Lake level variations from satellite radar altimetry with ... · PDF fileLake level variations from satellite radar altimetry with retracking of multi-leading edge Shirzad Roohi ([email protected]

Lake level variations from satellite radaraltimetry with

retracking of multi-leading edge

Shirzad Roohi([email protected])

andNico Sneeuw

University of Stuttgart, GermanyInstitute of Geodesy

Geodätische Woche, Essen, Germany 2013

Why waveform retracking?

I Improve the quality of water level measurementsI Increase the number of reliable observations particularly in the

shoreline and shallow water

44.9 45 45.1 45.2 45.3 45.4 45.5 45.6 45.7 45.8 45.937

37.2

37.4

37.6

37.8

38

38.2

38.4Sub−satellite points

Longitude [deg]

Latit

ude

[deg

]

In−situ Gauge

tracks 371

tracks 178

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Ascending tracks

Time [year]

Wat

er le

vel [

m]

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276

Residual = 42 cm

Ascending tracks

Time [year]

Wat

er le

vel [

m]

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Descending tracks

Time [year]

Wat

er le

vel [

m]

Water level time series and fitting the trend to the all values

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276

Residual = 83 cm

Descending tracks

Time [year]

Wat

er le

vel [

m]

Lake level variations from satellite radar altimetry with retracking of multi-leading edge 1

RADAR principle

http://www.altimetry.info


How can we have more precise water levelmeasurements?

I Increasing precision of range measurementsI Use more precise range correction, e.g. corrections included in

GDRsI waveform retracking, i.e. calculate another range correction from

SGDRs

∆Rretracking = (Gr −G0)×c2τ

Gr: retracked gate, G0: nominal retracking gate, c: light velocity, τ:pulse duration


Waveform retracking techniques

I Conventional retrackers

I Onboard retracker (Ice-1/2 and Sea-ice)I Offset Center Of Gravity (OCOG)I ThresholdI β- parameters

I Unconventional retrackers

I Multi-leading edgeI Modified waveform


Data and area of studyRA2 Geophysical and Sensor Data Records, i.e. RA2 GDRs and RA2

SGDRs of Envisat satellite altimetry from cycle 6 to cycle 113

Envisat satellite ground tracks from cycle 92


Conventional retrackersI Onboard retrackers:

Water level from RA2 GDRs data using median values of waterlevel in each satellite over pass in Ice-1 retracker algorithm

2002 2003 2004 2005 2006 2007 2008 2009 2010 20111270

1270.5

1271

1271.5

1272

1272.5

1273

1273.5

1274

1274.5

1275Water level from median values of all satellite tracks and fitting a trend

Time [year]

Wat

er le

vel [

m]

Residual = 27 cm

h(ti) = a+ bti + ct2i + dsin

(2πT

ti

)+ ecos

(2πT

ti

)a, b, c, d and e are unknown parameters to be estimated. T is the

annual period and h is the observed water height.Lake level variations from satellite radar altimetry with retracking of multi-leading edge 6

Conventional retrackers

I OCOG

2002 2003 2004 2005 2006 2007 2008 2009 2010 20111270

1270.5

1271

1271.5

1272

1272.5

1273

1273.5

1274

1274.5

1275

Time [year]

Wat

er le

vel [

m]

Water level time series from combined ascending and descending tracks

Residual = 27 cm

I Threshold

2002 2003 2004 2005 2006 2007 2008 2009 2010 20111270

1270.5

1271

1271.5

1272

1272.5

1273

1273.5

1274

1274.5

1275 Water level time series from combined ascending and descending tracks

Time [year]

Wat

er le

vel [

m]

Residual = 15 cm


Unconventional retrackersI Multi-leading edge

0 20 40 60 80 100 120 1400

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Gate

Pow

er


Unconventional retrackers

I Multi-leading edge

2002 2003 2004 2005 2006 2007 2008 2009 2010 20111270

1270.5

1271

1271.5

1272

1272.5

1273

1273.5

1274

1274.5

1275 Water level time series from combined ascending and descending tracks

Time [year]

Wat

er le

vel [

m]

Residual = 14 cm


Comparing different retrackers

I Onboard retracker

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Ascending tracks

Time [year]

Wat

er le

vel [

m]

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276

Residual = 42 cm

Ascending tracks

Time [year]

Wat

er le

vel [

m]

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Descending tracks

Time [year]

Wat

er le

vel [

m]

Water level time series and fitting the trend to the all values

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276

Residual = 83 cm

Descending tracks

Time [year]

Wat

er le

vel [

m]

I Other retrackers

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Ascending track

Time [year]

Wat

er le

vel [

m]

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Ascending track

Residual = 59 cm

Time [year]

Wat

er le

vel [

m]

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Descending track

Time [year]

Wat

er le

vel [

m]

Water level time series based on Threshold 10 % retracker

2002 2004 2006 2008 2010 20121268

1270

1272

1274

1276Dscending track

Residual = 74 cm

Time [year]

Wat

er le

vel [

m]


Comparing different retrackers

Water level standard deviation from different retrackersretracker standard deviation (cm) improvement

Ice-1 27 –OCOG 27 0 %

Threshold 10 18 33 %Threshold 20 15 44 %Threshold 50 17 37 %

Multi-leading edge 14 48 %

Improvement =σIce−1 − σRet

σIce−1× 100 %


Along track waveform variations

First ascending pass-Jun 2002

Latitude [deg]

Gat

e

37.2 37.4 37.6 37.8 38 38.2

20

40

60

80

100

120

0

0.5

1

1.5

2

2.5

3

x 104

Last ascending pass- Sep 2010

Latitude [deg]

Gat

e

37.2 37.4 37.6 37.8 38 38.2

20

40

60

80

100

120

0

500

1000

1500

2000

2500

3000

3500

4000


Validation

I OCOG

2002 2002.5 2003 2003.5 2004 2004.5 2005−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6 Water level anomaly from satellite and in−situ gauge measurements

Time [year]

Wat

er le

vel a

nom

aly

[m]

RMS = 41 cm

SatelliteIn−situ gauge

I Threshold

2002 2002.5 2003 2003.5 2004 2004.5 2005−0.8

−0.6

−0.4

−0.2

0

0.2

0.4


Time [year]

Wat

er le

vel a

nom

aly

[m]

RMS = 23 cm



Validation

I Multi-leading edge

2002 2002.5 2003 2003.5 2004 2004.5 2005−0.8

−0.6

−0.4

−0.2

0

0.2

0.4


Time [year]

Wat

er le

vel a

nom

aly

[m]

RMS = 26 cm



Conclusion

I Obviously waveform retracking techniques can improve the qualityof altimetry data.

I Due to the land and environmental effects on the return echoes tothe altimeter particularly in the shoreline the waveform retrackingis necessary.

I The quality of water level is dependent on the waveform retrackingtechniques.

I According to the result of data processing using both RA2 GDRand RA2 MWS (SGDRs) of Envisat, multi-leading edge andthreshold 20 % retrackers outperform the other retackers todetermine water level variations of Urmia lake.


Works under way

I Continuing waveform retracking using:I β–parameterI Modified waveform


Thank you for your attention


References

F. Frappart, S. Calmant, M. Cauhopé, F. Seyler and A. Cazenave, 2006,Preliminary results of Envisat RA-2 derived water levels validationover the Amazon basin. Remote sensing of environment 100, 2,252–264.

G. J. Yun, C. Xiaotao, G. Y. Gang, S.Jialong and H. C. Way, 2009, Lakelevel variations monitored with satellite altimetry waveform retracking,Ieee journal of selected topics in applied earth observations andremote sensing, 2, 2, 80–86.

G. J. Yun, G. Y. Gang, H. C. Way, and S. J. Long, 2009, Amulti-subwaveform parametric retracker of the radar satellite altimetricwaveform and recovery of gravity anomalies over coastal oceans,Science China, Earth Sciences, 53, 4, 610–616.

G. J. Yun, H. C. Way, C. Xiaotao and L. Yuting, 2006, Improved thresholdretracker for satellite altimeter waveform retracking over coastal sea,Progress in natural science, 16, 7.


References

H. Lee, C.K. Shum, K. H. Tseng, J. Y. Guo and C. Y. Kuo, 2010,Present-day lake level variation from Envisat altimetry over thenortheastern Qinghai-Tibetan plateau: links with precipitation andtemperature, Terr. Atmos. Ocean. Sci 22, 2, 169–175.

J. F. Crétaux and C. Birkett, 2006, Lake study from satellite radaraltimetry, International Geophysics (Applied Geophysics) 338, 14,1098–1112.

J. S. Silva, S. Calmant, F. Seyler, O. Corrêa, R. Filho, G. Cochonneauand W. J. Mansur, 2010, Water levels in the Amazon basin derivedfrom the ERS 2 and Envisat radar altimetry missions, Remotesensing of environment 114, 10, 2160–2181.

Y. Yuchan, A. V. Kouraev, C.K. Shum, V. S. Vuglinsky, J. F. Crétaux andS. Calmanti, 2012, The performance of altimetry retrackers at lakeBaikal, Terr. Atmos. Ocean. Sci 24, 4.


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Lake level variations from satellite radar altimetry with ... · PDF fileLake level variations from satellite radar altimetry with retracking of multi-leading edge Shirzad Roohi ([email protected]