NTIA Technical Report TR-11-473

Case Study: Investigation of Interference 

into 5 GHz Weather Radars from 

Unlicensed National Information 

Infrastructure Devices, Part I 

John E. Carroll

Frank H. Sanders

Robert L. Sole

Geoffrey A. Sanders

NTIA Technical Report TR-11-473

Case Study: Investigation of Interference 

into 5 GHz Weather Radars from 

Unlicensed National Information 

Infrastructure Devices, Part I 

John E. Carroll

Frank H. Sanders

Robert L. Sole

Geoffrey A. Sanders

November 2010

iii

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. 

v 

CONTENTS

PAGE

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

2 DYNAMIC FREQUENCY SELECTION OVERVIEW .............................................................4

3 TDWR MEASUREMENT RESULTS .........................................................................................5

3.1 TDWR LNA, RF Front-end Bandpass Filter, and IF Output Frequency Response .........5

3.2 PPI Azimuths of Interference Strobes ..............................................................................8

3.3 U-NII Signals at Antenna Waveguide Output, Bandpass Filter Output, and IF Output ..9

3.4 Summary of Interference Signals Coupling into the TDWR in San Juan ......................14

4 SIGNAL ENVIRONMENT MEASUREMENTS ......................................................................15

4.1 Measurements of TDWR Signal at Rooftops .................................................................15

5 MEASUREMENTS OF U-NII DEVICES AGAINST ACTUAL TDWR EMISSIONS AND 

BIN 1 WAVEFORM PULSES ................................................................................................19

5.1 DFS Tests on a Model 2 U-NII Device from Manufacturer A ......................................19

5.2 DFS Tests on a Model 1 U-NII device from Manufacturer B........................................21

5.3 DFS Tests on a Model 1 U-NII Device from Manufacturer C .......................................21

5.4 Overall Results of U-NII Device Performance Against the San Juan TDWR and Bin 1 

Waveform Pulse Bursts. .......................................................................................................22

6 SUMMARY OF RESULTS. ......................................................................................................23

6.1 General Results ..............................................................................................................23

6.2 TDWR Results ...............................................................................................................23

6.3 Signal Environment Results ...........................................................................................24

6.4 Results for the Identified U-NII Devices .......................................................................24

7 REFERENCES ...........................................................................................................................26

8 ACKNOWLEDGEMENTS ........................................................................................................27
vi

FIGURES

Page

Figure 1. An example of interference artifacts on the San Juan TDWR display. ........................... 2

Figure 2. TDWR component frequency response measurement points. ........................................ 5

Figure 3. TDWR RF front-end LNA frequency response. ............................................................. 6

Figure 4. TDWR RF front-end bandpass filter frequency response; normalized to 0 dB. ............. 7

Figure 5. TDWR IF output frequency response; normalized to 0 dB. ............................................ 7

Figure 6. Linearity measurement result for the TDWR front-end LNA. ........................................ 8

Figure 7. TDWR measurement point configuration. .................................................................... 10

Figure 8. U-NII signals at the TDWR antenna waveguide output at the 154 degree azimuth over 

a 1 GHz span. ................................................................................................................................ 11

Figure 9. U-NII signals at the TDWR antenna waveguide output at the 154 degree azimuth over 

a 50 MHz span. ............................................................................................................................. 11

Figure 10. Time-domain view of interference at the TDWR antenna waveguide output at 154 

degree azimuth tuned to 5610 MHz. ............................................................................................. 12

Figure 11. Time-domain view of interference at the TDWR IF output at 154 degree azimuth, 

tuned to 30 MHz. .......................................................................................................................... 13

Figure 12. TDWR bandpass filter effect on off-tuned signals in the radar receiver. .................... 13

Figure 13. The time-domain behavior of the San Juan TDWR measured through one complete 

volume scan at a distance of 8.6 miles. ......................................................................................... 16

Figure 14. A sequence of pulses in the TDWR backlobe (black) compared to the Bin 1 

Waveform sequence (red). ............................................................................................................ 17

Figure 15. A radiated single-pulse envelope from the TDWR in the backlobe of the antenna 

pattern (black) compared to Bin 1 Waveform pulse (red). ........................................................... 18
vii

TABLES

Page

Table 1. Azimuths and Corresponding Elevation Angles where Interference was Identified ........ 9

Table 2. Summary of Interference Measured Within the TDWR Receiver at San Juan .............. 14

Table 3. TDWR Maximum-Power Measurement Sites and Values in the San Juan Area ........... 15

Table 4. U-NII Devices Tested Against Actual TDWR and Bin 1 Waveform in San Juan ......... 19

Table 5. Summary of DFS Performance of U-NII Devices Tested In Situ Against the San Juan 

TDWR and Bin 1 Waveform Pulse Bursts. .................................................................................. 22

ABBREVIATIONS/ACRONYMS

CW Carrier Wave

dB decibel

dBi decibels relative to isotropic

dBm decibels relative to 1 milliwatt

DFS Dynamic Frequency Selection

DOC Department of Commerce

EIRP Equivalent Isotropically Radiated Power

FAA Federal Aviation Administration

FCC Federal Communications Commission

GHz gigahertz

IF Intermediate Frequency

I/N Interference-to-Noise Ratio

ITS Institute for Telecommunication Sciences

LNA Low Noise Amplifier

MHz megahertz

MSL Mean Sea Level

mW milliwatt

NTIA National Telecommunications and Information Administration

OSM Office of Spectrum Management

PPI Plan Position Indicator

PRI Pulse Repetition Interval

RF Radio Frequency 

RMS Root Mean Square 

TDWR Terminal Doppler Weather Radar

U-NII Unlicensed National Information Infrastructure

μs microsecond

VSG Vector Signal Generator

WISP Wireless Internet Service Provider 

ix

EXECUTIVE SUMMARY

In early 2009, the Federal Aviation Administration (FAA) became aware of interference to their 

Terminal Doppler Weather Radars (TDWR) that operate in the 5600–5650 MHz band, and 

provide quantitative measurements of gust fronts, windshear, microbursts, and other weather 

hazards for improving the safety of operations in and around major airports (45 airports at the 

time of this writing). Engineers from both the Institute for Telecommunication Sciences (ITS), 

part of the National Telecommunications and Information Administration (NTIA), and the FAA 

traveled to a site of reported TDWR interference in San Juan, Puerto Rico. They performed an 

extensive series of field measurements in the San Juan area to determine the cause of the 

interference. 

When ITS engineers arrived, they noted ten TDWR azimuths experiencing performance 

degradation due to interference. With the assistance of FAA engineers, ITS engineers 

systematically measured and traced the interference through the various information processing 

stages of the TDWR for all ten interference azimuths and measured individual component 

specifications of the radar. Through these tests, it was determined that the interference sources 

were unlicensed national information infrastructure (U-NII) dynamic frequency selection (DFS) 

devices, from different manufacturers, operating in the same frequency band as these Federal 

radar systems. These devices employ technology that are supposed to detect the presence of 

nearby radar systems and change operating frequencies to prevent interference with an

incumbent radar system. 

After determining the interference source, ITS engineers, along with the local Federal 

Communication Commission (FCC) field agent, began locating the DFS devices on nearby 

towers and surrounding building rooftops with the intent of documenting device manufacturers, 

model numbers, FCC IDs, and, to the extent possible, software versions, responsible for causing 

interference. ITS then performed in situ (outdoor) and indoor performance tests of the devices 

identified as causing interference to characterize the device behavior and the signal environment 

they operate in. These field measurements and performance tests are described in this report. 

This report is Part I of a series of three reports covering this interference mitigation effort.

After obtaining the initial results reported here, ITS performed additional testing in controlled 

environments to better understand how to resolve the interference, beyond the immediate, ad-hoc 

solution of having operators manually move their U-NII devices’ frequencies away from the 

TDWR frequency—a temporary solution that defeats the purpose of DFS. 

Part II of this series will detail additional, follow-up testing performed at the ITS laboratory in 

Boulder, CO, and the FAA’s Mike Monroney Aeronautical Center in Oklahoma City, to 

determine why some devices fail to detect TDWR signals, understand the interference 

mechanism into TDWRs, and explore at what interference levels the TDWR performance 

degrades. ITS obtained several U-NII devices from various manufacturers to test against an 

engineering TDWR system, with representatives from the respective manufacturers present to 

witness testing and provide engineering support. 

Part III of this series will contain engineering solutions that may be used as part of a strategic 

plan to resolve interference to TDWRs.

CASE STUDY: INVESTIGATION OF INTERFERENCE INTO 5 GHz WEATHER 

RADARS FROM UNLICENSED NATIONAL INFORMATION INFRASTRUCTURE 

DEVICES, PART I 

John E. Carroll1 1 , Frank H. Sanders , Robert L. Sole2 1 and Geoffrey Sanders

In early 2009, the Federal Aviation Administration (FAA) became aware of

interference to Terminal Doppler Weather Radars (TDWRs) that operate in the 5600–

5650 MHz band and provide quantitative measurements of gust fronts, windshear, 

microbursts, and other weather hazards for improved safety of operations in and 

around major airports. This report describes field measurements and results from an 

examination of interference to a TDWR in San Juan, Puerto Rico from unlicensed 

national information infrastructure (U-NII) dynamic frequency selection (DFS) 

devices operating in the same frequency band. Several U-NII devices from different 

manufacturers were found to be causing interference into the TDWR. These devices 

operate in the same bands as these Federal radar systems, but employ DFS technology 

that is supposed to detect the presence of nearby radar systems and change operating 

frequencies to prevent interference with incumbent radar systems. This is the first of a 

three-part series of reports that describe research efforts by the Institute for 

Telecommunication Sciences (ITS) engineers, with assistance from FAA engineers, 

to determine the cause of the interference, understand why some devices fail to detect 

TDWR signals, and engineer solutions. 

Key words: dynamic frequency selection; unlicensed national information 

infrastructure; spectrum sharing technology; radar interference; RF interference; radar 

performance degradation; terminal Doppler weather radar

1 INTRODUCTION

The Terminal Doppler Weather Radar (TDWR) system, introduced in 1993, operates in the 

5600–5650 MHz band, and provides quantitative measurements of gust fronts, windshear, 

microbursts, and other weather hazards that are used to improve the safety of operations at major 

airports (45 airports at the time of this writing). “During the period from 1964 to 1986, at least 32 

accidents and incidents have occurred in which windshear was identified as a contributing factor. 

These accidents resulted in over 600 fatalities and nearly 250 injuries. There is evidence to 

suggest that if undocumented “close calls” and general aviation statistics were included, these 

figures would be much higher.” [1] TDWRs improve the management of air traffic through 

forecasting of gust fronts and induced wind shifts, detection of precipitation, and detection of 

other hazardous weather phenomena including turbulence and tornados. 

Beginning in early 2009, the Federal Aviation Administration (FAA) became aware of 

interference to TDWRs, including one in San Juan, Puerto Rico. An informal investigation by 

 

1 Institute for Telecommunication Sciences, National Telecommunications and Information Administration (NTIA), 

U.S. Department of Commerce, Boulder, CO. 2 Office of Spectrum Management (OSM), NTIA, Washington, DC.
2 

the FAA concluded that Unlicensed National Information Infrastructure (U-NII) devices were 

responsible. U-NII systems can now operate in the 5250–5350 and 5470–5725 MHz bands, 

alongside many Federal radar systems, including the TDWR, but are required to employ 

dynamic frequency selection (DFS) technology to detect the presence of a nearby radar system 

and change operating frequencies to prevent interference to the radar. The Federal 

Communications Commission (FCC), the National Telecommunications and Information 

Administration (NTIA), other Federal agencies, and industry representatives collaborated to 

develop rules and compliance measurement procedures [2] for U-NII devices equipped with DFS 

technology. After three years of bench and field testing prototypes from many manufacturers, the 

DFS rules were finalized in March 2006 and the first DFS device certification was granted in 

August 2006. The U-NII devices are authorized to operate under the Part 15 rules for unlicensed 

devices, and as such they are not permitted to cause interference to allocated and protected 

systems.3

Interference sources cause interference strobes to appear on the San Juan TDWR plan position 

indicator (PPI) scopes. These strobes are sometimes referred to as “jam strobes” and appear as 

blacked-out zones on the PPI. Figure 1 shows an example of interference strobes, marked with 

white lines for graphical clarity. 

Figure 1. An example of interference artifacts on the San Juan TDWR display.

4

 

3. “Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful 

interference is caused and that interference must be accepted that may be caused by the operation of an authorized 

radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, 

or by an incidental radiator.” 47 CFR § 15.5(b) (2009-2010). 4 Map image © 2010 Google, Map Data © 2000 European Technologies, © 2000 TeleAtlas, NASA, © 2008 Digital 

Globe
3 

At the request of the FAA, ITS and OSM engineers participated in a TDWR interference study in

San Juan, Puerto Rico from March 9th through 20th, 2009 and performed a series of field 

measurements to determine the source of the interference. The results obtained are detailed in 

this report. 
4 

2 DYNAMIC FREQUENCY SELECTION OVERVIEW

In June 2006, the FCC released a Memorandum Opinion and Order [2] containing an Appendix 

specifying measurement procedures for certifying U-NII devices for compliance with the DFS 

requirements in the 5250–5350 MHz and 5470–5725 MHz frequency bands. U-NII devices 

operating in these bands do not have to follow any FCC guidelines for modulation or coding 

schemes, but must detect and avoid incumbent Federal radar systems. 

The operational behavior of a U-NII device operating in the 5250–5350 MHz and 5470–5725 

MHz frequency bands is:

1. U-NII devices must use DFS in order to detect nearby radar systems with received signal 

strength above the DFS Detection Threshold [2] of -62 or -64 dBm peak power5. The 

required Detection Bandwidth [2] is specified as a minimum of 80% of the device’s 99% 

transmission power bandwidth. 

2. Before a U-NII device can transmit on a channel, it must perform a Channel Availability 

Check [2] for 60 seconds to ensure no radar system is operating on the selected channel. 

3. The U-NII device then transmits Network Initiation [2] signals that allow other U-NII 

devices to associate.

4. During normal operation, the U-NII device monitors the channel (called In-Service 

Monitoring [2]) to ensure that there are no radar systems on the Operating Channel [2]. 

5. If the U-NII device detects radar systems during In-Service Monitoring, the Operating 

Channel must be vacated to prevent interference to nearby radar systems. 

6. Once the U-NII device has detected radar systems on a given channel, it must not utilize 

that channel for a minimum of 30 minutes. 

Sections 6 and 7 in the Appendix of [2] specify five sets of radar test waveform types and FCC 

certification procedures for U-NII devices requiring DFS. The waveforms were developed by 

ITS in coordination with several Federal agencies. The waveform sets simulate signals from 

various radars operating in the 5 GHz frequency band. All test waveforms are realized using a 

vector signal generator (VSG). 

The “Short Pulse Radar Type 1 Waveform,” hereafter referred to in this document as the “Bin 1 

Waveform,” was developed specifically to protect the TDWR. It consists of a pulse repetition 

interval (PRI) of 1428 microseconds (μs), a pulse width of 1 μs, and 18 pulses per burst [2].

For the purpose of the study documented in this report, in situ testing of U-NII devices in San 

Juan focused only on their Channel Availability Check and In-Service Monitoring modes. 

 

5 A detection threshold of -62 dBm applies to devices with less than 200 mW equivalently isotropically radiated 

power (EIRP). For devices with 200 mW or greater EIRP, the -64 dBm threshold applies. These values assume a 

0-dBi receive antenna.
5 

3 TDWR MEASUREMENT RESULTS

The TDWR can operate in a fixed number of 1-MHz channels in the 5600–5650 MHz band. The 

measurements reported in this section apply only to the San Juan TDWR, which operates on 

5610 MHz (at the time of this writing). The intermediate frequency (IF) of the TDWR is 30 

MHz; that is, a signal at the radar operating frequency of 5610 MHz is downconverted to an IF 

of 30 MHz in the radar receiver.

First, the radar’s PPI was used to locate the azimuths of interference. To determine the true 

characteristics of the interference and ultimately determine the interference source, it was 

necessary to understand the frequency response of various components of the radar system. This 

was done by disassembling the radar and performing measurements on various stages. ITS and 

OSM engineers measured the frequency response of the low-noise amplifier (LNA), radio 

frequency (RF) front-end bandpass filter, and the IF output (used to measure the IF filter 

response). On the radar azimuths where interference was occurring, measurements of signals in 

the ambient environment were then recorded at three different points in the radar system: at the 

antenna waveguide output, RF front-end bandpass filter output, and IF output. These 

measurement points are shown in Figure 2. 

Figure 2. TDWR component frequency response measurement points. 

Dashed lines indicate physical disconnections of waveguide. Waveguide to N-connector 

convertors were used to attach the test equipment to outputs A and B. Output C is bnc 

connectorized. 

3.1 TDWR LNA, RF Front-end Bandpass Filter, and IF Output Frequency Response

ITS and OSM engineers measured the frequency response of the TDWR’s LNA, RF front-end 

bandpass filter, and IF output by sweeping a carrier-wave (CW) signal in frequency and 

recording the results (Figures 3 through 5, respectively). The radar transmitter stage was disabled 
6 

during these measurements. A signal generator, used to inject the CW signal, was directly 

attached to 30-dB directional coupler before the TDWR limiter stage. 

The LNA frequency response presented in Figure 3 was recorded using a spectrum analyzer 

resolution bandwidth of 1 MHz with positive-peak detection at point A in Figure 2. During the 

measurement, a temporary malfunction of the signal generator caused three signal dropouts. 

These three data points do not reflect the actual response of the LNA.

Figure 3. TDWR RF front-end LNA frequency response. 

The data presented in Figures 4 and 5 were recorded using a spectrum analyzer resolution 

bandwidth of 300 kHz with positive-peak detection and were measured at points B and C, as 

shown in Figure 2. The frequency responses shown in Figures 4 and 5 have had their pass bands 

normalized to 0 dB for clarity. Figure 4 presents the TDWR RF front-end filter frequency 

response. 
7 

Figure 4. TDWR RF front-end bandpass filter frequency response; normalized to 0 dB. 

Figure 5 presents the overall TDWR frequency response through the receiver. Figure 5 indicates 

that the IF filter in the receiver stage is about 1 MHz wide at the 3-dB points, which is consistent 

with radar design theory for the TDWR pulse width of about 1 μs.

Figure 5. TDWR IF output frequency response; normalized to 0 dB. 
8 

ITS and OSM engineers also performed a measurement of the linearity of the LNA response as a 

function of input power, the results of which are shown in Figure 6. A CW signal was injected at 

the RF coupler with a peak power level of -50 dBm (a CW peak power level of -20 dBm minus 

30 dB coupler loss). The CW was incremented by 10-dB steps through -10 dBm. The amplifier 

output was found to be linear at inputs up to -20 dBm. This was well above the peak-power 

levels that U-NII devices were later found to be coupling into the radar receiver. The data 

presented in Figure 6 were recorded using a spectrum analyzer resolution bandwidth of 1 MHz 

with positive-peak detection at point A in Figure 2. 

Figure 6. Linearity measurement result for the TDWR front-end LNA. 

3.2 PPI Azimuths of Interference Strobes

The San Juan TDWR utilizes a parabolic antenna with a 0.55o 3-dB beamwidth and 50 dBi gain 

in its 3-dB beamwidth at its operational frequency of 5610 MHz. At the beginning of this effort, 

FAA engineers performed standard solar observation techniques. This allowed them to adjust 

and verify the accuracy of the TDWR beam-pointing angles. 

Next, the radar was operated in its typical volume-scanning mode (a combination of rotating in 

azimuth and varying the antenna elevation) so as to catalog the set of all azimuths and 

corresponding elevation angles at each azimuth where the interference strobes were occurring. A 

total of ten azimuths experiencing interference were identified as shown in Table 1. 
9 

Table 1. Azimuths and Corresponding Elevation Angles where Interference was Identified

Azimuth (Magnetic) Azimuth (True)6 Elevation Angle(s)

112o 99o +0.3

124o 111o +0.3, +1.0

131o 118o +0.3

143o 130o +0.3, +1.0

147o 134o +0.3

154o 141o +0.3, +1.0

161o 148o +0.3, +1.0

165o 152o +0.3, +1.0

175o 162o +0.3, +1.0

191o 178o +1.0

In addition to the azimuths listed in Table 1 where interference strobes were observed, measured, 

and recorded, a baseline azimuth of 0o magnetic was observed, measured, and recorded. This 

provided a reference against which effects on the interference azimuths could be compared. 

3.3 U-NII Signals at Antenna Waveguide Output, Bandpass Filter Output, and IF Output

All measurements were performed in a 1-MHz bandwidth so as to replicate the TDWR’s 

processing bandwidth and provide assurance that the peak power levels measured would 

replicate peak power levels as processed by the TDWR receiver stage. The radar transmitter 

stage was disabled during these measurements, except during the IF output measurements. The 

TDWR receiver expects a series of calibration pulses from the transmitter stage; without them,

the radar will disable itself as the lack of calibration pulses indicates a system fault has occurred.

ITS and OSM engineers measured the U-NII signals at various test points of the TDWR in the 

frequency and time domain at each of the azimuths that were receiving interference. These test 

points are shown in Figure 7. Dotted lines indicate physical disconnections of the radar 

waveguide. 

 

6 TDWR azimuths are reported in magnetic terms to maintain consistency with air traffic control operations, which 

use magnetic azimuths because pilots always fly on magnetic headings. For Puerto Rico, the conversion from 

magnetic to true azimuths is (True = Magnetic – 12.75o); a TDWR interference azimuth of 112o is 99.25o true. 

Listed azimuths are only reported to the nearest degree.
10

Figure 7. TDWR measurement point configuration. 

Figures 8 through 11 show the measured signal at an azimuth of 154 degrees from the radar 

location at each of the three test points of the TDWR. The frequency and time-domain 

measurement results yielded signal structures consistent with “frame-based” U-NII devices. 

Frame-based devices transmit data for a fixed period of time, followed by a fixed period of time 

during which they receive data. An example of this behavior is shown in Figure 10. A similar 

signal structure was observed on all but one of the azimuths listed in Table 1 (the exception was

one azimuth experiencing broadband noise emanating from a cruise ship). 

Each frequency-domain measurement at each point in the TDWR was performed across two 

frequency ranges: 5000–6000 MHz and 5595–5625 MHz. Examples of the two frequency range 

measurements are shown in Figures 8 and 9. This pairing provided a broad look at the spectrum 

on each azimuth, as well as a detailed examination of the spectrum near the radar center 

frequency of 5610 MHz. Each of these spectra was repeated with two spectrum analyzer 

detection modes: positive peak and Root Mean Square (RMS) average. This allowed the effect of 

the duty cycle of the interference signals to be observed, as high-duty-cycle signals would be 

observed with both detectors, while low-duty-cycle signals would be observed well with 

positive-peak detection but not with RMS average detection. The spectrum measurements 

allowed both the tuned frequencies and the transmitted spectrum shapes (spectrum width and 

roll-off) of the interference signals to be observed and recorded. 
11

Figure 8. U-NII signals at the TDWR antenna waveguide output at the 154 degree azimuth over 

a 1 GHz span. 

Figure 9. U-NII signals at the TDWR antenna waveguide output at the 154 degree azimuth over 

a 50 MHz span. 
12

Figures 10 and 11 show the time-domain measurements of the interference source at the 154 

degree azimuth at the TDWR antenna waveguide output(5610 MHz) and the TDWR receiver IF 

output (30 MHz), respectively. 

Figure 11 shows the magnitude of interference in the radar’s IF stage from an interfering signal 

produced by an outdoor U-NII device (Model 1) from Manufacturer A, at 5600 MHz (10 MHz 

below the radar center frequency). The +25 dB interference-to-noise (I/N) level of the 

interference on the TDWR center frequency caused visible strobes on the radar PPI. Taken 

together, Figures 8 through 11 indicate that the interference on the TDWR center frequency is 

due to a U-NII system. Other interference azimuths listed in Table 1 showed the same results 

(except the one azimuth with broadband noise emanating from a cruise ship). 

Figure 10. Time-domain view of interference at the TDWR antenna waveguide output at 154 

degree azimuth tuned to 5610 MHz. 
13

Figure 11. Time-domain view of interference at the TDWR IF output at 154 degree azimuth, 

tuned to 30 MHz. 

The output of the TDWR RF front-end bandpass filter (see Figure 7) is provided in Figure 12 as 

an example. It is important to note that the bandpass filter can mask the center frequency of the 

interferer. Therefore, the initial measurements must always be done at the radar’s waveguide 

antenna output (ahead of the RF bandpass filter) to ascertain the actual frequency of the 

interfering U-NII signal and to accurately observe its spectrum. Pseudo-peaks seen through the 

filter are not the true center frequencies of the signals.

Figure 12. TDWR bandpass filter effect on off-tuned signals in the radar receiver. 
14

3.4 Summary of Interference Signals Coupling into the TDWR in San Juan

Table 2 summarizes the results of the NTIA measurements of interference into the TDWR at San 

Juan. Out of ten interference azimuths, one was due to unknown broadband noise source from a 

cruise ship and nine were due to both co-channel and adjacent channel emissions from U-NII 

devices. Red lettering at 5610 MHz and 5615 MHz denotes U-NII systems whose intentional 

emissions were co-channel with the TDWR.

Table 2. Summary of Interference Measured Within the TDWR Receiver at San Juan

Az 

(mag)

Az 

(true)

Frequency or 

Frequencies 

(MHz)

Signal Type

Max I/N Level 

in Radar IF 

Stage

Comments on interference 

sources 

112o 99o 5610 Broadband noise About 6 dB Cruise ship; not U-NII

124o 111o 5600 Frame-based U-NII +6 dB U-NII device: 

Manufacturer A, Model 1

131o 118o 5600 Frame-based U-NII +10 dB U-NII device:

Unknown Manufacturer

143o 130o 5620 Frame-based U-NII +8 dB U-NII device:

Unknown Manufacturer

147o 134o 5600 and 5630 802.11 and 802.16 +12 dB U-NII device:

Unknown Manufacturer 

154o 141o 5610 Frame-based U-NII +25 dB U-NII device:

Unknown Manufacturer

161o 148o 5615 Frame-based U-NII +20 dB U-NII device:

Unknown Manufacturer

165o 152o 5610 Frame-based U-NII +20 dB U-NII device:

Unknown Manufacturer

175o 163o 5615 Frame-based U-NII +25 dB U-NII device:

Unknown Manufacturer

191o 178o 5600 Frame-based U-NII +18 dB U-NII device:

Unknown Manufacturer 
15

4 SIGNAL ENVIRONMENT MEASUREMENTS

The next step in the process was to discover why the U-NII devices were failing to detect the 

TDWR. This required measurements of the incident TDWR peak pulse power at known U-NII 

transmitter sites in the San Juan area. Then, the DFS functionality of selected U-NII devices7

4.1 Measurements of TDWR Signal at Rooftops 

was checked against both the actual TDWR signal as well as the Bin 1 Waveform used in FCC 

compliance testing. The Bin 1 Waveform provided a baseline signal that should have triggered 

the DFS algorithm to confirm proper DFS operation. All measurements performed in this series 

of tests used a 1-MHz resolution bandwidth. 

ITS and OSM engineers performed measurements of the maximum incident TDWR pulse power 

at the U-NII sites shown in Table 3. The measured levels exceeded the minimum DFS detection 

threshold of -64 dBm in a 1-MHz bandwidth by 47 dB to as much as 60 dB. 

Table 3. TDWR Maximum-Power Measurement Sites and Values in the San Juan Area

Azimuth 

(magnetic)

Distance 

from TDWR

Altitude 

(MSL)

Peak Radar Power Corrected to 0 

dBi Receiving Antenna

124o 8.6 miles 270 feet 0 dBm

147o 5.3 miles 300 feet -15 dBm

161o 7.9 miles 500 feet -11 dBm

The radar pulses were also well formed (sharp rise and fall times) in the time domain. Figures 13 

through 15 show examples of these frequency and time-domain measurements at the location of 

the interference source at the 124 degree azimuth; other azimuths exhibited the same results. 

Based on these measurements, the U-NII devices at these azimuths should have detected the 

radar signal and should have vacated their operating frequency as shown by these measurements.

Measurements taken at U-NII locations in San Juan show that the actual radar pulse environment 

differs somewhat from the environment originally assumed by DFS designers. It was assumed 

that radar pulses would occur in isolated, discrete bursts from the radar antenna main lobe only. 

However, measurements around the San Juan TDWR showed that pulse bursts are also received 

from radar antenna sidelobes, contradicting the model. The proof is provided by the data of 

Figure 13, taken 8.6 miles from the San Juan TDWR on a rooftop on the 124 degree azimuth.

 

7 These devices were made available to NTIA on a temporary basis by local wireless internet service providers 

(WISPs) who were using them on rooftops. Alterations were not made to the hardware or firmware of any of the 

devices that were examined in San Juan.
16

Figure 13. The time-domain behavior of the San Juan TDWR measured through one complete 

volume scan at a distance of 8.6 miles.

The measurement shown in Figure 13 was made through one complete volume-scan cycle of the 

radar. Every 20 seconds, the radar beam scanned past the azimuth where the measurement 

system was situated. But as the elevation of the beam varied from one scan to the next, variations 

occurred in the maximum power received at the rooftop from one radar rotation to the next. The 

TDWR beam in the first rotation of the volume scan actually hit the measurement location above

the building rooftop. The next two rotations of the TDWR volume scan reduced the elevation 

angle by a factor of two and caused the TDWR beam to hit the measurement location at 

approximately rooftop height. The calculations of the location of the TDWR beam on a given 

elevation angle were based on the San Juan TDWR scan strategy provided by the FAA. (It 

should be noted that scan strategies will vary from one TDWR location to the next.) After that, 

the radar beam elevation gradually increased higher and higher above the rooftop, and the 

measured power in the beam decreased steadily from one rotation to the next. The DFS detection 

threshold, indicated by the red line in Figure 13, is nearly always exceeded throughout the 

volume scan. 

By the twelfth rotation of the radar, the well-formed antenna pattern characteristic of the main 

beam was lost. Instead, the first sidelobe below that beam was seen during each TDWR rotation. 

After 18 radar rotations had elapsed (taking a total time of six minutes), the volume scan was

been completed and the next volume scan begun. 

For the purpose of this discussion, the most remarkable feature of the data in Figure 13 is the 

fact that never, during the entire volume-scan cycle of the TDWR, even when its beam was tilted 

60 degrees above the horizon and directed over the ocean away from the measurement location, 

did the stream of pulses from the radar cease to reach the measurement location where U-NII 

devices were positioned.
17

The peak backlobe and sidelobe emissions from the TDWR were nearly always above the DFS 

detection threshold at the measurement location. The measurements in Figure 13 were taken with 

a 10-dBi gain measurement antenna, but the data in the figure have been adjusted (by subtracting 

10 dB from each of the raw measurement point amplitudes) to the power that would have been 

measured by a 0-dBi gain measurement antenna. In general, the radar pulses never dropped

below -60 dBm with a 0-dBi gain measurement antenna at this location. Since the DFS detection 

threshold is either -62 or -64 dBm peak power, this means that a DFS device at this location 

should constantly detect radar pulses as the radar pulses do not occur in isolated burst. 

Figure 14 shows a set of individual pulses measured in the TDWR backlobes while the radar was 

rotating away from the measurement location at a high elevation angle. The structure of the 

backlobe pulses is the same as the pulses that occur when the radar beam is aimed at the 

measurement location, albeit at a lower amplitude.

Figure 14. A sequence of pulses in the TDWR backlobe (black) compared to the Bin 1 

Waveform sequence (red).

The TDWR pulse sequence and the individual TDWR pulse envelope are overlaid in Figures 14 

and 15 with a pulse sequence and the Bin 1 Waveform generated by a VSG, respectively. The 

actual TDWR and Bin 1 Waveform pulse envelopes are similar. The only significant difference 

between the actual TDWR emission and the Bin 1 Waveform is the PRI for the given elevation

scan (the TDWR PRI varies depending on elevation); the PRI of the Bin 1 Waveform sequence 

is almost three times longer than the TDWR PRI (for the elevation where the emission was 

captured).

The individual TDWR pulse envelopes are well-defined (sharp pulse rise and fall times), as 

exemplified by the individual radiated TDWR pulse shown in Figure 15. This pulse, like the 
18

sequence shown in Figure 14, was captured while the radar beam was pointing directly away 

from the measurement location and was tilted at a high elevation angle. The pulses in Figure 15

are offset slightly in time for graphical clarity. The pulse widths and shapes in this figure are 

virtually identical.

Figure 15. A radiated single-pulse envelope from the TDWR in the backlobe of the antenna 

pattern (black) compared to Bin 1 Waveform pulse (red).
19

5 MEASUREMENTS OF U-NII DEVICES AGAINST ACTUAL TDWR EMISSIONS 

AND BIN 1 WAVEFORM PULSES

The DFS functionality of selected U-NII devices8 was checked against both the actual TDWR 

signal as well as the Bin 1 Waveform. The devices that were tested are shown in Table 4. 

Firmware revisions for these devices were found to be critical to proper DFS radar-detection 

functionality.9

Table 4. U-NII Devices Tested Against Actual TDWR and Bin 1 Waveform in San Juan

Device Model Modulation

Manufacturer A Model 2 Frame-Based

Manufacturer B Model 1 WiMAX (802.16)

Manufacturer C Model 1 WiMAX (802.16)

5.1 DFS Tests on a Model 2 U-NII Device from Manufacturer A

Manufacturer A’s Model 2 device was individually correlated with the interference strobe on 124 

degree azimuth at the TDWR location (Table 2) by FAA engineers, the local FCC Resident 

Agent and a local WISP operator. FAA and FCC personnel located a U-NII device in use at 5600 

MHz (Table 2), which was 10 MHz below the TDWR frequency and contacted the WISP 

operator using the U-NII device. The local WISP operator voluntarily off-tuned Manufacturer 

A’s Model 2 device to 5590 MHz and, after the device was off-tuned, FAA engineers confirmed 

via the TDWR PPI that the strobes on the 124-degree azimuth disappeared; they were present 

minutes earlier. 

ITS and OSM measurements in the TDWR receiver showed this U-NII device, while off-tuned 

10 MHz from the TDWR frequency, produced enough unintentional energy on the TDWR 

frequency to produce an I/N level of +6 dB in the radar receiver (Table 2). In other words, this 

device was failing to sense the TDWR emissions when it was off-tuned by 10 MHz, yet its 

detection bandwidth, based on DFS compliance test results performed by the FCC, was listed as 

±11 MHz. Based on the FCC’s results, it should have detected the TDWR even though it was 

off-tuned by 10 MHz. 

A second Model 2 U-NII device from Manufacturer A was tested in situ on a rooftop with the 

help of a local WISP operator. This device had the same model number, and used the same 

firmware version as the Model 2 device tested by the FAA and FCC. On the rooftop, in the in 

situ signal environment that is shown in Figures 13 through 15, the U-NII device was mounted 

on a metal pipe approximately 1.8 m above the rooftop and was rotated to face toward the 

TDWR. It was tuned to the TDWR frequency (5610 MHz) and was verified to be operating with 

the proper regional/country code (some U-NII devices are sold to operate in different regions, 

with varying rules on DFS implementation). This U-NII device was operated without a client, 

and was not transmitting or receiving network traffic; it was only transmitting Network Initiation 

 

8 Firmware builds and FCC IDs were collected for these devices.

9 Firmware changes have the potential to alter or even disable DFS detection ability.
20

data. This should have given it a maximum probability of detecting radar pulses as it was not 

receiving data from a client and so could utilize the entire time it was not transmitting for radar 

detection. It correctly performed the 60-second Channel Availability Check before it transmitted, 

but then it began to transmit on the TDWR frequency. Because the TDWR pulses were, as 

shown in Figure 13, always above the DFS detection threshold at this location, the U-NII device 

should have detected the radar and moved to another frequency. 

After three minutes of operation on the TDWR frequency, while in In-Service Monitoring mode, 

it did detect the actual TDWR, declared this in its software interface, and ceased to transmit on 

the TDWR frequency. The test was repeated, and again the device began to transmit on the 

TDWR frequency after performing the 60-second Channel Availability Check. During this 

second trial, it transmitted for five minutes on the radar frequency, before it finally shut itself off 

after detecting the TDWR signal (as again indicated by a user-interface message).

Next, the same test procedure as described above was repeated with the U-NII device’s antenna 

rotated so that it faced 180 degrees away from the TDWR, and finally with the device’s antenna 

rotated 90 degrees away from the TDWR. These two cases should have resulted in reduced 

antenna directional gain and possibly reduced probability of detection of the TDWR signal. 

When facing 180 degrees from the TDWR, the device operated for 15 minutes but never detected 

the actual TDWR signal (FAA engineers did confirm that an interference strobe occurred during 

this test.) The device was manually powered off after 15 minutes. When facing 90 degrees from 

the TDWR, it did eventually detect the TDWR signal, but only after 12 minutes (two complete 

volume scans of the radar) had elapsed. FAA engineers confirmed interference strobes occurred 

with the device facing 90 degrees away from the TDWR.

The Detection Bandwidth of the U-NII device was checked by rotating its antenna to face the 

radar and then operating it on frequencies spaced 10 MHz and 15 MHz away from the TDWR 

frequency. It never detected the TDWR signal when it was off-tuned by 15 MHz. It continued to 

require many minutes to detect the TDWR when it was off-tuned by 10 MHz below the TDWR 

frequency (to 5600 MHz), but it eventually detected the radar after 5 minutes had elapsed. When 

it was tuned 10 MHz above the TDWR frequency (to 5620 MHz), however, it never detected the 

TDWR signal after 10 minutes of elapsed operation; it was finally turned off by the operator. 

A third Model 2 U-NII device from Manufacturer A was then tested indoors at the San Juan FCC 

office against the Bin 1 Waveform (Figures 14 and 15) in both Channel Availability Check and 

In-Service Monitoring modes. It should be noted that this was not the unit tested on the rooftop, 

but another, apparently identical, unit with the same model number provided by the same WISP 

operator that participated in the roof-top testing with ITS and OSM engineers. It is not known if 

the firmware version was the same as the one tested on the rooftop as the WISP operator lacked 

the means to check. In this indoor environment, which was radio-quiet but which probably 

caused a significant amount of multipath in the Bin 1 Waveform pulses, the U-NII device always 

immediately detected the radar bursts at peak amplitudes ranging from -62 dBm to 

approximately -30 dBm. The device was able to consistently detect the Bin 1 Waveform, both 

when the pulses occurred in bursts (as specified in section 7 of the Appendix of [1]) and when 

they occurred in a continuous stream (as a replication of the local TDWR pulse environment). It 
21

is noted, with reference to Figure 14, that the PRI of the TDWR pulses was about half as long as 

the PRI of the Bin 1 Waveform pulses. 

In summary, Manufacturer A’s Model 2 device was not able to consistently detect actual in situ 

TDWR pulses in either operating mode. However, it was able to consistently detect both bursts 

and continuous streams of the Bin 1 Waveform. In situ testing indicates that the device did not 

always achieve ±10 MHz Detection Bandwidth, even though FCC certification testing had 

indicated that this value should be ±11 MHz. 

Manufacturer A’s devices were found to be deployed with at least three different antenna 

configurations in the San Juan area.

5.2 DFS Tests on a Model 1 U-NII device from Manufacturer B

The Model 1 U-NII device from Manufacturer B lacked an FCC ID on its case. It was tested 

against TDWR pulses on a 300-ft-high rooftop on the 147-degree azimuth from the radar as well 

as the Bin 1 Waveform at a relatively radio-quiet outdoor location at the San Juan Federal 

Building. It failed all In-Service Monitoring tests performed by NTIA, as described in more 

detail below.

During the rooftop tests against the actual TDWR signal, the maximum received peak power 

level of the TDWR pulses, the radar beam-scanning pattern and the pulse formation at the 

rooftop location were very similar to those of Figures 13 through 15. The device was 

intentionally and repeatedly tuned to the TDWR frequency while it was on the rooftop and

operating in In-Service Monitoring mode. In no case did it detect the TDWR. Furthermore, it 

provided a user-accessible option for disabling DFS functionality. The device was tested on the 

rooftop in both DFS-enabled and DFS-disabled modes; in neither case did it detect the actual 

TDWR pulses. 

Subsequently, the Manufacturer B, Model 1 device was tested at an outdoor location at the 

Federal Building in San Juan against calibrated Bin 1 Waveform pulses (Figures 14 and 15, 

shown in red), directed at it via a horn antenna. The received power level of the Bin 1 Waveform 

was directly calibrated through the U-NII receiver antenna. The tests were performed with the 

calibrated pulse bursts adjusted to peak amplitude levels in the Manufacturer B receiver of -62 

dBm, -52 dB, -42 dBm, -32 dBm, -22 dBm, and -12 dBm, while the device was in In-Service 

Monitoring mode. In no case did the device detect any of the Bin 1 Waveform pulse bursts.

5.3 DFS Tests on a Model 1 U-NII Device from Manufacturer C

The Manufacturer C, Model 1 U-NII device was tested in its In-Service Monitoring mode in the 

same outdoor area as Manufacturer B’s device. Bursts of Bin 1 Waveform pulses were 

transmitted at peak amplitude levels in the Manufacturer C receiver of -62 dBm to -12 dBm. The 

DFS system of Manufacturer C, Model 1, consistently and immediately detected all of these 

radar pulse bursts. Due to logistical constraints, this device was not tested against the TDWR. 
22

5.4 Overall Results of U-NII Device Performance Against the San Juan TDWR and Bin 1 

Waveform Pulse Bursts.

A summary of the results from specific U-NII device tests are given in Table 5. Entries 

highlighted in red indicate failures to detect. 

Table 5. Summary of DFS Performance of U-NII Devices Tested In Situ Against the San Juan 

TDWR and Bin 1 Waveform Pulse Bursts.

Test Source Manufacturer A

Model 2

Manufacturer B

Model 1

Manufacturer C

Model 1

Channel 

Availability Check

Using actual 

TDWR signal

No radar 

detection Not tested1 Not tested1

Channel 

Availability Check

Using Bin 1 

Waveform

Consistent radar 

detection Not tested1 Consistent radar detection

In-Service 

Monitoring

Using actual 

TDWR signal

Inconsistent radar 

detection

No radar 

detection Not tested1

In-Service 

Monitoring

Using Bin 1 

Waveform

Consistent radar 

detection

No radar 

detection

Consistent radar 

detection

1 Not tested due to logistical constraints
23

6 SUMMARY OF RESULTS

At the request of the FAA, NTIA’s OSM and ITS provided technical assistance to examine 

interference into their San Juan, Puerto Rico TDWR. This was a two-week long, labor-intensive 

effort that required cooperation with local U-NII system operators and personnel from the ITS, 

OSM, FCC, and FAA. Measurements performed by these agencies ultimately confirmed that the 

interference was from U-NII devices operating in the 5 GHz band; specifically, devices operating 

between 5600-5650 MHz. 

A carefully developed test plan for the investigation of interference to TDWR receivers was 

implemented in San Juan by a combined team of engineers from ITS, OSM, FAA, and the FCC. 

A brief summary of the results follows, categorized by general results and by results that are 

specific to the interference into the TDWR and to the U-NII devices themselves.

6.1 General Results

1. The U-NII interference was located using the radar’s PPI display, combined with GPS 

mapping. The U-NII devices causing interference were found to be on the rooftops of tall 

buildings and towers located on hills in the San Juan area. 

2. Many of the rooftops and towers have installations of 5 GHz U-NII systems from various 

manufacturers. Leading examples are point-to-point and point-to-multipoint U-NII 

devices operated by WISPs. The deployment of U-NII devices has become increasingly 

dense in the San Juan area.

3. The U-NII devices can be manually tuned away from the TDWR if the operators can be 

found and contacted. This will eliminate or reduce the number of interference strobes. 

However, this mitigation technique is only temporary and defeats the purpose and 

function of DFS. 

6.2 TDWR Results

1. Interference to the TDWR receiver was due to U-NII signals at nine out of ten azimuths. 

2. The duty cycle of the U-NII devices (typically 40% or greater) is well above the TDWR 

receiver’s ability to process and mitigate that type of interference. With radar systems in 

general, mitigation is possible with duty cycles less than approximately 3%. 

3. The U-NII interference was either co-channel or adjacent channel to the TDWR operating 

frequency at those azimuths. Both conditions caused visible interference strobes on the 

TDWR’s PPI display. 
24

6.3 Signal Environment Results

1. The signal power of the radar at the U-NII locations on the nine affected radar azimuths 

was at least 40 dB above the U-NII detection threshold. 

2. The TDWR pulses were well-formed (sharp pulse rise and fall times) at the rooftop and 

tower locations where U-NII interference was originating.

6.4 Results for the Identified U-NII Devices

1. The Manufacturer B, Model 1 device case did not have an FCC ID label affixed. 

2. The Manufacturer B, Model 1 device did not detect the TDWR signal or the Bin 1 

Waveform radar pulse bursts in the In-Service Monitoring mode. 

3. The Manufacturer C, Model 1 device consistently detected Bin 1 Waveform pulse bursts 

in both In-Service Monitoring and Channel Availability Check modes. Due to logistical 

constraints, it was not tested against the actual TDWR pulses. 

4. The Manufacturer A, Model 2 device did not detect the TDWR during a pair of 60-

second Channel Availability Check tests on a San Juan rooftop. It sometimes failed to 

detect the TDWR when it was operating in its In-Service Monitoring mode. When it did 

detect the TDWR during its In-Service Monitoring operation, such detections only 

occurred after several minutes, during which time it continued to transmit. 

5. The Manufacturer A, Model 2 device consistently detected the Bin 1 Waveform pulse 

bursts. This waveform used pulses with the same width as those of the TDWR, but the 

PRI of these pulses differed from the PRI of the actual TDWR pulses. 

6. The Manufacturer B, Model 1 provides users with an option to deactivate DFS 

functionality.10 Some U-NII devices invoke DFS protocols that depend upon the 

geographic region in which the device is deployed. The operational region is a userdependent setting, and some geographic zones have no DFS rules.11 The extent to which 

individual users properly configure their DFS devices for geographic zones is not known. 

After obtaining the initial results reported here, ITS performed additional testing in controlled 

environments to better understand how to resolve the interference, beyond the immediate, ad-hoc 

solution of having operators manually move their U-NII devices’ frequencies away from the 

TDWR frequency—a temporary solution that defeats the purpose of DFS. 

Part II will detail additional, follow-up testing at the ITS laboratory in Boulder, CO, and the 

FAA’s Mike Monroney Aeronautical Center in Oklahoma City, to determine why some devices 

 

10 This is not permitted under the U-NII rules. 11 This is not permitted for operation in the United States.
25

fail to detect TDWR signals, understand the interference mechanism into TDWRs, and explore at 

what interference levels the TDWR performance degrades. ITS obtained several U-NII devices 

from various manufacturers to test against an engineering TDWR system, with representatives 

from the respective manufacturers present to witness testing and provide engineering support. 

Part III of this series will contain engineering solutions that may be used as part of a strategic 

plan to resolve interference to TDWRs.
26

7 REFERENCES

[1] U.S. Department of Transportation, Federal Aviation Administration, Advisory Circular 

00-54, Pilot Windshear Guide, Nov. 1988. 

<http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%20

00-54/$FILE/AC00-54.pdf>, accessed on Sept. 25, 2010. 

[2] Federal Communications Commission, Revision of Parts 2 and 15 of the Commission’s 

Rules to Permit Unlicensed National Information Infrastructure (U-NII) devices in the 5 

GHz band, ET Docket No.03-122, Memorandum Opinion and Order, 21 FCC Record 7672 

(2006). <http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-06-96A1.pdf>, accessed 

on Jul. 8, 2010. 
27

8 ACKNOWLEDGEMENTS

The authors wish to recognize local FCC Field Agent Ruben Jusino. Without his assistance, 

these measurements could not have been performed to this degree of accomplishment. The 

authors also wish to recognize Rajiv Gautam, Aaron Tuttle, and Tim Pigg from the FAA for their 

support in the characterization of the TDWR. Their knowledge proved critical to the success of 

this effort. 

FORM NTIA-29 U.S. DEPARTMENT OF COMMERCE 

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

BIBLIOGRAPHIC DATA SHEET 

1. PUBLICATION NO. 

TR-11-473 

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

4. TITLE AND SUBTITLE 

Case Study: Investigation of Interference into 5 GHz Weather Radars from 

Unlicensed National Information Infrastructure Devices, Part 1 

5. Publication Date 

November 2010 

6. Performing Organization 

Code 

7. AUTHOR(S) 

John E. Carroll, Frank H. Sanders, Robert L. Sole, and Geoffrey A. Sanders 

9. Project/Task/Work Unit No. 

3152011-300 

8. PERFORMING ORGANIZATION NAME AND ADDRESS 

Institute for Telecommunication Sciences 

National Telecommunications & Information Administration 

U.S. Department of Commerce 

325 Broadway 

Boulder, CO 80305 

10. Contract/Grant No. 

11. Sponsoring Organization Name and Address 

National Telecommunications & Information Administration 

Herbert C. Hoover Building 

14th & Constitution Ave., NW 

Washington, DC 20230 

12. Type of Report and Period 

Covered 

14. SUPPLEMENTARY NOTES 

15. ABSTRACT 

In early 2009, the Federal Aviation Administration (FAA) became aware of interference to Terminal Doppler 

Weather Radars (TDWRs) that operate in the 5600–5650 MHz band and provide quantitative measurements of gust 

fronts, windshear, microbursts, and other weather hazards for improved the safety of operations in and around major 

airports. This report describes field measurements and results from an interference examination of a TDWR in San 

Juan, Puerto Rico from unlicensed national information infrastructure (U-NII) dynamic frequency selection (DFS) 

devices operating in the same frequency band. Several U-NII devices from different manufacturers were found to be 

causing interference into the TDWR. These devices operate in the same bands as these Federal radar systems, but 

employ DFS technology that is supposed to detect the presence of nearby radar systems and change operating 

frequencies to prevent interference with an incumbent radar system. This is the first of a three-part series of reports 

that describe research efforts by the Institute for Telecommunication Sciences (ITS) engineers, with assistance from 

FAA engineers, to determine the cause of the interference, understand why some devices fail to detect TDWR 

signals, and to engineer solutions. 

16. Key Words 

dynamic frequency selection; unlicensed national information infrastructure; spectrum sharing technology; radar 

interference; RF interference; radar performance degradation ; terminal Doppler weather radar 

17. AVAILABILITY STATEMENT 

  UNLIMITED. 

18. Security Class. (This report) 

Unclassified 

20. Number of pages 

 27 

19. Security Class. (This page) 

Unclassified 

21. Price: 

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