Improvement of a Complete and Consistent TOPEX and Jason Orbit Time Series

GSFC Altimetry/POD GroupF.G. Lemoine, N.P. Zelensky, S.B. Luthcke, D.D. Rowlands, B.D. Beckley, D.S. Chinn

OSTST Meeting

March 12-15, 2007

Hobart, Tasmania

Outline

• Status

• ITRF2000 / ITRF2005 Comparisons & Validation.• Jason DORIS SAA Correction & Impact. • Jason Solar Radiation Pressure (SRP) model tests• Summary

Status

• We have completed and delivered a consistent time-series of ITRF2000 and ITRF2005-based SLR/DORIS dynamic orbits for TOPEX/Poseidon & Jason-1:

• Jason-1 cycles 1-183 ; 01/15/02 - 01/03/07

• TOPEX cycles 1-481; 09/25/92 - 10/09/05

• TOPEX SLR/DORIS orbits cycles 1-364, 365-446 Tandem• TOPEX SLR/Crossover orbits cycles 447-481 Tandem

• The TOPEX orbits used by Phil Callahan for GDR reprocessing (“tvg_dyn_2006aug28”) are based on ITRF2000 and other modelling improvements.

• These orbits are not the GSFC final product.

• ITRF2005-based orbits are preliminary and will soon be updated.

TOPEX/Jason GSFC Modeling Improvements

Models TOPEX GDR orbits GSFC Replacement Orbits:

March 2007

Gravity (static) JGM3 (70x70) GGM02C (120x120)

(Tapley et al., 2004)

Gravity

(time-variable)

C20dot, C21dot, S21dot

C20dot, C21dot, S21dot +

20x20 annual terms from GRACE KBRR data. (Luthcke et al., 2006)

Atmospheric gravity Not applied NCEP, 50x50 @6 hrs (22-year mean removed) (Petrov and Boy, 2004)

Ocean Tides Ray 94 + GEMT3X (T/P-derived) GOT00.2 (20x20)

(Ray and Ponte, 2003)

Solid Earth tides k2= 0.300; k3=0.093 + FCN special handling

IERS2003

Station Coordinates CSR95L02 (c001-359) ITRF2000 (c360-481)

ITRF2000 / DPOD2000

or ITRF2005

Tracking data SLR / DORIS same (+ Jason/DORIS SAA correction)

Parameterization CD/8-hrs + opr along+ cross track/day

Same: dynamic

Grace-derived time-variable gravity to 20x20 (annual terms only)

• Monthly solutions from KBRR data only• Fit mean, trend, & annual harmonic• 2 solutions: 1-yr & 3-yr data span

Jason SLR/DORIS orbit summary (cycles 1-176)TVG model

average RMS residuals

DORIS(mm/s)

SLR(cm)

Crossovers(cm)

atmosphere gravity + (no annual) 0.3699 1.482 5.580

atmosphere gravity + tvg1 (1-year annual) 0.3698 1.462 5.577

atmosphere gravity + tvg3 (3-year annual) 0.3698 1.461 5.577

Reference: ‘Monthly spherical harmonics gravity field solutions determined from GRACE inter-satellite data alone,’ Luthcke et al., Geophyscal Research Letters, 2006.

Jason cycles 008-020

Solution

DORIS

(mm/s)

SLR

(cm)

Crossover

(cm)

SLR and DORIS dynamic orbits

CNES GDR-A .4144 2.682 6.065

GSFC ITRF2000 replacement .3835 1.299 5.800

GSFC ITRF2005 replacement (newest) .3830 1.299 5.797

GSFC ITRF2005 SLR-rescale replacement (newest)

.3830 1.270 5.796

GPS-based reduced dynamic orbits

CNES GDR-B gps/slr/doris .4126 1.579 5.765

JPL gps release_6b .4131 1.403 5.719

GSFC gps rd3 tvg (ITRF2000) .4123 1.300 5.717

GSFC gps/slr tvg (ITRF2000) .4123 1.108 5.712

Jason-1 Tracking Data Summary Statistics Cycles 008-020 Evaluation Period

Evaluation of new Jason orbits based on ITRF2005

Jason Orbits

SLR/DORIS

Cycles 1-177

DORIS RMS

(mm/s)

SLR RMS

(cm)

SLR mean

(cm)

Altimeter

Crossover

RMS (cm)

ITRF2000 0.3740 1.440 -0.058 5.578

ITRF2005 0.3734 1.423 0.257 5.573

ITRF2005

SLR-Rescaled w. scale-rate

0.3734 1.365 -0.030 5.576

Evaluation of new TOPEX orbits based on ITRF2005

TOPEX

SLR/DORIS Orbits

Cycles 1-364

DORIS RMS

(mm/s)

SLR

RMS

(cm)

SLR

mean

(cm)

Altimeter

Crossover

RMS (cm)

GDR 0.5348 2.210 0.323 ---

ITRF2000 0.5104 1.815 0.161 ---

ITRF2005 0.5104 1.870 0.323 ---

ITRF2005 SLR-rescaled w. scale-rate

0.5102 1.771 0.195 ---

Subset Analysis:

21 TOPEX Cycles (344-364)

ITRF2000 0.4678 1.629 0.001 5.493

ITRF2005 0.4878 1.670 0.264 5.501

ITRF2005 SLR-rescaled

w. scale-rate

0.4676 1.572 0.099 5.494

Annual signal amplitude(TVG annual – none)

Geographically correlated error (GGM02C-JGM3)

TOPEX Radial Orbit Difference Trends(ITRF2005 – GDR (CSR95))

Jason (and TOPEX) radial orbit differences are driven by the difference in Z

Long-term Orbit Error and the Terrestrial Reference Frame (TRF)

Jason ITRF2005-ITRF2000 mean orbit differences (Jan '02 - Oct '06)

Z slope = +1.44 mm/yr

slope = -0.31 mm/year

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0 20 40 60 80 100 120 140 160 180 200

cycle

radial over water (mm)

0

2

4

6

8

10

12

14

16

18

Z (mm)

radialZLinear (Z)Linear (radial)

TOPEX Radial Orbit Difference TrendITRF2005 – GDR (CSR95)

Consequent error in estimated regional and global

MSL trends

TOPEX ITRF2005-GDR(CSR95) mean orbit differences (Jan '93 - Dec '01)

slope = -0.28 mm/yr

-5

-4

-3

-2

-1

0

1

2

3

4

5

0 50 100 150 200 250 300 350

cycle

radial (mm)

radial (over water)

Linear (radial (over water))

Jason GSFC TRF orbit trends

Jason GSFC SLR/DORIS ITRF2005 - ITRF2000 orbit differences

slope = +1.44 mm/yr

slope = +1.27 mm/yr

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 20 40 60 80 100 120 140 160 180 200

cycle

Mean Z (cm)

itrf2005-itrf2000

itrf2005s-itrf2000

Linear (itrf2005-itrf2000)

Linear (itrf2005s-itrf2000)

Jason TRF orbit trends – JPL GPS

Jason ITRF2005 / ITRF2000 differences with ITRF2000 JPL GPS-only orbits

slope = +2.24 mm/yr

slope = +0.20 mm/yr

slope = +2.38 mm/yr

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0 20 40 60 80 100 120 140 160

cycle

Mean Z (cm)

jpl_gps6b - slr/xover itrf2000slr/xover itrf2005 - jpl_gps6bslr/xover (itrf2005-itrf2000)Linear (jpl_gps6b - slr/xover itrf2000)Linear (slr/xover itrf2005 - jpl_gps6b)Linear (slr/xover (itrf2005-itrf2000))

Jason DORIS SAA correction

Jason Doris residuals: SLR/DORIS ITRF2000 orbits

0.28

0.33

0.38

0.43

0.48

0 20 40 60 80 100 120 140 160 180

cycle

rms residuals (mm/s)

GSFC

GSFC w SAA correction

GDR-A

Jason SLR/DORIS ITRF2000 orbits

cycles 1-177

average RMS residuals

DORIS(mm/s)

SLR (cm)

Crossover (cm)

GSFC 0.4078 1.482 5.585

GSFC w SAA corr. 0.3695 1.461 5.577

Jason DORIS SAA correction: Comparison of SLR/DORIS & SLR/Crossover Orbits

Jason (SLR/DORIS - SLR/Crossover) mean radial orbit differences over water

slope = -0.20 mm/yr

slope = -0.08 mm/yr

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 20 40 60 80 100 120 140 160 180

cycle

Mean radial (mm)

saa corrected cycles 1-90

saa corrected cycles 92-177

cycle 91

saa not-corrected cycles 1-90

saa not-corrected cycles 92-177

Linear (saa corrected cycles 1-90)

Linear (saa corrected cycles 92-177)Linear (saa not-corrected cycles 1-90)Linear (saa not-corrected cycles92-177)

~ 0.9 mm oscillator jump at cycle 91

Jason DORIS SAA correction

Jason SLR/DORIS (SAA uncorrected-corrected) orbit differences and DORIS points

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 20 40 60 80 100 120 140 160 180

Jason cycle

cm

65000

75000

85000

95000

105000

115000

125000

number DORIS pointsradial rms

mean Z

cycle 91

doris points

Jason Macromodel Scaling

Jason global CR estimate for

nominal SRP model using SLR/DORIS arcs

Jason solar radiation pressure coefficient (CR)

a-priori estimated

cycles 1-90 (45 arcs) 1.0 .9131

cycles 92-184 (46 arcs) 1.0 .9141

cycles 1-184 (91 arcs) 1.0 .9136

use CR = 0.914 for the estimated value in tests

Jason SRP model performancecycles 1-180

average RMS residuals

DORIS(mm/s)

SLR(cm)

Crossovers(cm)

nominal (CR =1.000) 0.3688 1.460 5.577

nominal (CR =0.914) 0.3688 1.433 5.585

UCL (untuned) 0.3632* 1.445* ----

* fewer points used (dynamic editing)

Jason SRP Model

Jason SLR/DORIS nominal(Cr=1.0) - test SRP model orbit differences

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 20 40 60 80 100 120 140 160 180

cycle

Mean Z (cm)

ucl

Cr=0.914

Jason SRP Model

Jason SLR/DORIS nominal SRP - JPL GPS6b orbit differences

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 20 40 60 80 100 120 140

cycle

Mean Z (cm)

Cr=1.000

Cr=0.914

Jason SRP Model120-day radial difference amplitude (mm)

SLR/DORIS (CR=1) – JPL GPS6b orbits

SLR/DORIS (CR=1) – (CR=0.914) orbits

POD Summary

=> Latest and most accurate available GSFC orbits ensure a uniform and accurate reference frame over 14 - years of Topex and Jason altimeter measurements, and show significantly improved DORIS, SLR, and altimeter crossover residuals. Consistency across missions is vital.

=> Orbit error due to Terrestrial Reference Frame (TRF) Z-drift directly affects MSL trend estimates on the order of 0.2-0.4 mm/year overall, and up to 1-2 mm/year regionally. Tests show ITRF2005 is the best TRF to date.

=> Latest model and method refinements anticipate a further improved orbit time series which will be generated upon completion of testing.

Current and Future Efforts=>Several models continue to be refined and tested

• Finalize ITRF2005 SLR/DORIS complement (multi-center effort)• Revisit SLR bias management and TOPEX LRA correction• Mean and Time varying gravity derived from GRACE and SLR.• DORIS SAA correction for Jason (?).• UCL radiation surface force model for Jason.• Atmosphere loading.• Ocean pole tide model.• GPS ITRF2005 issues and JPL GPS phase center maps• GPS processing enhancements (e.g. single difference, undifferenced, improved bias handling …).

=>Additional complete and consistent orbit time series:• Jason-1 GPS and GPS+SLR reduced dynamic orbits.• TOPEX and Jason-1 SLR+DORIS reduced-dynamic orbits.

Access to GSFC 2007 orbitsThe orbits are available on our anonymous ftp account in Unix compressed POE format.

 anonymous ftp dirac.gsfc.nasa.gov

ITRF2000 TOPEX and Jason orbits:

pub/earth/repro_topex/tvg_dyn_2006aug28/

gsfc_poe_dyn_tvg.$cycle.Z TOPEX (cycles 1-481)

pub/earth/repro_jason/swt07/

gsfc_poe_dyn_saa.$cycle.Z Jason (cycles 1-183)

The ITRF2005 and ITRF2005s (SLR rescaled) orbits are preliminary and will be updated:

  pub/earth/repro_topex/preliminary/

gsfc_poe_itrf2005_only.$cycle.Z TOPEX (cycles 1-481)

gsfc_poe_itrf2005s_only.$cycle.Z TOPEX (cycles 1-364)

  pub/earth/repro_jason/preliminary/

gsfc_poe_itrf2005_only.$cycle.Z Jason (cycles 1-183)

gsfc_poe_itrf2005_only.$cycle.Z Jason (cycles 1-183)

 

Backups

Jason TRF orbit trends – CNES GDRB

Jason ITRF2005/ITRF2000 difference with ITRF2000 CNES GPS/SLR/DORIS orbit

slope = +0.77 mm/yr

slope = +1.17 mm/yr

slope = +1.91 mm/yr

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0 20 40 60 80 100 120 140 160 180 200

cycle

Mean Z (cm)

gdrb - slr/xover itrf2000slr/xover itrf2005 - gdrbslr/xover (itrf2005-itrf2000)Linear (gdrb - slr/xover itrf2000)Linear (slr/xover itrf2005 - gdrb)Linear (slr/xover (itrf2005-itrf2000))

Jason-1 radial trend: SLR/DORIS (gsfc), vs. GPS(jpl).

No modelling of COM inSLR/DORIS orbits.

Include modelling of COM in SLR/DORIS orbits.

Jason-1 radial orbit difference trendSLR/DORIS (gsfc), vs. GPS(jpl)

Why do these differences occur?Why do we care?

GPS orbits for Jason-1 are based on GPS satellite orbits which are computed as free network solutions,

and rotated to ITRF2000.

SLR/DORIS orbits are computed using position & velocities fixed in ITRF2000.

Therefore Jason-1/GPS orbits follow more closely the COM (geocenter), whereas the SLR/DORIS orbits are more COF.

Lesson: Be consistent! Be careful when you create a sea level time series. Account for COM!

Orbit consistency across missions

Jason-1 - TOPEX Mean SSH

Improved agreement over Jason-1Verification period, seen in upper right image results from improved TOPEX and Jason-1 orbits. Improvement was realized by using the more current ITRF2000 reference frame and associated station complement in the orbit computations, the GGM02C gravity field, and other improved force and measurement models.

TP/Side A TP/Side B Jason-1

CSR95L02

ITRF2000

Improvement in orbit accuracy and consistency across missions is critical to long-term sea level studies …

Geophysical modeling and the Reference Frame are two issues that are critical to this problem.

Topex/Poseidon Orbit Performance Summary: Cycles 1-446

GSFC SLR/DORIS dynamic Orbits

DORIS SLR

Points Residual (mm/s)

Points Residual (cm)

GDR

T/P Standards (JGM3, Ray-94 tides, CSR95 stations to cycle 359, ITRF2000 from cycle 360)

24952608 0.5246 2065219 2.218

GGMO2C+GGM02C, ITRF2000 stations, Earth & Ocean (Got00) tides conform to IERS2003

25940300 0.5006 2415988 1.960

ATGRAVAs ggm02c+, plus IAU2000 reference, forward gravity modeling of the atmosphere

25944455 0.5018 2416363 1.895

TVGAs atgrav, plus Grace-derived (annual) time varying (20x20) gravity

25943984 0.5017 2415909 1.878

NCOMas tvg, plus CoM variations applied to station

25943761 0.5016 2415977 1.875