Geology Reference
In-Depth Information
sea ice is a complex and interwoven puzzle.” Today, a few
elements of the puzzle have been unraveled but the com-
plexity of the radar backscatter from snow‐covered sea ice
is still a challenge. It is manifested in the significant overlap
between backscatter from different ice and surface types.
The backscatter modeling approach using physical and
radiative transfer components has provided some clues on
the influence of certain snow and ice parameters [
Tonbeo
et al
, 2003]. Polarimetric SAR is another potential tool to
relate the backscatter observations to the surface composi-
tion, yet research on this subject is still at an early stage.
Many field and laboratory experiments followed the
pioneering TRAMAS program.
Onstott
[1992] listed 17
investigations conducted between 1977 and 1989 using
ground‐based scatterometers and airborne radars. The
author presented sets of graphs of backscatter coefficients
from the major ice types at different frequencies and inci-
dence angle. Two sets are shown in Figures 8.1 and 8.2 for
winter and summer ice conditions, respectively. Backscatter
8.1.1. Backscatter Databases from Ice Types
and Open Water
The first in situ scatterometer measurements of Arctic
sea ice using a surface‐based Frequency-Modulated
Continuous-Wave (FM‐CW) scatterometer was conducted
in May 1977 near Point Barrow, Alaska, during the
Transportable Microwave Active Spectrometer (TRAMAS)
[
Onstott et al.
, 1979]. The scatterometer was developed in
1976 at the University of Kansas. Backscatter measure-
ments were obtained over an incidence angle range from
10° to 70° at radar frequencies ranging between 1 and 18
GHz with co‐ and cross‐polarizations. The measurements
were used to identify a few trends that were confirmed later
in many studies. For example, backscatter from sea ice
decreases with increasing radar frequency and incident
angle. One conclusion that remains as valid today as it was
more than three decades ago was phrased in
Onstott et al.
[1979, p. 83] as follows: “Indeed the radar return from
1. 3 GHz, VV polarization
5.2 GHz, HH polarization
4
0
Calm water
L-band
Multiyear ice
C-band
Thick first-year ice
-4
-10
Pressure ridge
Freshwater lake
-12
-20
-20
-30
Calm water
First-year ice
-28
-40
Multiyear ice
-36
-50
15
30
45
60
0
20
40
60
80
Incidence angle
Incidence angle
9.6 GHz, HH polarization
13.6 GHz, HH polarization
0
20
X-band
Ku-band
10
-10
0
-20
-10
-20
Multiyear ice
First-year ice
-30
Multiyear ice
-30
First-year ice
Calm water
Calm water
-40
-40
10
20
30
40
50
60
0
10
20
30
40
50
60
70
Incidence angle
Incidence angle
Figure 8.1
Radar backscatter coefficients of Arctic sea ice at L‐, C‐, X‐, and Ku‐bands as a function of frequency
and incidence angle during winter [
Onstott
, 1992, Figure 5.19, with permission from AGU].
