Stuart Edwards and Philip Moore School of Civil Engineering and Geosciences

1. Calibration of ERS-2, TOPEX/Poseidon and Jason-1 Microwave Radiometers using GPS and Cold Ocean Brightness Temperatures Stuart Edwards and Philip Moore School of Civil Engineering and Geosciences University of Newcastle Newcastle upon Tyne NE1 7RU UK Email: s.j.edwards@newcastle.ac.uk

2. Calibration of ERS-2, TOPEX/Poseidon and Jason-1 Microwave Radiometers using GPS and Cold Ocean Brightness Temperatures Overview • Observed drift in Brightness Temperatures for ERS-2, T/P and Jason-1 radiometers are investigated • Modified wet tropospheric range inferred from ‘smallchange’ analysis of radiometric correction from GDR’s (T/P, ERS-2) • GPS derived wet tropsopheric delays used to validate correction accuracies • T/P altimetric range stability compared against time series form global network of tide gauges • Jason-1 radiometer measurements are compared with GPS • Original GDR (all cycles) • Rev_B corrections (cycles 22 - 120)

3. Outliers in tail removed Coldest Ocean Brightness Temperatures (TB) • Brightness Temperatures extracted from GDR’s • Recovery subjective and absolute values differ with differing selection criteria • Experimentally - selection criteria affect only TB base level • Negligible effect on observed trends • Simple technique of binning TBs over 0.1K bins • Outliers in lower tail removed • first bin where number of values in bin exceeded minimum value • Choice of cut-off arbitrary but negligible effect on temporal variability • Coldest over ocean temperature taken as • ERS-2 1st bin with 20 data points (less data, greater variation in tails) • T/P and Jason-1 1st bin with 10 data points (increased data, less variation in tails

4. Coldest TBs + 10 cycle running ave (red) 18 GHz channel clear temporal signature Quadratic fit (blue) Increase of 1.67 K for TB18 TOPEX/Poseidon Coldest TB Analysis

5. TOPEX/Poseidon Coldest TB Path Delay PDT/P = B0 + B18 loge(280 – TB18) + B21 loge(280 – TB21) + B37 loge(280 – TB37) T/P standard two stage process • Involves partials of path delay in PDT/P eqn. w.r.t. TB18, 21 and 37 • Partials complicated by dependence of coeffs B0 ect. on TB’s • Partial derivatives for all channels determined from forward differences at 1 K intervals Path Delay correction

6. T/P Path Delay change with observed drift in TB’s Results • Global ranges for partials • TB18 from -5.5 to -4.8 mm/K, TB21 from 5.5 to 9.0 mm/K, TB37 from -1.1 to -0.7 mm/K • Median values give: • Agrees well with others (e.g.Scharroo et al. 2004) • Highlights dependence of PD on respective radiometer channel • Note small contribution of 37 GHz channel • Partials of wet delay w.r.t. a TB channel multiplied by • Scaling of ΔTBc varies between 0 and 1 for TBc ≤ TB ≤ TBh • Regions of high ocean temperature associated with lower multipliers • Reduces overall spatial distribution of delay compared to underlying temperatures (cm) Path Delay Correction (TBh – TB)/( TBh –TBc) * ΔTBc

7. T/P Spatial variability of wet path delay correction • Correction derived up to T/P cycle 300 (7 years into mission) • 18 GHz channel average drift ~0.2 K/yr • Mean effect on sea-level change ~0.1 mm/yr • Spatial distribution varies from -0.80 mm/yr to -1.02 mm/yr • Variations amount to ~4% of global mean • Insignificant for global studies • May be significant over smaller scales

8. Gain fall (~10 K) Relaxation Linear Drift ERS-2 coldest TB Analysis • Fall approximated asymtotically • RMS of fit = 0.5 K • Constant fit pre gain fall • TB37 stable with increase ~0.05 K/yr • Good agreement with Scharroo et al. 2004, Eymard et al. 2005)

9. Jason-1 Coldest TB Analysis • Deficiency on all channels evident • TB18 potential linear fit • TB23 Anomalies cycles 26-32 and near cycle 70 • No simple algebraic fit • TB34 Large anomaly around cycles 60-80 • ΔPD c.f. MacMillan et al. (2004)

10. GPS wet path delay • GPS data from subset of global IGS sites • 13 sites for ERS-2, and T/P (long time series, < GPS data available) • 22 sites for Jason-1 (Launch 2001, > GPS data availability) • All IGS sites (coastal or island)

11. GPS wet path delay Processing strategy • Wet delay derived from 24 hr Precise Point Positioning batch solutions using GIPSY/OASIS software • Dual frequency ionosphere free carrier phase, decimated to 300 sec (5 min) • Precise ephemeris and clocks from JPL • ZWD estimated every 5 min • Niell (2000) mapping function • Elevation cut-off of 7 degrees GPS Radiometer comparison • GPS ZTD computed as combination of the ZWD and the a priori Dry delay • Dry delay corrected for atmospheric pressure and referenced to MSL using height offset of the GPS antenna and EGM96 geoid • Final estimate of GPS ZWD obtained from GPS ZWD = GPS ZTD – Corrected Dry Delay • GPS ZWD compared directly to radiometric measurements

12. Station mean differences - GPS, ERS-2 and T/P radiometers Relative to TMR - ERS-2 reading ~14 mm too short and JMR reading ~9 mm too short

13. GPS validation of radiometric brightness temperature corrections • GPS site bias removed from path delays at each site • Global mean offset subtracted form each location mean Results T/P • Black = 10 cycle running ave of diff GPS and T/P GDR • Max diff > 1 cm by cycle 300 • Green = global mean values of partials applied to T/P • Red = temperature dependent partials (all 3 channels) • Blue = temperature dependent partials for TB18 only • Better agreement 10 cycle running ave diff GPS

14. GPS validation of radiometric brightness temperature corrections Results ERS-2 • GPS - E2MR wet path delay difference. • ERS-2 GDR (dots) • ERS-2 TB23 corrected (triangles) • Effect of gain fall ~4 cm decrease in wet path delay for uncorrected TB23 • Application of correction accounts for gain fall • Possibility of residual long term trend remains

15. GPS v Jason-1 Results Perhaps Jason-1 model is best!!

16. Calibration of ERS-2, TOPEX/Poseidon and Jason-1 Microwave Radiometers using GPS and Cold Ocean Brightness Temperatures Conclusions • Drift in T/P and ERS-2 and JMR MWR’s formulated in terms temperature brightness • TMR = 18 GHz channel (dominant drift) • E2MR = 23.8 GHz channel • JMR = 18, 23 and 34 GHz channels appear affected • Correction to radiometer wet delay (TMR, E2MR, JMR_b) good agreement GPS (RMS of fit) • TMR = 0.37±0.61 cm • E2MR 1.78±0.61 cm • JMR 1.34±0.74 cm • TMR-E2MR mean difference = 1.37±0.35 cm. • E2MR reading ~14 mm too short relative to T/P • TMR-JMR mean difference = 0.93±0.62 cm • JMR reading ~9 mm too short relative to T/P • TMR 18 GHz channel correction + tide gauge comparison yields secular drift of only -0.27 ± 0.11 mm/yr