250 measurements occurred when the radar antenna pointed to
the RSMS van and the highest amplitude in the second set of
250 measurements occurred when the antenna had made one more
complete rotation. This means that the elapsed time for the
burst of 500 measurements should be about 20% greater than
the rotation time of the antenna and that the burst of measure-
ments should be started slightly before the mainbeam points
at the RSMS van. After all 500 measurements are made, the
analysis of the measurements begins. Various correction
factors are added to the measurements to give readings in
dBm. The highest amplitude in the first set of 250 measure-
ments is labeled "0°"; the highest amplitude measured in the
second group of 250 measurements is labeled "360°"; and all
the rest of the measurement points are assigned a rotation
angle by linear interpolation between these two fixed points.
If desired, the antenna patterns are plotted with the 0°
reading normalized to 0 dB, with the rest of the pattern
plotted in dB below mainbeam. For better plotting resolution,
the pattern is actually plotted in two graphs - one graph
between -20° and 180°, the other graph Letween 180° and

380°. These two graphs are intended to be pasted together
to get a single graph of the antenna pattern. (Figure G-7).

Along with the antenna pattern, other information is
written on the graph, including time, date, frequency, peak
received power, antenna rotation period, and some statistical
averages. The statistical averages describe the antenna
pattern in terms of average dB below the mainbeam gain. The
“Complete Average" is the level of the antenna pattern in dB
below mainbeam gain, averaging between 0° and 360°. The
“Sidelobe Average" is a similar average, except that the
pattern is averaged only from 3° to 25° on either side of
the mainbeam. The "Backlobe Average" is another similar
average, including only the backlobe region of the pattern
not within 25° of the mainbeam.

Instead of working with the amplitude as a function of
rotation angle, for some applications it is easier to work
with a statistical antenna pattern (Figure G-8). For this
function, the statistics of the antenna pattern are summarized
in a graph which plots amplitude in dB below mainbeam gain
versus percentage of the pattern which exceed this amplitude.
This kind of a pattern serves as a sort of figure-of-merit for
the antenna and indicates how much of the time the sidelobes
and backlobes of the antenna are likely to cause interference
to (or receive interference from) another radar. The analysis
of this data involves merely sorting the measured points of
the pattern (0 to 360°) into amplitude bins with 1 dB reso-
lution and plotting a cumulative distribution of the number
of measurements within each bin.

G-14


Pulse Blanking and AS-330 Pulse Train Separator

Since radar signals are very low duty, there is the
possibility of separating out only the signals of interest
by turning on the measurement system only during the instant
the signal of interest occurs. This process is accomplished
with the pulse-blanking equipment shown in Fiyure G-l. The
pulse-blanking card acts as a high-speed switch, connecting
the log detector to the peak hold only during the instant
when the desired radar pulses are present. The AS-330 pulse
train separator contains 3 independent PRF filter channels
that can be set up for up to 3 separate radars. These PRF
filters generate TTL-level switching signals whenever pulses
of the corresponding PRF occur. These switching signals
control the pulse-blanking card in an additive mode or ina
subtractive mode. A series of switches controls which mode
is used; the modes may be mixed. The switches on the pulse-
blanking card are each associated with a single PRF filter.
When the switch is in the additive position, the pulses
recognized by that PRF filter are passed through the pulse-
blanking card to be measured. Several PRF filters can be
switched to the additive mode at the same time, which will
cause pulses belonging to several PRF's to be measured. The
system can also be used in the subtractive mode. When a PRF
filter is switched to the subtractive mode, those pulses
belonging to the given PRF are subtracted from the total
number of pulses transmitted through the pulse-blanker card.
The pulse-train-separator card has been quite effective in
the additive mode, suppressing unwanted radars by as much as
40-50 dB, as shown in Figure G-3. The subtractive blanking
mode is not as effective, typically suppressing unwanted
radars by 5 dB. This is because the PRF filters are typically
"too choosy", and do not recognize all of the pulses belonging
to a given PRF. Thus, in the subtractive mode, not all of the
unwanted pulses are removed from the total number of pulses.
In the additive mode, it is not crucial that all of the pulses
be recognized, because the peak-hold hardware keeps only the
largest amplitude pulse anyway, and chances are good that a
typical sample contains 10-20 pulses which are all near the
peak amplitude. It is very unlikely that all of these pulses
would be missed by the PRF filter. For calibration, the pulse
blanker is turned on all the time and a normal KALIB is run
through the pulse-blanker card. Pulse blanking can be used
with most of the Radar Measurement Programs.

Application Programs

The following is a brief description of several RSMS
van application programs used during the Los Angeles and


Se ae Y Bene ne ee

it



SFS was chosen over other propagation measurement programs
because the 1 measurement/5 seconds characteristic gave less
chance for local radar signals to get through the pulse-
blanking equipment. Another use of SFS involved measuring
amplitude as a function of direction of arrival. For this
measurement, the graph scale is set up to match the -200°
to +200° headings on the dish azimuthal rotation limits.

By proper choice of steps and dwell, the graphing can be
made to track the 1°/second rotation of the dish, and the
graph gives a read-out of received amplitude versus heading
(Figure G-5).

Frequency Tracking Scan (FTS). Propagation Loss Measurements.

For long-term propagation measurements, the FTS program
offers some benefits over SFS. FTS is similar to SFS except
that FTS has some capability to continually track the slowly
drifting frequency of a desired signal. Thus, use of FTS
assures that any change in received amplitude is not caused
by transmitter frequency drift. When a graph is completed,
the graph is copied on the Versatec and a new graph is
started on the system. FTS was often used to gather data
on a radar overnight. In this mode a new graph was copied
each hour and 8-12 hours of data were obtained without operator
intervention. FTS tracks frequency by tuning over a small
range on either side of the nominal frequency and keeping
track of what frequency gives the highest received signal
amplitude. The highest amplitude is graphed and the corres-
ponding frequency is used as the nominal frequency for the
next set of measurements. SFS scans +2.5 MHz on either side
of the nominal frequency in 0.5 MHz steps. With a 5 second
dwell at each measurement frequency, the total set of 11
measurements requires almost exactly 1 minute. Thus each
measurement point on the graph requires about 1 minute, and
a 60-step graph requires 1 hour. Height-gain curves for
potentially interfering radars were measured at Long Beach
using FTS, while the RSMS tower was raised such that the
receiving antenna increased in height in 3-foot steps, from
waiting to several minutes at each height (Figure G-6).

Radar Antenna Patterns

The RSMS has a capability to make measurements of radar
antenna patterns which is particularly flexible because of
the large amount of mathematical and statistical work that
can be done on the raw data before it is plotted. Nevertheless,
the RSMS system has the same constraints as any system that
measures antenna patterns and some effort should be made to
account for the effect of reflections from the ground and

G-10


San Francisco area radar-to-radar interference investigation.

Wide-Band Scan - Radar Band Occupancy and Emission Spectra

The Wide-band (WB) scan allows a specification of many
operating parameters, including frequency range, number of
frequency steps within the specified range, dwell time on
each frequency step, bandwidth, RF attenuation, IF attenua-
tion, peak hold, and receiving antenna. The program makes
a measurement of peak amplitude received at each frequency,
starting at the lower end of the frequency range and con-
tinuing until the upper end of the frequency range is reached.
At each frequency, the system waits for a specified dwell
time (typically 5 seconds) continually measuring the peak
amplitude of the signal and updating its measurement when
a still stronger pulse occurs. If the dwell time is chosen
somewhat larger than the rotation period of the radar an-
tenna, one is assured that the measurement at each will con-
tain the instant when the radar antenna was aimed directly
at the RSMS van. This situation will give the largest peak
power reading, which is the power recorded for that frequency.
As the measurements are made, the measured data is graphed.
The operator may add or subtract attenuation in 10 dB steps
whenever action is required to keep the signal within the
linear dynamic range of the system. The system corrects
for input attenuation so that the graphed data is continuous
and appears to have been measured with a system with very
large dynamic range. For many of the measurements made with
this program, a 200 MHz frequency range is specified (usually
2700-2900 MHz), with 150 steps within this range. This means
that a measurement is made every 200/150 = 1.33 MHz, which
compares with measurement bandwidth of 1.5 MHz. Therefore,
WB SCAN is typically operated with frequency steps slightly
less than the measurement bandwidth.

As mentioned previously, WB SCAN is used for measuring
the emission spectra of a particular radar, as well as for
measuring general spectrum occupancy in the radar bands.

The chief difference between these two applications is in the
efforts made to select one radar only from out of several
radars that might be received in the frequency band. For
the general band occupancy measurement, WB SCAN is used with
the CBS antennas, often making a band scan with each of the
four CBS antennas to get an omnidirectional composite of
band occupancy (Figure G-4A). When making a measurement of
the emission spectra of a particular radar (Figure G-4B),
several techniques are used to assure that only the desired
radar is measured. First, the dish antenna is often used
for these measurements. The extra gain of the dish gives


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considerable angular discrimination against other radars,

in addition to giving the extra system gain necessary to measure
further down on the skirts of the radar spectra, The pulse
blanker capability may be used, generally in an additional

mode with the AS-330 set to the PRF of the desired radar.
Finally, for some of the radar emission spectra, the RSMS

van was moved line-of-sight to the radar and measurements

were made with sufficient input attenuation that other radars
in the band were generally not seen by the RSMS.

In order to assure that the 8580 receiver IF bandwidth
would not distort the WB SCAN and emission spectrum measure-
ments when operating in the peak hold mode, the receiver IF
bandwidth was set to meet the inequality (Reference 44)

2
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Largest pulse width measured

Single Frequency Scan. Propagation Loss and Direction of
Arrival.

The Single Frequency Scan (SFS) program is a close
relative of WB SCAN. The main difference is that SFS does
not change in frequency as it measures, but stays at the
same frequency. The same input parameters can be specified
as for WB SCAN except that frequency range is replaced by a
single frequency. The program plots peak received amplitude
at the single selected frequency versus time. Dwell time is
the amount of time spent collecting data for each measurement
point, and "steps" gives the number of measurement points
across the graph. The scale across the graph can be any
arbitrary scale, typically 0 to 150 or -200° to 200° or 200°
to -200°.

SFS is used for a variety of propagation loss measure-
ments, using the CBS or the dish antennas. Since dwell is
usually set at 5 sec (the typical rotation time of radars in
this band), one gets one measurement point every 5 seconds
and a typical graph might take 5-15 minutes. Measurements were
made of distance-gain curves using SFS with the RSMS van
driving along the runway at Los Alamitos. Some propagation
measurements were made with SFS alongside the Long Beach
ASR-5 at a frequency quite close to the ASR-5 frequency.


APPENDIX G

RSMS VAN MEASUREMENTS

INTRODUCTION

This appendix describes some of the RSMS measurement
procedures used during the Los Angeles and San Francisco area
radar-to-radar interference measurements. It is included
here to aid in interpreting the measured data. The appendix
includes a compendium of the RSMS van measurement system and
application programs related to radar measurements. Additional
information may be found in the RSMS Operations Manual or by
contacting RSMS personnel in Boulder, Colorado.

RSMS VAN SYSTEM DESCRIPTION

The RSMS was used in many different modes during this
measurement program. Most of the measurements were made
with the HP 8580 receiver and 8500 computer systems connected
as shown in Figure G-l. Two sets of RHCP receiving antennas
were used for these measurements - 4 fixed cavity-backed
spiral antennas on the tower and the dish antenna on the
roof of the van. These antennas can be connected to the
input of the RSMS, where signals are attenuated, if neces-
sary, by an RF attenuator, and then, filtered and amplified
by YIG-tuned preselectors and a preamplifier. The signal
is then further processed by a mixer, several IF's and log
detector. The output of the log detector is a DC-coupled
signal proportional to the logarithm of the envelope of the
input signal which is processed in several further ways.

For many of the measurements, a pulse blanking capability

is used to pass only the radar pulses belonging to the
desired radar. The peak amplitude of selected radar pulses
is held in a peak hold circuit, and digitized. Once digi-
tized, the peak amplitude information is processed by the
8580 computer which has several calibration algorithms, and
special software application programs. The specific appli-
cation program information may be displayed as part of a
graph, recorded on magnetic tape or cassette, or copied with
the Versatec plotter.

Antennas
Two sets of right-hand circular polarization antennas

were used in these measurements. A set of 4 cavity-backed
spiral antennas (CBS) on the tower were used more-or-less as


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G-12


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G-15


In the cases where antenna pattern measurements were
made through another radar antenna, the programs were run
as normal except that the RSMS was connected to the 30 MHz
IF of the radar system and tuned to 30 MHz. The radar trans-
mitter was turned off and the radar system was pointed
towards the interfering radar of interest. The rest of the
measurement is made routinely. One problem with this type
of measurement is that there is no easy way to attenuate
the interfering radar signal if transfer curve data show
that the interfering signal may be saturating in the radar
receiver.

G-17


omnidirectional antennas. These antennas have a 70-90°
beamwidth, 2 dB gain for a linearly polarized signal, and

gave relatively good coverage between the 4 of them which
were aimed 90° apart. A 36-inch diameter dish was also

used. This antenna could be pointed in different directions,
with an 8° beamwidth and a 20 dB gain for a linearly polarized
signal at 2800 MHz.

Noise Diodes and Calibration

Broadband noise diodes are used to calibrate the system
in terms of absolute amplitude at various points in the
system. In general, the particular noise diode is used
which is closest to the measurement point of interest. When
the CBS antennas are used for measurements, the noise diode
in the tower associated with those antennas is used for
system calibration. This assures the maximum likelihood
that the calibration procedure will account for all of the
various effects on signal gain which are seen by the signal
itself. By using the noise diode in the tower, for example,
one corrects for the approximately 4 dB of transmission line
and relay switching loss which attenuate the signal received
by the CBS antennas, as well as calibration errors within
the 8580. The diode at the front panel could also be used
as a reference point, but this diode would only correct for
calibration errors within the 8580. The 36-inch dish antenna
does not have a noise diode associated with it, which means
that it cannot be calibrated exactly. Instead, one can use
either the front panel noise diode or the CBS noise diode,
assuming that the CBS noise diode compensates for 4 dB line
loss and the front panel noise diode compensates for 0 dB
line loss. The measured line loss to the dish is around
2 GB at 2800 MHz, so that either diode is about equally poor,
one giving answers 2 dB too high, the other giving answers
2 dB too low.

In addition to the noise diode calibration procedure,
there is a 30 MHz system linearization procedure which tests
the system under different conditions and generates a cali-
bration curve for the log amplifier. The log amplifier
calibration curve is different, however, depending on
whether the peak-hold hardware is used (see below). If the
peak-hold hardware is used for the measurements, one gets
more linear system dynamic range if the system is calibrated
using the peak-hold hardware. Figure G-2 shows the system
dynamic range using peak-hold for measurements and for
calibration. This shows that the linear dynamic range using
peak-hold calibration is about 50 dB, before the cumulated
error exceeds 3 dB.


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other structures. Two types of antenna pattern measurements
were made. For one type of pattern we are concerned with
what the victim radar saw as an interferer antenna pattern,
knowing that this pattern would likely be not line-of-sight
and badly distorted by all sorts of propagation phenomena.
For these patterns, we try to duplicate the performance of
the victim radar as closely as possible. Occasionally, we
were actually able to use the victim radar antenna itself,
but usually we had to make measurements with dish or CBS
antennas on the RSMS van. In general, the CBS antennas

could be positioned closer to the victim radar antenna and

at about the same height. However, the dish antenna more
closely matched the performance of the radar antenna, as

well as offering better gain for the desired radar and more
rejection of the local radar. The other type of radar an-
tenna pattern measurement is an attempt to measure the free-
space pattern of the radar antenna. For these measurements,
great care is taken to locate the RSMS van so that it is
line-of-sight to the radar with open fields behind the radar
to eliminate reflections from the ground between the radar
and the RSMS van, several things were done. The spacing
between the radar and the van was considered carefully so the
dish would be in the proper zone, away from the possibility
of deep nulls. In addition, the dish was often aimed up
slightly to minimize gain in the direction of the ground re-
flection, though this was probably not too effectively done
because of uncertainty about the position of the null, the
levelness of the van, etc. In order to minimize the distor-
tion of the antenna pattern measurements due to receiver dynamic
range, variable attenuators are used to set the measured radar
peak received power level at the 8580 RF input to approximately
-30 dBm. This permits only a 3 dB compression of the antenna
pattern over a 50 dB range (Figure G-2). However, in some
cases, where good radar siting exists, the lower level of

the cumulative antenna distribution may be distorted due to
insufficient 8580 receiver linear dynamic range.

Once the receiving antenna position was judged satis-
factory, the radar antenna pattern program was run. This
program allows complete specification of 8580 operating
parameters in the first section of the program. The second
section of the program makes 500 measurements over a specified
period of time, minimum time about 4 seconds. The third sec-
tion of the program analyzes and plots the data. The pattern
program assumes a continuously rotating radar antenna, where
the peak signal received by the RSMS antenna occurs when the
radar antenna points at the RSMS antenna. The program also
assumes that the set of 500 measurements has been taken in
such a way that the highest amplitude received in the first

G-13


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G-16

SOLID

Figure G-8,