monitoring of lake water level variation from pulse- and ... · pdf filemonitoring of lake...

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Monitoring of lake water level variation frompulse- and doppler beam-limited as well as laser

altimeters

Shirzad Roohi1

([email protected])

andNico Sneeuw1, S. Dinardo2, J. Benveniste2

1- Institute of Geodesy, University of Stuttgart2- European Space Agency, Esrin

Geodätische Woche Sep 2015, Stuttgart, Germany

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Motivation

Evaluate the performance of different kinds of altimeters formonitoring inland bodiesHow much do

I waveform retrackingand

I waveform saturation

corrections improve the quality of water level monitoring?

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Altimetry missions

Pulse- limited:sending separated consequences electromagnetic pulses to thewater surface, e.g. Envisat, Jason-2, CS-2 (LRM)Doppler beam-limited:sending correlated consequences electromagnetic pulses to thewater surface and using delay doppler processing, e.g. CS-2(SAR and SARIn)Laser:sending separated laser pulses to the water surface, e.g. IceSat,Airborne LiDAR

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Pulse-limited and doppler beam-limited altimeters

(http://www.altimetry.info)

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Pulse-limited and doppler beam-limited altimetersPulse-limited Doppler beam-limited

(CryoSat-2 handbook, 2013)

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Laser altimeter

(http://icesat.gsfc.nasa.gov/icesat/links.php)

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How can we improve the water leveldetermination?

h = f (H,R)

H: Satellite orbit height R: RangePrecise orbit determination

Increasing precision of range measurementsI Use more precise range correction, e.g. media and geophysical

correctionsI waveform retracking correction

∆Rretracking = (Gr −G0)× c2τ

Gr: Retracked gate G0: Nominal retracking gatec: Light velocity τ: Pulse duration

I Waveform saturation correction: Ereceived > Ethreshold, E: Energy

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Waveform

The reflected signal from water surface back to the satellite, waveform,can be considered as:

Full-waveform

0 20 40 60 80 100 120 1400

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Nom. Bin

Sub-waveform

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Nom. Bin

Ret. Bin

Ret. Bin

Pulse-limited (CS-2 LRM), Qinghai lake, August 20107

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Waveform of different altimetersPulse-limited

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Doppler beam-limited (SARIn)

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Doppler beam-limited (SAR)

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Laser

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Along track waveform variations

Qinghai lake: Pulse-limited altimeter (CS-2 LRM), September 2012

Time or Bin

Latit

ude

[deg

]

20 40 60 80 100 120

36.7

36.75

36.8

36.85

36.9

36.95

37

37.05

37.1

37.15

37.2

Lough Neagh lake: Doppler beam-limited (CS-2 SAR), April 2012

Time or Bin

Latit

ude

[deg

]

20 40 60 80 100 120

54.55

54.6

54.65

54.7

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Nasser lake: Doppler beam-limited (CS-2 SARIn), August 2011

Time or Bin

Latit

ude

[deg

]

50 100 150 200 250 300 350 400 450 50023

23.1

23.2

23.3

23.4

23.5

23.6

23.7

23.8

23.9

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Lough Neagh lake: Laser (IceSat), October 2004

Time or Bins

Latit

ude

[deg

]

50 100 150 200 250 300 350 400 450 500

54.55

54.6

54.65

54.7

54.75

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Unsaturated waveform Saturated waveform

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Waveform retracking scenarios

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Retracked water level changes

Defining water level time series for each pass of satellite over thelake and rejecting outliers using linear trendCombining all of the short water level time series from eachsatellite over pass to build a long time seriesRejecting outliers from the long time series using following model:

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

(2πT

ti

)+ ecos

(2πT

ti

)a, b, c, d, e : Unknown parametersT : Annual periodh : Observed water heightValidation in front of available in-situ gauge data

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Data and area of studyData:

I Envisat: GDR, May 2002- Apr 2012I IceSat: Campaign L2 and L3 (GLA01 and GLA14), Oct 2003- Oct

2010I Jason-2: gdr-d, Jul 2008- Jun 2014I CS-2: L1b, L2I and L2 from Oct 2010- Jan 2014

Area of study:

Lake Surface area (km2) Depth (m) Latitude Longitude

Qinghai 4298 21 38.9◦ 100.12◦Lough Neagh 392 10 54.6◦ −6.4◦

Nasser 5250 25.2 22.7◦ 32.5◦

I Qinghai lake: Pulse-limited (CS-2 LRM), EnvisatI Lough Neagh lake: Pulse-limited (Envisat), Doppler beam-limited

(CS-2 SAR), Laser (IceSat)I Nasser: Doppler beam-limited (CS-2 SARIn), Pulse-limited

(Jason-2)

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Spatial and temporal resolutions

Repeat orbit of different missions

Mission β1 α2 Sub-cycle Inclination(day) (day)

Envisat 501 35 16 98.60◦IceSat 1354 91 33 94◦

Jason-2 127 10 3 66.00◦CS-2 5344 369 30, 85 92.00◦

1: Number of revolution 2: Repeat cycle

Ground track separation at the equator (km)

Mission 10 days Sub-cycle Full repeat cycle

Envisat 280.0 174.0 80.0IceSat 269.3 81.6 29.6

Jason-2 315.5 1051.8 315.5CS-2 277.0 92.2, 32.5 7.5

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Ground track after 30 days revolution

97 98 99 100 101 102 10336

36.5

37

37.5

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Longitude [deg]

Latit

ude

[deg

]

CS−2, LRMEinvisat

−9 −8.5 −8 −7.5 −7 −6.5 −6 −5.5 −5 −4.5 −454

54.5

55

55.5

Longitude [deg]

Latit

ude

[deg

]

CS−2, SAREnvisatIceSat

26 28 30 32 34 36 38 4020

21

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Longitude [deg]

Latit

ude

[deg

]

CS−2, SARInJason−2

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Water level from pulse-limited altimeter

Table: Standard deviation (cm) of water level from different retrackers

retracker full-waveform sub-waveform

first mean-all min-residual

Envisat ice-1 17–211 – – –CS-2 ESA 22 – – –CS-2 β-5 17 169 150 164

CS-2 OCOG 132 169 150 153CS-2 Threshold 10% 100 163 157 163CS-2 Threshold 20% 101 173 157 163CS-2 Threshold 50% 101 169 157 160

2010.5 2011 2011.5 2012 2012.5 2013 2013.5 2014 2014.53193

3193.5

3194

3194.5

3195

3195.5

Time [year]

Wat

er le

vel

[m]

σ: 17 cm σ: 22 cm

LRM1b retracked− β−5LRM L2Model

1- Roohi et al., Capability of pulse-limited satellite radar altimetry to monitor inland water bodies,

EGU 2014

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Water level from doppler beam-limited altimeterSAR

Table: RMS (cm) of water level from different retrackers



Envisat ice-1 32 – – –CS-2 ESA 18 – – –CS-2 β-5 19 48 74 77


CS-2 SAMOSA3 15.5 13 – –

2010.5 2011 2011.5 2012 2012.5 2013 2013.5 2014 2014.512.2

12.4

12.6

12.8

13

13.2

Time [year]

Wat

er le

vel [

m]

RMS = 13 cm RMS = 18 cm

SAR L1b retracked−SAMOSA3SAR L2In−situ

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Water level from doppler beam-limited altimeterSARIn

Table: RMS (cm) of water level from different retrackers



Jason-2 ice 54 – – –CS-2 ESA 44 – – –CS-2 β-5 97 40 92 64


2010.5 2011 2011.5 2012 2012.5 2013 2013.5168

170

172

174

176

Time [year]

Wat

er le

vel [

m]

RMS=40 cm RMS=44 cm

SARIn L1b retracked−β−5SARIn L2In−situ

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Water level from laser altimeter

2003 2004 2005 2006 2007 2008 2009 201012

12.2

12.4

12.6

12.8

13

13.2

Time [year]

Wat

er le

vel [

m]

RMS: 15 cm RMS: 13 cm

IceSat without saturation correctionIceSat with saturation correctionIn−situ

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Conclusion and discussionObviously waveform retracking techniques can improve the qualityof altimetry data especially over the shallow water bodies.Waveform saturation correction can improve the quality of waterlevel measurement.The quality of water level depends on the waveform retracking tech-niques and type of altimeters.For Qinghai lake the full-waveform outperforms the sub-waveformand Envisat shows better performance than CS-2 (LRM).For Neagh lake SAMOSA3 and threshold 10% with the accuracy of10–15 cm are the best retrackers for doppler beam-limited altime-ter, CS-2 SAR.Laser altimeter (IceSat) provides an RMS of 13 cm for water levelin the case of using waveform saturation correction.Doppler beam-limited altimeter (CS-2 SAR) has better performancethan laser altimeter and laser altimeter has better performance thanpulse-limited altimeter (Envisat) over Neagh lake.

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Conclusion and discussion

For Nasser lake the first detected sub-waveform retracked by β-5is the best reacking scenario for doppler beam-limited altimeter.Doppler beam-limited altimeter (CS-2 SARIn) has betterperformance than pulse-limited altimeter (Jason-2) over this lake.

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Many thanks for your attention

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monitoring of lake water level variation from pulse- and ... · pdf filemonitoring of lake...

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