WMOD00521

• Project Name: Project Skywater
• Title: Project Skywater SWR-75 Radars
• Date: 3/28/1978
• Weather Modification - Doc Type: Report

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A complete system calibration is performed on
each of the radars at the beginning and end of
each operational season. This calibration con-
tlsts of a measurement of the effective antenna
stem gain using the method described by
ith (1974), The indicated value of effective
antenna system gain is 43.7 dB (Smith, 1977).
This figure incorporates all waveguide, mis-
match. and similar losses beyond the directional
coupler as well as any radome loss (Smith.
1977). It also incorporates a Pre-. Mid-, and
Post-season antenna boresighting, elevation and
azimuth error determinations and corrections
using solar evaluations. These checks consist
of measuring with a gunner's quadrant the tilt
~rom the vertical of the azimuth rotation axis
of the antenna. The procedure used is an
adaptation of the Air Weather Service gunners'
quadrant procedure (Smith, 1977). Elevation
boresighting is accomplished by a solar method
of using the gunners' quadrant, a plumbline, and
solar azimuth and elevation data obtained from
a computer program called "Angles" that is
available on the Bureau of Reclamation Environ-
mental Data Network. Details of the solar
method for boresighting the antenna are given
in Section 4 of Whiton ~ al (1976). Boresight
errors as determined by this method are typi-
cally between 0.010 and 0.060 (Smith, 1977).

The azimuth orientation of the antenna is
checked by a solar method similar to that used
for elevation boresighting. The azimuth
orientation error is determined by noting the
differences between the antenna azimuth dial and
the true solar positions, The azimuth orienta-
tion error is determined as the average of a
series of 20 repeated measurements. The eleva-
~on orientation is accomplished by comparin~
~e elevation indications with the true solar
elevation angles. The details of this procedure
are outlined in the paper by Whiton ~ al (1976).

The weekly DVIP calibration consists basically
of injecting RF test signals of specified power
levels through the directional coupler into the
receiver. The offset and gain variable
resistors of the DVIP are set so that the given
test signals just trigger the appropriate DVIP
level thresholds. The test signals are varied
to determine the triggering thresholds of all
the DVIP levels, and the results are recorded
both on tape in DVIP units and on the calibra-
tion sheet in dBm. The total procedure is
described in Smith (1977), which is included in
the Operations Plans of each of the sites.

The data tape with the recorded DVIP values is
processed and the DVIP unit values for each of
the levels are combined with the recorded dBm
values in a computer program known as CALSUB
which resides in the Environmental Data Network.
This program calculates third-order equation
coefficients for a polynomial regression
analysis of DVIP/dBm calibration data. The
third order equation is of the form:
Y = A + B(X) + C(X2) + D(X3), where X is the
DVIP value and Y is the corresponding estimated
dBm value. These coefficients are then used in
the computer processing to determine reflectivity
values.

e

The daily checks conducted by the electronic
technician consist of measuring those values
needed for calculation of Ze in the Operational
torm of the Probert-Jones radar equation:

Ze

1024 dn2
, 'w!

PRF -1--
(aver. power)(Freq)2 g2e~

rz. Pr
/k12

The technician measures daily: '(1) PRF,
(2) Average Power, (3) Transmitter Frequency,
(4) Minimum Discernible Signal, and (5) checks
three levels of t~.DVIP using the procedure
described above. If the DVIP varies by more
than 1 dBm from the weekly calibration, a com-
plete recalibration is performed. The stability
of the DVIP during the 1977 seasons was such
that they did not require recalibration, and
calibration data indicate that the DVIP varied
only by 2 DVIP units (2/3 dB) over the season.

The technicians rigorously apply these calibra-
tion procedures, and their dedication to pro-
viding the most quantitative data possible has
resulted in an extremely high percentage of
on-the-air time at all sites.

References

Smith, P.L., Jr., 1974: The measurement of
antenna gain in radar systems,
Microwave J., 17, 4 (April), 37-40.

. 1977: Evaluation of Miles City
SWR-75 weather radar. Report 77-1,
Institute of Atmospheric Sciences, South
Dakota School of Mines and Technology,
April.

Whiton, R.C., P,L. Smith, Jr., and A.C. Harbuck,
1976: Calibration of weather radar
systems using the sun as a radio source.
Preprints, 17th Radar Meteorology Conf.,
Seattle, Washington, Amer. Meteor. Soc.,
60-65.