also require the observer to go out onto the ice or
area "illuminated" by the radar unit for measur-
for the instrument to make physical contact with
ing ice thickness depends upon height of the an-
the ice. Fortunately, there has been considerable
tennae above the ice surface and the velocity at
research into remote sensing of ice thickness and,
which the aircraft is moving (Arcone and Delaney
while the technology has still not evolved for easy
1987).
field use, advances in instrumentation continue
The use of a global positioning system (GPS)
that will likely allow field implementation soon.
unit in conjunction with the radar system would
Ford et al. (1991) report on the development and
be highly beneficial for tracking movement in the
field testing of a floating drogue equipped with a
horizontal plane. The GPS unit would need to send
pressure transducer and radio transmitter to mea-
its signal to the same data storage device as the
sure ice thickness beneath ice jams. The drogue is
ice thickness measurements, so that the two can
released into the water upstream from the jam and
be correlated. It would do little good to profile tens
floats downstream under the ice cover. The radio
of kilometers of stream and not know where the
transmitter in the drogue reports the hydrostatic
variations in ice thickness are. Another add-on to
pressure at the top of the drogue the pressure,
a radar system could be a video camera to main-
which allows estimation of jam thickness. The posi-
tain video coverage of the ice profiled. The GPS
tion of the drogue can be estimated from shore
unit could be set up to continuously query posi-
through the use of loop antennas. Two drawbacks
tion or determine position only on user demand,
are that the drogues may become stuck within the
depending on the needs (and data storage capa-
jam, and the speed and trajectory of the drogue
bility) of the observer.
through the jam cannot be controlled. However,
One limitation of the short-pulse system, be-
satisfactory results were obtained in the initial field
sides minimum detectable thickness, is difficulty
testing, and the method holds promise for the
in the measurement of frazil and brash ice thick-
future.
ness, and ice jams. The irregular surface of brash
Radar systems have been used in a variety of
ice and ice jams causes the radar signal to be scat-
geophysical applications for a number of years,
tered, and the high water content of frazil causes
including the measurement of sea and freshwater
the signal to be heavily attenuated. Daly and
ice thickness. Radar systems, in theory, detect ice
Arcone (1989) attempted to indirectly measure the
thickness by determining the distance to the air/
thickness of a brash ice jam by measuring the mean
ice interface and the ice/water interface and then
height of freeboard above the water surface using
subtracting the difference. The two most success-
a short-pulse radar from a helicopter. They accom-
ful types of radar have been short-pulse (or im-
plished this by measuring the weak, scattered sig-
pulse) and the millimeter-wave frequency-modu-
nal from the brash ice pieces and the strong signal
lated continuous-wave (MMW FMCW) systems
from the water surface. They concluded that it
(Yankielun 1992). Both are currently used by re-
would be possible to determine the relative
searchers at CRREL, and have advantages and
changes in brash depth, but more accurate abso-
disadvantages which are discussed below.
lute thickness determination would require some
Short-pulse systems have been used for a num-
type of empirical adjustment for brash ice poros-
ber of years. As overall radar technology has
ity, thickness and refractive index. The presence
grown, the ability to detect thinner layers of ice
of frazil (and brash) ice can be detected by radar
has increased. However, the best resolution of
at high power and low frequency, but this results
thickness to date has been about 10 cm, which is
in a loss of resolution of the ice thickness measure-
about twice the minimum thickness for safe tran-
ment (Arcone and Delaney 1987). In spite of this,
sit by one individual on an ice sheet (CRREL 1986).
Ismail and Davis (1992) report measuring the
Riek et al. (1990) state that it is theoretically pos-
thickness of a 7-m-thick ice jam from the ice sur-
sible under favorable conditions to measure thick-
face in New Brunswick using short-pulse radar.
nesses of 34 cm, using appropriate signal-process-
Another limitation of this radar system is that
ing algorithms. While units were originally devel-
interpretation of the data currently requires highly
oped and tested on the ice surface (e.g., Manula
skilled and experienced personnel (Dean 1981).
1987), most recent activity has centered on the use
Considerable work has gone into the automation
of the unit suspended from a helicopter (O'Neill
of processing the signal, but currently signal pro-
and Arcone 1991). The use of radar from a heli-
cessing is done after the data collection. If this sys-
copter has allowed long extents of river ice to be
tem is to be useful in the field, it would need to
profiled in a relatively short period of time. The
provide a real-time (or near-real-time) signal pro-
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