"Measuring the effective along-track resolution of CryoSat", presented at third CryoSat User Workshop. Topic: SAR altimetry.

1. Measuring the effective along-track resolution of CryoSat
Michele Scagliola, Aresys srl
Nicolas Tagliani, Aresys srl
Marco Fornari, ESA

2. Outline
 Along-track processing for CryoSat SAR/SARIn modes
 Theoretical model for along-track resolution
 Effective along-track resolution
 Azimuth window optimization
 Conclusions

3. Along-track processing
WGS84
Azimuth
window
Phase
weighting
FFT
64 pulses 64 beams
N samples
N samples
Along-track IRF Along-track resolution: main
beam width at -3dB with respect
to the peak of the power IRF

4. 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

5. Theoretical model
The along-track resolution of CryoSat for SAR/SARin IRF has been
derived:





 
 





  
R v
PRF h R h
wx f
( )
2 64
0.886


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
1 f w
1.486 f w

6. 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





 
 





  
R v
PRF h R h
wx f
( )
2 64
0.886


The along-track resolution has been evaluated as
function of operational time and as function of the
geographic position of the Level1b measurement

7. 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.

8. 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:

9. 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

10. 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

11. 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%

12. 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.

13. 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

14. 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

15. 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

16. 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