ørsted TEC profile, April 18 1999 Local time: 14:45 Latitude: 33 S Longitude: 13 W Solar flux: 113

13. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Ørsted estimated TEC residuals, April 18 1999

14. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Alternative estimate of straight line S-S TEC Corrects for first order dispersion effects Not sensitive to spherical asymmetry Drawback: clocks and POD needs to be solved (removed) traditional combination for comparison ( )

15. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Simulated TEC residuals, spherical asymmetry conditions

16. URSI XXVI General Assembly, Toronto, Canada, 13-21 August 1999 Summary Satellite-to-satellite TEC  L 1 - L 2 may give TEC errors of order 10 TECU Not a big problem at low solar activity or night time (error < 1 TECU) May be a problem at day time, solar maximum Dispersion correction reduce errors depending on spherical symmetry assumption Alternative dual frequency combination can be applied, but then one needs to solve for clocks and POD Results can be used in ionosphere tomography to obtain more accurate straight-line satellite-to-satellite TEC