and does not perform a true convolution of the victim and
interfering antenna patterns.

The mutual antenna gain threshold is a function of the
propagation path loss between the victim and interfering
radars and victim receiver Frequency-Dependent-Rejection (FDR) -
Therefore, changes in the mutual antenna gain threshold can
be interpreted as representing variations in the prediction
error of the propagation loss and FDR models. A study of
Table 3-1 shows that the degree of interference on the PPI
scope can be divided into several regions:

a. Region 1 (Mutual antenna gain threshold level
between +30 dBi and +20 dBi): This region of
mutual antenna gain threshold level can be de-
fined as transition region 1 where the inter-
ference is very infrequent and random in nature
and only occurs for mainbeam-to-major sidelobe
coupling of the victim and interfering antennas.

b. Region 2 (Mutual antenna gain threshold level
between +20 dBi and -5 dBi): In this region
the interference is periodic in nature from
scan to scan. The degree of interference on
the PPI display is also approximately the same
over the 25 dB range in mutual antenna gain
threshold level. This mutual antenna gain
threshold region occurs for mainbeam-to-backlobe
coupling of the victim and interfering radars.

c. Region 3 (Mutual antenna gain threshold level
between -5 dBi to -15 dBi): This region of
Mutual antenna gain threshold level is defined
as transition region 3. In this region, the
degree of interference changes from light strobes
of interference to interference covering approxi-
mately 70% of the PPI display. This sharp change
in degree of interference over a small mutual
antenna gain threshold range is due to the thres-
hold coupling level changing from mainbeam-to-
backlobe coupling to backlobe-to-backlobe coupling.

dad. Region 4 (Mutual antenna gain threshold level <
=I5 dBi): in this region the level of interference
ass. ppearret
is approximately constant and covers a significant
portion of the PPI display. This region is defined
as the backlobe-to-backlobe coupling region.

3-5




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SECTION 3

INTERFERENCE PREDICTION TECHNIQUE

3.1 INTRODUCTION

The following is a discussion of the method used to
predict potential radar-to-radar interference during the 2.7
to 2.9 GHz band investigation conducted in the Los Angeles
and San Francisco areas. The first step in predicting potential
radar-to-radar interference situations involved calculating the
interference-to-noise ratio (INR) at the victim radar receiver
IF output. These predicted INR values were then used to
determine the potential radar-to-radar interference conditions.
Once potential radar-to-radar interference conditions were
identified (INR 10 >dB), a PPI Simulation Program (Reference 12)
was used to predict the pattern of the interference on the
victim PPI display.

3.2 INR CALCULATION

The INR values were calculated for victim receiver main-
beam to interferer backlobe coupling conditions. That is, the
interference should appear on the PPI display when the victim
antenna is pointing in the direction of the potential interferer.
A three-level victim and interferer antenna pattern model
described in Appendix A was used to predict the interference.
The emission spectrum, FDR and propagation loss models used to
predict interference are discussed in Appendices B, C and D,
respectively. The INR for each radar in the environment was
calculated using the nominal characteristics of the victim
and interfering radars and the following equation:

4 = + + - - 5a ONG -
INR; 5 Pry Sq; Sy, Xp; FDR; 5 N; (3-1)
Where: Pe = The peak transmitter power of the ith radar,
ft in dBm
Gp = The interfering antenna median backlobe level
2 of the i radar (-10 dBi, Reference 13)

G = The nominal mainbeam gain of the jth radar
=4 minus correction factor for antenna tilt
angle (mainbeam gain -5 to -12 dB), in dBi

ed:


In summary, in predicting the form of the interference
on the PPI display, a total error of approximately + 10 dB
in the propagation and FDR models can still result in a
relatively accurate display of the interference if the pre-
dicted and actual mutual antenna gain threshold fall within
the center of Regions 2 or 4. However, if the predicted and
actual mutual antenna gain threshold level falls within the
transition regions 1 or 3, a significant difference between
predicted and actual observed interference can result due to
the high sensitivity in these regions to errors in the propa-
gation and FDR models.

The antenna patterns shown in Figures A-21 to A-22 are
line-of-sight. In order to predict the actual degree of
interference on the PPI display, the incorporation of environ-
mental effects on the interferer's antenna pattern as seen by
the victim radar is required in the mutual antenna gain dis-
tribution. These environmental effects are propagation related
phenomenon, and are discussed in detail in Appendix A.

3-7


Lp = Median propagation path loss exceeded 50% of
ij the time. Determined using RAPIT point-to-
point model (Reference 14), in dB

FDR = The frequency-dependent-rejection between the
ith transmitter and the jth receiver, in dB

Nj = Victim receiver inherent noise level, in dBm.

Once potential radar-to-radar interference situations
were identified (INR > 10 dB), the PPI simulation model was
used to predict the form which the interference would appear
on the victim PPI scope. The interference-to-noise ratio
criteria of 10 dB was obtained from Reference 47, and was
used for both the Los Angeles and San Francisco investigation.
During the Los Angeles investigation, closed system measure-
ments on an analog radar (ASR-5) and a digital radar (ASR-7)
were made to verify the interference threshold criteria. It
was found that nonsynchronous interfering pulses were just
perceptible on the PPI display at an INR of approximately 6 dB
and clearly visible at an INR of 10 dB.

3.3 PPI SIMULATION MODEL

The concept of the PPI simulation program is that
interference will appear on the victim PPI or RHI scope if
the interfering signal level exceeds the victim receiver
threshold criteria. One input parameter to the PPI simula-
tion program is the required mutual antenna gain to exceed
the interference criteria. This mutual antenna gain thres-
hold level is given by:

iy St Ge a Poy + Iyax + FDR; + Lei, 2, (3-2)

Gp; + Gry = Mutual antenna gain between victim and
interference antennas, in dB relative to
an isotropic antenna (dBi).

H
u

MAX Maximum allowable interfering power at victim
receiver input before the victim receiver
interference criteria level is exceeded, in dBm.


(q) = Victim receiver interference criteria,10 dB.
Py = Peak power loss of interfering signal due to

special radar signal processing circuitry, in dB.

Present PPI Simulation Program limitations are that the dis-
plays produced by the program are static in nature. That is,
the model does not take into account the complete statistical
variations in the victim and interferer antenna patterns since
the program only displays the interference for one scan of the
victim antenna. A similar PPI Simulation Program is being
developed on the HP 8500 minicomputer which uses the graphics
display to give a dynamic presentation of the interference

as it would appear on an actual PPI display. The new PPI
Simulation Program will use Monte Carlo techniques to simulate
the antenna pattern of the victim and interferer radars. The
antenna patterns used in the PPI Simulation Program for the
Los Angeles and San Francisco investigation are shown in
Figures A-21 and A-22 in Appendix A for the victim and inter-
fering radars, respectively.

Figure 3-1 shows sample PPI Simulation Program runs for
mutual antenna gain threshold ranges of +20.0 dBi to -18.8 dBi.
These runs were made to determine the variations in degree of
interference on the PPI display to parametric variations in
predicted propagation loss and FDR. The victim and interfering
radar parameters were held constant with only the mutual antenna
gain threshold being varied. The relative bearing of the inter-
ferer from the victim is 97 degrees.

The calculated mutual antenna gain threshold (Equation
3-2) can be related to the mean percentage of the PPI scope
covered by interference per scan. A mutual antenna gain
Probability distribution (Figure A-23) was calculated by
convolving the victim and interfering antenna gain probability
distributions shown in Tables A-2 and A-3 for the antenna
patterns used in the PPI Simulation Program. Using the mutual
antenna gain distribution shown in Figure A-23 for two of
the Los Angeles area FAA radars, a mutual antenna gain of 5 dBi
would indicate that the interference criterion would be ex-
ceeded approximately 3% of the time which implies that on the
average 3% of the scope or 10.8° (.03 x 360°) of the PPI
display would have interference. Table 3-1 shows a comparison
of predicted percentage of PPI scope covered by interference
using Figure A-23 and the PPI Simulation Program. The data
taken from the mutual antenna gain distribution and the PPI
Simulation Program are in close agreement considering the
fact that the PPI simulation program is static in nature,

3-3