2.2.3.3

1.0

A statistical comparison between measured and
modeled emission spectra indicated that the
average absolute mean difference between measured
and modeled emission spectra was 4.0 dB, and the
standard deviation between measured and modeled
emission spectra, 6.0 dB. The modeling accuracy
varied depending on the pulse width category of
the radar. The largest absolute mean (6 dB) and
steréerée Geviation (8 dB), between measured and
modeled emissions, occurred for 0.5 usec pulse
width category radars. The smallest absolute mean
(2.6 dB) and standard deviation (4.0 dB) occurred
for the 0.8 wsec pulse width category which in-
cludes FAA ASR radars.

The difference between modeled and measured emis-
sion spectra of radars with the same pulse width are

attributed to the following factors:

A. Magnetron Age

B. Modulating Pulse Shape

C. Magnetron Tube Type

D. Magnetron Anode Voltage and Current Setting
E. Magnetron Load

Since it is difficult to include the above factors
into a magnetron emission spectra model, it is
believed that the standard deviation of 6 dB
between measured and modeled emission spectra

may be the limit in obtainable accuracy of con-
ventional magnetron emission spectra modeling.

FDR Modeling

A statistical comparison of FDR computed from modeled IF
selectivities and emission spectra, and FDR computed from
measured IF selectivities and emission spectra, led to the
following findings:

1.0 Because of the slow fall-off rate of conventional

magnetron emission spectra relative to normal radar
receiver IF selectivity characteristics, the Frequency-
Dependent-Rejection model accuracy is limited by emis-

sion spectrum modeling accuracy.

San

Radar frequency measurements in the Los Angeles and

Francisco area indicate that radars are not always

Operated exactly at their assigned frequency. The error
contribution to FDR predictions due to the uncertainty

2-5


As previously mentioned, ducting can cause as much as 39 dB
less propagation loss than predicted by normal diffraction

loss theory. The ducting phenomena motivated the FAA to assign
frequencies to radars, within 20 miles of Los Angeles, 30 MHz
above the San Diego area Miramar ASR-5 radar upper channel fre-
quency of 2720 MHz.

Also, discussions with the FAA revealed that certain
manufactured 5586 magnetron tubes have inherently short
lifetimes when operated near the 2.7-2.9 GHz band edge, even
though the manufactured tuning range is specified as 2.7-2.9 GHz.
Consideration of this factor will be given in a subsequent OT
report. The previous discussion on the finding and effects
related to building attenuation amplifies the necessity to
include a building attenuation factor in the predicted propa-
gation path loss.

2.2.6 Interference Prediction Capability

The main objective of the Los Angeles and San Francisco
area measurement program was to assess the ability to predict
radar-to-radar interference. Measured data taken during the
investigation indicates that the standard deviation between
modeled and measured data was 4 dB for antenna pattern backlobe
level, 6 dB for emission spectrum/FDR, and 8 dB for propagation
loss incorporating a building attenuation factor. For these
three component models used to predict radar-to-radar inter-
ference, an estimate of the total standard deviation (c) would
be 10.7 dB since the models are independent. The worst case
limit that typically needs to be considered is the 20 region
which implies the predicted INR will not differ from the actual
INR by more than 21.4 dB 96% of the time. Due to uncertainties
in radar signal processing, plus the additional 10.7 dB standard
deviation associated with antenna patterns, emission spectrum/
FDR, and propagation loss, it appears that the interference
pattern on the victim radar PPI display cannot always be
accurately predicted. However, due to the inherent charac-
teristics of the mutual antenna gain distribution, INR errors
of approximately 25 dB can occur, but still result in relatively
accurate predictions of interference patterns of the victim
PPI display for conditions where mainbeam-to-backlobe antenna
coupling predominates, which is the most common type of inter-
ference antenna coupling that occurs in a radar environment.
(See Figure 3-1 and Table 3-1).

2.2.6.1 Los Angeles Area Investigation

For the 11 radar sites visited during the preliminary radar
site measurements, there were 27 predicted interference cases


2.0

caused by mountain reflection and diffraction,
which may result in less loss than the direct
path. Even though potential multipath wave
interference conditions can be identified, it is
difficult to include this factor in the predic-
tion process in other than a general way since
multipathing is a function of the interferer
and victim radar antenna azimuthal positions
and the surrounding terrain. To account for
this analytically would require an extremely
complex terrain model.

The result of man-made clutter attenuation measure-
ments by Turin (Reference 35) were incorporated

into the San Francisco loss predictions. This
involved identifying the degree of man-made clutter
over the propagation path and adding the appropriate
loss in dB to the point-to-point model loss predic-
tion. However, for line-of-sight paths the loss for
man-made clutter was simply added to the free space
loss. The absolute mean difference between predicted
and measured propagation was 7 dB which was a 5 dB
improvement over Los Angeles area propagation loss
predictions. The algebraic mean difference was -3 dB
which was also an improvement (7 dB) over Los Angeles
predictions.

Loss predictions by the area version of the propaga-
tion model were also statistically compared with
measurements. The point-to-point version of the
model had a lower average absolute prediction error
and standard deviation than the area version.

Superrefraction (ducting) conditions were found to
exist over one ocean path (from Miramar to Los
Alamitos) in the Los Angeles area and two San
Francisco Bay paths (from Oakland and Alameda to
Mountain View). The measured loss for the first
two paths were as much as 39 dB below the point-to-
point predicted loss. The measured loss for the
third path varied +25 dB relative to the predicted
loss, indicating that both ducting and radio holes
were likely present. Bean (Reference 54) reported
that during the summer months at San Diego and
Oakland, ducting occurs 50% of the time. Hitney
(Reference 43) states that radar range enhancement
occurs 30% of the time during the year in the off-
shore San Diego area.

2-7


2.2.3.4

of frequency separation between transmitter (interferer)
and receiver (victim) can be significant for frequency
separations less than 10 MHz.

Propagation Modeling

Comparison of predicted and measured propagation loss
over 20 paths in the Los Angeles and San Francisco area,
supported by an extensive literature search, led to the
following findings related to propagation loss predictions
and measurements:

1.0 A statistical comparison of the point-to-point

propagation model (Reference 14) loss predictions
with measurements over 13 Los Angeles nonducting
paths indicated that on the average, the measured
losses were 10 dB higher than predicted. The
average absolute difference between the predicted
and measured propagation losses was 12 dB. Factors
which are believed to have contributed to the
differences are:

A. Increased attenuation due to buildings and
man-made obstacles which were not included
in the prediction model and terrain data base.
Propagation loss measurements by researchers
(Reference 34,35) in suburban and urban environ-
ments have ranged from 12 to 50 dB below free
space, depending on the degree of building
congestion. Measurements over suburban area
paths, which constituted 80 percent of the type
radar-to-radar paths encountered in this in-
vestigation, were reported to be 12 dB below
free space loss.

B. Increased attenuation due to foliage is not
included in the model and terrain data base.
However, because the radar-to-radar paths
over which loss measurements were made did not
pass through heavy foliage, it is believed that
the contribution of error between predicted and
measured loss due to foliage was small relative
to other factors.

C. The point-to-point propagation model considers
only the terrain profile in the vertical plane
that cuts through the great circle line between
transmitter and receiver. Consequently, the
model does not account for multipath propagation,


Minimum Discernible Signal (MDS) level at an INR
of 10 dB for both the normal and MTI modes and
in the integrated MTI mode. The 10 GB INR cri-
terion is where the interference is clearly
visible, not at the point when the interference
is just causing speckling on the PPI display
which occurs at an INR of approximately 4-6 dB
for both normal and MTI modes.

2.0 The integrator on the FAA analog radars (ASR 4,5,6)
does not significantly suppress nonsynchronous
pulse interference. With the integrator on,
approximately 1 or 2 dB more interfering signal
power was required for the interference to become
just visible on the PPI display. However, the
interference was clearly visible with the inte-
grator on at an INR of 10 dB. The analog inte-
grator did enhance synchronous pulses by approxi-
mately 6 dB.

3.0 Closed system measurements made on a digital
radar (ASR-7) indicated that nonsynchronous pulse
interference was clearly visible on the PPI display
at an INR of 10 dB for both the normal and MTI
modes with the enhancer (integrator) off. With
the enhancer on, the interference was significantly
reduced. That is, for interfering signal levels
as high as -60 dBm, pulse width range of .5 usec
to 30 usec, and frequency separation of AF=0
interference was significantly reduced on the PPI
display. However, there were some interfering
signal PRF's (not including synchronous PRF)
which did result in light interference on the
PPI display. The dependence cf interference on
PRF will be investigated in a subsequent report.

2.2.5 Frequency Assignment Factors

As a result of the 2.7 to 2.9 GHz band investigation con-
ducted in the Los Angeles and San Francisco areas, factors were
discovered which are not normally considered in making frequency
assignments. These factors inciuded: ducting, decreased
magnetron lifetime when operating at the edges of the 2.7 to
2.9 GHz band, and building attenuation.

Discussions with the FAA Western Region Frequency Coordi-
nator and measurements made with the RSMS van in the Los Angeles
and San Francisco area confirm the necessity to include ducting
phenomena, when appropriate, in determining frequency assignments.

2-9


SECTION 2

FINDINGS AND RECOMMENDATIONS

2.1 -INTRODUCTION

The findings and recommendations resulting from the Los
Angeles and San Francisco investigations have evolved from a
combination of on-site observations of radar PPI displays,

RSME measurements, closed system measurements, and a litera-
ture search. The information obtained from the on-site radar
PPI display observations was used to compare actual observed
interference situations with predicted interference. The

RSMS van measurements were used to determine the error intro-
duced by the various component models used to predict potential
interference situations. Closed system measurements were made
to determine the non-synchronous pulse MDS level and the pro-
cessing of interfering signals for the various radar operational
modes.

2.2 FINDINGS

The measurement data and literature search information
obtained during the Lcs Angeles and San Francisco area investi-
gations were extensively analyzed. This led to the formulation
of the following findings, which are discussed in detail in
Sections 3 through 5 and Appendices A through E.

2.2.1 Los Angeles Environmental Observations

Ten of the eleven radar sites visited in the Los Angeles
area had some interference from other radars in the band on
their PPI display. The interference observed on the FAA radars
was generally very light. One military GCA (Ground Control
Approach) radar had very heavy interference on the PPI display.
Also three of the military range surveillance radars at Point
Mugu had very heavy interference on the PPI display. It should
be noted that the measurements were made with the FAA radars
operating on channels which resulted in minimum frequency sepa-
ration, and therefore maximum potential for interference. Closed
system measurements made during the Los Angeles investigation
revealed that the integrators on the analog radars (ASR-4,5,6)
do not suppress the level of interference. However, the ASR-7,
digital radar, enhancer did suppress the level of interference.
Considering all factors, it appears that frequency assignments
for the 2.7 to 2.9 GHz band radars deployed in the Los Angeles
area at the time of investigation have been engineered to the

2-1


This improvement was attributed to empirical modifications
to the interference prediction model.

2.2.6.3 Summary of Interference Prediction Capability

As a result of the extensive measurement program conducted
in the Los Angeles and San Francisco areas, it was concluded
that the statistical variations in antenna patterns, emission
spectrum/FDR, propagation loss and signal processing parameters,
that are inherently present, make it extremely difficult to
make precise predictions of the interference on the PPI display.
However, it is possible to provide a range of interference PPI
display patterns, which for many cases may appear tc have nearly
identical operational impact. In effect, the analytical radar-
to-radar interference techniques used in this investigation
can be used to:

1. Assess congestion

2. Determine the EMC of new radar systems

3. Evaluate alternate assignment plans

4. Develop spectrum engineering and management aids

5. Develop a spectrum-space metric for determining
band efficiency.

2.3 RECOMMENDATIONS

Considering the findings resulting from the Los Angeles
and San Francisco area measurements, the literature search,
and overall Spectrum Resource Assessment objectives, it is
recommended that the following action be taken to improve
radar-to-radar EMC prediction and analysis techniques, and
procedures required for assessment of band efficiency and
potential congestion:

1. Additional analysis or closed system measurements
should be made to investigate interfering signal
processing properties of existing and planned radar
deployments in the 2.7 to 2.9 GHz band. This in-
vestigation should also include interfering signal
processing characteristics of proposed new post
processing equipment such as the RVD-4, ARTS-III
Enhancement program and other new developments such
as the Moving Target Detector (MTD) radar presently
under consideration by the FAA. Findings from the
analysis should then be incorporated in the various

2-12


of mainbeam-to-backlobe coupling. The total number of actual
observed interference cases during the preliminary site visits
was 21. Of the 21 observed cases, there were 17 predicted and

4 unpredicted. Therefore, 63 percent of the predicted interferers
were observed on the victim PPI display, and 19% of the observed
interferers were unpredicted. The 4 unpredicted cases were due
to multipath phenomena. Most of the predicted interference
cases that were observed were not of the periodic nature that
was predicted. Therefore, in general, most of the interference
predictions were pessimistic when compared to the actually
observed interference.

It was estimated that in most cases the predicted inter-
fering signal power levels were approximately 10 to 20 dB too
high. The difference between predicted and actual interference
signal power level can be generally attributed to the statistical
nature and biases associated with the antenna and propagation
related variables that were not initially included in the models.
Based on measurements made during the Los Angeles investigation
and information obtained from a literature search, empirical
modifications to the propagation model to include building
attenuation and to the antenna model were made and incorporated
in the interference prediction technique used in the San Francisco
investigation.

2.2.6.2 San Francisco Area Investigation

For the eight radar sites visited during the San Francisco
area preliminary radar site measurements, there were 13 predicted
interference cases of mainbeam-to-backlobe antenna coupling.
However, due to some of the radars being inoperative during the
preliminary radar site measurement program, only 9 predicted
interference cases could be verified. All 9 of the predicted
interference cases that could be verified were observed during
the preliminary site visits. There was also one case of un-
predicted interference. Therefore, 100 percent of the predicted
interferers were observed on the victim PPI display, and 10%
of the observed interferers were unpredicted. In general, the
predicted form of the interference on the PPI display was close
to the actual observed interference with the exception of two
cases where the interference was lighter than predicted, and
one case where the interference was much heavier than predicted.
The disagreement between observed and predicted interference
for these three cases was attributed to the radars operating
on frequencies other than their assigned frequency.

The San Francisco area investigation showed a significant
improvement over the Los Anglees area investigation in the
correlation between predicted and actual observed interference.


5.0 The measured propagation loss as a function of
antenna height decreased more at some radar sites
than predicted. The degree of difference between
measured and predicted antenna height gain depended
on the density and proximity of clutter (trees and
man-made obstacles) around the radar site. The
measured propagation loss decreased 4 dB as the
receiving antenna was raised 15 feet on the sparsely
cluttered end of one path, and 11 dB on the densely
cluttered end of the same path. These measurement
results illustrated the importance of matching the
receiving antenna height to the radar antenna
in order to obtain accurate propagation loss measure-
ments.

6.0 Propagation loss measurements revealed that moving
the physical location of the receiving antenna by
a few feet can result in received signal variations
as great as 4.5 dB. For this reason, propagation
loss measurements were conducted at more than one
position at each radar site to average out the
effect of signal level variation due to multipath.

7.0 It was evident from propagation loss measurement
differences between using the RSMS omni, cavity
beck, and parabolic dish antennas, that measured
propagation loss is affected by the antenna beam-
width in a multipath environment. Antennas with
unlike patterns and beamwidth spatially weigh
incident multipath signals differently, which
affects the total power received by the antenna.
This illustrates the importance of using the
actual radar antenna, or at least a receiving
antenna whose beamwidth approximates the radar
antenna in the plane that multipathing is occurring.

2.2.4 Closed System Measurements

Closed system measurements were made on an FAA ASR-5
(analog) radar and FAA ASR-7 (digital) radar. The objective
of these measurements was to determine the nonsynchronous
pulse PPI display MDS level and the radar interfering signal
processing characteristics. Both the radar PPI display and
"A" scope were monitored during these closed system measurements
while various radar modes and fixes were turned on and off.
These measurements resulted in the following findings:

1.0 Analog radars similar to the FAA ASR-4, ASR-5
and ASR-6 have a nonsynchronous pulse PPI display


2.2.3.2

5.5 degrees, which results in mainbeam gain reduction
of 5 to 12 GB.

Antenna patterns of potential interfering radars
measured at the victim radar site revealed that line-
of-sight (free space) antenna pattern models will not
always result in realistic interferer pulse amplitude
distributions at the victim radar RF input. Propaga-
tion related phenomena, such as multipath wave inter-
ference arising from multiple scattering and diffrac-
tion of the waves by mountains, buildings and other
structures affect the interfering signal pulse ampli-
tude distribution at the victim radar RF input. The
effects of these propagation related pheomena are
dependent on the azimuthal position and pattern of
the victim and interfering radar antennas and,
therefore, are difficult to realistically represent
as separate models. Environmental antenna pattern
measurements (interferer antenna pattern measured

at the victim radar site) revealed the following
antenna-propagation related phenomena:

A. Apparent reduction of sidelobe and backlobe
radiation level relative to the mainbeam level
occurred on two measured paths. This was be-
lieved to be caused by radar site foreground
clutter attenuating the radiated spillover under-
neath and around the antenna reflector. This
phenomena occurred when there were low buildings
and trees between the radars, and no mountain
ranges between the interfering and victim radars
or in the vicinity. The apparent reduction in
sidelobe and backlobe level for the paths
measured was found to be approximately 6 dB.

B. Apparent broadening of the mainbeam and higher
backlobe levels of the interferer antenna pattern,
as seen by the victim radar. This propagation
related phenomena was measured over four paths
in the Los Angeles area where there were mountain
ranges between the victim and interfering radar
and also behind the interfering radar.

Emission Spectrum

Comparison of modeled emission spectra with measurements

conducted in the Los Angeles and San Francisco investigation,
and a literature search have led to the following findings
related to emission spectrum modeling:


San Francisco area in the 2.7 to 2.9 GHz band will be given in
a subsequent OT report.

2.2.3 Model Accuracies

One of the objectives of the Los Angeles and San Francisco
measurement program was to determine the error introduced by each
of the component models used to predict radar-to-radar interference.
The following is a discussion of the measurements made with the
RSMS van, and the findings relating to the accuracies of the
models used to predict radar-to-radar interference.

2.2.3.1 Antenna Pattern Modeling

Limited measurements made during the Los Angeles and San
Francisco area investigation and a literature search led to
the formulation of the following findings related to antenna
pattern modeling:

1.0 Because of the dynamic nature of radar antennas (i.e.,
rotating and nodding) and differences in radar site
clutter conditions, a statistical description of
radar antenna characteristics is the most realistic
approach to radar antenna pattern modeling for pre-
dicting radar-to-radar interference.

2.0 Line-of-sight (free space) antenna pattern measure-
ments made during the Los Angeles and San Francisco
area investigations on cosecant squared elevation
type antennas had an averaged median sidelobe (3° to
25° and 335° to 357°) level of -7.0 dBi and standard
deviation of 3.3 dB. The average median backlobe
(25° to 335°) level was found to be -12.8 dBi with
a standard deviation of 3.1 dB. Considering the
statistical antenna pattern data given in References
13 and 18, and the measured data resulting from the
Los Angeles and San Francisco investigation, it is
recommended that a median backlobe level of -13 dBi
with a standard deviation of 4 dB be used to model
the backlobe statistical characteristics of radio-
navigation radar antennas.

3.0 When predicting mainbeam-to-backlobe antenna coupling
interference, the nominal radar antenna mainbeam
gain should be adjusted to account for the reduction
of mainbeam gain on the horizon due to the antenna
tilt angle used to minimize clutter returns. Antenna
tilt argles associated with radars in the Los Angeles
and San Francisco areas varied from 2.75 degrees to

2-3


extent possible to provide satisfactory air traffic control
operating conditions.

Information on planned deployment changes by 1984 in the
2.7 to 2.9 GHz band in the Los Angeles area indicates that
four radars are planned for removal from the band, six radars
scheduled to be replaced, and three new radars added to the
environment. A detailed outline of the planned band changes
identifying which radars have frequency diversity capability
is given in Table 4-5. Considering the degree of interference
observed in the Los Angeles area during the measurement program,
the present operational frequency assignments in the band, and
the planned deployment changes by 1984, it is apparent that a
detailed Spectrum Resource Assessment of the Los Angeles area
will be required to determine if known planned deployment
changes can be accommodated. The accommodation of planned
future deployments in the Los Angeles area will be investigated
and reported in a subsequent OT report.

2.2.2 San Francisco Environmental Observations

Interference was observed on the PPI display of all eight
of the radar sites visited during the San Francisco area in-
vestigation. There was no consistent heavy interference observed;
however, three of the FAA radars and one GCA radar were considered
to have a moderate amount of interference on their PPI display.
The frequency assignment of 2.7 to 2.9 GHz band radars in the
San Francisco area (See Table 5-3) shows that all the radars in
the San Francisco Bay are assigned to operate in the 2.7-2.8 GHz
band, and all the radars in the Sacramento area are assigned to
operate in the 2.8 to 2.9 GHz band. It is believed that the
mountain range between the San Francisco Bay and Sacramento
areas would reduce interference potential sufficiently to allow
utilization of the whole 200 MHz band in each area. The present
assignments do not take into consideration this factor. The
significance of this factor will be investigated and reported
in a subsequent report.

Information on planned deployment changes by 1984 in the
2.7 to 2.9 GHz band in the San Francisco area indicates that
five radars are planned for removal from the band, two radars
are scheduled to be replaced, and one new radar added to the
environment. A detailed outline of the planned band changes
identifying which radars have a frequency diversity capability
is given in Table 5-5. Since the present frequency assignment
of 2.7 to 2.9 GHz radars provides 100 MHz of unused band in the
San Francisco Bay and Sacramento area, it is believed that known
planned deployment changes by 1984 can be accommodated. A
detailed study of known planned deployment changes in the


radar-to-radar interference prediction models
for improved prediction capability.

2. Additional analysis and measured data be obtained
on emission spectra of ASR-8 radars which employ
klystron output tubes, and height finding radars
(AN/FPS 6 and 90), which reportedly will use
tunable coaxial magnetrons.

3. Follow-up of the two on-going efforts* to establish
controller performance keyed PPI degradation criteria
to enable objective assessment of interference con-
ditions.

Information obtained from the results of the above
recommendations, and findings of the Los Angeles and San
Francisco area investigations will be incorporated in the
radar-to-radar interference prediction model(s) and techniques
to assure that the overall Spectrum Resource Assessment of
the 2.7 to 2.9 GHz band is based on sound technical procedures.
The established technical procedure to predict potential
radar-to-radar interference will then be applied in the eight
designated congested areas to assess the accommodation of
future radar deployments in the 2.7 to 2.9 GHz band, and as
frequency management aids to assist OTP and other government
agencies in achieving efficient spectrum management.

* Controller performance keyed PPI degradation studies are
currently being conducted by the FAA at NAFEC and ECAC at
CALSPAN.