Geology Reference
In-Depth Information
was measured using four frequencies in the L‐, C‐, X‐, and
Ku‐bands. All of them became operational in space‐borne
radar systems later. The Ku‐band is the operational fre-
quency of the scatterometer system. One purpose of the
measurements was to identify the optimum radar param-
eters (frequency, polarization, and incidence angle) that
offer the best discrimination between ice types. That was a
pressing question at that time when preparation of the
specifications for the future SAR systems (ERS and
Radarsat) was ongoing. The selection of the operational
C‐band on the succeeding SAR satellites was based on
these and similar measurements. Figure 8.1 shows the
decrease of backscatter with the incidence angle and the
capability of each frequency to discriminate between ice
and open water. The significant difference between back-
scatter from “calm water” and “freshwater lake” in the L‐
band measurements must have been caused by water
surface roughness not salinity. The above-zero backscatter
coefficient from calm water in the Ku‐band (13.6 GHz)
means that there is a focusing of backscatter energy
toward the radar. This usually happens with smooth
surfaces when the radar views it from a perpendicular
direction (i.e., nadir view) or at very small incidence angle
as shown in the figure. Note the sharp drop of the back-
scatter at moderate and high incidence angle because the
surface looks smoother to the incident radar beam at
shallower angles. Smoother surface scatters the signal
away from the viewing direction of the scatterometer. This
feature makes the Ku‐band useful in discriminating sea ice
from water at higher incidence angle (NASA QuikSCAT
scatterometer acquires data at fixed incidence angles of
46° and 54°). Comparison between Figure 8.1 and 8.2
shows also that the trend of backscatter variation with
incidence angle is not greatly affected by the seasonal vari-
ations of ice cover. The measurements were conducted in
summer on dry ice surface with no snow cover.
Lookup tables of backscatter coefficients from ice types
were compiled in order to be used in retrieving ice param-
eters from SAR data. One of the early tables was devel-
oped based on scatterometer measurements in the field in
different seasons for different ice types using three radar
frequencies: L‐, C‐, and X‐bands. The purpose was to
1. 3 GHz, VV polarization
5.2 GHz, HH polarization
5
0
Highest near-nadir
Multiyear ice
First - year ice
Interval of typical returns
-5
-10
Interval of typical returns
Calm water
Lowest ice returns
Calm water
-15
-20
-25
-30
-35
-40
L-band
C-band
-45
20
-50
30
40
50
60
0
20
40
60
80
Incidence angle
Incidence angle
9.6 GHz, HH polarization
13.6 GHz, HH polarization
10
10
Multiyear ice
First - year ice
0
First - year ice
Multiyear ice
0
Calm water
Calm water
-10
-10
-20
-20
-30
X-band
Ku-band
-40
-30
0
10
20
30
40
50
60
70
0
10
20
30
40
5
0
60
Incidence angle
Incidence angle
Figure 8.2
Radar backscatter coefficient of Arctic sea ice at L‐, C‐, X, and Ku‐ bands as a function of frequency
and incidence angle during summer [
Onstott
, 1992, Figure 5.20, with permission from AGU].
