Surface-Based Scatterometer Results of Arctic Sea Ice

IEEE TRANSACTIONS ON GEOSCIENCE ELECTRONICS, VOL. GE-17, NO. 3, JULY 1979

Surface-Based Scatterometer Results ofArctic Sea Ice

ROBERT G. ONSTOTT, STUDENT MEMBER, IEEE, RICHARD K. MOORE, FELLOW, IEEE,

AND W. F. WEEKS

Abstract-Radar backscatter measurements were made of shorefastsea ice near Point Barrow, AK, in May 1977, with a surface-based FM-CW scatterometer that swept from 1-2 GHz and from 8.5-17.5 GHz.The 1-2 GHz measurements showed that thick first-year and multiyearice cannot be distinguished at 10-70° incidence angles, but that un-deformed sea ice can be discriminated from pressure ridges and lake ice.Results also indicate that frequencies between 8-18 GHz have theability to discriminate between thick first-year, multiyear, and lake ice.Cross polarization was found to be a better discriminator than likepolarization. In addition, at these latter frequencies the differentialscattering cross section a0 was found to have an approximately linearlyincreasing frequency response.

I. INTRODUCTIONT he fact that radar is especially well suited as an operational

remote sensing tool for the polar oceans, because of itsall-weather, day-night capability, and its ability to sense largeexpanses of terrain, has been recognized for years. For example,side-looking airborne radar (SLAR) imagers have proven to beuseful aids in sea ice reconnaissance and mapping. Neverthe-less, development of the full potential for the application ofradar-to-polar oceanography is hindered by a general lack offundamental knowledge of the interaction of microwaves withthe various types of sea ice. Clearly, to obtain the greatestutility from such sensors, careful study must be made of thephysical phenomenon of radar return from sea ice. Thisknowledge should then be used in selecting the optimum param-eters for future designs of polar reconnaissance and mappingradars.The potential application of SLAR to sea ice research and

reconnaissance was recognized in 1962, when the U.S. ArmyCold Regions Research and Engineering Laboratory utilized aU.S. Air Force AN/APQ-56 K-band SLAR in sea ice experi-ments conducted during April and August [1]. This study in-dicated that various important sea ice features could beidentified on the K-band imagery, and that the SLAR systemprovided the capability of obtaining good-resolution imagemaps of large expanses of ice, independent of the incidentlight and weather conditions.Since that time, radar has been used extensively to monitor

sea ice by Rouse [2], Johnson and Farmer [3], Glushkov and

Manuscript received January 15, 1979; revised May 1, 1979. Thiswork was supported by the Office of Naval Research under ContractN00014-74-C-1 105.R. G. Onstott and R. K. Moore are with the Remote Sensing Lab-

oratory, University of Kansas Center for Research, Inc., KS 66045.W. F. Weeks is with the U.S. Army Cold Regions Research and En-

gineering Laboratory, Hanover, NH 03755.

Komarov [4], Ketchum and Tooma [5], Parashar et al. [6],Dunbar [7], Dunbar and Weeks [8], Gray et al. [9], andKetchum [10]. These studies have primarily been made at2- and 3-cm wavelengths, although a few studies were madeat 25-cm wavelengths. Most SLAR flights have used anglesof incidence near grazing. These studies tended to agree thatwith some reservations, the ability to detect and interpret iceage, ice drift, ice surface topography such as pressure ridgesand fractures, and to evaluate sea ice conditions, was verygood.The first attempt to expand the knowledge of the parametric

response of radar systems sensing sea ice began in 1967 whenthe National Aeronautics and Space Administration (NASA),Navy Oceanographic Office, U.S. Army Cold Regions Researchand Engineering Laboratory, Arctic Institute of North America,and the University of Kansas, Lawrence, undertook a study ofthe ability of a 2.25-cm wavelength (13.3-GHz) scatterometer(a calibrated instrument designed to measure the differentialscattering coefficient a' as a function of incidence angle [ 11 ] )to identify different ice types [2]. The results demonstratedthat a vertically polarized scatterometer operating at this fre-quency could indeed be used to discriminate ice types. Forinstance, multiyear returns were higher than thick first-year icereturns at all angles of incidence.In April 1970, another joint experiment was conducted by

NASA, the Naval Oceanographic Office, and the University ofKansas, in which systematic radar backscatter measurementsof sea ice were made at 400 MHz (HH, W, VH, and HVpolarizations; H = horizontal, V = vertical; the first letteridentifies transmitted polarization and the second letteridentifies received polarization) and 13.3 GHz (W polariza-tion). The results showed that multiyear ice gave the strongestreturn at 13.3 GHz, while first-year ice and open water gavethe strongest returns at 400 MHz [6]. Discrimination of icefrom open water was possible with both frequencies. Fourpolarization 1 6.5-GHz radar imagery were also analyzed. Theresults were consistent with scatterometer results in that0-18-cm, 18-90-cm, and 90-cm ice and open water werediscriminable. Cross polarization appeared to be better indiscriminating sea ice types, but problems with the imagesprevented a firm conclusion.During the winter and spring of 1975-1976, measurements

were made with a multipolarized 13.3-GHz scatterometer inconjunction with the AIDJEX and Beaufort Sea offshoreprograms [9], [15]. Results showed systematic changes inmicrowave backscatter which strongly correlated with grossice type categories. Multiyear ice showed significantly higher

0018-9413/79/0700-0078$00.75 © 1979 IEEE

78

ONSTOTT et a/.: SCATTEROMETER AND ARCTIC SEA ICE

backscatter than thick first-year ice. Typical differences inbackscatter ran 8-10-dB for like-polarization and 15-18 dB forcross polarization.

Finally, at roughly the same time an X-band SLAR experi-ment was conducted (May 1975) by the Naval OceanographicOffice over areas of the Lincoln Sea, Nares Strait, and BaffmBay [101. The ability to interpret sea ice features andevaluate sea ice conditions was considered to be generally goodin the more northerly Arctic areas. Multiyear ice was easy todiscriminate due to its consistent return and the well-definedcharacteristic shapes of the floes. However, discriminationamong the several stages of development of first-year ice was

not possible based either on backscatter information or theice patterns. New pressure ridges and hummocks gave returnsthat were similar to returns observed in undeformed old icesupporting the suggestion that volume scattering is the majorbackscatter mechanism of multiyear ice.

II. DESCRIPTION OF EXPERIMENT

Radar backscatter measurements were made during themonth of May 1977, on shorefast sea ice off the coast of PointBarrow, AK, and on two frozen fresh-water lakes near the coast.Backscatter measurements were conducted using a four-antennaFW-CW 1-2-GHz and 8-18-GHz scatterometer mounted on a

portable surface-based structure which placed the antennas atfixed ranges from the ice. Radar parameters included multiplefrequencies, cross and like transmit-receive polarizations, andnumerous angles of incidence. Surface truth measurementswere made describing snow, ice, and surface conditions at thetime of measurement.

A. Microwave Active Spectrometer

The microwave active spectrometer (MAS) usedwas a portablesurface-based wide-band frequency-modulated continuous-wave radar. The assembled system is shown prepared foroperation in Fig. 1. The L-band (1-2-GHz) radar is located atthe apex of the structure. The hom antenna transmits and thedish antenna receives. The Ku-X-band (8-18-GHz) parabolicdish antennas are located at the base of the structure, and are

aimed at a flat plate reflector which directs the radar signal toa focal point in the intersection of the apex and the groundplane. Nominal specifications are detailed in Table I and a

system block diagram is given in Fig. 2.The L-band radar was swept from 1.1 to 1.9 GHz for VV

and HH polarizations. Ku-X-band frequencies were centeredat 9.0, 10.0, *.., 17.0 GHz and swept ±0.5 GHz. Transmit-receive configurations were HH, HV, VH, and VV.Calibration of the scatterometer was achieved by frequently

passing the signal through a delay line of known attenuation,and by less frequently observing a standard radar target ofknown radar cross section. A corner reflector was used in thecalibration of the L-band radar, and a Luneberg lens reflectorin the calibration of the Ku-X-band radar.Reduction in the fading effects of constructive and destruc-

tive interference of the signal components of the scatteringelements within the resolution cell was accomplished by in-creasing the number of independent samples through volumeand spatial averaging. A single measurement of the backscatterfrom a particular target is almost meaningless. Several samples,

Type

Frequency Range

Modulating Waveform

FM Sweep: af

Transmitter Power

Interrrediate Frequency

IF Bandwidth

Antennas

Receive Type

Transmit Type

Feeds

PolarizationCapabi l ities

Target Distance

Transmit Beamiidth

Receive Beannwidth

Incidence Anigle Range

Cal ibration:

Internal

External

Operating TeeperatureRanrie

Ku-X-Band

FM-CW

8-18 GHz

Triangular

GHz

14-19 dBm

50 kHz

13.5 kHz

46 cm Reflector

31 cm Reflector

Dual Ridge Horn

HH,HV,VV,Vh

10.9 meters

8.20 at 8 GHz4.0° at 17.7 GHz

5.30 at 8 GHz2.30 at 17.8 GHz

10 -70

Signal Injection(delay line)

Luneberg SphereReflector

-50 C to -5SO C

L-Band

FM-CW

1.5 GHz

Triangular

800 MHz

19 dBm

50 kHz

13.5 kilz

91 cm Reflector

Standard Gain Horn

Log Periodic

VV,VH

6.5 meters

270

9.00

10°-700

Signal Injection(shorted delay line)

Square TrihedralCorner Reflector

-50° C to +500 c

preferably a relatively large number, must be measured andthe results averaged together to reduce the effects of the fad-ing. The precision of the radar scattering coefficient measure-

ment with a scatterometer is limited by these fading effects.Frequency averaging consisted of sweeping with bandwidthin excess of that required for resolution. Independent lookswere obtained through physically repositioning the structure.

B. Ground Truth

The task of the ground truth phase of the experiment was

to make measurements at the time of the backscatter measure-

ments that would describe snow and ice conditions in the area

of the radar footprint. The following parameters were used

Fig. 1. Microwave active spectrometer in operation.

TABLE INOMINAL SYSTEM SPECIFICATIONS

79


POWPER REGULATORSUPPL IES C IRCU ITS

Fig. 2. Block diagram of microwave active spectrometer system.

in the description of the snow pack: depth; density; snowsurface temperature; a general description of grain size, shape,and texture; and surface roughness. In describing the natureof the sea ice being studied, the following parameters wereused: ice type; thickness; surface roughness (both small andlarge scale); horizontal inhomogeneities (based on thin sec-tions from ice cores); ice surface temperature; ice salinityprofile; and vertical inhomogeneities (based on a visual coreexamination). At one site a statistical study was also made ofthe degree of preferred crystal orientation at two levels in theice sheet.

C. Summary ofMeasurement SitesFive sites of sea ice and two sites of fresh-water lake ice were

investigated. These sites included thick first-year and multiyearsea ice, a small pressure ridge composed of first-year sea ice, alake completely frozen to its bottom, and a lake with waterunderlying the ice. Each site is briefly described in Table II.The large variation in air and ice temperature at the same siteresults from the fact that a set of measurements commonlyrequired several hours, and in some cases more than one day.Appreciable variations in snow thickness also had a major effecton the temperature of the upper ice surface.

III. L-BAND RESULTS

A. a° Results

We initially believed that the returns from apparently feature-less thick first-year ice should yield simple trends. However, as

0

-4

o -8s

-12a

X -16

V-°- -20

m -24

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Thick First-Year (Sites 1,3)--TFYPolarization:

Vertical--VV

Site # (Look #) [Temp.]

1 (3 4, 5) [-4°C

3 11.2) 1-50C]

\ 1 (1,2) 1-60CI

(3,4,5,6)l-100CI

0 10 20 30 40 50 60 70Angle of Incidence (Degrees)

Fig. 3. Scattenng coefficient of thick first-year ice at 1-2 GHz.

shown in Fig. 3, we actually observed a pronounced clusteringof the observed returns into families. This separation is mostapparent at large incidence angles. Also, multiyear ice was ex-pected to be difficult to analyze using data collected by asurface-based system because of the characteristic nonuniformupper surface of such ice. A comparison of the two multiyearice sites and the coefficients of the individual looks obtainedat Site 5 (VV) is shown in Fig. 4. The data did not exhibit

-50

80

4r

ONSTOTT et al.: SCATTEROMETER AND ARCTIC SEA ICE

Multiyear (Sites 2, 5)--MYPolarization:

Vertical-- VV% Cross--VH

Z Theoretical 68% Confidence Interval*"1%%for Site 5 VV

,* * More than 1 Data Point

%-WVV (21

-0 10 20 30 40 50 60Angle of Incidence (Degrees)

70

Fig. 4. Scattering coefficient of multiyear ice at 1-2 GHz.

Multiyear (Sites 2,51--MYThick First-Year (Sites 1,31--TFYPolarization:

Vertical--VVCross--VH

z0

L;4 HP,1

"

m M- 0lwMY-VV

-32

- 10 20 30 40 50 60 70Angle of Incidence (Degrees)

Fig. 5. Scattering coefficient of thick flrst-year and multiyear ice at1-2 GHz.

the expected wide variation between independent looks andwere found to cluster more tightly than would be expected fromsurface scatter alone. The returns from both sites of lake icealso showed similar characteristics. At large angles ofincidencemultiyear ice returns decayed more rapidly than those fromthick first-year ice, as shown in Fig. 5.The ability to discriminate between thick first-year, multi-

year, pressure ridges, and lake ice is illustrated in Fig. 6. Multi-year and thick first-year sea ice were not found to bediscriminable between 100 and 600. Cross polarization providedthe greatest discriminatory capability and may be useable athigher angles. Sharp cornered pressure ridges and the flatsmooth surfaced parts of the pack ice could easily be distin-guished from each other at all but the highest angle ofincidence.Also, lake ice had a significantly lower return (6+ dB) than thesea ice types that were studied.

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v Multiyear (Sites 2, 5)--MY* Thick First-Year (Sites 1,3)--TFYo Fresh Water Lake (Sites 6, 7)--LAKE* Pressure Ridge (Site 4)--PR

Polarization:Vertical --VV

4

0

-4

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° -12lZ

* -16

20c 2X' -124:

TFY' PRMY

LAKE


Fig. 6. Average scattering coefficients of thick first-year, multiyear,fresh-water lake, and pressure-ridge ice at 1-2 GHz.

B. Discussion

Dielectric properties of sea ice are primarily a function ofthe brine volume in the ice, which is in turn controlled by icetemperature and salinity. Because the salinity profiles of thethick first-year ice sites were similar, ice temperature is adefinite index of brine volume. Temperature effects on radarreturn were demonstrated as shifts in return trends for thickfirst-year ice, as shown in Fig. 3. Each level of return wasdirectly relatable to air temperature at time ofbackscatter mea-surement. The largest returns were observed when air tem-peratures were highest. Correspondingly, weak returns wereobserved when temperatures were at their lowest. It should benoted, however, that the data set over which the temperatureeffect is described was small. Variation in snow depth was notfound to have a pronounced effect on the strength of thereturns.The first-year fast ice that was studied at Site 3 was observed

to have a very pronounced preferred c axis alignment in thehorizontal plane. It is presently believed that these alignmentsreflect the mean direction of the current at the ice-water inter-face with the resulting favored orientation having the c axisparallel to the mean current direction [12] . Antenna E-fieldorientation parallel to the c axis resulted in detectably higherreturns than when the orientation was perpendicular to the caxis. These results are in agreement with the results thatCampbell and Orange [14] and Kovacs and Morey [16] ob-tained with an impulse radar at 625 MHz. In 1978 experiments(not yet analyzed) were performed to provide a quantitativemeasure of the magnitude of this effect.For multiyear, for the pressure ridge, and for lake ice, the

individual looks clustered more tightly than would have beenexpected from fading produced by surface scatter alone;hence,we conclude that volume scatter plays a major role in thescattering process, thereby allowing the wide sweep width togive noncoherent averaging of numerous independent samplesfor each look. Clustering of the individual looks of multiyear

v Multiyear (Sites 2,5) --MY* Thick First-Year (Sites 1,3)--TFY

Polarization:Horizontal --HHVertical --VVCross--HV

* More than One Point

\

MY -VV

'MY-HH

ITFY-VVTFY -HH

-28- \NN MY-HV

32-~~~~N-

-36 -

-40-, >N..TFY HV0 10 20 30 40 50 60 70Angle of Incidence (Degrees)

Fig. 7. Scattering coefficient of thick first-year and multiyear ice at9 GHz.

ice Site 5 (VV) is shown in Fig. 4. The brackets indicate fortheoretical 68-percent interval about the mean at each differentangle of incidence.

IV. KU-X-BAND RESULTS

Scattering coefficient trends of thick first-year and multiyearice for frequencies in the Ku-X-band microwave region arepresented in Figs. 7-14. The radar parameters are 9-, 13-, and17-GHz frequencies and HH, HV, and W transmit-receivepolarizations. The response at 9 GHz is shown in Fig. 7. Theminimum spread (HH) which separated these ice types was 6dB allowing the ice types to be discriminated at all polariza-tions and angles. Cross polarization provided the best dis-crimination, affording 12 dB of separation which was indepen-dent of incidence angle from 20 to 700. W polarization wasslightly better than HH, afforded 8 dB of separation, and wasbest from 20 to 450. The 13-GHz response was similar (Figs.8 and 10). Angular response remained nearly identical, butincreased in absolute level. The 17-GHz response was similar,but with less separation of returns from different types of ice(Figs. 9 and 10). The average scattering coefficients of multi-year and pressure-ridged sea ice and oflake ice, for like polariza-tion and 13 GHz, are shown in Fig. 1 1. Discrimination amongpressure-ridged and smooth flat pack ice was not as good as1-2 GHz frequencies. However, discrimination of pressureridges from multiyear and thick first-year ice at the 8.5-17.5-GHz frequencies may be possible through the proper selectionof incidence angle. At large angles the pressure ridge producedreturns similar to retums for thick first-year ice, and at shallowangles it produced returns similar to returns for multiyear ice.

V. FREQUENCY RESPONSE

Trends show that radar cross section a°, increases approxi-mately linearly with increasing frequency in the 8.5-17.5-GHzregion (see Fig. 12). The 1.5-GHz response suggests that dif-

82

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t -16

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v Multiyear (Sites 2,5)--MY* Thick First-Year (1,3)--TFY

Polarization:Horizontal--HHVertical--VV

\ Cross--HV

F'

--* n- v- -m v-r T-_^n

"

HV-TFY-3 10 2 3 4 5 6


Fig. 8. Scattering coefficient of thick first-year and multiyear ice at13 GHz.

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-12

1 -16

° 0

m -24

-28

v Multiyear (Sites 2,5)--MY* Thick First-Year (Sites 1,3(--TFY

Polarization:Horizontal--HHVertical--VV

-> . Cross--HV

4-

-4

-12 -

B -20-0

C -28--

4-

L)-4 -tv

-12

-20 -

-28.-0

Fig. 10. Scattering

4

0

-4

X -80

-12c.-

i -16vn

0

°20-,

-24-321-

-28,. ---

-0 10 20 30 40 50 60 70Angle of Incidence (Degrees)

Scattering coefficient of thick first-year and multiyear ice at17 GHz.

ferent scattering mechanisms were at work in the 1-2-GHzregion. For like polarizations and incidence angles from 10to 700, discrimination was almost independent of frequency inthe 8.5-17.5-GHz region. Discrimination with cross polariza-tion was a function of frequency with the lower X-band fre-quencies affording the best distinction. The ability todiscriminate between thick first-year and multiyear ice as a

function of incidence angle and frequency is shown in Figs. 13and 14, where the decibel difference in the cra values for thesetwo ice types is plotted. For like polarization (VV) there is a

somewhat better discrimination at all angles at 9 GHz than atthe other three frequencies. 8.5-17.5-GHz frequencies discrim-inate well between the ice types at all but near-nadir angles.The 1.5-GHz frequency did not discriminate until anglesgreater than 600 were reached. Cross polarization discriminatedbetter than like polarization throughout all incidence angles. 9

-32

Multiyear (Site 5)--MYThick First-Year (Site 3)--TFYPolarization:

Vertical--VVFrequency (GHz):

*-- 1.5--9.0

1 13.0,_ ,_ ,_ ,_ ,_ 9.0

\v_~ ~~ R, _ ~ MY

_1-7.0>1709.0

1.5

10 20 30 40 50 60 70Angle of Incidence (Degrees)

;coefficient of thick first-year and multiyear ice at1.5, 9.0, 13.0, and 17.0 GHz.

v Multiyear (Sites 2,51--MY* Thick First-Year (Sites 1,3)--TFYo Fresh Mater Lake (Sites 6, 7)--LAKE* Pressure Ridge (Site 4)- PR

" ~~~~Polarization:\~~~~9_ ~~Horizontal- -HH


Fig. 11. Scattering coefficient of thick first-year, multiyear, fresh-waterlake, and pressure-ridge ice at 13 GHz.

and 13 GHz had more ability to discriminate than either 1.5 or17 GHz. At large angles of incidence 9 GHz was appreciablybetter (see Fig. 14).

VI. GENERAL CONCLUSIONS

The differential radar scattering coefficient which describesbackscatter is dependent upon many parameters. These includeradar parameters such as angle of incidence, polarization, andthe free-space wavelength. Environmental conditions also in-fluence backscatter. These may include temperature, snowdepth, ice dielectric properties, and surface roughness. Indeedthe study of radar return from sea ice is a complex and inter-woven puzzle.

Fig. 9.

-50'

83

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0

4

Multiyear (Site 5)--MY Polarization:Thick First-Year (Site 3) Horizontal--HH (MY-,, TFY-AlAngle of Incidence (Degrees): 70 Vertical--VV (MY- 0, TFY-

Cross--HV (MY-°, TFY-VH (MY- , TFY- )

_ A ____ MY-HH

MY-HVa _ ~~~~~= C _ _~~~~~ TFY-HH

- -_< I TFY-HV

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18Frequency (GHz)

Fig. 12. Scattering coefficient frequency response of thick first-yearand multiyear ice.

14

c 12 o o

,aa10 / - ^0 -Q

'Oi 10 2 0'" 4

Angle of Incidence (Degrees)

Fig. 13. Difference between radar cross section of thick first-year and

multiyear ice at 1.5, 9.0, 13.0, and 17.0 GHz with vertical polarization.

Thick first-year ice was found to have multiple families of

differential scattering cross section at the 1.5-GHz frequency.

Strong correlation was found between the level of the 1.5-

GHz scattering cross section and the air temperature at the

time of backscatter measurement, although the small size of

the data set precludes making this a general statement. Warmest

temperatures yielded the greatest cross section and coldest

temperatures yielded the lowest cross section (contrary to

findings for snow/earth [13]). The same effects were not

identifiable in the 8.5-17.5-GHz returns for thick first-year ice.

The effect of the orientation of the radar with respect to the

prominent preferred crystal orientation of thick first-year ice

was detectable at the L-band frequency. When comparisonswere made between returns when radar alignment was per-

pendicular to the c axis, and when the radar was parallel, the

strongest returns were noted when the incident radar beam

was aligned parallel to the c axis. Quantitative information

on this subject awaits further data. Ku-X-band returns appeared

free from orientation effects probably due to an inability to

penetrate to depths in the ice sheet where prominent crystalorientation was present.Volume scattering effects were quite noticeable in the returns

from multiyear, pressure ridge, and lake ice at the 1l.5-GHzfrequency. Measurements gave values closer together than

would have been expected from the fading for surface scatter

alone. Tight clusters of the returns from independent looks

well demonstrated the mechanism of volume scattering as a

main component of the backscatter returns of these ice types.

In the Ku-X-band region, the frequency response trends of

the scattering cross section indicate that discrimination between

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X 10

c: 8

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3-- o 0- v-A

__ - - M_ b9.0-0 v _13.0

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0 10 20 30 40 50 60 70

Angle of Incidence (Degrees)

Fig. 14. Difference between radar cross section of thick first-year andmultiyear ice at 1.5, 9.0, 13.0, and 17.0 GHz with cross polarization.

thick first-year and multiyear ice was reasonably independentof frequency for like polarization. For cross polarization therewas an additional 2-dB spread with a slight increase in slope ofthe thick first-year ice response. The frequency response ofthe three types of sea ice and the two types of lake ice demon-strate that in the Ku-X-band region the radar scattering cross

section increases in a linear manner with increasing frequency.The fact that this does not appear to be the case for the 1.5-GHz frequency suggests that different scattering mechanismsare in operation.

A. Application to Ice Type/Thickness Identification

The 1.5-GHz frequency did not have the ability to discrim-inate between thick first-year and multiyear sea ice for anglesbetween 12 and 700. However, the data suggest that discrimina-tion may be possible at angles larger than 700. Cross polariza-tion was found to have more ability to distinguish these ice

types than like polarization. Returns at this frequency were

found to distinguish easily between the flat areas of the packice and the more prominent features, a pressure ridge. Lakeice returns were much lower than the three sea ice types.Ku-X-band results indicate that frequencies between 8-18

GHz have the ability to discriminate between thick first-year,multiyear, and lake ice. Like or cross polarization are bothuseable. 9 GHz was the best like-polarization (VV) frequencyfor discrimination of thick first-year and multiyear ice andworked best at 20-400. 9 GHz was also the best cross-polarized(HV) frequency and worked best from 35 to 700. Again, cross

8-O

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84


polarization was found to have more capability to discriminatethese ice types than like polarization.

REFERENCES

[1] V. H. Anderson, "High altitude side-looking radar images of seaice in the arctic," in Proc. Fourth Symp. on Remote Sensing ofEnvironment, University of Michigan, Ann Arbor, 1966, pp.845-857.

[21 J. W. Rouse, Jr., "Arctic ice type identification by radar,"Proc.IEEE, vol. 57, pp. 605-614, 1969.

[31 J. D. Johnson, and L. D. Farmer, "Use of side-looking airborneradar for sea ice identification," J. Geophys. Res., vol. 76, no. 9,pp. 2138-2155, 1971.

[4] V. M. Glushkov and V. B. Komarov, "Side-looking imaging radarsystem TOROS and its application to the study of ice conditionsand geological explorations," Proc. Seventh Int. Symp. on RemoteSensing ofEnvironment, Univ. of Michigan, Ann Arbor, 1971.

[51 R. D. Ketchum and S. G. Tooma, "Analysis and interpretationof airborne multi-frequency side-looking radar sea ice imagery,"J. Geophys. Res., vol. 78, no. 3, pp'. 520-538, 1973.

[6] S. K. Parashar, A. W. Biggs, A. K. Fung, and R. K. Moore, "In-vestigation of radar discrimination of sea ice," in Proc. Ninth Int.Symp. on Remote Sensing of Environment, Univ. of Michigan,Ann Arbor, 1974.

[7] M. Dunbar, "Interpretation of SLAR imagery of sea ice, NaresStrait, and the Arctic ocean," J. Glaciology, vol. 15, no. 73, pp.193-213, 1975.

[8] M. Dunbar and W. F. Weeks, "The interpretation of young iceforms in the Gulf of St. Lawrence using radar and IR imagery,"DREO Rep. 711, pp. 1-41, 1975.

[9] L. Gray, R. 0. Ramseier, and W. J. Campbell, "Scatterometerand SLAR results obtained over Arctic sea ice and their relevanceto the problems of Arctic ice reconnaissance," in FourthCanadian Symp. on Remote Sensing, P. Q., Canada, May 1977,pp. 424-443.

[10] R. D. Ketchum, "An evaluation of side-looking radar imagery ofsea ice features and condition in the Lincoln Sea, Nares Strait, andBaffin Bay (U)," NORDA Tech. Note 57, 1977.

[11] R. K. Moore, "Radar scatterometry-An active remote sensingtool," Fourth Symp. on Remote Sensing of Environment, Univ.of Michigan, Ann Arbor, pp. 339-373, April 1966.

[12] W. F. Weeks and A. J. Gow, "Preferred crystal orientations inthe fast ice along the margins of the Arctic ocean," J. Geophys.Res., vol. 83, no. C10, pp. 5105-5121, 1978.

[13] F. T. Ulaby and W. H. Stiles, "Backscatter and emission of snow,"in Proc. Microwave Remote Sensing Symp., Houston, TX, 1977.

[14] K. J. Campbell and A. S. Orange, "The electrical anisotropy ofsea ice in the horizontal plane," J. Geophys. Res., vol. 79, no. 3,pp. 5059-5063, 1974.

[15] R. 0. Ramseier, L. Gray, and W. J. Campbell, "Scatterometer andimaging radar results obtained over Big Bear, AIDJEX 1975," SeaIce Processes and Models. (Proc. AIDJEX/ICSI Symp., R. S.Pritchard, Ed.), Seattle, WA: Univ. of Washington Press, inprint.

[16] A. Kovacs, and R. M. Morey, "Radar anisotropy of sea ice due topreferred azimuthal orientation of the horizontal C-axes of icecrystals," J. Geophysical Research, to be published.

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Surface-Based Scatterometer Results of Arctic Sea Ice

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