Research relative to weather radar measurement techniques, which involves some investigations related to measurement techniques applicable to meteorological radar systems in Thailand, is reported. A major part of the activity was devoted to instruction and discussion with Thai radar engineers, technicians, and meteorologists concerning the basic principles of radar meteorology and applications to specific problems, including measurement of rainfall and detection of wind shear/microburst hazards. Weather radar calibration techniques were also considered during this project. Most of the activity took place during two visits to Thailand, in December 1990 and February 1992

Smith, Paul L.

English

NASA Technical Reports Server

NASA-CR-190637
SDSMT/IAS/R-92/08
RESEARCH RELATIVE TO WEATHER
RADAR MEASUREMENT TECHNIQUES
August 1992
By: Paul L. Smith, Principal Investigator
Prepared for.'
National Aeronautics and Space Administrati0n
Goddard Space Flight Center
Greenbelt, MD 20771
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Final Technical Report on Grant No. NAG 5-1477
'od Covered: 15 November 1990 - 14 August 1992
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Institute of Atmospheric Sciences
South Dakota School of Mines and Technology
501 E. St. Joseph Street
Rapid City, South Dakota 57701-3995
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SDSMTIIASIR-92108
RESEARCH RELATIVE TO WEATHER
RADAR MEASUREMENT TECHNIQUES
By: Paul L. Smith, Principal Investigator
August 1992
Prepared for:
National Aeronautics and Space Administration
Goddard Space Flight Center
Greenbelt, MD 20771
Final Technical Report on Grant No. NAG 5-1477
Period Covered: 15 November 1990- 14 August 1992
Institute of Atmospheric Sciences
South Dakota School of Mines and Technology
501 E. St. Joseph Street
Rapid City, South Dakota 57701-3995

TABLE OF CONTENTS
1. INTRODUCTION ........................................ . . . . ... 1
2. SUMMARY OF PROJECT ACTIVITIES ................................... 2
2.1 Details of Visits to Thailand ........................................ 2
2.2 Facilities at Phuket .................................................... 2
2.3 Specific Activities and Findings ................................... 3
REFERENCES .............................• , .................... • ....................... 9
APPENDIXA ............................................................... .. ... A1
APPENDIX B ................................. B 1
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111 PRECEDING PAGE BLANK NOT FILMED
LIST OF TABLES
Numb¢r Title
Test Equipment Needed for Radar
System Calibration .................................................... 7
iv
1. INTRODUCTION
This is the final technical report on NASA Grant No. NAG 5-1477,
involving research relative to weather radar measurement techniques. This
grant provided for some investigations related to measurement techniques
applicable to meteorological radar systems in Thailand. Quality data are
needed from those systems to support the Tropical Rainfall Measuring
Mission (TRMM) and other scientific investigations. A major part of the
activity under the grant was devoted to instruction and discussion with
Thai radar engineers, technicians, and meteorologists concerning the
basic principles of radar meteorology and applications to specific problems,
including measurement of rainfall and detection of wind shear/mlcroburst
hazards. Weather radar calibration techniques were also considered during
this project. Most of the activity took place during two visits to Thailand,
in December 1990 and February 1992.
2. SUMMARY OF PROJECTACTIVITIES
2.1 Details of _ to Thailand
Each trip was about two weeks in duration, with most of the time
being spent at the Thai Meteorological Department (TMD) facilities near the
Phuket airport. Visits to TMD headquarters and the facilities of the Royal
Rainmaking Research and Development Institute in Bangkok also occurred
during the second trip.
For the first visit, the principal investigator departed from Rapid City
on 5 December 1990 and arrived in Phuket on 8 December. Visits to the
local weather radar site and instruction/discussion sessions occurred over
the period 9-15 December. Dr. Daniel Rosenfeld of the Hebrew University
of Jerusalem and Mr. Curt Hartzeli of the U.S. Bureau of Reclamation also
participated in those sessions. Return travel to the U.S. took place on
17-18 December.
For the second visit, the principal investigator departed from Rapid
City on 28 January 1992 and arrived in Bangkok in the early morning hours
on 31 January. He and Dr. Rosenfeld presented a seminar on operating
strategies for meteorological radars to officials of the Thai Meteorological
Department on the morning of 3 February. Travel to Phuket occurred
on the afternoon of 3 February, and visits to the local radar site and
instruction/discussion sessions took place over the period 4-12 February.
Mr. Hartzell again participated in the early parts of this visit, and
Dr. Rosenfeld was involved over almost the whole period. The return
trip to Bangkok, and a visit to the Royal Rainmaking Research and
Development Institute, occurred on 13 February, and the return trip to
the U.S. took place on 14 February.
2.2 Facilities at Phuket
The weather radar at Phuket is an Enterprise Electronics Corporation
(EEC) DWSR-88S S-band Doppler system with the following principal
characteristics:
Peak transmitted power
Antenna diameter
Pulse duration
Pulse repetition frequencies
500 kW
6.1 m (beamwidth approximately 1.25 °)
2 ps (reflectivity); 0.8 ps (velocity)
250 s-1 (reflectivity); 625/935 s-1
(velocity)
2
The antenna and transmitter/receiver units are located on a hilltop a few
kilometers away from the Phuket airport. An S-band airport-surveillance-
type radar (ASR) with similar power output is located only 50 m away from
the weather radar installation. This is a dual frequency/dual feed system
which would be expected to cause some interference with the operation
of the weather radar system.
The weather radar operator consoles are located in a tower room
a few kilometers away, adjacent to the Phuket airport, with the radar data
being sent by microwave relay from the antenna location to the operator
console. Most of the signal processing takes place at the radar site proper,
and only the products are remoted down to the console. Thus one cannot
view the actual radar signals themselves at the airport workstation. The
data processing there occurs on MicroVAX computers using the EEC-
furnished Interactive Radar Information System (IRIS) software.
There is a classroom-type facility (which was still under construction
at the time of the first visit) in a building adjacent to the tower housing the
radar operator consoles. The lecture/discussion sessions were held in this
room. During the first visit, there was no provision for darkening the room to
permit showing of 35-mm slides. The physical working arrangements were
much improved during the second visit, and we were also granted freer
access to the innards of the radar system for calibration measurements.
A printed monograph containing the basic background materials
for the presentations and discussions was distributed to all participants.
Appendix A contains a copy of the cover page of this document. The
copies of the monograph arrived late during the first visit, which com-
plicated the proceedings to some extent. Also, some copies of a supple-
mental document containing additional material on radar calibration
methods and rainfall measurement techniques were distributed during
the second visit.
2.3 Soecific Activities and Findinas
a. Group sessions: During the first visit, five formal sessions with
the Thai personnel were conducted on a lecture/discussion basis. About
25 people participated in all or part of these sessions. The topics for the
sessions were as follows:
1. Radar Fundamentals
2. Radar Equations
3. Doppler Principles
4. Z-R Relationships and Rainfall Measurements
5. Radar System Calibrations and Echo Pattern Analysis
/
3
Dr. Rosenfeld conducted a similar number of sessions dealing with cloud
and precipitation physics, storm structure, echo pattern interpretation,
and rainfall measurements.
The level of understanding of the discussions among the various
Thai personnel seemed to be quite uneven. This was probably due to a
combination of factors, including some difficulties with the English language
as well as the problem of dealing with a mixture of meteorologists, radar
engineers, and technicians in a single group.
During the second visit, six formal half-day sessions with the
Thai scientists, engineers, and technicians were again conducted on a
lecture/discussion basis. About 33 people, Including several who had
participated during the earlier visit, were present for much of these
sessions. Topics for the sessions were generally the same, but the
coverage was broadened, additional details were introduced, and radar
system calibration issues were covered in much greater depth.
Dr. Rosenfeld again conducted a similar number of sessions on topics
inclUding Cloud and precipitation physics, storm structure, echo pattern
interpretation, and rainfall measurement. As this was the second time
through much of this material for several of the participants, the level of
understanding appeared to be much enhanced. The presence of three
men who had recently completed M.S. programs at U.S. universities
was no doubt beneficial in this regard.
b. Workshop sessions in radar operations room: It was intended
to review some archived radar data as a means of illustrating some of the
points under discussion. The radar data display system at Phuket includes
a playback capability, but its operation is rather slow. Therefore, it was
important to set up in advance some example cases for examination during
the workshop sessions. An attempt was made to do this during the first
visit, but the limited time available hampered the effectiveness of this
approach. Moreover, little locally-recorded data was available for the
purpose; fortunately, a good set of evening storms occurred during the
visit to provide additional data for discussion. Much of the example data
available had been collected by the University of North Dakota radar at
Huntsville, Alabama; the recorded data included examples of a microburst
and second-trip echoes, which were quite useful. The presence of some
anomalous-propagation cases in the examples available for illustration
would have been helpful. The radar operations room was small and could
only hold about half the group at one time, so repeat sessions had to be
held to accommodate everyone.
During the second visit, Dr. Rosenfeld used the data playback
capability to recall the recorded radar data from a small squall line which
4
we had observed during the previous visit. The data were still resident
in the computer system, and with some effort could be recalled for the
purposes of this demonstration. The observations illustrated several of the
basic principles of echo pattern interpretation that were discussed in the
group sessions.
During the first visit, the solar microwave signal was visible on some
of the RHI displays; this was useful because it gave a direct indication of the
operation of the range-normalization function. It was also possible to make
at least a rough estimate of the antenna beamwidth from looking at the dis-
played solar signal. In addition, this observation suggested the possibility
of using the solar signal for antenna orientation checks, and perhaps even
for antenna gain measurement.
During the second visit, we found the ANTENNA and A-SCOPE
utilities in the IRIS system to function quite satisfactorily for conducting
solar scans to check the orientation of the weather radar antenna. Such
scans were carried out on the afternoon of 7 February and the morning of
11 February. Mr. Hartzell supplied a computer program for determining the
true solar azimuth and elevation angles. Results of an analysis of the solar
observations showed that the antenna angle indications coming from the
Phuket weather radar were in error by about + 10 o in azimuth and + 1 e in
elevation. The table in Appendix B provides a synopsis of the observations
leading to this conclusion. We were not successful in obtaining permission
to take the necessary steps to remove the errors from the indicated angles,
but hopefully this was later taken care of by local personnel.
Usually a PPI Doppler velocity display tends to show weaker echoes
than are visible on the corresponding reflectivity display, because velocity
patterns can often be recognized even in _clear-air" situations when the
echoes are otherwise too weak to be identified. This occurs largely because
of the increase in sensitivity associated with the Doppler processing of the
echoes (Smith, 1986). In the Phuket radar, there seemed to be some thres-
hold limit in effect that screened out most of this "clear-air-type" echo so
that it did not show up on the velocity display. This is probably an adjust-
able threshold in the radar system or in the software, but we did not succeed
in pinpointing the source of this threshold or modifying the system to get rid
of it during the visits. It is also possible that some kind of radar system
coherence problem may be contributing to this difficulty.
c. Radar site activities: During the first trip, the visit to the radar
site itself was interesting but somewhat nonproductive. Access to even a
simple A-scope display could not be obtained at the site because of the
Thais' reluctance to do anything in the face of the perceived warranty
5
limitations in effect. Thus it was not practical to review the radar
calibration procedures in any detail.
During the second trip, visits to the radar site were much more
productive. We were allowed to conduct several basic radar calibration
exercises, being limited mainly by the test equipment available. Table 1
lists the equipment needed for a reasonably complete set of calibration
measurements; however, not all of the equipment listed turned out to be
available in the field at Phuket. Also, work inside the radome at Phuket is
not possible unless the neighboring ASR can be shut down for the time
period required. We could not arrange to have the ASR turned off to
permit such work inside the radome. Communications from the radar
site to the operator console were quite good.
Smith (1977) contains an overview of most of the necessary
calibration procedures. Some difficulties arose in trying to adapt those
procedures to the radar in Thailand. For example, the IRIS system had to
be incorporated into the methodology; this turned out to be fairly success-
ful. The hilltop location presents a difficult challenge to use of the standard-
gain-horn or calibration-sphere methods for measuring antenna gain, and
the solar alternative would appear to be most promising. However, the
suitability of the DWSR-88S system configuration for solar antenna gain
measurements is uncertain.
We were able to conduct measurements of the transmitted power,
and noticed that the peak power output appeared to be somewhat below
the nominal value (but not enough to be of concern). The lack of a Type N
barrel adapter made it impossible to check the test cables to be sure that
the loss values were correct. No frequency meter was available for deter-
mining the transmitted frequency. For the linear receiver, the signal-
generator test indicated about a 30 dB dynamic range, which is quite
reasonable. The ZAUTO utility worked quite well for calibration of the
logarithmic receiver, although the system configuration was such that
we could not drive it all the way to the saturation point. Apparently, this
was the first time that such a calibration had been made on the system
at Phuket; the operators in charge were willing to incorporate the new
calibration data into their operational routine. A check of the VSWR on the
antenna showed that to be satisfactory. No test equipment was available
for determining the antenna gain, or for examining the duration and shape
of the transmitted pulse.
During the first visit, there was some discussion of a receiver problem
which was leading to periodic dropouts of the Doppler velocity data. The
problem was evidently related to the gating of samples of the transmitted
pulse for the purpose of locking the STALO. It may be that the locking
6
TABLE 1
Test Equipment Needed for Radar System Calibration
a. Instruments
bo
Cl
d,
1) Microwave power meter with power sensor
2) Frequency meter or counter
3) Microwave signal generator
4) Oscilloscope
5) Coaxial attenuator set (3/6/10/20 dB)
6) Crystal detector
7) Multimeter
Cables and Connectors
1) RF cable (RG-214, RG-9 or equivalent) about 1.5 - 2 m long with
Type N (M) connectors each end
2) Type N barrel adapter (F/F)
3) Several IF cables 1.5 - 2 m long, with BNC M connectors each
end. Cable impedance should match the radar system video
output impedance
4) Miscellaneous BNC connectors (T's, F/F adapter, etc.)
5) BNC termination matched to the video output impedance
Other Items
a) A small compass
b) A plumb line that can be hung over the antenna
c) A gunner's quadrant, or equivalent clinometer which can be
clamped onto the antenna support structure
d) A computer program to give sun position in horizon coordinates
(azimuth, elevation)
e) A digital clock or watch that can be set to UTC (via WWV radio
receiver, or whatever)
Also Desirable
a) An S-band standard-gain horn with waveguide-to-coax adapter
b) A small spirit level
c) Portable radios of some kind for two-way communication over
distances of a few kilometers
7
pulse, which is only available for the 0.8 ps of the transmitted pulse in the
Doppler mode, is not adequate for discriminator operation. The problem
could also be related to difficulties with the AGC applied to the linear
receiver for the Doppler processing of the echoes. Without suitable test
equipment and extensive time to troubleshoot the problem, it could not be
definitely resolved. However, it does point to the possibility of a general
coherency problem.
d. Other comments: The presence of the ASR radar so near the
weather radar site raises concerns, because at those power levels the
filter/limiter arrangement in the weather radar is not likely to provide
adequate receiver system protection. Synchronization of the transmitted
pulses might be possible, but the radar PRF's are not well matched (the
ASR has a nominal 880 s -1 PRF, which does not match any of the standard
weather radar PRF's noted above). However, no conspicuous indications
of a significant problem were present in the products displayed at the
operator's consoles.
Sea clutter in some of the recorded data had managed to pass
through the clutter filter because of the fact that it had non-zero Doppler.
8
REFERENCES
Smith, P. L., Jr., 1977: Evaluation of Miles City SWR-75 Weather Radar.
Report 77-1, Institute of Atmospheric Sciences, S.D. School of Mines
and Technology, Rapid City, SD. 55 pp.
Smith, P. L., 1986: On the sensitivity of weather radars. J. Atmos.
Oceanic Tech., 3, 704-713.
9

NASA
National Aeronautics and
Space Administration
APPENDIX A
Fundamentals of Weather Radar
Part I and H
Paul L Smith
South Dakota School of Mines and Technology
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