Some Remaining Issues in Satellite Altimetry

Some Remaining Issues in Satellite Altimetry

Lee-Lueng Fu

Jet Propulsion Laboratory

NASA Sea Level Workshop

November 2-4, 2009, Austin, Texas

Present-Day Sea Level ChangePresent-Day Sea Level Change

-30

-20

-10

0

10

20

30

40

1990 1995 2000 2005 2010

TOPEX

Jason-1

Jason-2

M

SL

(m

m)

YearNerem, 2009

Pow

er d

ensi

ty (

mm

^2/

cyc/

day

)

Global mean sea level spectrum

Frequency (cyc/day)

Period(days)

Pow

er d

ensi

ty (

mm

^2/

cyc/

day

)

60-day

Period Power

65.0745 85.1839 64.3895 76.6947 63.7187 179.475 63.0619 115.847 62.4184 171.108 61.7879 75.2514 61.1700 412.935 60.5644 192.179 59.9706 398.861 59.3883 4128.00 58.8173 2384.98 58.2571 3428.01 57.7075 974.379 57.1682 447.638 56.6389 1183.94 56.1193 154.271 55.6091 143.652

60-day

Spectral peak at 58.7 days

p

2

1

7.58 12//f

f

ffpdfp

Comparison to S2 tide error estimate

From R. Ray

58.7 day period

Iono correction at 58.7 day period

Global mean wet tropo correction

Period (days)

Pow

er d

ensi

ty (

mm

^2/

cyc/

day

)

Wet tropo correction at 58.7 day period

Jason-1

GRACE +ARGO (Willis et al, 2008)

A slightly different analysisLeuliette and Miller (2009)

Systematic errors in sea level measurement

Sea level trend estimated from an ocean model constrained by 2.1 billions observations. The altimetry global mean was not used.

Global mean: 1.6 mm/yr

Estimated errors of the sea level trend.

(Wunsch et al, 2007)

Consistent with other measurements?

Source of error for the MSL calculation

MSL trend uncertainties from 1993 to 2009

Minima Maxima

Orbit : Cnes POE (GDR B) for Jason-1 and GSFC (ITRF2000) for T/P.

0.10 mm/yr 0.15 mm/yr

Radiometer Wet troposphere correction: JMR (GDR B) & TMR

(with drift correction). 0.20 mm/yr 0.30 mm/yr

Dynamical atmospheric and dry troposphere corrections using

ECMWF pressure fields. 0.05 mm/yr 0.10 mm/yr

Sigma0 drift impacting altimeter wind speed and sea state bias

correction 0.05 mm/yr 0.10 mm/yr

Bias uncertainty to link TP A / TP B, TOPEX and Jason-1, Jason-1 and

Jason-2 0.10 mm/yr 0.25 mm/yr

Finally the total error budget of GMSL is :

0.6 mm/yr in a confidence interval of 90%

Ablain et al, 2009

How realistic is the current uncertainty estimate ?

Measurement stability

• OSTM/Jason-2 Level One Requirements– Maintain the stability of the global mean

sea level measurement with a drift less than 1 mm/year over the life of the mission.

• This requirement was not strictly propagated to the subsystems

• The radiometer subsystem (AMR) took the entire 1 mm/year as its stability requirement and allocated it to the ground system.

Radiometer Long Term Calibration IssuesRadiometer Long Term Calibration Issues

• mm-level long term stability is a demanding requirement for the radiometer

– < 0.1 K brightness temperature stability

• Radiometers on Topex, Jason-1 and Jason-2 use internal calibration technique– Disadvantage: Do not view calibration sources through same path

as Earth scene

– Vulnerable to calibration instability from hardware changes requiring periodic post-launch re-calibration

Radiometer Receiver

Calibration Source

Emission from Earth

Front-end path loss correction required

TA

Not calibrated by on-board sources

S. Brown/JPL

Observed TMR and JMR PD InstabilityObserved TMR and JMR PD Instability

• TMR drifted at a rate of about 1 mm/year over the first 6 years of the mission

Brown et al. MicroRad08-10

-5

0

5

10

15

20

0 20 40 60 80 100 120

JMR-ECMWFJMR-SSMIJMR-TMIJMR-GPS

Vers

ion

A G

DR

JM

R D

iffer

ence

s w

.r.t.

Inde

pend

ent O

bser

vatio

ns (m

m)

Jason Repeat Cycle

Jaso

n-1

-Gro

un

d T

ruth

(m

m)

-10

-5

0

5

10

15

20

0 20 40 60 80 100 120


Vers

ion

A G

DR

JM

R D

iffer

ence

s w

.r.t.

Inde

pend

ent O

bser

vatio

ns (m

m)

Jason Repeat Cycle

Jaso

n-1

-Gro

un

d T

ruth

(m

m)

• JMR exhibited two jumps of about 5 mm then an additional 8 mm• 6mm/year when treated

as driftJMR GDR-A

Observed instability significant compared to

sea level rise signal

S. Brown/JPL

Recalibrated JMRRecalibrated JMR

• JMR tuned to on-Earth brightness temperature references for GDR-B

• Eliminates large jumps in PD record

-20

-15

-10

-5

0

5

10

15

20

0 20 40 60 80 100 120


Re

ca

libra

ted

JM

R D

iffe

ren

ce

s w

.r.t

.In

de

pe

nd

en

t O

bs

erv

ati

on

s (

mm

)

Jason Repeat Cycle

-10

-5

0

5

10

15

20

0 20 40 60 80 100 120


Ve

rsio

n A

GD

R J

MR

Dif

fere

nc

es

w.r

.t.

Ind

epe

nd

ent

Ob

se

rva

tio

ns

(m

m)

Jason Repeat Cycle

GDR-A

GDR-B

S. Brown/JPL

Limitations of On-orbit RecalibrationLimitations of On-orbit Recalibration• On-orbit references sensitive to climate variability; require corrections; risk of aliasing

geophysical signals

• Need to acquire sufficient data to reach mm-level– 30+ days of data required to reach 2-4 mm level

• Validation of recalibrated product at mm/yr level against other models/sensors challenging– Uncertainty near + 1mm/yr level, decreasing with record length.

Observed TMR 18.0 GHz Amazon TBsTB Reference Regions in

Amazon

S. Brown/JPL

Radiometer Long Term CalibrationRadiometer Long Term Calibration

• Option for Jason-3: Eliminate reliance on periodic on-orbit recalibration by supplementing internal calibration system with external calibration system

– On-board blackbody and cold-sky reflector calibration targets can be

added to existing radiometer design– Periodic observations of on-board external calibration

targets used to maintain the long term stability

(e.g. once per pass or cycle over land)

• Calibration is traceable to known physical quantities that are independent of the climate system and other sensors or models

• External calibration approach is well established and used scanning Earth observing radiometers

– MSU, AMSU, SSM/I, TMI on TRMM, WindSat, AMSR-E, SSMIS

• Combination internal/external calibration approach has the potential to produce a long term calibration stability that exceeds that of each system individually

– expect sub-mm/year inherent stability from such as system– 0.01 K long term TB stability estimated for MSU (Spencer et al., 1990): ~0.1mm/yr

S. Brown/JPL

Harvest SSH Calibration Time Series

June 22, 2009 OSTST Meeting

-200

-100

0

100

200

300

400

500

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Bia

s (m

m)

B

-76.059998Minimum

108.62Maximum

2583.62Sum

154Points

16.776753Mean

15.495Median

35.644953RMS

31.552622Std Deviation

995.56793Variance

2.5425827Std Error

0.082236129Skewness

0.5277976Kurtosis

B

-47.509998Minimum

119.19Maximum

1249.46Sum

81Points

15.425432Mean

12.98Median

36.381962RMS


1099.2745Variance

3.6839232Std Error

0.35396602Skewness

0.17171038Kurtosis

B

-46.73Minimum

85.43Maximum

100.45Sum

22Points

4.5659092Mean

3.85Median

29.56274RMS


893.73225Variance

6.3737111Std Error

0.58295717Skewness

0.79963724Kurtosis

B

11.16Minimum

180.32001Maximum

20830.32Sum

209Points

99.666603Mean

99.790001Median

103.6355RMS


810.7653Variance

1.9695837Std Error

-0.017166805Skewness

0.56008765Kurtosis

B

155.12Minimum

244.82001Maximum

3616.62Sum

19Points

190.34842Mean

190.92Median

191.50276RMS


465.27468Variance

4.9485494Std Error

0.61435917Skewness

0.52827396Kurtosis

D

54.389999Minimum

107.29Maximum

1285.3Sum

16Points

80.33125Mean

76.469997Median

81.793538RMS


252.8781Variance

3.9755354Std Error

0.28338511Skewness

-0.73005023Kurtosis

E

7.3579998Minimum

8.1990004Maximum

146.651Sum

19Points

7.7184737Mean

7.684Median

7.7217748RMS


0.053802148Variance

0.053213645Std Error

0.6405714Skewness

-0.38858046Kurtosis

SlopeMeanNAltimeter+5 ± 1+1 ± 332154TOPEX ALT-A+3 ± 3–10 ± 73122POSEIDON-1–1 ± 3+14 ± 43381TOPEX ALT-B–2 ± 1+94 ± 228208JASON-1 (GDR-C)

–5 ± 23+174 ± 52727OSTM/JASON-2

-200

-100

0

100

200

300

400

500

2008.5 2008.8 2009.1

Bia

s (m

m)

B

-76.059998Minimum

108.62Maximum

2583.62Sum

154Points

16.776753Mean

15.495Median

35.644953RMS


995.56793Variance

2.5425827Std Error

0.082236129Skewness

0.5277976Kurtosis

B

-47.509998Minimum

119.19Maximum

1249.46Sum

81Points

15.425432Mean

12.98Median

36.381962RMS


1099.2745Variance

3.6839232Std Error

0.35396602Skewness

0.17171038Kurtosis

B

-46.73Minimum

85.43Maximum

100.45Sum

22Points

4.5659092Mean

3.85Median

29.56274RMS


893.73225Variance

6.3737111Std Error

0.58295717Skewness

0.79963724Kurtosis

B

11.16Minimum

180.32001Maximum

20830.32Sum

209Points

99.666603Mean

99.790001Median

103.6355RMS


810.7653Variance

1.9695837Std Error

-0.017166805Skewness

0.56008765Kurtosis

B

155.12Minimum

244.82001Maximum

3616.62Sum

19Points

190.34842Mean

190.92Median

191.50276RMS


465.27468Variance

4.9485494Std Error

0.61435917Skewness

0.52827396Kurtosis

JASON-2 — JASON-1 SSH BIAS:

FROM COMMON OVERFLIGHTS:+80 ± 4 mm (N = 16, = 16 mm, R = .76)

FROM GLOBAL ANALYSIS:+77 ± 1 mm (N = 19, = 2 mm)

Harvest site, Haines et al.

Altimeter Bias

Assessment of Jason-1 and OSTM Global Verification Phase Sea Surface Height Collinear Residuals,

Beckley et al.

Relative biases from tide gauge calibration (Leuliette et al)

Tide gauge calibration has order 1 cm uncertainty?

• The cross-calibration between T/P and Jason-1 has been a moving target, changing from 144 mm to 100 mm depending on the SSB corrections.

• Has this issue been resolved?

Jason‑2 range bias per Cycle (with respect to Jason-1) during the cross-calibration phase

Dettmering et al.

• I was told that the Jason-2/Jason-1 cross-calibration was not a function of wave height, suggesting the original Jason-1 instrument algorithm problem had been solved.

Intermission Differences

Jason1 – T/P Jason2 –Jason1

Without SSB

With SSB

Beckley, 2009

Some Remarks

• 60 day error/signal• Measurement stability requirement and

allocation to subsystems• Radiometer calibration• Altimeter bias• Importance of a cross-calibration between

successive missions• Importance of maintaining a dedicated

science team

BackupBackup

Brown et al. OSTST Seattle

10 11 12 13 14 15 16 17 18 19 20 21 22

Complementary Missions - Medium accuracy/Higher inclination

Broad-Coverage Mission

0908

GFO-FO USA

Reference Missions - Higher accuracy/Medium Inclination

Jason-CS/Jason-4 Europe/USA

LaunchDate

12/01

2/98 GFO USA

3/02

Jason -1 Fr./USA

ENVISAT Europe Sentinel -3C/D

Jason-3 Europe/USA

Altimetry Missions

Wilson et al.

External Calibration

Stow Calibrate

Hot Target

Cold sky reflector

AMR PD Stability AssessmentAMR PD Stability Assessment

• Comparisons between AMR and model and other radiometers

AMR-ECMWF AMR-AMSRE

AMR-TMI

1 mm

No conclusive evidence of long term PD instability or drift

Comparison to M2 tide error estimate

Some Remaining Issues in Satellite Altimetry

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