factors to consider for intersystem emc itu-r radar seminar geneva 24 september 2005 frank sanders...

Factors to Consider for Intersystem EMC

ITU-R Radar SeminarGeneva

24 September 2005

Frank SandersChief, ITS Telecommunications Theory Division,

U.S. Department of Commerce, NTIA325 Broadway, Boulder, CO

[email protected]

Institute for Telecommunication Sciences – Boulder, Colorado


Introduction

-Proposals have been made to operate communication systems in radar bands.

-These proposals represent a change from the historical approach of separate band allocations due to perceived electromagnetic compatibility issues between radar and communication services.

-This talk addresses the technical challenges analyzing intersystem EMC.


Some justifications offeredfor sharing

1) Radar systems make little use of existing spectrum allocations.

2) Radar receivers are inherently robust against interference.

3) Perhaps interference to radars might be accommodated on a statistical basis.

4) Spectrum might be shared by communication devices that sense radar signals and avoid local radar frequencies.


Evaluation of these ideas…

…may create opportunities for new patterns of spectrum use. But careful technical consideration of EMC is needed to test these concepts.

We will consider the four ideas from the previous slide in this presentation…


Measured spectrum utilization of radars

Radars usually consist of high power transmitters operating in concert with very sensitive receivers. EIRP transmit levels are often more than 1 GW, while receiver noise figures are often only a few decibels above the theoretical thermal noise limit.

Mission Pulse

width (us)

Pulse rate (Hz)

Peak

power (MW)

Antenna

gain (dBi)

Peak

EIRP (GW)

Short range air search

1 1000 0.8 33 1.6

Long range air search

3-10 300 1 33 2

Maritime navigation

0.08-0.8 10000 0.02 30 0.02

Weather 1-5 300-1300 0.75 45 24

Representative radar emission parameters as a function of mission.


Measured spectrum utilization of radars, continued

-Some spectrum surveys have not observed radar emissions. -Some spectrum surveys have not observed radar emissions. Is this evidence that radars don’t utilize spectrum allocations Is this evidence that radars don’t utilize spectrum allocations very much?very much?

-But published NTIA spectrum surveys show significant -But published NTIA spectrum surveys show significant spectrum utilization by radars on an ongoing basis at every spectrum utilization by radars on an ongoing basis at every location in the US where such measurements have been done location in the US where such measurements have been done (for example, in Denver, San Diego, Los Angeles, and San (for example, in Denver, San Diego, Los Angeles, and San Francisco).Francisco).

Why might this be?



San Diego, California measured radar utilization of L-band (2 weeks)

Maximum

Mean

Minimum

Source: NTIA spectrum survey report



San Diego, California measured radar utilization of C band (2 weeks)

Maximum

Mean

Minimum

Source: NTIA spectrum survey report


Why don’t some spectrum surveys show radar signals whereas the NTIA results do show substantial usage?

Radar emissions don’t show up with conventional survey techniques. Instead, radars need to be observed with specialized, computer-controlled techniques. NTIA publications (e.g., a recent NTIA Report on RSEC measurement procedures) explain how. See also ITU-R New Recommendation M.1177.

Specialized RF front ends are also needed…


M.1177 radar meas. front-end


Radar receiver performance in the presence of interference

In some EMC studies that have been performed by NTIA, radars have been intentionally subjected to low levels of interference from various waveforms.

Protocols:Radar receiver is disconnected from its antenna. In place of live

targets, test targets are injected into the radar receiver.

Target power level is adjusted so that the probability of detection of these targets is as close as possible to 90% (Pd=0.9).

Interference is injected with a selected modulation. Target losses are counted as a function of interference power level relative to radar inherent receiver noise level (I/N).



Example radar plan position indicator (ppi) display during interference testing.

The bright wedge is a time-lapse effect of the radar ppi scan line.



Equipment room of a radar station Examination of a radar receiver circuit card during interference testing

Radar receiver EMC performance in the presence of interference

Example data showing target losses as a function of I/N ratio for a search radar.


Example data showing target losses as a function of I/N ratio for a maritime search radar.

Pulsed signals representative of otherRadar emissions.

Communication-typesignals



Radar type CW interference

QPSK interference

Pulsed interference

Air search -10 dB -10 dB +30-40 dB

Maritime navigation and

search

-10 dB -10 dB +30-40 dB

Weather surveillance

-12 dB -12 dB +30-40 dB

Typical I/N ratios at which measurable target losses begin to occur. Based on results of measurements to date and grouped by radar mission.


Do target losses only occur at the edge of radar coverage?

Interference causes loss of targets within some number of decibels (call it X dB) of radar inherent noise...

…therefore target losses don’t depend upon range, per se. To the extent that distant targets are weaker than closer targets, losses would correspond somewhat to range. But target cross sections vary…

…so some targets are weak and close. Since X dB is X dB, target losses don’t strictly translate into range reduction. Weak targets are not necessarily unimportant targets.



Target losses at low I/N levels of interference are insidious.

Low-level interference doesn’t make any weird features appear. Targets simply fade away, so there is no way to tell that targets have been lost.


10 targets with no interference presentLow-level interference has caused all targets to disappear. But there is no overt indication of Interference.


Example of ppi screen display effects of high-level interference (such as dramatized in movie portrayals).



Technical consideration of a statistical basis for radar interference

The statistical approach raises several questions:

1) Which targets is it acceptable to lose?

3) How can target losses be correlated with communication system interference?

2) How can communication systems coordinate activities with radars to assess target losses?


Mitigation of interference by avoiding radar frequencies

-In ‘dynamic frequency selection’ (DFS), communication units sense radar signals on the communication systems’ frequency, and then move operations to another frequency.

-No theoretical reason has been shown why this approach cannot work; it is possible in principle. But it needs to be tested.

-In the USA, NTIA, other Government agencies, and industry are currently working together to test this technology in the 5 GHz part of the spectrum.

-DFS depends upon communication systems to sense radar signals and then avoid the conflicted frequencies.


Summary-Radar activity fills allocated spectrum bands at surveyed locations in the US. But proper monitoring techniques are necessary for successful surveys.

-In tests performed to date, radars typically begin losing targets when interference is at about -10 dB I/N. (That is, at 10 dB average power below radar receiver inherent noise.)

-Proposals to allow interference on a statistical basis are in early stages; the technical feasibility of this approach is being studied.

-An alternative approach, dynamic frequency selection, may be feasible. It typically requires substantial amounts of listening time. The concept is undergoing tests at this time.

-Effects of low-level interference are insidious; targets fade away without overt indications of interference.

factors to consider for intersystem emc itu-r radar seminar geneva 24 september 2005 frank sanders...

Documents

search radar

radar signals

radar systems

radar receivers

radar stationexamination

radar bands

colorado slide

radar receiver emc performance