Measuring the effective along-track resolution of CryoSat

Michele Scagliola, Aresys srl

Nicolas Tagliani, Aresys srl

Marco Fornari, ESA

Outline

Along-track processing for CryoSat SAR/SARIn modes

Theoretical model for along-track resolution

Effective along-track resolution

Azimuth window optimization

Conclusions

Along-track processing

WGS84

Azimuth window

Phase weighting

FFT

64 pulses 64 beams

N s

amp

les

N s

amp

les

Along-track IRF Along-track resolution: main beam width at -3dB with respect to the peak of the power IRF

Azimuth window The azimuth windowing has been included in the beam forming to mitigate the effect of clutter on the off-nadir beams due to specular backscattering of the ice surface from the nadir. Hamming window is implemented in IPF1 VK1.0: low sidelobes to

suppress the clutter mainlobe widening that

decreases the resolution

Rectangular Hamming

Theoretical model The along-track resolution of CryoSat for SAR/SARin IRF has been derived:

vR

hRhPRFw fx

)(

642886.0

Parameters fixed by instrument design

Quantities that are function of the position of the satellite along the orbit

Widening factor due to the window function used in the processing: For IPF1 vk1.0, with

It’s the only term that can be modified in IPF1

1fw

486.1fw

Experimental approach

Level1b products dataset

About 3000 SAR L1b

About 4000 SARIn L1b

From July 2011 to

October 2012 R

v

h Quantities that are function of the position of the satellite along the orbit are read from L1b products

vR

hRhPRFw fx

)(

642886.0

The along-track resolution has been evaluated as function of operational time and as function of the geographic position of the Level1b measurement

Effective along-track resolution Mode Minimum [m] Average [m] Maximum [m]

Along-Track

resolution

SAR 434.39 448.83 465.13

SARin 434.46 450.42 465.76

SAR+SARin 434.39 449.64 465.76

The along-track resolution has been here represented as

function of the position of the satellite

Moreover it has been verified

that the along-track resolution is approximately constant along the

time.

Effective along-track resolution The along-track resolution as function of the Latitude has been reported and a polynomial model of the along-track resolution has been evaluated to compute the average resolution as function of the latitude of the Level1b measurement:

Validation of the results

9

To verify the analysis on the effective along-track resolution, a point scatterer has been found in a SAR L1b product and the -3dB width of the along track IRF has been measured.

450 m

The along-track resolution is about 450 m at Latitude 85.61 deg

Azimuth window optimization The average along-track resolution for CryoSat results to be approximately equal to 450 m.

Aiming at improving the along-track resolution, the azimuth window has to be optimized to reduce the widening factor

Trade-off analysis between along-track resolution and clutter suppression as function of the azimuth window.

fw

Azimuth window optimization

Average along-track resolution [m] PSLR [dB]

No window 303 13.26

DT PSLR= 37.5dB 404 37.50

Hamming c1=0.2 402 31.60

Hamming c1=0.08 450 42.45

The following azimuth windows have been considered in the optimization analysis:

Dolph-Tchebyshev (DT): its Fourier transform exhibits the

narrowest mainlobe width for the specified sidelobe level

All the sidelobes have the same level

Improvement of about 11%

Clutter suppression To verify if the clutter is sufficiently suppressed using the proposed azimuth windows, a SAR product of the 19th of August 2011 has been processed up to Level1b using the four different windows.

The clutter suppression has been evaluated on

Bright scatterer area for sea ice

Average L1b waveform over sea ice

Average L1b waveform over ocean

Moreover, the Signal-to-Clutter-and-Noise Ratio (SCNR) has been measured for the average L1b waveforms.

Sea Ice

Bright scatterer Weak scatterer

DT has higher clutter than Hamming

Clutter suppression w.r.t. no window is noticeable

The L1b waveform is not dependent of the azimuth window

Sea ice: average waveform About 5 seconds of Level1b waveforms over sea ice have been averaged:

The shape of the average L1b waveform is not dependent on the azimuth window

Window SCNR No window 33.39 dB DT PSLR= -37.5dB

36.48 dB

Hamming c1=0.2 37.38 dB Hamming c1=0.08

37.61 dB

Ocean: average waveform About 5 seconds of Level1b waveforms over ocean have been averaged:

The shape of the average L1b waveform is slightly dependent on the azimuth window

Window SCNR No window 25.98 dB DT PSLR= -37.5dB

29.14 dB

Hamming c1=0.2 29.52 dB Hamming c1=0.08

29.83 dB

Conclusions

Effective along-track resolution for CryoSat SAR/SARIn modes has been evaluated

The current Level1b measurements are characterized by an average resolution approximately equal to 450 m

By optimizing the azimuth window used in along-track processing, it is possible to improve the resolution of about 11% with a very limited loss in clutter suppression