Interference in VLBI Observations

1
Interference in VLBI Observations

• Jon Romney
• NRAO, Socorro
• 2002 June 12

2
Overview

• 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

3
Background Interferometry

• Basic Concept
• Synthesis of large aperture by combining signals
  from multiple receiving antennas. Spatial
  resolution (synthesized beam) determined by
  maximum extent of synthesized aperture.
• Fundamental Detail
• Signals from individual elements must be delayed
  (and phase-shifted) to align desired wavefront
  before combination.
• Analogy
• Interference susceptibility corresponds to that
  of single antenna with synthesized beam.

4
Background VLBI

• Very Long Baseline Interferometry
• Basic Concept
• Extension of interferometry to continental or
  global scale.
• NRAO operates worlds only dedicated VLBI
  instrument.
• Other formal and informal VLBI arrays exist on
  part-time bases.
• Fundamental Details
• Distances 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.

5
Interference Immunity of VLBI

• Fundamental Consideration for Interference
• Interference almost always independent at
  individual antennas.
• Independent Interference Signals
• Generally do not appear in output of VLBI system.
• Independent signals do not correlate.
• Washed out by differential delay / phase
  compensation.
• Exceptions
• Strong interference may cause gain compression.
• Simultaneous (but independent) interference has
  been observed.

6
Regulatory Status of VLBI

• ITU Recommendation RA.769
• Recognizes 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 (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.

8
Special Regulatory Concept 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 NRAOs VLBA instrument.
• Other VLBI-only stations exist, but do not
  operate in RAS-allocated bands.

9
Impact of Interference on VLBI Calibration

• Single-Dish Measurements
• Used 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.

10
Interference in VLBI Calibration (Continued)

• Template Method of Gain Calibration
• Uses 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.

11
Interference in VLBI Calibration (Continued)

• Consider Advocating Single-Dish Protection
• For VLBAs two highest frequency bands 43 86
  GHz
• to maintain usability of template method in SiO
  maser observations.

12
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