The National Oceanic and Atmospheric Administration (NOAA) Wave Propagation Laboratory (WPL) presently operates five dual-channel microwave radiometers, one triple-channel microwave radiometer, and one six-channel microwave radiometer. The dual-channel radiometers operate at frequencies of 20.6 or 23.87 GHz and 31.4 or 31.65 GHz. The triple-channel radiometer operates at 20.6, 31.65, and 90.0 GHz. The six-channel radiometer operates at frequencies of 20.6, 31.65, 52.85, 53.85, 55.45, and 58.8 GHz. Recent brightness temperature measurements and attenuation values from some of the above radiometers are presented. These radiometric measurements, taken in different locations throughout the world, have given WPL a diverse set of measurements under a variety of atmospheric conditions. We propose to do a more complete attenuation analysis on these measurements in the future. In addition, a new spinning reflector was installed recently for the dual-channel radiometer at the Platteville, Colorado site. This reflector will extend our measurement capabilities during precipating conditions. Locating the three-channel and portable dual-channel radiometers at or near Greeley, Colorado to support the Advanced Communications Technology Satellite (ACTS) program is discussed

Jacobson, Mark D.

Snider, J. B.

Westwater, E. R.

English

NASA Technical Reports Server

N94-14660
Radiometric observations of atmospheric attenuation at
20.6 and 31.65 GHz: The Wave Propagation Laboratory data base
M. D. Jacobson, J. B. Snider, and E. R. Westwater
NOAA/ERL/Wave Propagation Laboratory
Boulder, Colorado 80303
Abstract
The National Oceanic and Atmospheric Administration (NOAA)
Wave Propagation Laboratory (WPL) presently operates five dual-
channel microwave radiometers, one triple-channel microwave
radiometer, and one six-channel microwave radiometer. The dual-
channel radiometers operate at frequencies of 20.6 or 23.87 GHz and
31.4 or 31.65 GHz. The triple-channel radiometer operates at 20.6,
31.65, and 90.0 GHz. The six-channel radiometer operates at
frequencies of 20.6, 31.65, 52.85, 53.85, 55.45 and 58.8 GHz.
Recent brightness temperature measurements and attenuation values
from some of the above radiometers are presented here. These
radiometric measurements, taken in different locations throughout
the world, have given WPL a diverse set of measurements under a
variety of atmospheric conditions. We propose to do a more
complete attenuation analysis on these measurements in the future.
In addition, a new spinning reflector was installed recently for
the dual-channel radiometer at the Platteville, Colorado site.
This reflector will extend our measurement capabilities during
precipating conditions. We also discuss locating the three-channel
and portable dual-channel radiometers at or near Greeley, Colorado
to support the Advanced Communications Technology Satellite (ACTS)
program.
I. Introduction
Between 1985 and 1988, WPL operated a limited network of four
dual-channel microwave radiometers in northeastern Colorado. These
radiometers, operating at 20.6 and 31.65 GHz, were deployed for
meteorological purposes to measure precipitable water vapor (PWV)
and integrated cloud liquid water (CLW) . Data from ground-based
microwave radiometers are also commonly used to derive attenuation
statistics (Ortgies et al. 1990; Vogel et al. 1991; Fionda et al.
1991) useful to communication engineers. Using the radiometric
technique, these authors have shown that attenuation levels up to
about 12 dB can be derived with good accuracy.
Data from two of the Colorado stations, Denver and
Platteville, have previously been used to derive single-station and
two-station attenuation statistics for two 3-month periods during
1988 (Fionda et al. 1991). Recently, Westwater et al. 1992,
greatly extended that work by deriving attenuation and attenuation-
diversity statistics for an entire year's data taken between 1
November 1987 and 31 October 1988 at each of the four stations of
61
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the Colorado network. We present the results of some new
brightness temperature and attenuation measurements here.
A promising technique for measuring attenuation in precipating
conditions is also discussed. This technique involves spinning the
exposed reflector. This will help to minimize contaminated
atmospheric data caused by the wet reflector (Jacobson et al. 1986)
during precipating conditions.
Future attenuation studies that are related to the ACTS
program are addressed.
Finally, WPL has also recently developed an airborne dual-
channel radiometer which can view up and down (Fedor et al. 1988).
The first experiment for this instrument will occur in September
1993, near San Clemente Island, California.
II. Description of Radiometers in WPL
A. Radiometer characteristics
WPL's radiometers were designed, constructed• and field-tested
by WPL (excluding the portable radiometer which was designed and
built by Radiometrics Corporation); a complete description of the
systems is given by Hogg et al. (1983) and Westwater et al. (1987).
A sketch of the triple-channel system is shown in Fig. i. The WPL
instruments were designed to operate continuously• unattended, in
almost all weather conditions. The salient characteristics of the
instruments are shown in Table I. Note that the antenna beamwidths
for the Denver radiometer, the three-channel radiometer, and one of
the transportable radiometers differ from those of the other four
stations. Field experiments (Snider et al. 1988), in which the
three-channel radiometer with a beamwidth of 2.5 degrees was
compared with the network radiometers with their beamwidths of 5
degrees, showed a correlation of 0.99 between the systems. The
receivers of all seven radiometers are of the same construction.
The radiometers are internally calibrated by switching between the
antenna and two hot blackbody loads, and are externally calibrated
approximately every two weeks using the "tipping curve" method
(Hogg et al. 1983; Decker and Schroeder, 1991).
B. Methodology to derive attenuation from emission
The basic quantity measured by a radiometer is brightness
temperature, Tb, which is closely related to input power present at
the antenna (Ulaby et alo 1981). Although the probability
distributions of T b are of interest in themselves, the quantities
needed by communication engineers are the distributions of
attenuation. We derive the attenuation, Z(dB), from T b by using the
well-known formula (Westwater et al. 1990)
(dB) = 4.34 In{ (Tm - T=)/(T m - T b) }
where
T m = mean radiating temperature (K),
• (I)
62
and
T c = cosmic background temperature = 2.75 K.
In deriving _, we used monthly mean values of T m calculated from
our radiative transfer and cloud models.
C. Single-station attenuation statistics
Data are taken by radiometers that operate unattended,
although bimonthly, on-site calibrations are made. For the most
part, the data are of high quality, although occasional outliers
have to be removed. Such outliers can arise from liquid and ice
buildup on the antennas, spurious signals of electromagnetic
origin, calibration drifts in the receivers, and data transmission
errors.
After the quality control methods discussed by Westwater et
al. (1992) are applied to the data, cumulative distributions of
brightness temperature and radiometrically derived attenuation are
obtained for each station. An example of the four-station
composite values of the Colorado Research Network are shown in Fig.
2. It is interesting to compare these radiometrically derived
values of attenuation with forthcoming beacon measurements from
ACTS (Chakraborty and Davarian, 1991).
III. Recent Radiometric Data
A. Data from Norman, Oklahoma
We operated a dual-channel radiometer at Norman, Oklahoma from
February 1991 through February 1992 at frequencies of 20.6 and
31.65 GHz. Brightness temperature measurements for one month,
October 1991, are shown in Fig. 3. Attenuation statistics for this
period are currently being derived. An infrared radiometer, with
a wavelength of operation around 10.6 micrometers, was also
operated here. This instrument can sense clouds and is used to
estimate cloud base temperature. Initial tests have shown
agreement with radiosonde indications of cloud base temperature to
within a degree Celsius (Shaw et al. 1993).
B. Data from Coffeyville, Kansas
We operated the three-channel radiometer near Coffeyville,
Kansas from November through mid-December 1991 at frequencies of
20.6, 31.65, and 90.0 GHz. Attenuation measurements for three days
are shown in Fig. 4. Notice the high attenuation measured by the
90.0 GHz channel relative to the attenuation at the two lower
frequencies. This increased attenuation is due to the factor of
five increase in cloud liquid attenuation coefficient (Westwater et
al., 1990). An infrared radiometer, with a wavelength of operation
around 10.6 micrometers, was also operated here. Furthermore, we
operated our infrared spectrometer system (Shaw et al, 1991), known
as FIRS (Fourier-transform infrared _ounder), at this location.
63
This instrument measures the atmospheric temperature and humidity
profiles.
C. Data from Porto Santo, Portugal
This location provided attenuation measurements for the marine
environment in the north Atlantic Ocean. We operated the three-
channel radiometer on Porto Santo Island, Portugal, in June 1992.
One of the transportable dual-channel radiometersmade measurements
aboard the NOAA ship Malcom Baldridge in June 1992. This ship
cruised across the North Atlantic Ocean during this trip. The
radiometer was then taken off the Malcom Baldridge ship and located
on Porto Santo Island, Portugal in July 1992. It will be operating
there until the end of June 1993, The dual-channel frequencies are
20.6 and 31.65 GHz. Two days of attenuation measurements from the
three-channel system are shown in Fig. 5. As before, the 90.0 GHz
attenuation measurements are much greater than the other two
channels. Infrared radiometers, with a wavelengths of operation
around 10.6 micrometers, were also operated in these locations.
D. Data from Kavieng, Papua New Guinea
This location provided an extremely high water vapor
environment for the marine environment in the South Pacific Ocean.
We operated one of the transportable dual-channel radiometers in
Kavieng, Papua New Guinea during January and February 1993 at
frequencies of 23.87 and 31.65 GHz. Two days of these attenuation
measurements are shown in Fig. 6. In addition, we operated our
FIRS instrument at this location.
IV. Spinning r@fl@ctor
A dry reflector surface is required during precipating
conditions to avoid contamination of the atmospheric data (Jacobson
et al., 1986 and Stankov et al., 1992). Although a high velocity
air stream has been used successfully to prevent rain from falling
on small optical apertures of a few inches diameter, the technique
is impractical for the large apertures used by WPL the microwave
radiometers. An alternate solution is to spin the exposed
reflector so that particles falling on the rapidly rotating disk
will be thrown off, thereby reducing the liquid water layer buildup
on the exposed flat. The Desert Research Institute (DRI), and the
Commonwealth Scientific and Industrial Research Organization
(CSIRO), which is located in Melbourne, Australia, both have
implemented spinning disks on their radiometric systems. DRI has
found that the spinning reflector has been effective in shedding
liquid water an d snow accumulations (Demoz et al. 1993) .
We have recently designed and built a spinning flat reflector
for the Platteville radiometer. The exposed circular flat is
positioned at 45 ° with respect to the horizontal (see Fig. 7). The
flat reflector is 3 feet in diameter and is currently rotating at
300 revolutions per minute (RPM), although the speed can be
64
adjusted. A controlled experiment will be performed in the near
future to determine how well the spinning flat sheds water.
V. Future Plans
We propose to analyze and publish attenuation statistics for
the locations listed in Section III. In addition, we propose to
locate the triple-channel radiometer near Greeley, Colorado,
adjacent to the radar operated by the University of CHicago and the
ILLinois State Water Survey (CHILL), beginning in the fall of 1993
for a period not to exceed 1 year. We plan to locate the portable
dual-channel radiometer next to the ACTS receiver in Greeley,
Colorado, also in the fall of 1993 for a maximum of one year.
Measurements from this instrument would provide valuable
information for the ACTS project.
VI. Acknowledgements
The authors wish to thank Duane Hazen, Bill Madsen, Anthony
Francavilla, Mike Nunnelee and Paul Schmidt for their work on the
hardware and software for the various radiometric systems.
Referonces
Chakraborty, D., and F. Davarian, " The ACTS Propagation
Program," Proc. NAPEX XV, JPL Publ. 91-31, pp. 50-54, 1991.
Decker, M. T., and J. A. Schroeder, "Calibration of ground-based
microwave radiometers for atmospheric remote sensing,"
NOAA Tech. Memo. ERL WPL-197, 16 pp., 1991.
Demoz, B. B., A. W. Huggins, J. A. Warburton, and R. L. Smith,
"Field performance of a spinning-reflector microwave radiometer,"
J. Atmo$. Oceanic Tech., vol. I0, no. 3, pp. 420-427, 1993.
Fedor, L. S., M. D. Jacobson, A. J. Bedard, Jr., E. R. Westwater,
D. C. Hogg, and R. T. Nishiyama, "Dual-channel microwave radiometer
for airborne meterological applications," NOAA Tech. Memo. ERL WPL-
157, 1988.
Fionda, E., M. J. Falls, and E. R. Westwater, "Attenuation
statistics at 20.6, 31.65, and 52.85 GHz derived from emission
measurements by ground-based microwave radiometers," IEE Pro¢.-H,
vol. 138, pp. 46-50, 1991.
Hogg, D. C., M. T. Decker, F. O. Guiraud, K. B. Earnshaw, D. A.
Merritt, K. P. Moran, W. B. Sweezy, R. G. Strauch, E. R. Westwater,
and C. G. Little, "'An automatic profiler of the temperature, wind,
and humidity in the troposphere," J. Appl. Meteor_, vol. 22, pp.
807-831, 1983.
6S
Jacobson, M. D., D. C. Hogg, and J. B. Snider, "Wet Reflectors in
Millimeter-Wave Radiometry-Experiment and Theory," IEEE Trans.
Geosci. Remote Sensing, vol. GE-24, no. 5, pp. 784-791, 1986.
Ortgies, G., F. Rucker, and F. Dintelmann, "Statistics of clear air
attenuation on satellite links at 20 and 30 GHz," Electron.
Lett., vol. 26, pp. 358-360, 1990.
Shaw, J. A., J. H. Churnside, and E. R. Westwater, "An infrared
spectrometer for ground-based profiling of atmospheric temperature
and humidity," Proc. SPIE, vol. 1540, Infrared Tech. XVII, pp.681-
686, 1991.
Shaw, J. A, L. S. Fedor, "Improved calibration of infrared
radiometers for cloud temperature remote sensing," Optical
Engineering, vol. 32, no. 5, pp. 1002-1010, 1993.
Snider, J. B., "Verification of the accuracy of a network
of water-vapor radiometers," Proc. IGARSS '88 Symposium,
Edinburgh, Scotland, 13-16 September, 1988, pp. 19-20, 1988.
Stankov, B. B., E. R. Westwater, J. B. Snider, and R. L. Weber,
"Remote measurements of supercooled integrated liquid water during
WISP/FAA aircraft icing program," Jour. Aircraft, vol. 29, no. 4,
pp. 604-611, 1992.
Ulaby, F. T., R. K. Moore, and A. K. Fung, Microwave Remote
Sensing: Active and Passive; Volume I: Microwave Remote Sensing
Fundamentals and Radiometry_ Reading, MA: Addison-Wesley, 456 pp.,
1981.
Vogel, W. J., G. W. Torrence, and J. E. Allnutt, "Estimating
satellite beacon attenuation from radiometer data: error
statistics based on two year's low elevation angle measurements at
11.2 GHz," Proc. ICAP 31, York (UK), pp. 362-365, 1991.
Westwater, E.R.,and J.B. Snider, "Microwave Radiometer
Facilities at the Wave Propagation Laboratory," Proc. NAPEX XI,
Virginia Polytechnic Institute and State University, Blacksburg,
Virginia, pp. 24-27, 1987.
J. B. Snider, and M. J. Falls, "Ground-based radiometric
observations of atmospheric emission and attenuation at 20.6, 31.65,
and 90.0 GHz: A comparison of measurements and theory," IEEE Trans.
Antenna_ Propagat,, vol. 38, pp. 1569-1580, 1990.
M. J. Falls, and E. Fionda, "Brightness Temperature and
Attenuation Diversity Statistics at 20.6 and 31.65 GHz for the
Colorado Research Network," Proc. NAPEX XVI, Houston, Texas, pp. 81-
96, 1992.
66
Figure and Table Captions:
Fig. i. Side view of the transportable triple-channel radiometer.
Fig. 2. Composite brightness temperature (A) and zenith attenuation
(B) statistics for the four stations of the Colorado Research
Network. These measurements were taken between November 1987 and
October 1988 at frequencies of 20.6 and 31.65 GHz.
Fig. 3. Dual-channel radiometer observations at Norman, Oklahoma,
taken for the month of October 1991 at frequencies of 20.6 and 31.65
GHz.
Fig. 4. Triple-channel radiometer observations near Coffeyville,
Kansas. These measurements were taken during 1991, on (a) November
15, (b) November 19, and (c) November 21, at frequencies of 20.6,
31.65, and 90.0 GHz.
Fig. 5. Dual-channel radiometer observations from Porto Santo
Island, Portugal. These measurements were taken on (a) 15 June 1992
and (b) 27 June 1992, at frequencies of 20.6 and 31.65 GHz.
Fig. 6. Dual-channel radiometer observations at Kavieng, Papua New
Guinea. These measurements were taken on (a) 23 January 1993 and (b)
27 January 1993 at frequencies of 23.87 and 31.65 GHz.
Fig. 7. The spinning reflector structure, designed and built by WPL.
Table i. Characteristics of the WPL radiometers.
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Radiometric observations of atmospheric attenuation at 20.6 and 31.65 GHz: The Wave Propagation Laboratory data base

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