Philip Moore Jiasong Wang School of Civil Engineering and Geosciences University of Newcastle upon Tyne Newcastle upon Tyne NE1 7RU ENVISAT Orbit & RA2 CCVT in Newcastle University

ENVISAT Precise Orbit Determination ForceModel Gravity model 3-body perturbation Earth tides Ocean tides Atmospheric density SRP + Albedo + Infrared Relativity Param (Dynamic) Param (Reduced Dyn) GRIM5-C1(120×120) DEC403 JPL Eph IERS96 GOT99 MSIS86 Macro/ESA; SRP factor = 1 Schwarzschild 6-hour drag+daily 1cpr along/cross track 6-hour drag+hourly 1cpr along/cross track Sigma=5.E-9m/s/s Correl. Time = 3 hours

Altimeter Range model adopted Model Source ENVISAT / ERS2 Total geocentric ocean tide (1) + pole tide Solid earth tide Dry troposphere Wet troposphere Ionosphere Inverse Barometer Sea state bias Orbit GOT99: ENVISAT from GDR; model for ERS2 Model (from GDR): both satellites GIM + scale from IRI95: both satellites ENVISAT from GDR; inferred for ERS2 Model (from GDR): both satellites From GDR or NCL POD

RMS of SLR (cm) and DORIS RMS (0.1mm/s) residuals in NCL POD

RMS radial orbit difference (cm) for ENVISAT cycles 10 and 11

RMS cross-track orbit difference (cm) for ENVISAT cycles 10 and 11

Radial orbit difference between reduced dynamic and dynamic POD: RMS values over 4º long. by 2º lat. bins

Radial orbit difference between DEOS and NCL (cyc10) RMS values 4º long. by 2º lat. bins

Radial orbit difference between DEOS and NCL (cyc11): RMS values over 4º long. by 2º lat. bins

Altimeter statistics FlagRejected Cyc 10 Rejected Cyc Land Shallow water (reject if > -1000m) SWH = Tide (reject if > 20m) Peakiness (reject if 1.8) Total rejected Accepted

RMS of crossover residuals cycles 10-11: CNES v NCL reduced SLR+DORIS orbit

Sea State bias and altimeter time tag estimation from single crossovers orbit 1 = CNES; orbit 2 = NCL (Reduced dynamic); Max time diff=5day, rejection criterion 20cm OrbitUsed/rejectedRMS change  t(ms) SSB: %SWH 1/Cyc 10 2/Cyc / / —7.25cm 9.23—6.94cm 0.01 ± ± ± ± /Cyc 11 2/Cyc / / —7.29cm 8.81—6.80cm ± ± ± ± 1.03 Note:SSB given as % of SWH Conclusion:Results independent of orbit

Sea State bias and altimeter time tag estimation from single crossover Max time diff = 5day, NCL orbit applied, for Cyc Reject level Used/rejectedRMS change  t(ms) SSB: %SWH 60cm11347/ —7.71cm-0.10 ± ± cm11154/ —6.87cm-0.09 ± ± 0.73 Conclusion: Rejection level unimportant

ENVISAT - ERS2 SSH: Ascending passes Corrections applied: sea state bias and USO drift (ERS2) only

ENVISAT - ERS2 SSH: Descending passes Corrections applied: sea state bias and USO drift (ERS2) only

ENVISAT - ERS2 SSH: All passes All Corrections applied:

Note: All corrections applied: descending pass mean = 118.1cm ; ascending pass mean = 115.0cm

Differences between ENVISAT (cyc 10) and ERS2 (cyc 78) solid Earth tide corrections: Note mean = -1.1cm

Differences between ENVISAT (cyc 11) and ERS2 (cyc 79) solid Earth tide corrections: Note mean = -0.8cm

Differences between ENVISAT (cyc 10) and ERS2 (cyc 78) total tidal corrections. Note mean = 1.4cm. Noise from total geocentric ocean tides, see next slide

Differences between ENVISAT (cyc 10) and ERS2 (cyc 78) inverse barometer corrections: Note mean = -3.0cm

Differences between ENVISAT (cyc 11) and ERS2 (cyc 79) inverse barometer corrections: Note mean = -2.8cm

Differences between ENVISAT (cyc 10) and ERS2 (cyc 78) iono+wet+dry+inverse bar. corrections: Note mean = -3.1cm

Differences between ENVISAT (cyc 10) and ERS2 (cyc 78) wet trop. Correction: Note mean = -0.4cm

Differences between ENVISAT (cyc 10) and ERS2 (cyc 78) iono (GIM) corrections: Note mean = 0.14cm

ENVISAT (cyc 10): mean = 1.237m

ENVISAT (cyc 11): mean = 1.237m again!!

Conclusions Orbits –agree to near 2cm RMS radially between analysis centres –Reduced dynamic is better than dynamic OD Sea-state bias –Zero values (for very low wave heights??) –Assuming wave heights correct first look SSB  4.6% SWH –Non-parametric approximation not yet applied

Conclusions Altimetry correction (Cyc10, 11) –inverse barometer correction has a bias of -3.0cm with ERS2 –tide effect is different from ERS2 with mean=-1.4cm Crossover residuals –RMS about 7cm; shows benefit of DORIS tracking and to a lesser extent dual frequency altimeter. Great results c.f. 6.0cm for TOPEX/Poseidon

Sea surface height (SSH) difference with ERS2 (Cyc10, 11) –With only SSB correction applied, bias=112cm –With all corrections for both satellite, bias=117cm –For Ascending pass and descending pass, 3cm difference for this relative bias calculation. Within error bars? SSH compared with mean sea surface (CLS01) (Cyc10) – Bias=123.7cm (ENVISAT Cyc 10) – Bias=5.4cm (ERS2 Cyc 078) – relative bias = 118.3cm between two satellites Conclusions