radio frequency interference and how to deal with it

Atacama Large Millimeter/submillimeter ArrayExpanded Very Large Array

Robert C. Byrd Green Bank TelescopeVery Long Baseline Array

Radio Frequency Interference and How to Deal With It

Toney Minter NRAO

2

What can RFI do to your observations?

• Cover your spectral line

3

What can RFI do to your observations?

• Cover your spectral line • Create baseline issues

4

What can RFI do to your observations?

• Cover your spectral line • Create baseline issues• Create non-linear responses

What can you do?

Ignore the RFI Flag data after observations Get worse results than desired Waste lots of telescope time

Modify your observations Try to avoid known RFI Observe when RFI is off (night-time, etc.)

Eliminate the RFI transmitter Not always possible Only have rights in protected bands Quiet Zone Cooperation of transmitter

Cancel RFI signal Need reference antenna Subtract reference antenna signal from data

Planning Your Observations

• There is no magic bullet for dealing with RFI• Understand the RFI

– What frequencies– How strong– How often

• Constant• Periodic• Random

– Signal direction• Look at Spectrum Surveys

– Arecibo• http://www.naic.edu/~rfiuser/smarg-hplots.html

– Green Bank• http://www.gb.nrao.edu/IPG/

Planning Your Observations

• Coordinate the observations if possible– Is RFI less prevalent at night?– Are emitters willing to participate?

• IF system– Set filters and frequencies in IF system to remove RFI

• Use narrow filters• Offset spectral line from bandpass center

– Watch your power levels• Strong RFI can saturate parts of the IF

• Backends– Higher level sampling– Shorter integration times

Ringing from Strong RFI

Removing RFI from Data

• Automated removal of RFI very difficult– Pulsars

• Very short integration time in each sample• Majority of samples do not have “signal”• “Signal” < noise level in sample• Determine noise level• Clip signal well above noise

– Spectral line• Longer integration times

– Fewer samples– Lower noise levels – easier to see RFI

• How do you tell difference between RFI and line?

Flagging Spectral Line Data

• “Types” of RFI– Narrow lines that vary in time– Narrow lines that are persistent

Narrow, Time Varying RFI

Flagging Spectral Line Data

• “Types” of RFI– Narrow lines that vary in time– Narrow lines that are “constant” in time– Strong occasional or periodic RFI

Strong RFI

Flagging Spectral Line Data

• “Types” of RFI– Narrow lines that vary in time– Narrow lines that are “constant” in time– Strong occasional or periodic RFI – Occasional strong out of band RFI

Out of Band RFI

Conclusions

• No Magical solutions• Do your homework before you observe• Do your best to avoid RFI during your observations• Understand your data

– Where is the RFI– When is the RFI

• Flag your data accordingly