CryoSat-2 SARIn mode success to determine lakelevel variations

Sh. Roohi, N. Sneeuw

Institute of Geodesy, University of Stuttgart, Germany

Abstract

To determine realistic lake water level variations, corrupted SARIn waveformsneed to be corrected. We use different retracking algorithms to retrack thewaveforms and to correct water level variations. We assessed the performanceof SARIn mode of CryoSat-2 against external data, e.g. in-situ gauge. In thisstudy we use SARIn level 1 and 2I data from October 2010 to 2014 over Nasserlake, located in Southern Egypt. We analyzed different retracking scenarios overfull and sub-waveforms and based on external validation the performance of eachretracker was evaluated.

Introduction

CryoSat-2 mission was originally designed to study the ice fluctuation in conti-nental ice sheet and ice covered marine of the Earth, especially in Arctic region.But nowadays it is being used to measure water level changes even over in-land water bodies. CryoSat-2 is equipped with the SIRAL radar altimeter thatcollects measurements from the Earth surface in three modes. Over the openocean, flat ice sheets and land it plays the role of pulse limited altimeters, LRMmode. SIRAL is running in SAR mode over sea-ice, coastal zone and some inlandwater bodies. Generally, the mission operates in SARIn mode over continentalsloped ice sheets but it also makes observations over other areas such as lakesand coastal zones. In this mode of observation instead of a pulse, a burst ofpulses is emited from the radar therefore we have more samples during a givenecho, 512 samples. Even though SARIn mode footprint size is small, some ofthe reflected signal back to the radar (waveform) can be contaminated by landand vegetation effects.

CryoSat-2 coverage [CryoSat-2 handbook,2013]

Launch 8 April 2010Mission duration 3 years

Orbit height 717 kmInclination 92 ◦

LRM Ocean and landSAR Coastal zone and sea ice

SARIn Slopped ice-sheet

Retracking

SARIn waveforms were retracked by following retracker algorithms:I OCOG (Offset Center Of Gravity)I Threshold with different threshold valuesI 5-β parameterI Brown model

∆Rretracking = (Gr − G0) ×c

2τ

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

Retracked water level

Water level anomaly estimation:I Defining water level time series from median values of water level for each

satellite passI Rejecting outliers from the long time series by fitting the following model:

h(ti) = a + bti + ct2i + d sin

(2π

Tti

)+ e cos

(2π

Tti

)a, b, c, d, e : Unknown parameters T : Annual period h : Retrackedwater height

I Validation in front of available in-situ gauge data

Data and area of studying

Data: SARIn mode level 1B and 2I (Oct 2010 – 2014) Area: Nasser lake

31 31.5 32 32.5 33 33.521.5

22

22.5

23

23.5

24CS−2 SARin mode sub−satellite ponits on the Nasser lake

Longitude [deg]

Latit

ude

[deg

]

32.4 32.45 32.5 32.5522.66

22.68

22.7

22.72

22.74

22.76

22.78

22.8 CS−2 SARin mode sub−satellite ponits on the Nasser lake

Longitude [deg]

Latit

ude

[deg

]

Along track waveform variations

0100

200300

400500

600

22.5

23

23.5

24

24.5

25

25.50

1

2

3

4

5

6

7

x 104

Time or Bin Latitude [deg]

Pow

er

0100

200300

400500

600

22.5

23

23.5

24

24.5

25

25.50

1

2

3

4

5

6

7

x 104

Time or Bin Latitude [deg]

Pow

er

Along track waveform variations

Time or Bins

Latit

ude

[deg

]

0 50 100 150 200 250 300 350 400 450 50022.715

22.72

22.725

22.73

22.735

22.74

22.745

0

1

2

3

4

5

6

x 104

Along track waveform variations

Time or Bins

Latit

ude

[deg

]

0 50 100 150 200 250 300 350 400 450 500

22.72

22.725

22.73

22.735

22.74

22.745

22.75

0

1

2

3

4

5

6

x 104

Backscatter coefficient of SARIn mode over Nasser lake

Longitude [deg]

Latit

ude

[deg

]

32.48 32.49 32.5 32.51 32.52 32.53 32.54 32.55 32.5622.7

22.8

22.9

23

23.1

23.2

23.3

23.4

23.5

10

15

20

25

30

35

40

45

50

0100

200300

400500

600

22.5

23

23.5

24

24.5

25

25.50

1

2

3

4

5

6

7

x 104

Time or Bin Latitude [deg]

Pow

er

0100

200300

400500

600

22.5

23

23.5

24

24.5

25

25.50

1

2

3

4

5

6

7

x 104

Time or Bin Latitude [deg]

Pow

er

Along track waveform variations

Time or Bins

Latit

ude

[deg

]

0 50 100 150 200 250 300 350 400 450 500

22.72

22.725

22.73

22.735

22.74

22.745

1

2

3

4

5

6

x 104

Along track waveform variations

Time or Bins

Latit

ude

[deg

]

0 50 100 150 200 250 300 350 400 450 500

22.72

22.725

22.73

22.735

22.74

22.745

0

1

2

3

4

5

6

x 104

Peakiness of SARIn waveform over Nasser lake

Longitude [deg]

Latit

ude

[deg

]

32.48 32.49 32.5 32.51 32.52 32.53 32.54 32.55 32.5622.7

22.8

22.9

23

23.1

23.2

23.3

23.4

23.5

20

40

60

80

100

120

140

Rretracking result

RMS (cm) of different retracking scenarios respect to the in-situ gauge data

retracker full-waveform sub-waveform

mean-all first min-residual

ESA 44 – – –OCOG 89 104 96 94

Threshold 10% 87 93 65 53Threshold 20% 71 92 79 72Threshold 50% 51 92 102 945-β parameter 97 62 40 64

Brown 64 194 64 57

0

50

100

150

200Full−waveform

RM

S [c

m]

ES

A

OC

OG

Th1

0

Th2

0

Th5

0

Bet

a5

Bro

wn

0

50

100

150

200Sub−waveform (mean all)

RM

S[c

m]

OC

OG

Th1

0

Th2

0

Th5

0

Bet

a5

Bro

wn

0

50

100

150

200Sub−waveform (first)

RM

S [c

m]

OC

OG

Th1

0

Th2

0

Th5

0

Bet

a5

Bro

wn

Performance of different retrackers

0

50

100

150

200Sub−waveform (optimized)

RM

S [c

m]

OC

OG

Th1

0

Th2

0

Th5

0

Bet

a5

Bro

wn

2010.5 2011 2011.5 2012 2012.5 2013 2013.5−6

−4

−2

0

2

4

Time [year]

Wat

er le

vel a

nom

aly

[m]

Water level anomaly from CS−2 SARIn mode and in−situ gauge measurements

RMS=40 cmSatelliteIn−situ gauge

Conclusion

I Obviously waveform retracking techniques can improve the quality of altime-try data over inland water bodies.

I The quality of water level is dependent on the waveform retracking techniques.I Backscatter coefficients are changed too much over the lake surface which

leads to variety of waveform variations.I Full-waveform retracking: ESA retracker provides the minimum RMS.I Sub-waveform retracking: First sub-waveform retracked by 5-β parameter is

the best scenario.

Geodetic Week 2014, Berlin, Germany http://www.uni-stuttgart.de/gi shirzad.roohi@gis.uni-stuttgart.de