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Interference in VLBI Interference in VLBI ObservationsObservations

Jon Romney

NRAO, Socorro=====================================

2002 June 12

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OverviewOverview

Background: Interferometry

Background: VLBI

Essential Point: Interference “Immunity” of VLBI

Examples of Interference in VLBI Observations

Regulatory Status of VLBI Observations

Secondary Impact of Interference: Calibration

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Background: InterferometryBackground: Interferometry

Basic ConceptSynthesis of large aperture by combining signals from multiple

receiving antennas. Spatial resolution (“synthesized beam”) determined by maximum extent of synthesized aperture.

Fundamental DetailSignals from individual elements must be delayed (and phase-

shifted) to align desired wavefront before combination.

AnalogyInterference susceptibility corresponds to that of single antenna

with synthesized beam.

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Background: VLBIBackground: VLBI

Very Long Baseline Interferometry

Basic ConceptExtension of interferometry to continental or global scale.

NRAO operates world’s only dedicated VLBI instrument.Other formal and informal VLBI arrays exist on part-time

bases.

Fundamental DetailsDistances & bandwidths too great to transmit signals to central

site in real time; must be recorded, with time tags.Distances also too great to transmit reference frequency; each

antenna must have independent frequency standard.

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Interference “Immunity” of Interference “Immunity” of VLBIVLBI

Fundamental Consideration for Interference Interference almost always independent at individual antennas.

Independent Interference SignalsGenerally do not appear in output of VLBI system.Independent signals do not correlate.“Washed out” by differential delay / phase compensation.

ExceptionsStrong interference may cause gain compression.Simultaneous (but independent) interference has been

observed.

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Regulatory Status of VLBIRegulatory Status of VLBI

ITU Recommendation RA.769Recognizes some interference immunity even for local

interferometers:“... compared to a single radio telescope, the interferometer

has a degree of immunity to interference which, under reasonable assumptions increases with the array size expressed in wavelengths.”

But recommends special treatment only for VLBI:“The greatest immunity from interference occurs for

interferometers and arrays in which the separation of the antennas is sufficiently great that the chance of occurrence of correlated interference is very small (e.g. for very long baseline interferometry (VLBI)).”

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Regulatory Status of VLBI Regulatory Status of VLBI (Continued)(Continued)

Instead, RA.769 specifies alternative protection for VLBI:“The tolerable interference level is determined by the

requirement that the power level of the interfering signal should be no more than 1% of the receiver noise power.”

These levels, specified in terms of SPFD, are 40-55 dB higher than for non-VLBI observations.

A few other ITU Recommendation specifically mention VLBI, but only very special cases: Space VLBI, and “Shielded Zone of the Moon”.

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Special Regulatory Concept: Special Regulatory Concept: “VLBI Observatories”“VLBI Observatories”

“VLBI Observatories”Concept appears in other ITU documents, derived from RA.769

category of VLBI observations.Considered to make no non-VLBI observations.Entitled only to VLBI-level protection in site-dependent

agreements.

Class is Limited (so far)… to the 10 stations of NRAO’s VLBA instrument.Other VLBI-only stations exist, but do not operate in RAS-

allocated bands.

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Impact of Interference on Impact of Interference on VLBI CalibrationVLBI Calibration

Single-Dish MeasurementsUsed to calibrate antennas used in VLBI observations.Were once invoked to suggest VLBI “immunity” to interference

should be dropped from RA.769; effort was dropped.Pointing and gain measurements:

Need not be impacted severely by interference.Must be done using strong sources.Can be done at frequencies where interference is minimal.

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Interference in VLBI Interference in VLBI Calibration Calibration (Continued)(Continued)

“Template Method” of Gain CalibrationUses strength of spectral feature in single-dish data from VLBI

tapes to derive time-dependent antenna gain.Useful when gain is highly variable:

Pointing errors at high frequencies.… or when gain cannot be measured directly at all:

Weak sources.Small antennas.Low- or variable-sensitivity receivers..

Was essential in early VLBI, still may be in some cases.BUT cannot be used if interference corrupts single-dish spectra

seriously enough.

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Interference in VLBI Interference in VLBI Calibration Calibration (Continued)(Continued)

Consider Advocating Single-Dish ProtectionFor VLBA’s two highest frequency bands: 43 & 86 GHz;to maintain usability of template method in SiO maser

observations.

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Q u e s t i o n s ?Q u e s t i o n s ?

Background: Interferometry

Background: VLBI

Essential Point: Interference “Immunity” of VLBI

Examples of Interference in VLBI Observations

Regulatory Status of VLBI Observations

Secondary Impact of Interference: Calibration