The electromagnetic scattering from ice coated structures is examined. The influence of ice is shown from a measurement standpoint and related to a simple analytical model. A hardware system for the realistic measurement of ice coated structures is also being developed to use in an existing NASA Lewis icing tunnel. Presently, initial measurements have been performed with a simulated tunnel to aid in the development

Beard, L.

Dominek, A.

Walton, E.

Wang, N.

English

NASA Technical Reports Server

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0610 rn 
UNIVERSITY 
Electromagnetic Properties 
by 
A. Dominek, E. Walton, N. 
of Ice Coa.ted 
Wang and L. Beard 
The Ohio State University 
ElectroScience l a  bora tory 
Department of Electrical Engineering 
Columbus, Ohio 43212 
/kASA-CE-1E47&?0)  ELECIRCCACLISIC €bOPE83JES 
C F  I C E  C C A I E L :  5 C E k A C E :  5 e r i a r r l ; z l  SEport 
( C b i o  S t a t e  L c i o . )  1s CSCL 2ou 
63/32 
Semi-Annual Report No. 720964-2 
Grant N@ 3-91 3 
February 1989 
National Aeronautics and Spa.ce Administration 
Lewis Research Center 
Cleveland,OH 44135 
Surfaces 
N89-2C355 
U n c l a s  
01901t.t) 
https://ntrs.nasa.gov/search.jsp?R=19890010984 2020-03-20T03:53:13+00:00Z
NOT ICES 
When Government drawings, specif icat ions,  o r  other data a re  
used f o r  any purpose other than i n  connection w i t h  a def in i te ly  
re la ted Government procurement operation, the United States  
Government thereby incurs no responsibi l i ty  nor any obligation 
whatsoever, and the f a c t  t ha t  the Government may have formulated, 
furnished, o r  i n  any way supplied the said drawings, specif icat ions,  
o r  other data ,  is not t o  be regarded by implication o r  otherwise a s  
i n  any manner l icensing the holder o r  any other person o r  corporation, 
or conveying any rights or  permission t o  manufacture, use, o r  s e l l  
any patented invention tha t  may i n  any way be related thereto.  
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Contents 
List of Figures 
1 Introduction 
2 Scattering Mechanisms 
3 Tunnel Simulation 
4 Conclusions 
PRECEDING PAGE BLANK NOT FILMED 
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13 
List of Figures 
2.1 Rough ice simulation with artificial ice on almond test body. 
Test region is approximately 18” from tip to tip. . . . . . .  4 
2.2 Swept frequency measiirement and transient signature for 
rough ice simulation. Solid line - measurement, Dashed line 
. analytical model calculation. . . . . . . . . . . . . . . . .  5 
3.1 Ice tunnel simulation for RCS measurements. . . . . . . . .  8 
3.2 Stability test of the simulation tunnel. . . . . . . . . . . . .  10 
3.3 Measured response of a corner reflector 10 feet from test horns. 11 
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3.4 Measured response of a corner reflector 14 feet from test horns. 12 
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1 
Chapter 1 
Introduction 
The presence of ice formations on an aerodynamic structure affects not only 
the aerodynamic performance but also the anticipated radar cross section 
(RCS) of the structure. Few studies have been performed to investigate 
the influence of ice on the RCS values for a structure. It is important to 
understand the mechanisms involved that control the RCS signature. Re- 
search to characterize the impact of ice related RCS signatures is presently 
underway. So far, all of the studies have involved simulations with artificial 
ice geometries (both experimentally and analytically) as one area of effort. 
Another area of effort is to provide the measurement capability of mon- 
itoring the RCS of a structure in an existing NASA Lewis existing icing 
tunnel. 
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Chapter 2 
Scattering Mechanisms 
The scattering from a structure can many times be modeled in terms of 
various mechanisms. The scattering mechanisms may not be only due to  
the ice itself but also to the mechanical structure involved when deicing 
approaches are also considered. The conventional deicing approach is to 
flow ethylene glycol over the structure. This approach is not invasive to 
the aerodynamic contour and hence, does not impose a significant concern 
from this viewpoint. A more modern deicing approach is to use a pneu- 
matic boot which involves the mechanical stressing of ice to break it off. 
The stressing force is developed by the expansion of parallel rubber tubes 
internal to the the aerodynamic contour. Such an approach naturally has a 
corrugated surface contour due to the discrete internal rubber tubes. The 
effect of a similar metal surface is discussed in [l]. The major concern from 
a corrugated surface is the generation of very strong grating lobes if the sur- 
face variation has a half wavelength periodicity. Such a surface variation 
has been modeled for a corrugated ice surface in (21. 
The initial RCS measurements made to demonstrate the significance o f  
scattering from a rough ice surface were made with a material (supplied by 
NASA Lewis) which simulated ice in terms of its bulk electrical properties. 
Figure 2.1 illustrates the tested geometry. The artificial ice was randomly 
positioned on a metal plate that was flush mounted into a low cross section 
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test body. The measured return was solely due to the positioned material 
since the cross section of the test body was much lower (31. A typical 
measured swept frequency RCS response is shown in Figure 2.2. The sample 
was illuminated from 20" from grazing with the electric field in the plane of 
incidence. Also shown is a bandlimited impulse signature generated from 
the measurement which demonstrates the surface scattering of the material. 
The material particles were generally .25 to .5 inches across and the time 
domain signature indicates a positional randomness among the particles 
since there is no evidence of any distinct scattering centers. The RCS 
levels are in the -20 to -30 dBsm range for a surface area of 47.3 square 
inches. These values can be scaled based upon area to estimate the return 
for larger areas when the scattering between scattering centers is random. 
For example, if the measured return is -20 dBsm for a given area, the return 
from an area twice that size would be -17 dBsm or 3 dB more. 
The measured response was analyzed as being generated from electri- 
cally small, random, dielectric hemispheres on a perfectly conducting sur- 
face. The dashed line in Figure 2.2 is the result. One hundred scatter- 
ers were used in this calculation with diameter size variations from .25 to 
.5 inches. The scattering from electrically small (characteristic dimension 
much smaller than a wavelength) objects is essentially geometry indepen- 
dent. The calculation required the constitutive parameters of the material 
which were obtained with X-band waveguide measurements. The measured 
constitutive parameter values are e, = (2.9,j.O) and p, = (l., j0.).  Similar 
measurements for a frozen sample of deionized water resulted in values of 
e, = (3.25,jO.) and p, = ( l . , j O . ) .  
The scattering from ice so far has concerned itself with electrically small, 
random ice particules in both size and position. However, physically larger 
ice formations are geometry dependent and have to be studied on an indi- 
vidual basis. An example of this is when an inch or two of ice may form an 
3 
ORJGlNAL PAGE %S 
OF POOR QUALrPY 
Figure 2.1: Rough ice simulation with artificial ice on almond test body. 
Test region is approximately 18” from tip to tip. 
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Figure 2.2: Swept frequency measurement and transient signature for rough 
ice simulation. Solid line - measurement, Dashed line - analytical model 
calculation. 
5 
extension on the rim of a jet inlet. Other scattering mechanisms may exist 
when all the ice is not successfully removed through a deicing process. For 
example, the pnuematic boot approach may successfully remove the i'ce on 
the surface itself but it will leave some ice to form along the boot. This 
process will form an edge of ice which acts as an electrical discontinuity in 
which scattering can occur. 
Another potential scattering mechanism involves the presence of surface 
waves. Ice, especially glaze ice, may guide energy to other areas of the 
surface to  scattering from a discontinuity. Ice itself, when formed from 
deionized water, is very lossless and can provide a good guide of energy. A 
similar guide of energy may be the residual ethylene glycol fluid to prevent 
ice build up. However, if the fluid is not sufficiently lossy, it can readily be 
made so. 
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Chapter 3 
Tunnel Sirnulat ion 
A major effort of this research is to develop the capability to perform real 
time RCS measurements in the NASA Lewis icing tunnel. The benefit 
gained is the ability to obtain actual RCS values for various realistic ice 
formations. A system is presently being designed and tested using a HP 
8510 network analyzer as the core piece of instrumentation with accompa- 
nying software for data collection and signal processing. A 24 foot long 
tunnel with metal walls was erected at OSU to aid in this development to 
simulate the actual electromagnetic conditions that would exist in the icing 
tunnel. The internal cross section of the test tunnel is 9 feet wide and 6 
feet high. Figure 3.1 is a photograph an interior portion of this tunnel with 
a styrofoam mount used to support the test scatters. 
The ability to accurately acquire RCS measurements is dependent upon 
two factors. The first factor is the electrical stability of the radar and test 
tunnel. The measurement procedure generally requires a vector subtraction 
of data measured when the ice is present and absent o n  t hp strurt u r t .  T h i s  
eliminates most returns that are ice independent. This procedure will no t  
work if the subtraction of two like measurements does not result in a low 
level. The other factor is to be able to distinguish between desired and 
undesired terms which are left after the subtraction process. This aspect is 
very important since multiple images of the structure which supports the 
7 
ORlGlNAL PAGE IS 
OF POOR QUALlTY 
Figure 3.1: Ice tunnel simulation for RCS measurements. 
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a 
ice formation can readily exist. The elimination of these undesirable returns 
can be accomplished in a variety of signal processing techniques. The most 
basic technique is the time domain (down range) gating of desired terms 
from the total signature from a swept frequency measurement (41. Another 
technique is through modified (near field) synthetic aperture radar (SAR) 
techniques [5,6] for cross range gating. These SAR techniques require a 
significant amount of measured data. 
The inititial evaluation of the system was performed using the HP 8510 
network analyzer configured in a two horn transmit-receive mode with the 
horns positioned 3 feet above the floor of the tunnel and centered between 
the side walls. Swept frequency measurements were taken between 2 and 
10 GHz. Figure 3.2 illustrates the stability of this system when two similar 
measurements are subtracted which were generated 7 hours apart. This 
result is calibrated by scaling it with the measured and known responses of 
a 9” trihedral corner reflector. 
Figures 3.3 and 3.4 illustrate the measured response of a 9 inch tri- 
hedral corner reflector positioned 10 and 14 feet from the horns. These 
responses were generated by subtracting measurements which had the re- 
flector present and absent from the tunnel. Note the variation of the desired 
signal level as a function of range and the other smaller returns delayed in 
time from the main return of the corner reflector. These other terms may 
be due to multiple reflections from the metal walls of the test chamber. 
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Figure 3.2: Stability test of the simulation tunnel. 
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Figure 3.3: Measured response of a corner reflector 10 feet from test horns. 
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Figure 3.4: Measured response of a corner reflector 14 feet from test horns. 
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Chapter 4 
Conclusions 
Future work for analytical modeling will be centered around calculating the 
response from shaped ice structures that are naturally formed (those that 
are not electrically small) and thus formed from the residual ice left on the 
aerodynamic surface after a pnuematic deicing approach has been applied. 
The major work for the tunnel simulation is to improve the noise level 
of the system to measure the low signal levels which are present. The ap- 
proaches available involve either hardware modification or software imple- 
mentation. The hardware modification incorporates the use of a modulated 
radar configuration (pulsed-cw) to achieve the necessary improvement. The 
software approach uses signal processing of a modified SAR imaging con- 
cep t which requires multiple measurement s. 
13 
Bibliography 
(11 A.K. Dominek, H.T. Sha.mansky and R.M. Wood, “Backscatter from a 
Periodic Rough Surface at a Near Grazing Incidence,” Report 716148- 
24, The Ohio State University ElectroScience Laboratory; Prepared un- 
der Grant NSG 1613, NASA Langley Research Center, October 1987. 
[2] K.M. Lambert and L. Peters, Jr., “Calculation of the effects of ice on the 
backscatter of a ground plane,” Report No. 720964-1, The Ohio State 
University Elect roScience Laboratory, prepared under Grant NAG 3-91 3 
for NASA Lewis Research Center, September, 1988. 
[3] A.K. Dominek, H.T. Shamansky, R.M. Wood and R. Barger, “A Low 
Cross Section Test Body,” Conference Paper, Antenna Measurements 
Techniques Association, Seattle WA, Sept. 1986. 
[4] A.K. Dominek, L. Peters, Jr. and W.D. Burnside, “A Time Domain 
Technique for Mechanism Extraction,” IEEE Trans. on Antennas and 
Propagation, Vol. AP-35, No. 3, pp. 305-312, March 1987. 
(51 D.R. Koberstein, “Near Field Synthetic Aperture Imaging of Probe 
Data for Scattering Studies of the ElectroScience Laboratory Compact 
Range,” Report 718331-7, The Ohio State University ElectroScience 
Laboratory; Prepared under Contract GR9300, Boeing Aerospace Com- 
pany, September 1986. 
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[6] D.L. Mensa, High Reaohtaon Radar Imaging, Dedham, MA, Artech 
House, 1981, pp. 103-118. 
14 
60172-101 
T A v a i l a b i l t y  Statement 10. Security Class (This Report) 
Unclassified 
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I 21. No. of Pages 14 
REPORT DOCUMENTATION 1. REPORT NO. 
PAGE 
~ 
4. Title and Subtitle 
ELECTROMAGNETIC PROPERTIES OF ICE COATED SURFACES 
A. Doiilinek, E. Walton, N. Wang and L. Beard 
The Ohio State University 
Elec troS cience Laboratory 
1320 Kinnear Road 
Columbus, OH 43212 
7. Author(.) 
8. Performing Organisation Name and Address 
12. Sponsoring Organisation Name and Address 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, OH 44135 
. ___ 
16. Supplementary Notes 
S. Recipient's Accession No. 
6. Report Date 
February 1989 
6. 
8. Performing Org. Rept. No. 
720964-2 
10. ProJect/Task/Work Unit N 
11. Contraet(C) or Grant(G) N 
(C) 
(G) N1)GI-SlI 
13. Report Type/Period Coven 
Semi- Annual 
~ 
14. 
~ ~ ~~ 
16. Abstract (Limit: 300 words) 
The electromagnetic scattering from ice coated structures is examined. The influence of 
ice is shown from a measurement standpoint and related to a simple analytical model. 
A hardware system for the realistic measurement of ice coated structures is also being 
developed to se in an existing NASA Lewis icing tunnel. Presently, initial measurements 
have been peformed with a simulated tunnel to aid in the development. .(.' 
h 
17. Docunient Analysis a. Descriptors 
b. IdentiflerrlOpen-Ended Terms 
30. Security Class (This Page) I Unclassified 22. Price 
(See ANSI-ZI9.18) Scc Inrtruetionr on Rcvcrrc OPTIONAL FORM 372 (4-77) 
Department of Commerce 


Electromagnetic properties of ice coated surfaces

Abstract

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