X + ∆X
Xgrid
Table 1. Doppler system configurations.
vgrid =
=
= v + ∆v ,
(13)
∆t
∆t
Event
Breakup
Frazil run
where X is the actual length scale, ∆t is the transit
Radar band
C
Ka
time of a floe over X, and ∆v is a velocity error
5 GHz
30 GHz
corresponding to length-scale error ∆X. The
6 cm
1 cm
actual video velocity v can then be obtained as
Antenna gain
15 dB
27 dB
20
9
3 dB antenna beam width
Xgrid - ∆X
Microwave power
20 dBm
20 dBm
CXgrid
v=
=
= Cvgrid ,
(14)
∆t
∆t
was underway at a river flow of approximately
850 m3/s when measurements began. For most
where C = 1 indicates no grid distortion, and
increasing grid error is indicated as C departs
of this record, the ice was densely packed rubble
from 1.
that provided strong radar reflections and many
video targets. Toward the end of the sampling
period, the surface ice concentration diminished
Ice breakup--March 1993
A 30-minute Doppler radar ice velocity record
and the ice velocity increased.
and a simultaneous video ice velocity record
The ice velocitytime trace obtained from the
were obtained for the Connecticut River at the
Doppler frequency band is presented in Figure 7.
CornishWindsor bridge on 30 March 1993.
Digital signal processing software identified the
Specifications that characterize the radar system
peak return from each radar scan in this record.
used are given in Table 1. The radar antenna was
These peak detector data, representing the high-
mounted about 6 m above the ice and aimed di-
est magnitude power reflections from within the
rectly upstream with an angle β of 25 below hor-
footprint illuminated by the radar, are also plot-
izontal. Concurrently, a video camera was
ted in Figure 7. Both of these data sets were
mounted at the top of the left bank and aimed
processed by first identifying the upper v+ and
normal to the direction of the flow. The stream-
lower v edges of the trace at 20- or 30-s intervals
line where video data were obtained passed
throughout the record; the results are shown in
through the footprint of the radar. Ice breakup
Figure 7. The peak returns generally originated
2.5
Raw Radar Bounds
Raw Data
2.0
V+
1.5
V
1.0
0.5
0
2.5
Peak Radar Bounds
Peak Data
2.0
V+
1.5
1.0
V
0.5
0
30 0
5
10
15
20
25
30
0
5
10
15
20
25
Time (min)
Figure 7. Data band of raw Doppler velocities and peak returns for the ice breakup event and
boundaries that were drawn for each of these data sets.
7
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