The acquisition and interpretation of ice thickness data from a mobile platform has for some time been a goal of the remote sensing community. Such data, once obtainable, is of value in monitoring the changes in ice thickness over large areas, and in mapping the potential hazards to traffic in shipping lanes. Measurements made from a helicopter-borne ice thickness profiler of ice in Lake Superior, Lake St. Clair and the St. Clair river as part of NASA's program to develop an ice information system are described. The profiler described is a high resolution, non-imaging, short pulse radar, operating at a carrier frequency of 2.7 GHz. The system can resolve reflective surfaces separated by as little as 10 cm. and permits measurement of the distance between resolvable surfaces with an accuracy of about 1 cm. Data samples are given for measurements both in a static (helicopter hovering), and a traverse mode. Ground truth measurements taken by an ice auger team traveling with the helicopter are compared with the remotely sensed data and the accuracy of the profiler is discussed based on these measurements

Gedney, R.

Heighway, J. E.

Vickers, R. S.

English

NASA Technical Reports Server

N A S A  T E C H N I C A L  
M E M O R A N D U M  
AIRBORNE PROFILING OF ICE THICKNESS USING 
A SHORT PULSE RADAR 
by R. S Vickers 
Dept of Electrical Engineering 
Colorado State University 
J. Heighway and R. Geaney 
Lewis Research Center 
Cleveland, Qhio 44135 
TECHNiCAL PAPER proposed for presentation at 
Advanced Cancepts and Techniques in the Study of 
Snow and Ice Resources 
Mo'iterep California. December 2 -6, 1973 
NASA TM X- 71481 
Unclas  
G 3 / 1 j  21380 
https://ntrs.nasa.gov/search.jsp?R=19740003091 2020-03-23T14:02:12+00:00Z
AIRBORNE PROFILING OF ICE THICKNESS 
USING A SHORT PUISE RADAR 
R. S. Vickers 
Dept. of Electrical Engineering 
Colorado State University 
J. E. Heighway 
R. T . Gedney 
NASA /Lewis Research Center 
ABSTRACT 
The acquisition and interpretation of ice thickness data from a 
mobile platform has for some t ime been a goal of the remote 
sensing c~mmunity.  Such data, once obtainable, i s  of value in 
mo~i to r ing  the changes in ice thickness over large areas ,  and in 
mapping the potential hazards to traffic in shipping lanes. 
This paper describes measurements made from a helicopter- 
borne ice thickness profiler of Ice in Lake Superior, Lake St. Clair 
and the St. Clair r iver as  part of NASA's program to  develop an 
ice irformation system. 
The profiler described i s  a high resolution, non-ixxaging, 
short pulse radar,  operating at  a ca r r i e r  frequency of 2.7 GHz. 
The system can resolve reflective surfaces separkted by a s  little 
a s  10 cm. and permits measurement of the distance between re- 
solvable surfaces with an accuracy of about 1 cm. Data samples 
a r e  given for measurements both in a static (helicopter hovering), 
a x !  a trc-, x s e  mode. Ground truth measurements taken by an 
ice auger team traveling with the helicopter a r e  compared with 
the remotely ~ e n s e d  ata and the accuracy of the profiler is dis- 
cussed based cn these measurements. 
Techniques for the remote measurement of ice thickness 
have been stzdied for many years. In some cases,  the thickness 
is inferred frcm the surface temperature as in the case of 
thermal infrared scanning, o r  from the microwave brightness 
temperature (Meier and Edgerton, 1971). A more direct method, 
that of profiling with an active radar  system was used in 1966 by 
the U.S. Army (Rinker, 1966) to map the thickness of the 
Greenland Ice Cap. This technique proved successful in t e rms  
of penetration through a great thickness of ice, but the resolution 
was not suitablf for other applications involving relatively thin 
ics  due to the small  04 MHz) bandwidth em loyed. 
In this paper, a radar system is descr i  g ed which has been 
used to profile lake ice down to a thickness of 10 cm. Data from 
the system both in a static and mobile mode a r e  given. The 
platform used was a Sikorski H-52 helicopter furnished through 
the cooperation of the U.S. Coast Guard stations at  Selfridge 
Field and Traverse City. 
3IICROWAVE PROPAGATION THROUGH ICE 
-4 sunllllary of the microwave properties of ice and snow is 
given by Holter and h u 7 e  0966) based on data published by 
Von Hippel 0954) 311d Cummins 0952) A t  3 GHz the value given 3 for the loss tangent of ice is 0-9 x 10' , corresponding to an 
attenuation of 0.44 db'meter for ice with a dielectric constant 
of 3.2. This figure sssurxes no losses from scattering processes. 
The losses indicate that a plane parallel layer of ice could 
clrsily s i re  rise to multiple returns when illurniwted by a radar 
pulse. due to energy being multiply reflected within the layer with 
r;linin~al absorption. This phenonienon was noted in some of the 
drit;? obtained over smooth ice. One advantage of the low absorp- 
tion is the attendant high penetration capability of the radar. 
C'sing 11 dielectric itpprosinlation for the calculation of the reflec- 
tion coefficient. the values to be espected at 3 GHz 9-e 3s follouvs: 
Air 'Ice interface 2 8r: 
Air Snow interface lCT* 
Snox :Ice interface 
Ice Wnter interface 677 
T h e  presence of surface roughness would of course modify these 
figures, a s  u-ould also the presence of surface fi1r.s of water in 
the liquid phase. The calculations do indicate however that in 3n  
 deal case of horizontallv stratified snow.'ice in-ater, a substantial 
reflection \~ould result from each boundary. Previous work 
@ickers iind Rose, 1972) on layer& snow and ice supports this 
contention. 
SYSTEM DESIGN 
A diagram of the system is given in Figure 1. The generator 
is a solid state source capable of tuning from 2. 5 to 2.9 GHz. 
The frequency was adjusted to 2.7 GHz to minimize leakace fron? 
the  minimize leakage from the mixer. The C.W. signal from 
the oscillator was then switched with a double balanced mixer 
driven by a 1 ns  pulse to give a short burst of 2.7 GHz carrier. 
This signal was amplified :a a peak power of 1W with a Travelling 
Wave Tube amplifier (TWT) and then transmitted through a double 
ridged horn antenna. The TWT also had an amplitude modulation 
capability, thereby allowing range dependant gain to be employed 
if necessary. 
The receiver chain consisted of two solid state amplifiers with 
a total gain of 70 db feeding an envelope detector. The output from 
this detector was then displayed on a sampling oscilloscope and 
recorded on either polaroid film (static mode) or on continuous 
strip film (traverse mode). 
Options available to the operator included triggering the dis- 
play on the first return, which removed helicopter altitude vari- 
ations from the data, and an intensity modulation display mode. 
The system was designed to operate at altitudes around 40m 
above the ice, although the gain was sufficient for greater alti- 
tudes i f  necessary. The detected return pulse, shown in Figure 2, 
had a half width of 1.3 ns. A photograph of the completed system 
appears in Figure 3. 
A protype of the eouipnient, with some minor differences, had 
already operatee for one field season in the measurement of snow 
depth in the Colorado Rockies (Vickers and Rose, 1972). The 
present system u as therefore given brief preliminary testing by 
using it to view a vertical ice wall, of adjustable thickness built 
from commercial ice blocks. This experimer~t was used to 
determine the minimum measurable thickness of ice, under 
relatively controlled conditions. This minimum was less than 
10 cm. Also evaluated were the effects of having the first surface 
(air/ice interface) covered with water to simulate melting ice zon- 
ditions. The thin film of water retained on the vertical face of the 
ice wall had no appreciable effect. An effect was observed during 
these tests which later gave s ~ m e ~ e x p r i r n e n t a l  difficulty, namely 
the beiamu-idth of the antennas (45 ) was large enough to allow 
returns frsm a ~ u n l b e r  of paths to interfere with that from the 
line-of-tight pati. from the target. This interference effect be- 
tween the returns caused significant amplitude modulation of the 
total signal as  the r a T e  \\-as varied, and made data interpretation 
difficult. The sar,?e effect was later noted in the helicopter in- 
stallation where the return coming from the ice interfered with a 
reflection froni the whicle  landing gear. It was eventually re- 
moved by lowering the antennas. An example of the test d a t ~  from 
a 19 cm. thick wall of ice i s  given in Figure 4. 
EXPEREtIEhTA L PROCEDURE AXD RESULTS 
After a number of trial  flights over Lake St. Clair, the system 
was taken over Whitefish Bay in Lake Superior for demonstration. 
The procedure employed was to fly the system to a suitably thick 
region of ice and land the helicopter. An ice auger team then 
took a sounding of the ice thickness, and t h e  radar system was 
subsequently flown over the area a t  an altitude of 20 to 30 m. A 
typical return from these tests  is shown in Figure 5 where the 
two pulses correspoild to the upper and lower surfaces of ice 
30 cm. in thickness. The si tes  were photographed from the ground 
and from the helicopter for later correlation with the data. The 
radar data itself was recorded on polaroid film. In addition some 
traverse data was recorded on s t r ip  film. 
An exa.mple of data taken in the t raverse mode is given in 
Figure 6, where each excursion of the t race to the left indicates 
a target. An alternative and more powerful presentation in te rms 
of visual impact is to detect the minima in each trace, and show 
these bright p i n t s  i n  an  intensity modulated display a s  shown in 
Figure 7. Visual separation of the ice stratigraphy from random 
returns due to roughness is then more easily achieved. 
It was found that the ice thickness could be accurately mea- 
sured ir, the case of a single unbroken layer thicker than 10 cm. 
and smooth on both its upper and lower surfaces. The accuracy 
of measurement was about 2 cm. for such sheets of ice. Quite 
often, however, one encounters ice that exhibits roughness on 
one o r  both of i t s  surfaces, o r  possesses a layered structure. 
Surfacc roughness can be caused by 1) the freezing of slush formed 
from the melting of snow, o r  2) the freezing of spray and droplets 
generated in the leads between storm-tossed floes, o r  3)the u p  
thrusting of edges in pressure ridges. 
Lower surface roughness is observed in pressure ridges, of 
course. Besides this, one frequently encounters a "steppedw 
lower surface in re-frozen brash in which relatively thick floes 
a r e  held together by 3 matrix of thinner ice. 
Upper surface roughness causes difficulty because signals 
reflected from bumps located off the axis can interfere with the 
signal of primary interest, namely, that reflected from the ice- 
water interface. An example of such a case is  given in figure 8. 
The "steppedw lower surface encountered in refrozen brash 
seems to produce a complex return as off-axis signals a r e  eifi- 
ciently returned from surfaces forming corner reflectors, a s  it  
were. These reflections make interpretation of the return dif- 
ficult. If the most straightforward interpretation is used, 
(identifying the largest peak affer the first peak as that reflected 
from the ice-water interface) significant e r rors  a r e  made. 
The problem of itlterierence by off-axis reflections is judged 
to be the most serious impediment to the development of an 
operational radar ice thickness system. 
Because of the large attenuation one would expect that i f  ice 
sheets a re  separated by more than a few millimeters of liquid 
water, lower sheets would be undetectable. No data bearing upon 
this issue was taken. 
In the case of smooth sheets of ice, it was found that multiple 
reflections were often observable. While these multiple reflec- 
tions caused no difficulty in inferring ice thickness for ideal ice 
sheets, they obviously increase the difficulty of interpreting the 
returns from more complex ice structurps, This problem is  con- 
sidered to be minor as  compared with that of off-axis reflections. 
Figure 9 presents data from I4 sites comprising a variety of 
ice types. The R. M. S. er ror  of these measurements is about 
15%. Previous measurements on ice and snow show that with 
perfectly smooth surfaces an accuracy of better than 10% is easily 
achievable (Vickers and Rose, 1972). 
CONCLUDING REMARKS 
In summary, it has been demonstrated that a short pulse 
radar system can make accurate measurement of the thickness 
of simple, smooth ice sheets, and at least give an indication of 
the structure of rough-surfaced o r  broken ice. Furthermore, 
the data indicate that a system capable of making thickness mea- 
surements of nearly all types of ice is feasible, provided that 
interference from off-axis reflections can be overcome. The 
most natural attack on this problem is to reduce the beamwidth 
of the system. For the 1973-74 season, an array of four re- 
ceiving antennae will be used, reducing the beamwidth from 45 
degrees to sclnewhat less than 20 degrees. In addition, the use 
of a higher carrier  frequency (up to 5 GHz) will be studied to 
determine whether the higher attenuation will reduce the ampli- 
tude of multiple reflections. 
ACKNOWLEDGEMENT 
The authors v i sh  to acknowledge the generous cooperation 
of the United States Coast Guard Stations at Selfridge Field and 
a t  Traverse  City. 
REFERENCES 
Cummins, W. A. , 19 52. The Dielectric Properties of Snow and 
Ice a t  3 . 2  ems. J, Appl. Physics, vol. 23, No. 7 p.768. 
Holter, M. R. and Lowe, D. S. ed, 1966. Peaceful Uses of Eai-th 
Observation Spacecraft. vol. III, Report No. 7219-1- F 
0. Willow Rvr, Labs., University of Michigan. 
Meier, bI. F. and Edgerton, A. T. , 1971. Microwave Enliss n 
frcjm Snow, a Progress  Report. Proc. 7th Int. S;lmp. 
on Remote Sensing of Environment, University of 
Michigan. pp. 1155-1163. 
Rinker, J. N. , 1966, Radio Ice Sounding Techniques, Proc. 4th 
Int. Symp. on Rexnote Sensing of Environment. University 
of Michigan. 
Vickers, R, S. and k s e ,  G o  G,  , 1972. High Resolution Measure- 
ments of Snowpack Stratigraphy using a Short Pulse Radar. 
Proc. 8th Int. Symp. on Remote Sensing of Envir0nmer.t. 
vol. 1, University of Michigan. 
Von Hippel, A ,  R., 1954. Dielectrics and Wdves, John Wiley & 
Sons, Inc. 
Ftourp 2. - Waveform I1 ns perd~vls~onl  as passed Ihrouoh complet~ svrlpm, w ~ t h  
rccelvlnq horn fx tng the transm~tttnq horn. 
!: RAND 
SOURCE 
F~gure  1. - Block d~agram of ice profiler. 
Figure 2. - Waveform (1 ns per division) as passed through complete system, with 
receiving horn  facing the transmifling horn. 



F I Q U ~ P  A. - Radar r p l u r n  f rom 1 m e k r  ICP with 30 c m  ol debrts, 5 n s  per dlvtsion. 
0 S I M P L E  SMOOTH ICE: 4: THREt 
l DENTICAL READINGS 
A REFROZEN BRASH, THIN 
COMPONENT 
P 
a D
v 1 I I 
0 20 40 60 80 100 
THICKNESS, AUGER 
Figure 9. - Comparison of radar derived thickness lcml 
wi th  ice auger  readings. 


Airborne profiling of ice thickness using a short pulse radar

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