NTIA Technical Report TR-11-479

Case Study: Investigation of Interference 

into 5 GHz Weather Radars from 

Unlicensed National Information 

Infrastructure Devices, Part II

John E. Carroll

Geoffrey A. Sanders

Frank H. Sanders

Robert L. Sole

NTIA Technical Report TR-11-479

Case Study: Investigation of Interference 

into 5 GHz Weather Radars from 

Unlicensed National Information 

Infrastructure Devices, Part II

John E. Carroll

Geoffrey A. Sanders

Frank H. Sanders

Robert L. Sole

U.S. DEPARTMENT OF COMMERCE

July 2011

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 

recommendation 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 U-NII DEVICES TESTED.......................................................................................................4

3 TEST WAVEFORMS..............................................................................................................5

4 ITS LABORATORY MEASUREMENT RESULTS................................................................6

4.1 Test Results of DFS Detection Capability with Emulated TDWR Waveforms 

and with the Bin 1 Waveform......................................................................................7

4.2 Effects of Varying Master Device Talk/Listen Ratio for Manufacturer A’s 

Device.........................................................................................................................8

4.3 U-NII Device Emission Spectra ...................................................................................9

5 TDWR PSF TEST AND MEASUREMENT RESULTS.........................................................11

5.1 Measurements of Actual TDWR Emissions in the Time Domain................................13

5.2 U-NII DFS Performance Against Actual TDWR Waveforms .....................................15

5.3 Qualitative Determination of U-NII I/N Levels that Cause TDWR PPI Strobes..........16

5.4 Effects of TDWR Antenna Sidelobes on PPI Displays ...............................................19

5.5 U-NII Device Off-Tuning For Strobe Elimination ......................................................20

6 SUMMARY OF TEST RESULTS.........................................................................................22

7 REFERENCES ......................................................................................................................24

8 ACKNOWLEDGEMENTS....................................................................................................25

APPENDIX: INTERFERENCE-TO-NOISE RATIO IN RADAR RECEIVER 

PERFORMANCE TESTS ...................................................................................27
vi

FIGURES

Page

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

Figure 2. ITS Laboratory Test Setup............................................................................................6

Figure 3. Radiated emission spectra, normalized to zero dB at the fundamental frequency 

(itself normalized to zero hertz) of several 5-GHz U-NII devices...............................10

Figure 4. Geometry of the PSF TDWR and the FAA warehouse facility where the U-NII 

master devices were located. ....................................................................................12

Figure 5. Block diagram for injection of U-NII interference waveforms into the TDWR 

receiver. Items shown in red were provided by NTIA. ...............................................13

Figure 6. Power received from the TDWR at the FAA warehouse facility U-NII location 

throughout one complete monitor-mode scan. ...........................................................14

Figure 7. A single TDWR pulse in the time domain at the U-NII location. ................................15

Figure 8. TDWR meteorological display PPI showing effects of frame-based interference 

(Manufacturer E) injected on a hardline from a VSG at baseline (no 

interference), and I/N = -12 dB, -9 dB, and -6 dB, respectively..................................17

Figure 9. TDWR meteorological display PPI showing effects of frame-based interference 

(Manufacturer E) injected on a hardline from a VSG at I/N = -3 dB, 0 dB, +3 dB, 

and +25 dB, respectively. ..........................................................................................18

Figure 10. TDWR 360-degree-scan interference strobes generated by a U-NII transmitter 

(Manufacturer D, Model 1) on a rooftop 2.97 miles (4.78 km) from the TDWR, 

at an azimuth indicated by the red arrows and at a height of 35 feet (10.6 m) 

above the ground.......................................................................................................20

TABLES

Page

Table 1. FCC-certified U-NII devices used for DFS performance testing.....................................4

Table 2. Waveform test categories...............................................................................................5

Table 3. U-NII device laboratory test results ...............................................................................8

Table 4. Results of Manufacturer A, Model 2, talk/listen ratio tests.............................................9

Table 5. Summary of DFS detection of selected, actual TDWR waveforms...............................16

Table A-1. Effective radar receiver noise-limit increase, (I+N)/N, as a function of I/N ..............28
vii

ABBREVIATIONS/ACRONYMS

AWG Arbitrary Waveform Generator

AGL Above Ground Level

CBS Cavity Backed Spiral

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

IEEE Institute of Electrical and Electronics Engineers

IF Intermediate Frequency

I/N Interference-to-Noise Ratio

ITS Institute for Telecommunication Sciences

LNA Low Noise Amplifier

MHz megahertz

NTIA National Telecommunications and Information Administration

OSM Office of Spectrum Management

Pd Probability of Detection

PPI Plan Position Indicator

PRI Pulse Repetition Interval

PSF Program Support Facility

RF Radio Frequency

TDWR Terminal Doppler Weather Radar

U-NII Unlicensed National Information Infrastructure

μs microsecond

VSA Vector Signal Analyzer

VSG Vector Signal Generator

ix

EXECUTIVE SUMMARY

In early 2009, the Federal Aviation Administration (FAA) reported 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 and determined the interference to be from 

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 U-NII devices employ technology that is supposed to detect the presence of 

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

radar systems. Part I of this report series details the results of this measurement effort.

After determining the interference source in San Juan, Puerto Rico, ITS engineers tested these 

U-NII devices in controlled environments at the ITS laboratory in Boulder, Colorado and at an 

engineering TWDR system at the FAA’s TDWR Program Support Facility (PSF) in Oklahoma 

City, Oklahoma. ITS tested several U-NII devices, some of which were provided by 

manufacturers, against emulated TDWR waveforms and FCC certification waveforms at the ITS 

laboratory, and waveforms generated by an actual TDWR at the PSF. Representatives from the 

respective U-NII manufacturers were present to witness testing and to provide engineering 

support. The primary goals of these tests were to: examine DFS detection performance of U-NII 

devices in the presence of TDWR signals; explore the interference levels at which TDWR 

performance visibly degrades; examine the U-NII device frequency off-tuning required to 

eliminate interference artifacts on the TDWR display from incorrectly configured U-NII devices; 

and explore how U-NII device energy coupling into TDWR antenna sidelobes impacts azimuthal 

direction finding. These performance tests and field measurements are described in this report.

In laboratory tests with emulated TDWR waveforms, four U-NII devices performed as intended

off-the-shelf. One U-NII device did not detect the TDWR waveforms off-the-shelf (although, it 

did detect waveforms used in FCC certification). Eventually, this U-NII device detected 

emulated TDWR signals with prototype firmware, provided by the manufacturer that altered the 

DFS detection algorithms. A different U-NII device was tested against FCC DFS certification 

waveforms and it did not detect any. It also failed to detect any emulated TDWR waveforms. 

The device, however, was FCC certified, meaning it had passed these same tests when the FCC 

laboratory tested its DFS performance. A firmware update from the manufacturer eventually 

rectified this problem, illustrating how firmware upgrades can lead to DFS non-compliance.

In radiated tests with waveforms generated by an actual TDWR, U-NII device radar detection 

performance was found to be in agreement with laboratory testing. U-NII devices that failed to 

detect and avoid the TDWR generated interference artifacts on the TDWR display. Properly 

operating U-NII devices did not generate interference artifacts on a TDWR display when they 

operated on the nearest available frequencies (relative to the TDWR fundamental frequency) 

where their DFS functionality did not detect the TDWR. With the antenna geometry and 

distances tested in this report, a nominal minimum 20-MHz center-to-center frequency 
x

separation between the TDWR and the U-NII device was found to eliminate TDWR display

interference artifacts. 

Part III of this report series describes engineering solutions including frequency separation.

These solutions 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 II

John E. Carroll1, Geoffrey A. Sanders1, Frank H. Sanders1, and Robert L. Sole2

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. Institute for Telecommunication Sciences (ITS) engineers, 

with assistance from FAA engineers, determined the interference to be from 

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

(DFS) devices, from several manufacturers, operating in the same frequency band as 

TDWR systems. 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 report describes measurements and results from 

controlled laboratory and field testing of these U-NII devices. This is the second of a 

three-part series of reports that describe research efforts by the ITS engineers, with 

assistance from FAA engineers, to determine the cause of the interference, examine 

the effects of the interference on TDWR systems, 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, Colorado.

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 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 Part 15 rules for unlicensed 

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

systems.3 References to DFS operational behavior and a brief overview of DFS appear in [2] and 

[3].

Interference sources cause interference strobes to appear on the 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 from the TDWR in San 

Juan, Puerto Rico, marked with white lines for graphical clarity. 

Figure 1. An example of interference artifacts on the San Juan TDWR PPI 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 that confirmed 5 GHz U-NII devices, from various manufacturers, as the 

interference source.

Additional testing was performed in 2009 at the ITS laboratory in Boulder, Colorado and the 

FAA Mike Monroney Aeronautical Center TDWR program support facility (PSF) in Oklahoma 

City, Oklahoma, with various 5-GHz U-NII devices in controlled laboratory and field 

environments. Laboratory testing consisted of synthesizing 24 unique TDWR waveforms whose 

parameters were provided by the FAA. The 24 waveforms were generated using an Agilent 

E4438C vector signal generator (VSG). Supplemental laboratory testing of the U-NII devices 

against the “Bin 1 Waveform” [2] used in the FCC certification process was also performed. 

Field testing took place at the TDWR PSF. The TDWR PSF possesses an engineering TDWR 

that is not a part of the national TDWR network and it was made available for these tests.

These laboratory tests examined DFS detection performance of U-NII devices, from various 

manufacturers, in the presence of emulated TDWR signals. The field tests examined DFS 

detection performance of the same U-NII devices in the presence of an actual TDWR system,

explored the interference levels at which TDWR performance visibly degrades, examined U-NII

device frequency off-tuning required to eliminate interference strobes from incorrectly 

configured U-NII devices, and explored how U-NII device energy coupling into TDWR antenna 

sidelobes impacts azimuthal direction finding.
4

2 U-NII DEVICES TESTED

The U-NII devices shown in Table 1 were tested at the ITS laboratory in Boulder, Colorado and 

at the TDWR PSF, in Oklahoma City, Oklahoma, with an engineering TDWR system. As

indicated in the table, some devices were procured by NTIA, while others were provided by 

manufacturers. IEEE 802.11, IEEE 802.16 and proprietary frame-based signal architectures were 

tested. 

Table 1. FCC-certified U-NII devices used for DFS performance testing

Device

Manufacturer5 Model Ownership Signal 

Architecture

Modulation 

Bandwidth

Manufacturer A Model 2 NTIA Frame-Based 20 MHz

Manufacturer A Model 3 NTIA Frame-Based 20 MHz

Manufacturer A Model 46 Manufacturer Frame-Based 21 MHz

Manufacturer C Model 1 NTIA IEEE 802.16720 MHz

Manufacturer D Model 1 NTIA Frame-Based 20 MHz

Manufacturer E Model 1 NTIA Frame-Based 20 MHz

Manufacturer F Model 1 manufacturer IEEE 802.11 20 MHz

Manufacturer G Model 1 manufacturer IEEE 802.11 40 MHz8

In the 802.11 architecture, data is transmitted at the first available opportunity, so the channel is 

accessed on a per-packet basis. In 802.11 networks, channel time is randomly competed for. In 

802.16 and other proprietary frame-based architectures, channel access is on a per-frame basis 

and may contain more than one data packet. In these network types, channel time is partitioned 

into dedicated transmit and receive segments. In this report, the term “frame-based” only refers 

to proprietary frame-based architectures. The 802.16 protocol (commonly referred to as 

WiMAX) is frame-based, however, it is not proprietary. 

All tested U-NII devices used publicly available firmware unless otherwise noted. 

Manufacturer A supplied a modified device for testing, which was loaded with firmware 

developed specifically for these tests; the modified firmware is not currently deployed in 

production Model 4 units from that manufacturer. 

A Model 1 U-NII device from Manufacturer F was also modified for the testing. The 

modification allowed it to operate in the 5600–5650 MHz frequency range for the purpose of this 

testing; this manufacturer does not market U-NII devices that operate in this frequency range in 

North America.

 

5 Manufacturer B did not provide, nor would they supply to NTIA, a device for this testing.

6 The modified unit from Manufacturer A had no FCC ID because it was a prototype that had not yet been tested by 

the FCC.

7 At the time of testing, this device was not WiMax-Forum certified, but nominally 802.16-compliant devices are 

commonly referred to as “WiMax” and that usage is adopted here. 802.16 is a frame-based signal architecture.

8 The unit from Manufacturer G had more than one bandwidth mode available; 40 MHz was selected for this testing.
5

3 TEST WAVEFORMS

Three waveform categories, listed in Table 2, were used to test the DFS detection performance of 

the U-NII devices in Table 1. The first two categories are generated using a VSG. The first 

category is the “Short Pulse Radar Type 1 Waveform [3],” hereafter referred to in this document 

as the “Bin 1 Waveform.” This waveform is currently used by the FCC in their certification 

process for 5-GHz U-NII devices. It was developed specifically to protect the TDWR.

For the second waveform category, ITS engineers created 24 unique VSG waveforms based on 

TDWR waveform parameters provided by the FAA. These waveforms replicate the 24 available 

waveforms in use by the TDWR. For the sake of clarity, these waveforms are referred to as 

“emulated TDWR waveforms,” in that they have pulse parameters that are identical to those of 

an actual TDWR system, but are generated using a VSG. Only two parameters in the 24 

emulated TDWR waveforms vary: pulse repetition interval (PRI) and pulses per burst (pulses per 

burst is a function of PRI). 

Waveforms transmitted by an actual TDWR system constitute the third category. These 

waveforms are referred to as “actual TDWR waveforms” in this document. 

Table 2. Waveform test categories

Waveform 

Category

Pulse 

Width PRI Pulse Type Pulses per burst

Bin 1 

Waveform 1 μs 1438 μs Unmodulated 18

emulated 

TDWR 1 μs 518–3066 μs (24 discrete values) Unmodulated 18–102

actual 

TDWR 1 μs 518–3066 μs (24 discrete values) Unmodulated N/A9

There are two key differences between the actual TDWR waveform category and the emulated 

TDWR waveform category. First, the emulated TDWR waveforms are a product of low-power,

solid-state electronics. The actual TDWR waveforms are a product of high-power klystron 

vacuum tube amplifiers. 

Secondly, unlike the emulated TDWR waveforms, the actual TDWR system will not transmit 

discrete bursts of pulses in boresight10 mode or monitor-mode11. In monitor-mode, the TDWR 

system will vary PRI values as a function of antenna elevation angle. A given external location 

will observe a constant stream of pulses for a given PRI with varying amplitude (due to antenna 

sidelobe coupling) while the radar completes one azimuthal scan. This pulse sequence will 

continue with varying PRIs and amplitudes while the TDWR completes an entire scan, then the 

cycle will repeat.

 

9 Due to TDWR antenna sidelobe coupling and changes in antenna elevation angle, an external location will 

experience a continuous sequence of pulses with varying PRIs and amplitudes.

10 Boresight refers to the configuration where the TDWR antenna is fixed in both azimuth and elevation.

11 A second scan mode, known as “hazardous mode” is used by the TDWR but was not tested in this study. 
6

4 ITS LABORATORY MEASUREMENT RESULTS

The objectives of these laboratory tests were to:

 Test off-the-shelf 5-GHz U-NII devices for their DFS performance against the Bin 1 

Waveform [2] and against emulated TDWR waveforms.

 Examine the effects on probability of detection (Pd) under different transmit and receive 

duty cycles. Some U-NII devices allow the user to modify the talk/listen ratio [2] of the 

device and, consequently, the duty cycle.

 Measure the emission spectra of the U-NII devices. 

The test setup is shown in Figure 2.

Figure 2. ITS Laboratory Test Setup.

Due to time constraints, it was not feasible to test each U-NII device against all 24 emulated

TDWR waveforms. Rather, the U-NII devices were tested against the lowest, midmost, and 

highest available PRI of the 24 waveforms. These PRIs correspond to 518 μs, 838 μs (some tests

inadvertently used 898 μs), and 3066 μs, respectively. At the request of the FAA, U-NII device 

response to additional PRIs was also examined during laboratory testing. 
7

4.1 Test Results of DFS Detection Capability with Emulated TDWR Waveforms and with 

the Bin 1 Waveform

U-NII device response to the Bin 1 Waveform was first tested for each U-NII device. Emulated 

TDWR waveforms were subsequently tested. All waveforms were configured to transmit on the 

U-NII device operating frequency. 

To reduce measurement uncertainty, trials for each waveform category were repeated 30 times. 

During each trial, the device’s response (detection or no detection) to a given waveform was 

recorded. A successful detection was determined in one of two ways. The first was to monitor 

the U-NII device operating frequency with a spectrum analyzer and observe. If the U-NII device 

transmission ceased after a radar waveform was transmitted, the trial was considered to be a 

successful detection. The second method of determining a successful detection was to monitor 

the U-NII device’s diagnostic information. Some manufacturers include notification of radar 

waveform detection in their device firmware.

The overall Pd for each tested waveform is shown in Table 3. The Pd was calculated by dividing 

the number of successful detections by the number of trials. A minimum successful Pd of 60% 

against the Bin 1 Waveform is required to attain FCC certification [3]. Each U-NII device was 

tested in the In-Service Monitoring mode, with data being sent across the U-NII network. 

Some U-NII master devices [3] allow their talk/listen ratio to be configured. On master devices 

that allow their talk/listen ratio to be configured, the ratio was set to 45%/55% [3] for the results 

shown in Table 3. At this ratio, the U-NII device will, in any one frame duration, transmit during 

45% of the frame time and listen for the remaining 55%. Other frame-based systems tested had a 

fixed ratio of 50%/50%, which is not user-accessible. 
8

Table 3. U-NII device laboratory test results

Manufacturer Model Bin 1 

Waveform

Emulated TDWR Waveform 

PRI Trials Comments

518 μs 838 μs 3066 μs 

Manufacturer A Model 212 100% Pd 0% Pd 0% Pd 0% Pd

Manufacturer A Model 3 100% Pd 73% Pd 0% Pd 0% Pd

Manufacturer A Model 4 100% Pd 0% Pd 66% Pd 43% Pd

Manufacturer A Model 413 100% Pd 100% Pd 73% Pd 93% Pd modified firmware

Manufacturer C Model 1 0% Pd 0% Pd 0% Pd 0% Pd original firmware

Manufacturer C Model 1 100% Pd

14 100%15 100%15 100%15

new firmware

Manufacturer D Model 1 >60% >60% >60% >60% informal tests16

Manufacturer E Model 1 >60% >60% >60% >60% informal tests16

Manufacturer F Model 113 90% Pd 100% Pd 100% Pd

17 76% Pd

Manufacturer G Model 118 100% Pd 100% Pd 100% Pd

17 100% Pd

The Manufacturer C, Model 1 device results in Table 2 illustrate how changes in firmware can 

lead to non-compliant U-NII devices. This U-NII device was tested against many FCC DFS 

certification waveforms, including the Bin 1 Waveform, and it did not detect any. It also failed to 

detect any of the 24 emulated TDWR waveforms. However, the device was FCC-certified,

meaning it had passed these same tests when the FCC laboratory tested the DFS performance. 

The Manufacturer C, Model 1 device tested in this report had the same model number and FCC 

ID number as appeared in the FCC DFS certification database. The difference is in the device 

firmware; the non-compliant firmware tested by ITS was a later firmware revision than the 

firmware version tested by the FCC. It is believed that the DFS radar-detection failure of this 

device was somehow introduced during the firmware evolution. The FCC notified Manufacturer 

C of the problem and they provided a firmware update. The device was tested again after 

applying the firmware update and it detected all 24 emulated TDWR waveforms, as well as the 

Bin 1 Waveform.

4.2 Effects of Varying Master Device Talk/Listen Ratio for Manufacturer A’s Device

Manufacturer A employs a frame-based architecture and allows operators to configure the

talk/listen ratio of their master devices to various ratios. The FCC Memorandum Opinion and 

Order [3] currently specifies using a talk/listen ratio of 45%/55% for certification testing. 

 

12Additional PRI values were tested for this model: 538 μs (0% Pd), 638 μs (0% Pd), 758 μs (0% Pd), 898 μs (0% 

Pd), and 938 μs (0% Pd).

13 Modified firmware provided by the Manufacturer was utilized for this test; this firmware was not publicly 

available at the time of testing.

14 This test was performed using only 10 trials, instead of 30.

15 This device was later tested against all 24 emulated TDWR waveforms, each a single time. It detected all 24 of 

them. The reported percentages are based on a single trial at the stated PRI.

16 These tests were conducted separately from the other tests and only success (Pd > 60%) was recorded.

17 Manufacturer F and G testing inadvertently used a PRI of 898 μs instead of 838 μs. The reported Pd is for 898 μs.

18 Additional PRI values were tested for this model: 638 μs (100% Pd), 698 μs (100% Pd), 758 μs (100% Pd), and 

938 μs (100% Pd).
9

Manufacturer A user’s manual suggests setting the talk/listen ratio to 75%/25% as a typical

configuration. ITS engineers examined the overall Pd as a function of varying the talk/listen ratio 

to progressive reduce the duration of “listen time.”

The test results show that the Manufacturer A device’s Pd was severely degraded when the 

talk/listen ratio was set to the 75%/25% talk/listen ratio suggested in the user manual. The results 

for the Manufacturer A, Model 2 device, tested against the Bin 1 Waveform, are presented in

Table 4.

Table 4. Results of Manufacturer A, Model 2, talk/listen ratio tests

Talk/Listen Ratio Bin 1 

Waveform Pd

45%/55% 100%

60%/40% 80%

75%/25% (user manual) 40%

95%/5% (highest allowed) 0%

As mentioned earlier, the minimum successful Pd required to obtain FCC certification is 60% for 

the Bin 1 Waveform. Therefore, if tested with a 75%/25% talk/listen ratio or a 95%/5% 

talk/listen ratio, the Model 2 device from Manufacturer A would fail to obtain FCC certification.

4.3 U-NII Device Emission Spectra

Radiated emission spectra were measured for several 5-GHz U-NII devices at the ITS laboratory

and are presented in Figure 3. The U-NII device spectra generally drop 60–70 decibels (dB) from 

the peak power at the fundamental frequency within ±25 MHz of the fundamental frequency. 

Beyond that they tend to flatten out, sometimes for several hundred additional megahertz of 

spectrum on either side of the fundamental frequency. Note that in the figure, all of the U-NII 

fundamental-frequency power levels have been normalized to zero dB; actual equivalent 

isotropically radiated power (EIRP) emission levels of U-NII transmitters vary by tens of dB.
10

Figure 3. Radiated emission spectra, normalized to zero dB at the fundamental frequency (itself 

normalized to zero hertz) of several 5-GHz U-NII devices.
11

5 TDWR PSF TEST AND MEASUREMENT RESULTS

The objectives of the work at the FAA’s TDWR PSF were to: 

 Determine whether selected U-NII devices can reliably detect the TDWR signal in 

boresight mode and monitor-mode.

 Use emulated U-NII signals produced by a vector signal generator (VSG) coupled into 

the TDWR receiver via a hardline coupler to determine, qualitatively, the visible effects 

on the TDWR PPI at calibrated levels of interference power, I, to radar-receiver noise 

power, N, or (I/N), in the TDWR receiver.

19 These data were gathered for both the 

existing (legacy) TDWR receiver and for a new model of TDWR receiver that is 

currently in development but has not yet been deployed at operational sites, known as the 

“Sigmet RVP8.”

 Determine the effect of TDWR antenna sidelobes on the reported interference azimuths 

on the PPI display.

 Measure the emissions of an actual TDWR system in the time and frequency domains.

 Determine the amount of required frequency off-tuning of a incorrectly functioning 

U-NII device to eliminate strobes on the PPI display.

To meet the first two test objectives, a suitable location for U-NII operation was identified where 

the signal power of the TDWR exceeded the -62 to -64 dBm FCC DFS detection threshold. An 

FAA warehouse located 2.97 miles (4.78 km) northeast of the TDWR was used for this purpose 

(Figure 4). The U-NII devices were positioned on the rooftop of the warehouse, 35 feet (10.6 m)

above the ground, with a clear line-of-sight to the 80-foot-high (24.3-meter-high) TDWR

antenna. The NTIA/ITS RSMS-4 vehicle, positioned next to the warehouse, provided electrical 

power and housing for computer control of the U-NII devices via cabling that was draped over 

the side of the building.

 

19 See the Appendix of this report. Also, refer to [4] for the definition and meaning of this parameter.
12

Figure 4. Geometry of the PSF TDWR and the FAA warehouse facility where the U-NII master

devices were located. 20

 

20 Map image © 2010 Google, Map Data © 2000 European Technologies, © 2000 TeleAtlas, NASA, © 2008 Digital 

Globe
13

To meet the third test objective, the interference injection and intermediate frequency (IF)

monitoring equipment, shown in red in Figure 5, were located in the TDWR equipment shelter 

located at ground level directly adjacent to the elevated TDWR antenna.

Figure 5. Block diagram for injection of U-NII interference waveforms into the TDWR receiver. 

Items shown in red were provided by NTIA.

5.1 Measurements of Actual TDWR Emissions in the Time Domain

A spectrum analyzer and measurement antenna (placed on the roof of the warehouse in the same 

position as the U-NII devices) were used to measure the TDWR signal power in the time 

domain. The spectrum analyzer was located inside the RSMS-4 vehicle, as was the control 

computer. Measurements were made with the TDWR boresighted on the warehouse and in its 

monitor-mode scan. The analyzer resolution bandwidth was set to 1 MHz and the detection mode 

was set to positive-peak. The 1-MHz resolution bandwidth matched the TDWR receiver’s IF 

processing bandwidth. The measurement antenna was a cavity-backed spiral (CBS) antenna that 

has a gain of 6.5 dBi and is circularly polarized.

Figure 6 shows that the incident TDWR power at the warehouse rooftop was well above the -

64 dBm threshold, even when the TDWR main beam was not aimed directly at the rooftop. For 

low-elevation angle scans, the incident peak power in a 1-MHz bandwidth was as much as 

-20 dBm. The power out of the antenna sidelobes was also usually above the DFS detection 

threshold. The power from the TDWR has been corrected to the level that would have been 

measured with a 0-dBi gain antenna with linear polarization.
14

Figure 6. Power received from the TDWR at the FAA warehouse facility U-NII location 

throughout one complete monitor-mode scan. 

Measurements were also made of individual pulses in the time domain to show that the radar 

pulse-shape envelopes were without distortion at the U-NII location. A typical pulse envelope 

measured at that location is shown in Figure 7. As shown in this figure, the pulse is very similar 

to the transmitted rectangular pulse shape.
15

Figure 7. A single TDWR pulse in the time domain at the U-NII location.

5.2 U-NII DFS Performance Against Actual TDWR Waveforms

Each of the U-NII devices in Table 1 was tested in the presence of actual TDWR emissions, with 

the U-NII master device placed on the warehouse rooftop. For all but one case, the U-NII client 

device [3] was nearly co-located on the rooftop.21 An MPEG test file was streamed between the 

master and client device to simulate a loaded U-NII channel. U-NII device detection capability 

was tested while the TDWR operated in monitor-mode scan where the PRI varies with the 

antenna elevation angle. Fixed PRI testing was performed with the antenna beam boresighted on 

the warehouse rooftop with PRI values of: 518, 598, 798, 838, and 3066 μs. Table 5 summarizes 

the results for U-NII detection of selected TDWR PRI values, spanning the range of PRI values

available in the radar design. Each PRI was tested between five to ten times per device.

 

21 In one exception, a Manufacturer D U-NII device was located about 2.3 miles (3.7 km) from the warehouse and 

0.72 miles (1.15 km) from the TDWR, on the tower of an L-Band radar system. This separation between the U-NII 

device pair was necessary to ensure the proper functioning of the U-NII devices.
16

Table 5. Summary of DFS detection of selected, actual TDWR waveforms

U-NII

Device

TDWR Waveform PRI Trials Monitor

518 μs 598 μs 798 μs 838 μs 3066 μs Mode

Mfr. A 

Model 2 100% 0% 0% 0% 0% no

Mfr. A 

Model 422 100% 100% 100% 100% 100% yes

Mfr. C

Model 1 0% 0% 0% 0% 0% yes23

Mfr. D

Model 1 100% 100% 100% 100% 100%24 yes

Mfr. E

Model 1 100% 100% 100% 100% 100% N/A

Mfr. F

Model 1 100% 100% 100% 100% 100% N/A

Mfr. G

Model 1 100% 100% 100% 100% 100% N/A

The Model 2 device from Manufacturer A with the off-the-shelf firmware available at the time

and the Manufacturer C device did not reliably detect the TDWR waveforms. Five U-NII 

devices, including the Model 4 device from Manufacturer A (using modified firmware not 

publicly available) reliably detected all of the actual TDWR waveforms.

5.3 Qualitative Determination of U-NII I/N Levels that Cause TDWR PPI Strobes

Emissions of selected U-NII devices were digitized using an Agilent 89601 vector signal 

analyzer (VSA) at the ITS laboratory in Boulder, Colorado. The recordings were injected 

(Figure 5) into the TDWR receiver, generated with an Agilent E4438C VSG, at the RF stage to 

determine the Interference-to-Noise (I/N) levels at which strobes appear on the meteorological 

PPI display. The simulated U-NII emissions were co-channel with the TDWR. The PPI screens 

were recorded in JPEG format for each I/N level25 for each U-NII waveform. The test equipment 

was set up so that the Arbitrary Waveform Generator (AWG) would trigger the VSG to output 

U-NII device emissions such that 10 azimuths of interference would be visible on the PPI when 

the strobe threshold was exceeded. This process was repeated for both the existing (legacy) 

TDWR receiver as well as the newly developed TDWR receiver (Sigmet RVP8) which has not 

yet been deployed operationally. Tests in Oklahoma City with the RVP8, showed corresponding 

levels of effects at I/N levels that were 3 dB lower than observed with the legacy receiver.

 

22 Modified firmware was used for this device; this firmware was not publicly available at the time of testing.

23 The device took approximately 12 minutes to detect the TDWR. In general, the device did not detect the TDWR 

waveforms.

24 The device took anywhere from 45 seconds to 135 seconds to detect the 3066 μs PRI. 

25 See the Appendix to this report. Additionally, the I/N measurement method described here is detailed in [3].
17

Regardless of the U-NII signal type (frame-based, 802.11, or 802.16), strobes begin to appear 

very faintly (at a level that is almost invisible to human observers) on the TDWR PPI with the 

legacy receiver at I/N = -6 dB and on the same display with the Sigmet RVP8 receiver at I/N = 

-9 dB. Such displays are shown in Figures 8 and 9. The strobes were observed when emissions

replicating the modulation of Manufacturer E’s U-NII device were injected into the radar 

receiver.

Figure 8. TDWR meteorological display PPI showing effects of frame-based interference 

(Manufacturer E) injected on a hardline from a VSG at baseline (no interference), and I/N = -12 

dB, -9 dB, and -6 dB, respectively. 
18

Figure 9. TDWR meteorological display PPI showing effects of frame-based interference 

(Manufacturer E) injected on a hardline from a VSG at I/N = -3 dB, 0 dB, +3 dB, and +25 dB, 

respectively. 

It is important to note that the possibility of TDWR data corruption exists at I/N levels below the 

point at which visible strobes occur on the PPI [4]. The effects of this are insidious; data 

gathered from certain azimuths may be corrupt, but no indication to the user exists. During these 

tests, NTIA and FAA engineers had no readily available mechanism to confirm if this was 

occurring.
19

5.4 Effects of TDWR Antenna Sidelobes on PPI Displays

For this portion of testing, some of the U-NII devices were forced to not vacate the TDWR 

channel when the radar was detected.26 The purpose was to observe interference strobes on the 

PPI display in such a way as to replicate the behavior of U-NII devices that might not, due to a 

failure of DFS sensing, vacate a TDWR’s frequency.

The results showed that co-channel U-NII emissions at a distance of 2.97 miles (4.78 km) from 

the TDWR receiver can produce strong (meaning well-defined and opaque) interference strobes 

on the radar’s PPI display.

It was also observed that single U-NII sources produce strobes on multiple azimuths, as shown in 

Figure 10. The PPI in Figure 10 shows areas of non-U-NII clutter; these areas of clutter were 

visible on the PPI with the U-NII devices powered off. The U-NII device strobes are due to 

coupling from the U-NII to the sidelobes of the TDWR antenna. As the TDWR antenna main 

beam elevation angle increases from 0.3 degrees to 30.6 degrees, the coupling of the U-NII 

signal into the TDWR antenna side lobes causes the intensity and azimuths of the strobes to vary 

considerably. Strobe width increases as interference power in the TDWR antenna pattern 

increases, resulting in variation in the interference azimuthal widths. In this geometry, a single 

strobe occurring on the U-NII azimuth is never witnessed, instead strong strobes occur on 

azimuths that are anywhere from tens of degrees to as much as 180 degrees away from the U-NII 

azimuth, and sometimes are not observed on the U-NII azimuth at all. (It is emphasized that this 

U-NII device had to be forced, via the use of an engineering mode not accessible to users, to 

operate co-channel with the TDWR for this strobe test. Operated under normal DFS software 

control, it would not tune closer than 41 MHz to the TDWR frequency, and did not produce 

strobes under that condition.)

The implications of this sidelobe-coupling behavior are:

1. A single U-NII device can and will produce strobes on multiple azimuths on a TDWR 

display.

2. The strobe azimuths will vary as a function of the TDWR antenna main beam elevation 

angle.

3. At some TDWR antenna main beam elevation angles, no strobes will occur on the actual 

azimuth of the U-NII device, but some strobes will occur on azimuths on either side of the 

true azimuth of the U-NII device.

4. Locating incorrectly operating U-NII devices based on observed strobe azimuths on the 

TDWR PPI display will generally be a difficult problem to solve.

 

26 This non-vacating behavior was accomplished by accessing U-NII test-and-engineering modes that are not 

accessible to users.
20

5. If any U-NII device that is producing strobes at a TDWR field site can be located, 

multiple strobe azimuths may be eliminated.

Figure 10. TDWR 360-degree-scan interference strobes generated by a U-NII transmitter 

(Manufacturer D, Model 1) on a rooftop 2.97 miles (4.78 km) from the TDWR, at an azimuth 

indicated by the red arrows and at a height of 35 feet (10.6 m) above the ground. 

5.5 U-NII Device Off-Tuning For Strobe Elimination

Some of the U-NII devices were again forced to not vacate the TDWR channel when the radar 

was detected. The purpose was to observe interference strobes on the PPI display in such a way 

as to replicate the behavior of U-NII devices that might not, due to a failure of DFS sensing, 

vacate a TDWR’s frequency. U-NII device frequencies were then manually tuned away from the 
21

TDWR operating frequency until PPI strobes disappeared. During the field tests at the TDWR 

PSF, the coupling between the U-NII device antennas and the TDWR antenna was mainbeam-tosidelobe.27 In this geometry, a frequency separation of 10 MHz (or more) between the TDWR 

frequency and the 3-dB point of the U-NII device emission spectra or a nominal minimum 

20-MHz center-to-center frequency separation between the TDWR and the U-NII device was 

found to eliminate PPI strobes. This observation is based on measurements of a limited number 

of U-NII devices (five) operating line-of-sight to the TDWR in this geometry.

As was earlier observed in San Juan, PR (Part I of this series), the coupling between the U-NII 

transmitter antennas and the TDWR antenna can often actually be mainbeam-to-mainbeam. This 

interference coupling scenario could not be empirically examined at the TDWR PSF location due 

to logistics. Report III of this series discusses mainbeam-to-mainbeam coupling in further detail.

 

27 The TDWR antenna height above ground level (AGL) was 98 feet (29.8 m) and the U-NII height was 40 feet 

(12.2 m) AGL, at a separation distance of 2.97 miles (4.78 km).
22

6 SUMMARY OF TEST RESULTS

1. A Model 2 U-NII device from Manufacturer A detected the Bin 1 Waveform used in FCC 

certification but did not detect the emulated or actual TDWR waveforms at the ITS 

Laboratory or at the TDWR PSF.

2. A Model 2 U-NII device from Manufacturer A with modified firmware (provided by the 

manufacturer) successfully detected the emulated and actual TDWR waveforms at the ITS 

lab and at the TDWR PSF.

3. The Manufacturer A U-NII devices’ ability to detect the radar signal in the ITS laboratory 

was severely degraded using the Bin 1 Waveform for in-service monitoring when the 

talk/listen ratio was set to the user manual suggested value of 75%/25%.

4. A Model 1 device from Manufacturer C failed to detect the actual TDWR signal at the 

TDWR PSF during boresight testing, only detected the actual TDWR signal at the TDWR 

PSF on two out of four monitor-mode scans, failed to detect the Bin 1 Waveform, and 

failed to detect all 24 emulated TDWR waveforms during laboratory testing. This device 

was certified for DFS functionality with a different firmware revision number than the one 

that failed both the laboratory and field tests, although it was still using the original FCC ID 

number. After new firmware was loaded onto this device, it successfully detected the Bin 1 

Waveform and all 24 emulated TDWR waveforms during subsequent laboratory testing.

5. Four off-the-shelf U-NII devices from Manufacturers D, E, F and G reliably detected the 

actual TDWR waveforms at the TDWR PSF; a device from Manufacturer A reliably 

detected the actual TDWR signals after a firmware upgrade provided by the manufacturer

(see item 2 above).

6. Properly operating U-NII devices did not generate strobes on the TDWR PPI display when 

they operated on the nearest available frequencies (relative to the TDWR fundamental 

frequency) and when their DFS functionality did not detect the TDWR emission.

7. A frequency separation of 10 MHz (or more) between the TDWR center frequency and the 

3-dB point of the U-NII device emission spectra or a nominal minimum 20-MHz center-tocenter frequency separation between the TDWR and the U-NII device was found to 

eliminate PPI strobes in interference geometries with U-NII device mainbeam to TDWR 

antenna sidelobe coupling. This observation is based on measurements of a limited number 

of U-NII devices operating line-of-sight to the TDWR.

8. The results of laboratory versus field measurements and tests of the DFS functionality of 

the seven U-NII devices that were evaluated are in agreement with each other. Therefore, 

any additional testing with U-NII devices (for their ability to detect the actual TDWR radar 

signals) can be done in the ITS or FCC laboratories with emulated TDWR signals. 

Additional in-situ tests at TDWR facilities are not necessary.
23

9. Strobes appear very faintly on the TDWR PPI at some elevation angles at an I/N of -6 dB, 

regardless of the type of U-NII signal modulation, for the existing (legacy) TDWR

receiver. They appear faintly at an I/N of -9 dB for the newly developed, but not-yetdeployed RVP828 TDWR receiver. Strobes become progressively stronger in appearance as 

the I/N level increases.

10. The TDWR sidelobe interaction with a single U-NII interference source will generate 

strobes at multiple azimuths on the PPI display. The response depends on the TDWR 

antenna main beam elevation angle: this sidelobe coupling occurs even when the TDWR 

main beam is elevated substantially above the angle where it would directly illuminate a UNII location. In this geometry, little or no strobe activity occurs solely on the U-NII 

azimuth; instead, strong strobes occur on azimuths that are anywhere from tens of degrees 

to as much as 180 degrees away from the U-NII azimuth. This increases the difficulty of 

identifying interference sources based on strobe-azimuth data, but also means that 

elimination of a single U-NII interference source may eliminate multiple TDWR strobes.

 

28 Two new TDWR receiver designs, the 14-bit RVP8 and the 16-bit RVP9, are currently in progress. The RVP9 is 

predicted to show interference effects at I/N levels that are around -12 dB, but no RVP9 was yet available for 

evaluation when the U-NII interference tests were performed.
24

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] Carroll, J. E., F. H. Sanders, R. L. Sole, and G. A. Sanders, “Case Study: Investigation of 

Interference into 5 GHz Weather Radars from Unlicensed National Information 

Infrastructure Devices, Part I”, NTIA Report TR-11-473, November 2010. Downloadable 

at: <http://www.its.bldrdoc.gov/pub/ntia-rpt/11-473/>.

[3] 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.

[4] Sanders, F. H., R. L. Sole, B. L. Bedford, D. Franc, and T. Pawlowitz, “Effects of RF 

Interference on Radar Receivers”, NTIA Report TR-06-444, September 2006. 

Downloadable at: <http://www.its.bldrdoc.gov/pub/ntia-rpt/06-444/>.
25

8 ACKNOWLEDGEMENTS

The authors 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.
26
27

APPENDIX: INTERFERENCE-TO-NOISE RATIO IN RADAR RECEIVER 

PERFORMANCE TESTS

Microwave radar receivers detect objects in space (generically referred to as targets) by 

processing the power in successive transmitted-pulse echoes in such a way as to reinforce a 

series of echoes from a single object or weather phenomenon into a power level that exceeds a

detection threshold. Non-meteorological radars perform this target-detection function for discrete 

targets such as aircraft, while meteorological radars detect spatially-extended atmospheric 

phenomena. All microwave radars are ultimately limited in their target-detection capability by 

electronic noise that is generated by thermal electrons in the receiver circuitry. In many radar 

receivers, the performance of a single component, the RF front-end low-noise amplifier (LNA), 

determines the overall thermal-noise limit for the entire receiver, and thus is critically important 

to the overall performance of the entire receiver. The thermal noise generated by an LNA is 

equal to the fundamental noise limit imposed by quantum-atomic physics, multiplied by a 

coefficient called the noise factor:

 ( ) (1)

where:

n = noise power generated by the LNA (W);

k = Boltzmann’s constant, 1.38 x 10-23 J/K;

T = temperature of the receiver (K);

B = bandwidth of the receiver circuitry (Hz);

nf = noise factor of the LNA (dimensionless).

Rendered into decibel terms, (1) becomes:

 ( ) ( 

 ) (  ) (2)

where:

N = noise power generated by the LNA (conventionally dBW or dBm);

NF = 10log10(nf) = so-called noise figure of the LNA.

For T = 290 K and B is 1 Hz, 10log10(kTB) = -204 dBW = -174 dBm. For a TDWR with a 

1 MHz-wide receiver bandwidth, the value of 10log10(kTB) is (-174 dBm + 60 dB) = -114 dBm. 

If the noise figure, NF, of the LNA is 1 dB, then the thermal noise limit, N, against which 

meteorological phenomena must be detected is N = -113 dBm.

Due to engineering limitations on the EIRP that radar transmitters can generate, along with the 

relatively small reflection cross sections of most radar targets, the pulse-echo energy from most 

radar targets, including meteorological phenomena, is actually lower than N. Such energy can 

nevertheless be detected for two reasons: first, since the pulse-echo energy adds linearly to the 

thermal noise power in the receiver, the sum of the two power levels can sometimes be detected 
28

directly; second, radar receivers do not generally detect the echoes of single pulses, but instead 

use the coherently correlated power in successive pulse echoes to integrate a final power level 

that does exceed N.

Radars that detect and track aircraft typically need 15-20 successive pulse echoes to form a 

detectable target in the receiver circuitry, while meteorological radars need about 30-50 

successive pulse echoes to form detectable patterns of atmospheric phenomena. This requirement 

for a minimum number of pulse echoes to detect targets, combined with the minimum requisite 

interval between pulses to meet maximum-range requirements, ultimately determines the rate at 

which a radar beam can be scanned through space.

The most important point to understand about this entire process is that all microwave radars 

operate against their own receiver’s thermal noise. But just as echo power from desired targets or 

meteorological phenomena adds to N, so too does RF interference energy in the receiver add to 

N. As RF interference power, i, adds to n, desired targets are obscured in the radar receiver 

detection circuitry. In the presence of interference power the radar power level against which 

targets must be detected is not n, but instead is (i + n). Consider, for example, a situation in 

which the level of i is 9 dB lower than n. In that case, the sum of the two power levels, expressed 

in decibels, is:

 ( ) ( 

 

 

 

 ) ( )

 

where N is normalized to 0 dB, and i and n are the linear power levels corresponding to the 

decibel values of I and N.

If the interference power is 6 dB lower than the radar receiver’s inherent thermal noise floor, 

then the sum of the two quantities raises the effective limit against which targets must be 

detected by 10log10(0.25+1) = 1 dB. If the interference power is equal to the radar receiver’s 

inherent noise floor, then the effective limit against which targets must be detected increases by 

10log10(1+1) = 3 dB, and so on. Table A-1 shows how effective radar receiver noise,

 (

 

 

) 

 

 

(in decibels), varies as a function of I/N.

Table A-1. Effective radar receiver noise-limit increase, (I+N)/N, as a function of I/N

I/N Effective Increase in Radar Receiver Noise, (I+N)/N

-9 dB 0.5 dB

-6 dB 1 dB

0 dB 3 dB

10 dB 10.4 dB (notice that (I+N)/N approaches I/N as I N)

Because the ratio, I/N, of the interference power level to the radar receiver’s inherent noisepower level is the quantity of concern in this analysis, measurements of radar receiver 

performance in the presence of interference are always quantified by NTIA in terms of radar 

target-detection performance as a function of that ratio. NTIA studies [1] have shown that when 

I/N = -6 dB, most radars begin to show measurable losses in target-detection performance.
FORM NTIA-29 U.S. DEPARTMENT OF COMMERCE

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

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION NO.

TR-11-479

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 II

5. Publication Date

July 2011

6. Performing Organization 

Code

7. AUTHOR(S)

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

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 safety of operations in and around major 

airports. Institute for Telecommunication Sciences (ITS) engineers, with assistance from FAA engineers, determined 

the interference to be from unlicensed national information infrastructure (U-NII) dynamic frequency selection 

(DFS) devices, from several manufacturers, operating in the same frequency band as TDWR systems. 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 report describes measurements and results from controlled laboratory and field testing of these U-NII 

devices. This is the second of a three-part series of reports that describe research efforts by the ITS engineers, with 

assistance from FAA engineers, to determine the cause of the interference, examine the effects of the interference on 

TDWR systems, and 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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