Radiometric Measurements of

Powerline Cables at 94 GHz

ARL-TR-837 February 2001

David A. Wikner and Thomas J. Pizzillo

Approved for public release; distribution unlimited.
The findings in this report are not to be construed as an

official Department of the Army position unless so

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ARL-TR-837 February 2001

Army Research Laboratory

Adelphi, MD 20783-1197

Radiometric Measurements of

Powerline Cables at 94 GHz

David A. Wikner and Thomas J. Pizzillo

Sensors and Electron Devices Directorate

Approved for public release; distribution unlimited.
When pilots are flying at low altitudes, they need a sensor that can help

them detect and avoid wires; this need remains a high priority for all U.S.

military services. Many different sensors have been considered to fulfill

this need. This report presents data that were collected on powerline

wires of various diameters with the use of a 94-GHz radiometer. These

measurements were conducted at short range in an effort to quantify the

wire signatures and to determine if a millimeter-wave radiometer could

be used to help pilots avoid wires. Data are presented for seven sizes of

wire as well as for the horizon background of each wire. The results show

that wires down to 1/4 in. in diameter can probably be detected reliably

at ranges up to 200 m with a 94-GHz radiometer that uses a 3-ft

antenna, provided that the signature of the horizon background can be

characterized during flight. Beyond 200 m, the results show that an

image-enhancement algorithm, if proven to be adequate, or a larger

antenna will probably be necessary to reliably detect most standard

powerline wires.

Abstract

ii
iii

Contents

1. Introduction 1

2. Experiment Description 2

3. Measurement Results/Discussion 4

4. Comparison With a Cylinder Model 7

5. Conclusions 10

References 11

Distribution 13

Report Documentation Page 15

Figures

1. Block diagram of ARL 94-GHz radiometer .............................. 2

2. Measurement scene from point of view of radiometer ............... 3

3. Image of 11/16-in.-diam wire taken with ARL 94-GHz radiometer

at a range of 4 m .................................................................. 4

4. Left graph shows vertical profile of brightness temperature for

both 11/16-in. wire and background versus zenith angle ............ 4

5. Left graph shows vertical profile of brightness temperature for

both 1/2-in. wire and background versus zenith angle ............... 4

6. Brightness temperature of background scene versus zenith angle

for various background data runs .......................................... 6

7. A calculation of expected wire contrast versus range .................. 9

Tables

1. Physical characteristics of all wires measured during

experiment ........................................................................ 3

2. Summary of all data collected and results of cylinder calculation ... 6

1

1. Introduction

The U.S. military is greatly interested in finding a sensor that can help pilots detect and avoid wires when flying at low altitudes. The U.S. Army is

particularly interested in preventing wire collisions with its helicopters,

which often fly at an altitude of 200 ft or less during missions. The required

sensor should operate effectively in the daytime, nighttime, and under

reduced-visibility conditions. The type of sensor that ultimately will be used

to fill this need is currently undetermined. Millimeter-wave (MMW) radar,

ladar, and passive MMW imagers are all being considered [1–4]. We made

the measurements presented in this report to assess the potential of an MMW

radiometer to detect powerline wires by empirically determining the limits

of the technology. From this information, we can weigh the benefits of radiometry against its limitations in a meaningful way.

An ARL 94-GHz radiometer was used to measure the signature of a variety

of wire types and sizes at short range. In this report, we present these results and extrapolate them with range and antenna size to show results for

many cases of interest. In addition, we have made theoretical calculations

of the passive MMW signature of a bare metal cylinder for comparison.
2

2. Experiment Description

The experiment reported here measured the 94-GHz radiometric signature

of a variety of powerline wires of known size at a known range. We accomplished this with the use of the Army Research Laboratory’s 94-GHz Dickeswitched radiometer [5]. This instrument is used to measure the low-level,

thermal MMW radiation emitted and reflected naturally by its environment.

The voltage data collected are converted to a noise-equivalent temperature

or brightness temperature by calibration software. Figure 1 shows a circuit

diagram of the radiometer. The radiometer’s noise figure of about 8 dB,

instantaneous bandwidth of 6 GHz, and typical signal-integration time of

30 ms result in a system sensitivity of 0.2 K. For this experiment, the radiometer received vertically polarized radiation and was fitted with a

3-in.-diam horn-fed lens antenna that results in a 3-dB beamwidth of 2.75°.

The switch in front of the mixer (as shown in the figure) is a ferrite device

that operates over the bandwidth 91 to 97 GHz with a loss of 0.6 dB. It was

switched at a rate of about 1 kHz during data collection to remove offset

drift in the receiver circuit. The radiometer was calibrated before each measurement with the use of liquid-nitrogen-soaked radar-absorbing material.

The radiometer is housed in a temperature-controlled box that is heated to

95 °F to minimize gain drifts between calibrations. This results in an absolute accuracy of about ±1 K for the system.

Wires of various diameters and materials were collected, and the signature

of each one was carefully measured with the radiometer at short range so

that the results could be extrapolated to longer range. Table 1 shows the

wires that were measured. Each wire was about 2 m in length and was

mounted in a fixture that held it horizontally in front of the radiometer at a

range of about 4 m (fig. 2). This meant that the radiometer 3-dB far-field

beam spot on each wire was approximately 6.2 in. (15.7 cm). In addition,

each piece of wire was rotated relative to the radiometer such that the angle

between the axis of the wire and the radiometer line of sight was about 45°.

Switch

Load

Mixer

 24 dB

1.1 dB (6–12 GHz)

37 dB

85 GHz

3-in.

antenna 

(91–97 GHz)

Detector

Switch

control

Positioner LO

83 GHz

IEEE-488 PC

A/D card 

37 dB

50 kHz 

Figure 1. Block

diagram of ARL

94-GHz radiometer.
3

This was done to minimize the reflections off the wire emitted by the building behind the radiometer; these reflections would have increased the measured brightness temperature of the wire. The 15/16-in. wire was clad in

polystyrene (as indicated in the table). All other wires were bare metal.

We collected data on each wire by raster scanning the scene and forming an

image of the wire and its background. The data were collected in halfbeamwidth steps across the scene. A raster-scanned image of just the background with the wire removed was taken after each wire measurement to

allow a comparison of the two images and to create background statistics.

In addition, the zenith sky brightness temperature was measured after each

wire measurement so that its effect on the scene could be quantified.

Period of

Wire diameter Strand diameter spiral

(in.) (in.) (in.) Description

1/4 1/16 3 3/8 Steel

5/16 1/8 4 15/16 1 center steel wire

7 Al outer wires

1/2 5/32 6 Al

11/16 1/8 8 15/16 1 copper center wire

18 Al outer wires

13/16 3/32 9 7/8 37-strand Al

15/16 1/16 N/A 5/32 cross-linked

 polystyrene outer layer

17/16 1/8 11 1/2 37-strand Al wire

Table 1. Physical

characteristics of all

wires measured

during experiment.

Figure 2. Measurement

scene from point of

view of radiometer.

Shown is 15/16-in.-

diam wire mounted in

fixture.
4

3. Measurement Results/Discussion

As stated in the previous section, we collected the data of each wire by

raster scanning the radiometer’s antenna beam over the scene. Figure 3

shows an example of such an image. The data were processed by extracting

the portion of each image between the vertical wooden beams that supported the wire. This was done so that the beams did not enter into the data

statistics. From this portion of the image, we averaged each horizontal line

of data to a single value. This process results in a one-dimensional array

that corresponds to an averaged vertical brightness-temperature profile of

the image. We performed the process on each wire image and also for each

background image taken without the wire present. Given a vertical profile

for both the wire and its background, one can make a subtraction and plot

the difference. Figures 4 and 5 show examples of data for two different wires

that have been processed in this manner.

Wire

Zenith angle (°)

84 86 88 90 92 94 96 98

Brightness temperature (K)

235

240

245

250

255

260

265

270

Wire

Background

Background–wire (K) –4

–2

0

2

4

6

8

10

12

Zenith angle (°)

84 86 88 90 92 94 96 98

Zenith angle (°)

84 86 88 90 92 94 96 98

Brightness temperature (K)

230

235

240

245

250

255

260

265

Wire

Background

84 86 88 90 92 94 96 98

Background–wire (K) –10

–8

–6

–4

–2

0

2

4

6

8

10

Zenith angle (°)

Figure 3. Image of

11/16-in.-diam wire

taken with ARL

94-GHz radiometer at

a range of 4 m. Dark

vertical structures are

due to vertical wooden

pieces of fixture. Wire

can be seen as a thin

white line across

middle of scene.

Figure 4. Left graph

shows vertical profile

of brightness

temperature for both

11/16-in. wire and

background versus

zenith angle. Right

graph shows

difference between

two plots.

Figure 5. Left graph

shows vertical profile

of brightness

temperature for both

1/2-in. wire and

background versus

zenith angle. Right

graph shows

difference between

two plots.
5

The left-hand graph of each figure shows the vertical profile of the brightness temperature of each scene versus the angle from zenith. These graphs

have two plots; one is the profile with the wire present and the other is the

profile without the wire. The wire was located at a zenith angle around 88°

and is seen in the figures to have a lower brightness temperature than the

background. This is more apparent in the difference graph on the righthand side of each figure. The background plots show that as the zenith

angle increases above 93° (3° below the horizon), the standard deviations

become larger. This is mostly because the radiometer is pointed at asphalt

at these angles. A relatively large variability in brightness temperature that

is often seen in asphalt is due to changes in solar loading and wind speed.

On days that the data were taken, it was variably cloudy and cold; therefore, the physical temperature of the asphalt was not stable. Table 2 presents a summary of all wire data as well as sky-temperature data. The wire

data shown are the maximum value of the difference between the background and the wire. The last two columns of the table show calculated

model results that will be described later.

The differences seen in the table from wire to wire are due to a combination

of a changing fill factor (the fractional area of the 3-dB spot occupied by the

wire), variations in the background, and a changing sky temperature. The

changing fill factor is obviously due to the varying wire size. The changing

background brightness temperature is due mostly to variations in its physical temperature. Despite all these variables, one can see some trends in the

data. The 1/2- and 11/16-in. wires show more contrast than the smaller

wires, as expected, but the 13/16-, 15/16-, and 17/16-in. wires do not. The

smaller than expected contrast for the 15/16-in. wire is likely due to attenuation caused by the outer polystyrene insulation around the cable. Additionally, it is likely that the 13/16- and 17/16-in. cables have a smaller than

expected contrast because of an increased sky temperature. What is more

significant, though, is that the contrast magnitudes are fairly small overall,

especially when compared to the amount by which the background changes

versus angle. If one studies all the background profiles taken during the

test (fig. 6), one can clearly see that the variation is quite large compared to

the signature of the wires. What is important to note, though, is that the

shape of the curves is very similar near the horizon. (The shape below the

horizon varies more as was discussed earlier.) Therefore, to detect wires, it

may be necessary to collect a running brightness temperature template of

the horizon during flight and look for changes in its shape to pull the small

wire signal out from the horizon clutter.
6

Table 2. Summary of all data collected and results of cylinder calculation.

Measured Calculated

brightness brightness

Background temperature temperature

Zenith sky brightness (background- Weighted sky (backgroundWire diameter temperature temperature wire) temperature cylinder)

inches (m) (K) (K) (K) (K) (K)

1/4 (0.00635) 48 262 4.3 111 3.0

5/16 (0.00793) 33 262 4.5 100 4.0

1/2 (0.01270) 56 260 7.0 115 5.7

11/16 (0.01746) 42 266 7.5 108 8.6

13/16 (0.02064) 79 265 6.4 133 8.5

15/16 (0.02381) 39 264 5.5 105 —

17/16 (0.02699) 89 257 4.3 138 10.0

Zenith angle (°)

84 86 88 90 92 94 96 98

Temperature (K)

230

235

240

245

250

255

260

265

270

Data run 1

Data run 2

Data run 3

Data run 4

Data run 5

Data run 6

Data run 7

Figure 6. Brightness

temperature of

background scene

versus zenith angle for

various background

data runs.
7

4. Comparison With a Cylinder Model

A simple model can be used to estimate the expected brightness temperature of metal cylinders of sizes that match those of the measured wires.

Imagine a perfectly reflecting metal cylinder that is much longer than the

radiometer’s antenna beam spot is wide and that runs across the middle of

the beam spot. If the diameter of the circular beam spot is much larger than

the diameter of the cylinder, then the shape of the wire can be considered

rectangular and the fraction of the spot filled by the cylinder (fill factor) can

be expressed as a ratio of the areas

 

f Rd

R

d

R =









= Θ

Θ Θ

π

π

2

4

2 . (1)

In this expression, f is the fill factor, d is the diameter of the cylinder, Θ is the

radiometer 3-dB beamwidth in radians, and R is the range from the radiometer to the cylinder. If the radiometer is about the same height above the

ground as the cylinder, then the top half of the cylinder will reflect MMW

energy from the sky and the bottom half will reflect MMW energy from the

ground (assuming other objects are not nearby, e.g., trees, buildings). Therefore, the contrast between a pixel that contains the cylinder and a pixel without the cylinder can be expressed

 

∆T b f s f =− + − b 



 



 



 



 2

1

2 * , (2)

where b is the background-brightness temperature without a wire present

and s is the sky temperature. The fill factor, f, has been divided by two because only half the cylinder reflects energy from the sky. The skybrightness temperature increases as a cosine function from the zenithbrightness temperature to the horizon-brightness temperature. Therefore,

the s used in this equation is a cosine-weighted average of those two values,

since the cylinder reflects all sky angles toward the radiometer. Table 2 shows

these values. Inserting equation (1) into equation (2) gives

 

∆

Θ

T bs d

R = − ( ) 2

π . (3)

Equation (3) shows the expected 1/R dependence of the contrast and a linear dependence on wire diameter that is valid for d << ΘR. Note that signal

path attenuation has been ignored, since it is negligible at the ranges

involved.

Table 2 shows the calculated ∆Ts for the cylinders along with the measured

data for the same-sized wire. We made each calculation for a particular cylinder size, sky-brightness temperature, and background-brightness temperature, and the values we used are shown in the table. We made the
8

calculations with the use of the same range and antenna size as the measurements. Several points can be made about the calculated data. One is that

the brightness temperature differences do not go up linearly with wire diameter, as equation (3) would seem to imply since b and s were not constant. This is simply because the sky and background temperature values

used in the model were varied with cylinder size according to the measured values (e.g., the 1/2-in. cylinder calculation uses the 1/2-in. wire skybrightness temperature data). The 11/16- and 13/16-in. cylinders show about

the same signature despite their size difference because a warmer sky temperature was used for the 13/16-in. cylinder calculation as shown in the

table. The 15/16-in. wire had an attenuating outer layer of polystyrene that

was not modeled, so no calculated results are given for it. In general, though,

the model results trend upward with increasing cylinder size as expected.

A comparison of the model with the calculated values, however, shows significant differences. The measured wire data show no meaningful trends

with wire size, which can be explained by one difficulty with the measurement technique. As figure 6 shows, the background brightness temperature

can vary appreciably between the time the scene is measured with the wire

and the time the scene is measured without it. If nothing changed during

that period of time, then the corresponding background and wire curves

should lie on top of one another except at angles where the wire is present.

Clearly this is not always the case. Figure 4 shows a good correspondence

between the wire and background data, but figure 5 does not. In effect, this

changing background adds about 5 K of noise to the ∆T measurement. This

means that because of the way the data were collected, they are not sensitive enough to determine the effect of diameter on the MMW brightness

temperature of a wire. The data do show, however, the order of magnitude

of the signal that can be expected under the test parameters given here. This

conclusion is supported by the fact that the cylinder calculations are close

to the measured data.

Given the magnitude of the brightness-temperature differences shown by

table 2 and the fact that the data were taken at a 4-m range with a 3-in.

antenna, we can extrapolate the data to longer ranges and larger antennas.

These data provide a rough measure of what is possible with this type of

sensor at ranges of interest. Based on the data in this report, figure 7 shows

the temperature differences that would be expected versus range for a radiometer with a 3-ft-diam antenna. After recalculating the fill factor, we see

that the measured values have simply been extrapolated out in range. The

curves follow the expected 1/R dependence evident in equation (3). The

graph shows that, under the environmental conditions of the measurement,

the brightness-temperature change caused by a wire is on the order of 1 K

between 200 and 400 m. A radiometer would require a sensitivity of about

0.2 K to detect this change, which is realistic for a W-band radiometer that

has a 30-ms integration time.
9

A significant challenge in the use of an MMW radiometer to detect wires is

clearly the variation and fluctuation of the background. The data show that

the background temperature changes by ±4 K at angles, near the horizon.

This problem is complicated by the fact that an airborne radiometer may be

viewing the horizon scene from a variety of angles, depending on its orientation. The simplest way to solve this problem may be to have the sensor

continuously take data of the horizon during flight and store an averaged

template to which each new piece of data could be compared. Testing this

idea would require either sophisticated modeling of this scenario or flight

testing.

Before this effort is undertaken, however, one should thoroughly address

the issue of varying weather. The data in this report show qualitatively the

effect of a changing sky-brightness temperature on the wire-brightness temperature. Larger wires sometimes showed lower than expected contrast because of a higher sky-brightness temperature. Overall, though, the sky temperatures measured during this test were fairly low. It is not unusual to have

zenith sky temperatures of 125 to 175 K on humid summer days. This would

tend to reduce wire contrast. On the other hand, a humid summer day would

raise the background-brightness temperature and therefore tend to increase

wire contrast. The seasonal effects on contrast should certainly be considered more thoroughly to assess the range of contrasts that might be

encountered.

It should also be noted that no sophisticated image processing has been

applied to the data. Some algorithms such as maximum entropy and 2-mu

have demonstrated an ability to enhance contrast for some types of objects

in an image. Future work could involve trying these or similar algorithms

to determine if a contrast improvement can be made that might extend the

usable range of a radiometric sensor.

Figure 7. A calculation

of expected wire

contrast versus range.

Curves are a

calculated

extrapolation based on

data collected for each

wire. Radiometer

antenna diameter was

taken to be 3 ft.

Range (m)

100 200 300 400 500

Background pixel

–wire pixel (K)

0

1

2

3

4

5

6

7

8

1/4 in.

5/16 in.

1/2 in.

11/16 in.

13/16 in.

15/16 in.

17/16 in.
10

5. Conclusions

We have presented the results of 94-GHz radiometric measurements of

powerline wires. We collected data on wires of a variety of diameters and

compared them to the background of each wire. The background data

showed a large variation in brightness temperature around the horizon,

but measurable differences were detected when a wire was present in the

scene. We made calculations to determine the MMW brightness temperature of cylinders under the same conditions as those of the wire experiment. The results showed that such a method comes close to predicting the

magnitude of the actual wire data. We extrapolated the wire data to a longer

range scenario simulating a radiometer with a 3-ft antenna. The extrapolated data show that wire-to-background contrast is marginally sufficient

for detection out to ranges of 400 m. The use of an 8-dB signal-to-noise ratio

criterion shows that reliable detection would occur out to about 200 m. We

can conclude that individual wires down to 1/4 in. in diameter can probably be detected reliably at ranges less than 200 m with a 94-GHz radiometer that uses a 3-ft antenna, with the condition that the MMW emission

of the horizon background can be characterized during flight. Beyond 200 m,

the results show that an image-enhancement algorithm or a larger diameter

antenna will probably be necessary to reliably detect most standard

powerline wires. The possibility exists, however, that groups of wires could

be detected reliably at greater ranges (>400 m) because of the increased pixelfill factor. Since wires are usually found in groups of two, three, or more,

this scenario should be considered in future measurements. Additional

measurements with the use of polarimetry could also be considered to explore possible improvements in wire-to-clutter ratio.
11

References

1. N. A. Salmon, “W-band real-time passive millimeter-wave imager for helicopter collision avoidance,” Aerosense 99, Proc. SPIE 3703 (April 1999),

pp 28–32.

2. P. Cerchie, B. Shipley, R. Aust, J. Wasson, and D. Reago, “Manned simulation results concerning design parameters for an effective obstacle avoidance system (OASYS),” J. Am. Helicopter Society 37, No. 2 (April 1992),

pp 3–10.

3. R. J. Grasso, A. C. Pratty, C. M. Vann, C. G. Stimson, and J. E. Ackleson,

“OASYS laser radar characterization of natural and manmade terrestrial

features,” Proc. SPIE 3870 (April 1999), pp 620–631.

4. L. D. Almsted, R. C. Becker, and R. E. Zelenka, “Affordable MMW aircraft

collision avoidance system,” Proc. SPIE 3088 (April 1997), pp 57–63.

5. D. Wikner and T. Pizzillo, “Measurement of nadir and near-nadir 94-GHz

brightness temperatures of several tactical scene clutter materials,” Aerosense

99, Proc. SPIE 3703 (7 April 1999), pp 102–113.

13

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NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Radiometric Measurements of Powerline Cables at 94

GHz

February 2001 Final, 1 Jan to 31 Jan 2000

When pilots are flying at low altitudes, they need a sensor that can help them detect and avoid

wires; this need remains a high priority for all U.S. military services. Many different sensors have

been considered to fulfill this need. This report presents data that were collected on powerline

wires of various diameters with the use of a 94-GHz radiometer. These measurements were

conducted at short range in an effort to quantify the wire signatures and to determine if a

millimeter-wave radiometer could be used to help pilots avoid wires. Data are presented for seven

sizes of wire as well as for the horizon background of each wire. The results show that wires down

to 1/4 in. in diameter can probably be detected reliably at ranges up to 200 m with a 94-GHz

radiometer that uses a 3-ft antenna, provided that the signature of the horizon background can be

characterized during flight. Beyond 200 m, the results show that an image-enhancement

algorithm, if proven to be adequate, or a larger antenna will probably be necessary to reliably

detect most standard powerline wires.

Radiometry, millimeter-wave, collision avoidance, powerline

cables

Unclassified

ARL-TR-837

0NEMHH

622120.142

A142

62120A

ARL PR:

AMS code:

DA PR:

PE:

Approved for public release; distribution

unlimited.

19

Unclassified Unclassified UL

2800 Powder Mill Road

Adelphi, MD 20783-1197

U.S. Army Research Laboratory

Attn: AMSRL-SE-RM wikner@arl.army.mil

U.S. Army Research Laboratory

15

David A. Wikner and Thomas J. Pizzillo

email:

2800 Powder Mill Road

Adelphi, MD 20783-1197