02/6/ jdr1 Interference in VLBI Observations Jon Romney NRAO, Socorro ===================================== 2002 June 12

02/6/ jdr2 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

02/6/ jdr3 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.

02/6/ jdr4 Background: VLBI Very Long Baseline Interferometry Basic Concept Extension 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 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.

02/6/ jdr5 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.

02/6/ jdr6 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)).”

02/6/ jdr7 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 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”.

02/6/ jdr8 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 NRAO’s VLBA instrument. Other VLBI-only stations exist, but do not operate in RAS- allocated bands.

02/6/ jdr9 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.

02/6/ jdr10 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.

02/6/ jdr11 Interference in VLBI Calibration (Continued) Consider Advocating Single-Dish Protection For VLBA’s two highest frequency bands: 43 & 86 GHz; to maintain usability of template method in SiO maser observations.

02/6/ jdr12 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