NTIA Technical Memorandum TM-06-439 

Resolving Interference from an Airport 

Surveillance Radar to a Weather Radar 

Frank H. Sanders 

J. Randy Hoffman 

Yeh Lo 

NTIA Technical Memorandum TM-06-439 

Resolving Interference from an Airport 

Surveillance Radar to a Weather Radar 

Frank H. Sanders 

J. Randy Hoffman 

Yeh Lo 

U.S. DEPARTMENT OF COMMERCE 

Carlos M. Gutierrez, Secretary 

John M. R. Kneuer, Acting Assistant Secretary 

for Communications and Information 

April 2006

ACKNOWLEDGEMENTS 

The work described in this report was performed under an other-agency agreement between 

NTIA and the National Weather Service (NWS). The results reported here were obtained as a 

result of close coordination and cooperation between many personnel of NWS, NTIA, and the 

Federal Aviation Administration (FAA). The work was performed at a field location of an airport 

surveillance radar and a weather radar at Ft. Smith, AR, during the dates of January 24-26, 2006. 

During those tests, the assistance of the following NWS personnel was essential to the successful 

outcome of the work: Mr. Russell Cook, Mr. T. Lynn Allmon, Mr. Glenn Secrest, and Ms. 

Felicia Willard. FAA personnel who made important contributions included Mr. Timothy 

Pawlowitz, Mr. Dan Hicok, and Mr. Allen Cloud. Ms. Iris Tobias of NTIA provided important 

on-site support during the measurements, and also drove the RSMS-4 measurement truck to and 

from the site. 

DISCLAIMER 

Certain commercial equipment and materials are identified in this report to specify adequately 

the technical aspects of the reported results. In no case does such identification imply 

recommendations or endorsement by the National Telecommunications and Information 

Administration, nor does it imply that the material or equipment identified is the best available 

for this purpose. 

iii 

CONTENTS

Page 

FIGURES.............................................................................................................................. vi 

TABLES ..............................................................................................................................vii 

ACRONYMS/ABBREVIATIONS.................................................................................... viii 

1 INTRODUCTION.........................................................................................................1 

2 INTERFERENCE MECHANISMS AND DIAGNOSTICS.........................................4 

 2.1 Interference Mechanisms......................................................................................4 

 2.2 Mitigation Techniques..........................................................................................5 

2.3 Diagnostics for Interference Mechanisms............................................................6 

2.4 Measurements to Determine Interference Mechanism.........................................8 

 2.5 Supporting Measurements....................................................................................9 

2.6 Approach to Measurements..................................................................................9 

3 ON-SITE INTERFERENCE RESOLUTION MEASUREMENTS ...........................11 

3.1 Measurements of the Airport Radar’s Radiated Emissions................................11 

3.2 Measurements Inside the Weather Radar RF Front-End....................................14 

3.3 Measurements Inside the Weather Radar IF Stage.............................................22 

3.4 Isolation of the Interference Mechanism............................................................25 

3.5 Experiments with Notch Filtering of the Airport Radar.....................................27 

3.6 Experiments with Variation of the I/N Level in the Weather Radar Receiver...29 

4 SUMMARY.................................................................................................................30 

5 REFERENCES............................................................................................................31 

v
FIGURES 

Page 

Figure 1. Interference strobe on the diagnostic display of the weather radar......................2 

Figure 2. Radar antenna propagation geometry..................................................................3 

Figure 3. Schematic depiction of the front-end overload mechanism. ...............................4 

Figure 4. Schematic depiction of the co-channel interference mechanism. .......................5 

Figure 5. Example gain compression of an LNA output for varying levels of overload....7 

Figure 6. Example comparative pulse shapes for diagnosis of co-channel interference. ...8 

Figure 7. NTIA/ITS RSMS-4 vehicle...............................................................................11 

Figure 8. Block diagram of radar spectrum measurement system....................................12 

Figure 9. Radiated emission spectra of the airport radar with RSEC mask......................13 

Figure 10. Detected time waveform envelopes of a radiated pair of airport radar pulses.14 

Figure 11. Simplified block diagram schematic of the weather radar receiver. ...............15 

Figure 12. Weather radar receiver passive diode limiter..................................................15 

Figure 13. Weather radar receiver EMI bandpass filter....................................................15 

Figure 14. Airport radar spectrum measured at the weather radar antenna coupler.........16 

Figure 15. Airport radar spectrum measured at the output of the passive diode limiter....16 

Figure 16. Spectrum measured at output of EMI filter.....................................................17 

Figure 17. PDL and EMI output spectra overlaid on each other. .....................................17 

Figure 18. Spectrum at EMI filter output when PDL was removed from the system. .....18 

Figure 19. Spectrum at PDL output when PDL was placed after the EMI filter..............19 

Figure 20. Comparative results for various PDL and EMI configurations.......................19 

Figure 21. Response of the LNA to the airport radar with PDL after the EMI filter.... ...20 

vi
Figure 22. Time domain waveform at LNA output with PDL in original position..........21 

Figure 23. Spectrum at LNA output with a notch filter in front of the PDL. ...................22 

Figure 24. Weather radar IF spectrum when no interference was present........................23 

Figure 25. IF spectrum when RF front-end was arranged in non-linear combination......24 

 

Figure 26. Weather radar IF spectrum when PDL was placed after the EMI filter..........24 

Figure 27. Comparison of curves from Figures 25 and 26. ..............................................25 

Figure 28. Diagnostic confirmation of co-channel interference. ......................................26 

Figure 29. Comparative weather radar responses to reductions in unwanted emissions..27 

Figure 30. Comparison of the effect of the notch filter on the airport radar output. ........28 

Figure 31. Envelopes of interference pulses with notch filter on airport radar output. ....28 

TABLES 

Table 1. Selected technical characteristics of the airport surveillance radar. .....................3 

Table 2. Selected technical characteristics of the weather radar. .......................................3 

Table 3. Mitigation solutions as a function of the interference mechanism. ......................6 

vii
ABBREVIATIONS/ACRONYMS

ASR airport surveillance radar 

EMI electromagnetic interference mitigation filter 

FAA Federal Aviation Administration 

I interference power level 

IF intermediate frequency stage of a receiver 

I/N interference to noise ratio in receiver IF stage 

ITS Institute for Telecommunication Sciences 

LNA low noise amplifier 

N noise power level

NTIA National Telecommunications and Information Administration

NWS National Weather Service 

PDL passive diode limiter 

RSEC Radar Spectrum Engineering Criteria 

RSMS-4 Radio Spectrum Measurement System, fourth generation 

RBW resolution bandwidth 

RSL received signal level 

OOB out-of-band emission type 

YIG yttrium iron garnet filter technology 

viii
RESOLVING INTERFERENCE FROM AN AIRPORT SURVEILLANCE 

RADAR TO A WEATHER RADAR 

Frank H. Sanders, J. Randall Hoffman, and Yeh Lo1

In response to interference from an S-band (2700-2900 MHz) airport 

surveillance radar (ASR) to a meteorological (weather) radar in the same 

band, measurements were performed at the field location of the two radars 

to determine the interference mechanism and any possible mitigation 

options. Measurements included emission spectra of the ASR and 

observations of the interference energy in the RF front-end and IF stages 

of the weather radar. Measurement results showed that interference energy 

originated in the unwanted emissions of the ASR (i.e., front-end overload 

was not occurring in the weather radar). But the problem was exacerbated 

by the placement of a passive diode limiter ahead of a bandpass filter in 

the weather radar receiver’s RF front-end. The interference could not be 

mitigated unless the front-end configuration of the weather radar was 

modified. With the necessary modification completed, the interference 

was successfully mitigated by installing a conventional notch filter on the 

ASR’s output stage, the notch being tuned to the weather radar frequency. 

It is recommended that the front-end configuration of all weather radars of 

the type in question should be immediately changed in the same way as 

the weather radar in this study, and that appropriate output filters should 

be installed in ASRs that are located in close proximity to these weather 

radars to mitigate interference effects at all sites in the U.S. 

Key words: airport surveillance radar interference; radar co-channel interference; radar 

emission spectrum measurements; radar interference mechanisms; radar 

interference mitigation; Radar Spectrum Engineering Criteria (RSEC); RF 

front-end overload; weather radar interference 

1 INTRODUCTION 

When an S-band (2700-2900 MHz) airport surveillance radar was recently installed in 

close proximity (930 m separation distance) to a weather radar that operates in the same 

band, interference occurred to the weather radar. The interference caused a visible strobe 

to be displayed on the weather radar’s diagnostic plan-position indicator screen, as shown 

in Figure 1. The strobe pointed in the direction of the ASR, making identification of the 

interference source straightforward. A solution for the interference problem was required. 

Since the type of solution that needs be implemented in such cases depends critically 

 

1 The authors are with the Institute for Telecommunication Sciences, National 

Telecommunications and Information Administration, U.S. Department of Commerce, Boulder, 

CO 80305.
upon the interference mechanism, a study was required to ascertain the type of 

interference and the solutions that would be viable. 

This report describes the measurements and tests that were performed to determine the 

interference mechanism and the mitigation solutions that were determined to be 

appropriate. The results of tests of those solutions are presented, along with the 

description of a novel type of interference exacerbation that was found to result from the 

RF front-end configuration of the weather radar receiver. 

The pertinent technical characteristics of the two radars are listed in Tables 1 and 2. Each 

radar tower was positioned on a hilltop, and the propagation between the antennas was 

line-of-sight with no obstructions between them (Figure 2). The two radar antennas were 

at virtually the same height relative to sea level. 

Figure 1. Photographic image of an interference strobe on the diagnostic display of the 

weather radar. 

2
Table 1. Selected Technical Characteristics of the Airport Surveillance Radar. 

Parameter Value

Fundamental frequency 2860 MHz (normally used) or 2775 MHz 

Transmitter type Solid state, consisting of 8 power modules and 2 drivers 

Modulations used Interleaved unmodulated and FM-modulated (chirped) pulses 

Pulse chirp width, Bc 4 MHz 

Pulse sequencing Pulses emitted in pairs, each pair consisting of an unmodulated 

pulse followed by an FM-modulated (chirped) pulse 

Pulse widths, τ 1 μs (non-chirped); 89 μs (chirped) 

Emission bandwidth (Bc/τ)

1/2 = (4 MHz/89 μs)1/2 = 212 kHz 

Pulse intervals 105 μs between the non-chirped pulse and the chirped pulse in 

each pair; 1.117 ms interval from the start of one pair to the 

start of the next pair 

Overall duty cycle 8.05 percent 

Transmitter peak power +73 dBm (20 kW) 

Waveguide loss 

between transmitter and

antenna 

5 dB 

Antenna gain +34 dBi 

Table 2. Selected Technical Characteristics of the Weather Radar. 

Parameter Value

Fundamental frequency 2895 MHz (35 MHz above the ASR frequency) 

Pulse modulation Pulsed Doppler 

Pulse widths, τ 1.5 μs, 4.5 μs, depending upon operational mode 

IF bandwidths 

corresponding to pulse 

widths 

670 kHz, 220 kHz 

Antenna gain +44 dBi 

Figure 2. Radar antenna propagation geometry. 

3
2 INTERFERENCE MECHANISMS AND DIAGNOSTICS

2.1 Interference Mechanisms 

RF interference problems can be caused by a number of mechanisms, each with a 

corresponding mitigation technique. When considering possible solutions to a particular 

RF interference problem, it is essential that the type of interference mechanism that is 

occurring is positively identified first. For interference between radars, there are two 

major types of interference mechanism: front-end overload of the victim receiver low 

noise amplifier (LNA), and co-channel interference from the spurious or unwanted 

emissions of the interference-causing radar on the fundamental frequency of the victim

receiver. These two mechanisms are shown schematically in Figures 3 and 4, and are 

described in more detail in [1]. 

Figure 3. Schematic depiction of the front-end overload mechanism. The receiver loses 

its desired signal when its front-end LNA is gain-compressed by a strong signal that is far 

from the receiver’s tuned frequency. 

When front-end overload occurs, the gain of the LNA in the RF front-end of the victim 

receiver is driven into compression by the presence of a strong signal that is tuned 

somewhere within the frequency response range of the LNA. Since modern LNAs often 

have frequency response ranges of several gigahertz (e.g., 1-18 GHz), it is possible for 

the interfering signal and the victim receiver’s desired (tuned) frequency to be hundreds 

of megahertz or even some gigahertz apart. The gain reduction in the LNA causes loss of 

the desired signal in the victim receiver. 

4
Co-channel interference occurs when out-of-band (OOB) or spurious emissions (together 

referred to as unwanted emissions) from a radar transmitter occur on the tuned frequency 

of a victim receiver at a power level that is high enough to cause degradation of the 

victim receiver’s output.

Figure 4. Schematic depiction of the co-channel interference mechanism. 

Other types of RF interference exist, but they are not so easily defined as broad 

categories. They include such problems as inadequate grounding of the power supply or 

RF circuitry of the victim receiver; case penetration due to inadequate RF shielding of 

critical components; non-linear diode-rectification effects of incident signals near the 

victim receiver (the so-called rusty-bolt or rusty-fence effect); and non-linear receiver 

effects (other than the front-end overload effect described above) caused by poor receiver 

design. In this study, a receiver design problem was discovered to be a contributor to the 

overall interference problem, and it had to be resolved before any other mitigation 

solutions could be effectively implemented. 

2.2 Mitigation Techniques 

Filtering solutions are effective for each of the two major interference mechanisms

described above, but the type and location of the filtering that must be implemented will 

depend critically on the type of interference that is occurring. The solutions are 

summarized in Table 3. 

5
Table 3. Mitigation Solutions as a Function of Interference Mechanism.

Interference Mechanism Mitigation Solution 

RF front-end overload (due to lack of 

bandpass filtering in the receiver) 

Bandpass filter installation on the victim

receiver RF front-end 

Co-channel (due to unwanted emissions 

from the transmitter) 

Notch, highpass, or lowpass filter 

installation on the transmitter

Other categories referred to in the text Various, innovative solutions depending 

upon the problem

The critical difference between the solutions for the two major interference mechanisms

is that RF front-end overload interference can be mitigated only by installing bandpass 

filtering on the RF front-end of the victim receiver. Co-channel interference, in contrast, 

can be mitigated only by installing filtering (usually either notch, highpass, or lowpass) 

on the output stage of the transmitter that is generating the interfering energy. 

2.3 Diagnostics for Interference Mechanisms 

Given the criticality of identifying the interference mechanism that is occurring so that 

the proper solution can be tested most efficiently (and that valuable time and effort not be 

wasted on mitigation approaches that will not work), it is essential to understand the 

observations that must be performed to identify the interference mechanism that is 

occurring. These observations are described in detail in [1], and are summarized here 

from that source. 

2.3.1 RF Front-End Overload Diagnostics 

The diagnostic observation for RF front-end overload due to radar pulses is gain 

compression behavior of the front-end LNA. This effect can be observed in the time

domain at the LNA output, and also in the IF stage of the victim receiver (assuming that 

the LNA excess noise output determines the receiver noise level in the IF stage). This 

behavior is manifested as a decrease in the noise floor of the LNA, and is easily observed 

with a spectrum analyzer. An example of this effect is shown in Figure 5 (extracted 

from [1]). 

In Figure 5, an LNA was subjected to high-amplitude inputs from a pulsed signal source 

in a laboratory, and its responses were measured as a function of interference amplitude. 

The result was a series of curves showing gain compression and recovery as a function of 

interference amplitude. On these curves, interference pulses are injected at time t=0 and 

the gain response is compressed to greater and greater degrees as interference amplitude 

increases. The gain compression is observed as a decrease in the LNA output noise by 

amounts ranging from 10 dB to 40 dB. Subsequently, the LNA gradually recovers over 

time intervals that range from 100 μs to 900 μs. Other LNAs will exhibit quantitative 

variations on this response, as documented in [1], but qualitatively the overload effects 

will always be the same as those shown in Figure 5. 

6
Figure 5. Example of gain compression at an LNA output for varying levels of 

overload. 

2.3.2 Co-channel Interference Diagnostics 

The diagnostic observation for co-channel interference due to radar unwanted emissions 

is ragged or irregular pulse shapes, as measured on the victim receiver’s frequency. The 

physical phenomenon is as follows. When a measurement of pulse shape is performed at 

the fundamental frequency of a pulsed source (such as a radar), the Fourier lines 

convolved by the measurement filter will produce a good approximation of the nominal 

pulse shape envelope in the time domain. An example of this behavior is shown in the top 

row of Figure 6 for long-pulse and short-pulse modes of a radar transmitter. 

However, when the same measurement filter is tuned to any given frequency in the 

unwanted (OOB or spurious) portions of the pulsed-emission spectrum, the Fourier lines 

that it convolves will fail to produce a smooth, clean pulse shape envelope in the time 

domain. Instead, the observed pulse envelopes will either appear ragged, as shown in the 

lower right-hand corner of Figure 6, or else will exhibit a pair of spikes (with a spacing 

equal to the nominal pulse width), as shown in the lower left-hand corner of Figure 6. 

When a victim receiver is affected by co-channel interference due to unwanted emissions 

from a radar, its bandpass characteristic will convolve a subset of Fourier lines in the 

portion of the interferer’s spectrum that corresponds to its tuned frequency. The result 

will be that the interference pulses at either the receiver’s LNA output or the victim

receiver’s IF stage will have the same appearance as the pulse envelopes in the lower row 

of Figure 6. More details on the physics of this phenomenon are provided in [1]. 

7
Figure 6. Examples of comparative pulse shapes at a radar fundamental frequency in 

long pulse and short pulse modes (left and right, respectively, in the upper row), versus 

examples of the corresponding pulse shapes that are observed at off-tuned frequencies in 

the unwanted emission spectrum in the same modes (lower row). The ragged, or ‘rabbit 

ears’ shapes of the pulses at the off-tuned measurement frequencies are the result of 

measurement bandwidth-limiting of the Fourier lines in the radar unwanted emission 

spectrum.

2.4 Measurements to Determine Interference Mechanism 

Given the clear-cut differences between the two types of interference mechanisms, the 

observations that need to be performed to determine which mechanism is occurring are 

straightforward. At the output of the LNA in the receiver’s RF front-end, or else (or in 

addition) in the IF stage of the victim receiver, the interference pulses should be observed 

in the time domain. In particular, the pulse envelopes should be resolved. Inspection 

needs to be performed for the presence of gain compression (such as depicted in 

Figure 5) in the intervals immediately following the interference pulses, and the pulse 

envelopes should be inspected for a ragged or double-spiked (so-called rabbit ears) 

appearance, as depicted in Figure 6. Depending upon which effect is observed, the 

interference mechanism is diagnosed and the method for resolving the problem is as 

specified in Table 3. 

8
In the event that neither effect is observed, it is likely that some sort of non-linear 

behavior is occurring in the victim receiver, and the receiver’s design and construction 

need to be examined for possible sources of such behavior. As will be seen, such an 

effect was found to occur in the course of this case study. 

2.5 Supporting Measurements 

Additional measurements need to be made when interference is caused by a radar 

transmitter (i.e., co-channel interference due to a radar’s unwanted emissions). In 

particular, the emission spectrum of the radar that is suspected as the source of

interference needs to be measured to determine whether it complies with applicable 

emission mask limits. For radar transmitters inside the U.S., the NTIA Radar Spectrum 

Engineering Criteria (RSEC), as described in [2], describe the applicable mask limit. 

Conformance with the RSEC mask limit by a radar’s emissions does not guarantee that 

interference will not be caused by that radar’s unwanted emissions. But conformance 

versus non-conformance with the mask may be an important factor in determining which 

entities may be responsible for providing the necessary resources to filter the emissions 

of the radar in question. 

When such emission spectrum measurements need to be performed, the procedures given 

in [3] should be followed. The emission measurements need to be performed on radiated 

energy from the transmitter, rather than through hardline coupling from the transmitter.2

Selection of the proper measurement bandwidth is absolutely critical. For transmitters 

with unmodulated pulses, the proper measurement bandwidth is less than (1/pulse width). 

For chirped-pulse emissions, the proper measurement bandwidth is less than (Bc/τ)

1/2, 

where Bc = chirp bandwidth and τ = pulse width. See [3] for additional information about 

proper measurement bandwidths, measurement algorithms, measurement system

hardware design, and other critical aspects of RSEC-compliance measurement 

techniques. 

In addition to the radar emission spectrum, the radar pulse waveforms, pulse repetition 

sequence, and antenna pattern should all be measured and recorded as described in [3]. 

2.6 Approach to Measurements

Given the goals for measurements and diagnostic observations as outlined above, the 

overall approach that is most often successful is to measure the interfering energy 

beginning with the radiation from the transmitter that is thought to be causing 

interference. Then the energy from that transmitter needs to be traced through the victim 

receiver starting at its antenna output, progressing through each stage of its RF front-end 

after the antenna, and then continuing to trace the energy through its IF stages up to the 

detector (or equivalent signal-processing terminal point). At each of these stages, the 

 

2

 Hardline-coupled measurements have been found to contain modes that do not occur in 

radiated emissions. See [3] for more details on the trade-offs between radiated and 

hardline-coupled measurements. 

9
spectrum and time waveform characteristics of the interfering energy need to be 

measured, with care given to thorough documentation. The diagnostics described above 

for front-end overload and co-channel interference need to be applied at each stage. 

Although unexpected behaviors may occur in the victim receiver, as described in this 

case study, this approach should nevertheless ultimately yield a successful conclusion to 

the effort to determine the interference mechanism and resolve the interference problem. 

Unexpected behaviors, if any occur, will inevitably be revealed by carefully tracing the

interfering energy through the stages of the victim receiver. 

10
3 ON-SITE INTERFERENCE RESOLUTION MEASUREMENTS 

On-site interference resolution measurements were performed at Ft. Smith, Arkansas 

January 24-26, 2006. The purpose of the measurements was to ascertain the interference 

mechanism and implement one or more solutions for the problem. Broadly, the 

measurement sequence was as described in Section 2.6. Emission spectrum and time 

waveform measurements were performed on the radiated energy from the airport radar, 

and then the spectrum and time waveforms of the energy from the airport radar were 

measured inside the RF front-end and IF stages of the weather radar. The diagnostic 

criteria described in the previous section were used to ascertain the interference 

mechanism, as well as likely solutions. This section describes the interference resolution 

measurements and their results. 

3.1 Measurements of the Airport Radar’s Radiated Emissions 

The NTIA/ITS Radio Spectrum Measurement System, fourth generation (RSMS-4), as 

shown in Figure 7, was used to perform radiated emission spectrum and time waveform

measurements on the airport radar. 

Figure 7. NTIA/ITS RSMS-4 vehicle. Major elements include an RF-shielded 

equipment enclosure, two telescoping masts, and an on-board power generator. 

For these measurements, the RSMS was positioned on the same hilltop as the weather 

radar, where clear line-of-sight propagation was available to the airport radar antenna.

The separation distance between the RSMS and the radar was determined with a laser 

range finder to be 489 m (530 yds). 

The measurements proceeded in accordance with the techniques specified in [3]. Two 

independent measurement systems, one that had been used for many years and another 

that was new, were used to gather data. Each measurement system consisted of a 1-m 

11
diameter parabolic dish antenna, a low-noise, frequency-selective RF front-end, a 

spectrum analyzer, and a controller computer. Figure 8 shows a block diagram of the 

measurement system RF front-ends. It is emphasized this design for a measurement 

system is absolutely essential for successful and accurate measurements of radar emission 

spectra, as described in [3]. The emission spectrum measurement rate was accelerated by 

halting the rotation of the airport radar antenna and boresighting it directly on the RSMS 

location. 

Figure 8. Block diagram of radar spectrum measurement system, based on ideal design 

given in [3]. 

The measurements were performed in several bandwidths, and two of those bandwidths 

in particular were 680 kHz and 200 kHz. Each of these bandwidths corresponded to one 

of the IF bandwidths of the victim receiver. By coincidence, the 200 kHz bandwidth also 

happened to be nearly the same as (Bc/τ)

1/2, the ideal measurement bandwidth for the 

RSEC compliance measurement for this radar (computed as 212 kHz in Table 1). Thus 

two measurement bandwidths satisfied three measurement needs. The results of the 

spectrum measurement are shown in Figure 9. 

12
Figure 9. Radiated emission spectra of the airport radar with RSEC mask. 

As shown in Figure 9, the radiated emission spectrum levels at the frequency of the 

weather radar (35 MHz above the frequency of the airport radar) are -94 dB relative to 

the radiated power level at the fundamental in the 680 kHz bandwidth, and -103 dB 

relative to the radiated fundamental power level in the 200 kHz bandwidth. The roll-off 

rate seen in Figure 9 is considered to be good compared to emission spectra that have 

been measured from other radars with a similar mission and design. It is noted that the 

110-dB dynamic range of the measured spectrum in Figure 9 is only achievable with the 

kind of measurement system hardware and software that were used at this location, as 

described in [3]. 

Based on a pulse rise time measurement (radiated) of 0.15 μs, a pulse width of 89 μs, a 

chirp range of 4 MHz, and a roll-off rate of 40 dB per decade, the RSEC emission mask 

is as drawn in Figure 9. The mask applies to the data taken in 200 kHz. The radar 

emission meets the RSEC emission mask limit. 

Figure 10 shows the time waveform envelope that is emitted by the airport radar. The 

pulses occur in pairs spaced 1.117 ms apart. Each pair consists of a 1-μs, unmodulated 

pulse followed by an 89-μs pulse that is chirped across 4 MHz of bandwidth. 

13
Figure 10. Detected time waveform envelopes of a radiated pair of airport radar pulses. 

The first pulse is unmodulated, and the second is FM-modulated (chirped). 

3.2 Measurements Inside the Weather Radar RF Front-End 

Figure 11 shows a highly simplified block diagram schematic of the weather radar 

receiver; monitoring points are noted in that diagram. RF protection was provided in the 

front end by a passive diode limiter (PDL) and an electromagnetic interference (EMI) 

bandpass filter, shown in Figures 12 and 13, respectively. 

The spectrum at the outputs of the weather radar antenna front-end 20-dB coupler and 

passive diode limiter (PDL) are shown in Figures 14 and 15, respectively. Nothing 

surprising was noted in these spectra. But when the spectrum was measured at the output 

of the EMI filter, the curve of Figure 16 was observed. This result was surprising, as 

indicated by the superposed curves of the PDL and EMI output spectra (from Figures 15-

16) in Figure 17. 

It was concluded that a non-linear interaction was occurring between the PDL and the 

EMI filter, such that when the PDL was connected to the EMI filter input, and highamplitude interference energy was injected into the PDL, the result was a non-linear 

output at the EMI filter output. The injected energy level of +5 dBm at 2860 MHz was 

redistributed across a wide spectrum range (at a density of -25 dBm in the measurement 

bandwidth), illuminating the bandpass shape of the EMI filter (and limited by that 

bandpass, as shown in Figures 16-17). 

14
Figure 11. Simplified block diagram schematic of the weather radar receiver. 

Figure 12. Weather radar receiver passive diode limiter. 

Figure 13. Weather radar receiver EMI bandpass filter. 

15
Figure 14. Airport radar spectrum measured through the weather radar receiver’s RF 

front-end 20-dB coupler. 

Figure 15. Airport radar spectrum measured at the output of the passive diode limiter. 

Nothing surprising had been noted up to the point that the spectrum was measured at the 

output of the PDL. But when the spectrum was measured at the output of the 

electromagnetic interference suppression bandpass filter (EMI filter), the result was as 

shown in the next figure (Figure 16). 

16
Figure 16. Spectrum measured at output of EMI filter. This should have looked like a 

band-limited version of the curve in Figure 15, but it did not. 

Figure 17. PDL and EMI output spectra overlaid on each other, from Figures 15 and 16. 

17
To ascertain the nature of the effect, the PDL was removed from the system and the 

spectrum was again measured at the output of the EMI filter, the result being shown in 

Figure 18. Then the PDL was reintroduced into the system immediately after the EMI 

filter. The resulting spectrum for this configuration at the PDL output is shown in 

Figure 19. The results obtained when the PDL was removed entirely versus when the 

PDL was placed after the EMI filter were nearly identical. In Figure 20, all of the curves 

from the previous set of figures are overlaid to demonstrate more clearly the relationships 

between the results obtained for all of these configurations. 

The explanation for this behavior probably lies in the way that PDL’s are supposed to 

operate. In principle, a PDL output should be connected to the input of an LNA to 

prevent physical damage to the LNA due to high-power signals. The PDL output 

impedance is ordinarily supposed to be high. But when the PDL is subjected to a signal 

power level that exceeds a certain threshold, the PDL output impedance is supposed to go 

to a low value, as close to zero as possible. The impedance change should then cause the 

high-power signal to be reflected backward out of the PDL’s (nominal) input port. In 

effect, the PDL is supposed to function like a fast-response, pop-up electromagnetic 

mirror. 

In reality, when the PDL output was connected to the EMI filter input, the PDL output 

impedance may not have dropped as low as desired. As a result, when high energy levels 

impinged on the PDL, some of that energy was still passed forward (with a wideband 

spectrum distribution). This non-linear response was eliminated by moving the PDL to a 

location after the weather radar EMI filter. The response of the LNA to the energy from

the airport radar was as shown in Figure 21. 

Figure 18. Spectrum at EMI filter output when the PDL was removed from the system. 

18
Figure 19. Spectrum at PDL output when PDL was placed after the EMI filter; the 

result is virtually identical to the data curve of Figure 18. 

Figure 20. Comparative results for various PDL and EMI configurations. 

19
Figure 21. Response of the LNA to the airport radar energy when the PDL was placed 

after the EMI filter. 

Even when the PDL was in its original location, ahead of the EMI filter, time-domain 

pulse envelope waveforms showed no indication of front-end overload, as shown in 

Figure 22. In this figure, neither front-end overload gain compression nor the ragged, 

rabbit-ears envelopes of unwanted emissions are observed—a seemingly paradoxical 

result that could not be resolved until the non-linear behavior of this front-end 

configuration was understood. This paradox had apparently confounded previous 

attempts to resolve the interference problem between these radars. 

With the non-linear front-end combination in place (that is, with the PDL in front of the 

EMI filter), the power injected into the LNA input, as high as it was, was still not 

sufficient to push the LNA into gain compression. But the time waveform going into the 

LNA and being amplified was not that of the airport radar’s unwanted emissions 

occurring on the weather radar fundamental frequency. Instead, the airport radar’s 

fundamental frequency energy (separated by 35 MHz from the weather radar receiver’s 

tuned frequency) was spectrally redistributed inside the weather radar receiver’s RF 

front-end across a range of frequencies that included the weather radar fundamental 

frequency. That was the effect of the non-linear element, the PDL, acting at the input port 

of the EMI bandpass filter. 

20
Figure 22. Time domain waveform at the output of the LNA when the PDL was located 

in its original position, in front of the EMI filter. 

Until this effect was finally understood, any attempts to resolve the interference problem 

would be futile. For example, the installation of any sort of filter on the output of the 

airport radar would have been completely ineffective. 

Other than moving the PDL behind the EMI filter (or removing it from the system

entirely), only one other technical solution was possible: To place a notch filter in front of 

the PDL, the notch frequency being tuned to the fundamental frequency of the airport 

radar. This experiment was tried, and the resulting spectrum is shown in Figure 23. 

With the notch filter in place, the LNA output spectrum was the same as when the PDL 

was either removed entirely or was placed behind the EMI filter. But this would not be 

considered a satisfactory solution for the weather radar, inasmuch as the insertion loss of

the notch, which was several decibels, would add directly to the noise figure of the 

weather radar and would be expected to considerably degrade its performance. In the end, 

the notch filter experiment served mainly to confirm that the PDL must not be allowed to 

experience a high input signal level, lest it interact with the EMI bandpass filter in such a 

way as to generate the output of Figure 16. 

21
Figure 23. Spectrum at LNA output when a notch filter was placed in front of the PDL. 

The PDL was in its normal location in front of the EMI filter. 

3.3 Measurements Inside the Weather Radar IF Stage 

With the non-linear behavior in the RF front-end understood, and no indication at the 

LNA output that front-end overload was occurring (even with the non-linear behavior, 

much less when the non-linear behavior was corrected), the next step was to perform

measurements within the IF stage of the weather radar. Figure 24 shows the radar IF 

spectrum when no interference was present (i.e., when the airport radar transmitter was 

turned off), as measured at the “first available IF monitor point used in this report” shown 

in Figure 11.3, 4

 This spectrum was generated with a peak detector; the RMS average 

noise level would have been about 10 dB lower. 

 

3

 Subsequent IF-stage measurements were also performed at this point, unless otherwise 

indicated. 

4

 It is noted that the average level of the weather radar noise floor is -80 dBm; the IF 

noise floor observed in Figure 24 is peak-detected rather than average, and has not been 

corrected for the receiver’s gain at that point. The level shown in Figure 24 is used only 

as a reference for the data shown in subsequent figures (such as Figure 30) in this report. 

22
Figure 25 shows the weather radar IF spectrum with the same measurement configuration 

as for Figure 24, but when the airport radar transmitter was radiating. The RF front-end 

was configured in its non-linear combination, with the PDL ahead of the EMI filter. Here, 

the interference level is about 85 dB higher than the IF noise level from the previous 

figure. 

In Figure 26, the weather radar IF spectrum is shown when the PDL was placed after the 

EMI filter; Figure 27 overlays these curves for comparison. When the PDL was placed 

after the EMI filter, the interference to noise (I/N) ratio in the weather radar IF stage was 

reduced by about 45 dB relative to the level in Figure 26, with the original front-end 

configuration. 

Figure 24. Weather radar IF spectrum when no interference was present (i.e., the airport 

radar transmitter was turned off). The elevated noise level that is observed is due to the 

normal output of the weather radar IF amplification stage. The elevated region of interest 

is between approximately 45-70 MHz. IF noise level is peak-detected, and has not been 

corrected for receiver gain. This (uncorrected) IF noise level shown in this figure is only 

supposed to be used as a reference for subsequent depictions of the weather radar IF 

spectrum in this report. The origin of the bump at 35 MHz was not investigated; it may be 

an image frequency. 

23
Figure 25. IF spectrum in the presence of interference from the airport radar, with the 

RF front-end arranged in its non-linear configuration. 

Figure 26. Weather radar IF spectrum in the presence of interference from the airport 

radar, when the PDL was placed after the EMI filter. The peak interference level has 

been reduced by 45 dB relative to the level in Figure 25. The origin of the bump at 

24 MHz was not investigated. 

24
Figure 27. Comparison of curves from Figures 25 and 26. 

3.4 Isolation of the Interference Mechanism 

It had become clear that, with the non-linear problem discovered and solved, and lacking 

the presence of front-end overload gain compression in the RF front-end, the operative 

interference mechanism had probably been reduced to simple co-channel interference 

from the airport radar’s unwanted emissions on the weather radar’s frequency. This 

hypothesis was confirmed by measurement of the interference pulse time waveform

envelope, as shown in Figure 28. Here, the 89-μs pulse displays the classic rabbit-ears

diagnostic signature of an unwanted spectrum emission observed in the time domain. The 

waveform of Figure 28 was recorded when the PDL had been placed after the EMI filter. 

Since this was the case, the (approximately) 40-dB I/N level that was achieved with this 

front-end configuration would now behave linearly, which is to say that every decibel of 

attenuation that could be achieved in the external interference input would translate into a 

corresponding reduction in the I/N level inside the receiver. This behavior is 

demonstrated in the comparative data curves of Figure 29. In this figure, the response of 

the weather radar IF stage is observed when the unwanted emissions of the airport were 

reduced under two cases: 1) the PDL located in front of the EMI filter and 2) the PDL 

located after the EMI filter. In the first case, attenuating the airport radar unwanted 

emissions (with a notch filter on the airport radar’s output) caused no effect on the I/N

25
level, as evidenced by the identical blue and red curves in the figure. In the second case, 

attenuation of the airport radar signal did result in a reduction in the I/N level, as 

evidenced by the difference between the black and green curves in the figure. 

When a notch filter was installed in the airport radar output (with the weather radar RF

front-end in its modified configuration), the level of the interference strobe was reduced. 

This is the configuration of the airport radar and the weather radar that is represented by 

the green data curve in Figure 29. 

Above all else, the data in Figure 29 demonstrate that the interference problem cannot be 

solved unless the weather radar RF front-end is reconfigured, and furthermore that after 

such a reconfiguration, the problem becomes one of merely filtering the unwanted 

emissions of the airport radar to a sufficient degree to eliminate interference effects in the 

weather radar. 

Figure 28. Diagnostic confirmation of co-channel interference due to unwanted 

emissions. These data were collected when the PDL had been placed after the EMI filter. 

26
Figure 29. Comparative weather radar responses to reductions in unwanted emission 

interference energy, as a function of weather radar configuration. 

3.5 Experiments with Notch Filtering of the Airport Radar 

Considering that reductions in the unwanted emission levels of the airport radar would 

reduce the I/N level of the interference in the weather radar IF stage, an experiment was 

performed in which the airport radar was tuned from its initial frequency of 2860 MHz to 

a new frequency 85 MHz lower, at 2775 MHz. Such off-tuning was found to provide a 

10-dB reduction in the unwanted emission level on the weather radar frequency. When

the notch filter was installed on the airport radar output, an improvement was noted in the 

I/N level in the weather radar’s IF stage, as shown in Figure 30. 

In Figure 30, the noise floor that is seen in the center of the notch shape is in fact the 

noise floor of the weather radar IF stage5

; this is a desirable result. The notch is simply 

not quite wide enough to fully reduce the interference energy across the full bandwidth of 

the IF stage.

 

5

 The IF noise level shown in Figure 30 is peak-detected and uncorrected for receiver 

gain, as previously noted in Figure 24. The weather radar’s true (corrected, average) IF 

noise floor level is actually -80 dBm; the IF noise floor level shown in Figure 24 should 

be used as a reference for Figure 30. 

27
Figure 30. Comparison of the effect of the notch filter on the airport radar output when 

the tuned frequencies were 2860 MHz vs. 2775 MHz. 

Figure 31. Time domain envelope of interference pulses when notch filtering (as shown 

in Figure 30) was installed on the airport radar output. 

28
The effect of the notch filter was very pronounced in the time domain as well. There, the 

effect was to shorten the pulse pairs, from the original 1-μs+89-μs to a new set that looks 

more like 1-μs+1-μs. This is seen in Figure 31. The data in this figure show that, with the 

notch installed, the effective duty cycle of the interference in the weather radar receiver 

has been reduced from a nominal value of 8% (the duty cycle of the true airport radar 

pulse repetition sequence) to a new value that is about 0.2%. 

3.6 Experiments with Variation of I/N Level in the Weather Radar Receiver 

The final stage of the field work was to ascertain the critical I/N level at which 

interference from the notch-filtered airport radar would be reduced to a level at which no 

adverse effects were noted in the weather radar performance. Such experiments are 

difficult to perform because the weather radar is a data collection system that feeds its 

outputs into an off-site data processing system. That system generates weather ‘products’ 

that represent successively more complex layers of data processing. It is impossible to 

obtain immediate feedback on the results of testing with varying I/N levels; field teams 

who perform such work are therefore forced to work largely blind. 

With that said, preliminary experiments were performed in which attenuation was 

inserted into the weather radar receiver, and also in which individual power output 

modules in the airport radar were taken out of service, thereby gradually reducing that 

radar’s output power level. At the time of this report’s completion, the results of those 

experiments indicate that more such experimental work will need to be performed at the 

weather radar site to ascertain definitive results on that topic. Among other concerns, data 

sets need to be collected under a wide range of weather conditions, whereas only a single 

day, with clear weather, was available at the field site for the first round of I/N tests that 

were performed. 

29
4 SUMMARY

Although RF interference problems are often difficult and expensive to solve, a 

methodical approach, combined with a thorough understanding of the possible 

mechanisms of such interference, can (and almost inevitably will) yield positive results. 

As demonstrated by the results of this case study, even the occurrence of unexpected 

interference mechanisms should be revealed by a well-considered approach. 

In this case, the interference was caused by unwanted emissions of an ASR on the 

operating frequency of a weather radar. But due to an unexpected, non-linear behavior in 

the RF front-end of the victim receiver, no workable solution could be implemented until 

the configuration of the victim receiver’s RF front-end had been modified. Specifically, 

the PDL in that front-end had to be placed after the EMI bandpass filter before any 

technically workable solution could be implemented. 

With the RF front-end configuration of the weather radar having been modified in this 

way, the interference problem was reduced to a straightforward exercise of determining 

the amount of attenuation that will be required in the levels of the unwanted emissions 

that cause interference. It is anticipated that such attenuation will be implemented by 

installing filtering on the output of the ASR that is causing interference at the location in 

question. This is the conventional solution to co-channel interference problems. 

The determination of the necessary amount of attenuation that the output filter must 

achieve will await the completion of additional, definitive measurements by the NWS 

with the cooperation of the FAA. If a notch filter (as opposed to either a highpass or 

lowpass filter) is ultimately used on the ASR output, it will need to be about twice as 

wide as the one used for the experimental testing. 

30
5 REFERENCES 

[1] F.H. Sanders, R.L. Hinkle, and B.J. Ramsey, “Analysis of electromagnetic compatibility 

between radar stations and 4 GHz fixed-satellite Earth stations,” NTIA Report 94-313, 

Jul. 1994. 

[2] Manual of Regulations and Procedures for Federal Radio Frequency Management, May 

2003, Revisions January 2006, NTIA Office of Spectrum Management, U.S. Government 

Printing Office, Stock No. 903-008-00000-8. 

[3] F.H. Sanders, R.L. Hinkle, and B.J. Ramsey, “Measurement procedures for the Radar 

Spectrum Engineering Criteria (RSEC),” NTIA Report TR-05-420, Mar. 2005. 

31
FORM NTIA-29 U.S. DEPARTMENT OF COMMERCE 

(4-80) NAT’L. TELECOMMUNICATIONS AND INFORMATION ADMINISTRATION 

BIBLIOGRAPHIC DATA SHEET 

1. PUBLICATION NO. 

TM-06-439 

2. Government Accession No. 3. Recipient’s Accession No.

5. Publication Date 

April 2006

4. TITLE AND SUBTITLE 

Resolving Interference from an Airport Surveillance Radar to a Weather Radar 

6. Performing Organization Code 

NTIA/ITS

7. AUTHOR(S) 

Frank H. Sanders, J. Randy Hoffman, and Yeh Lo

9. Project/Task/Work Unit No.

611 6611000-300

8. PERFORMING ORGANIZATION NAME AND ADDRESS 

NTIA/ITS.T 

U.S. Department of Commerce 

325 Broadway

Boulder, CO 80305

10. Contract/Grant No.

11. Sponsoring Organization Name and Address 12. Type of Report and Period Covered 

14. SUPPLEMENTARY NOTES 

15. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.)

In response to interference from an S-band (2700-2900 MHz) airport surveillance radar (ASR) to a meteorological

(weather) radar in the same band, measurements were performed at the field location of the two radars to determine 

the interference mechanism and any possible mitigation options. Measurements included emission spectra of the 

ASR and observations of the interference energy in the RF front-end and IF stages of the weather radar. 

Measurement results showed that interference energy originated in the unwanted emissions of the ASR (i.e., frontend overload was not occurring in the weather radar). But the problem was exacerbated by the placement of a

passive diode limiter ahead of a bandpass filter in the weather radar receiver’s RF front-end. The interference could 

not be mitigated unless the front-end configuration of the weather radar was modified. With the necessary 

modification completed, the interference was successfully mitigated by installing a conventional notch filter on the 

ASR’s output stage, the notch being tuned to the weather radar frequency. It is recommended that the front-end

configuration of all weather radars of the type in question should be immediately changed in the same way as the 

weather radar in this study, and that appropriate output filters should be installed in ASRs that are located in close 

proximity to these weather radars to mitigate interference effects at all sites in the U.S. 

16. Key Words (Alphabetical order, separated by semicolons)

airport surveillance radar interference; radar co-channel interference; radar emission spectrum measurements; radar 

interference mechanisms; radar interference mitigation; Radar Spectrum Engineering Criteria (RSEC); RF front-end 

overload; weather radar interference 

18. Security Class. (This report) 20. Number of pages 

31 

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