MethodObservingUpdated 2026-08-15

Very Long Baseline Interferometry

Time a wavefront across two distant antennas, and get a reference frame out of the difference.

Evidence status

Checked against 3 inspected sources

3 sources were retrieved, identified and read, and the claims below are tied to specific passages at the scope those passages state. Each source also records what it cannot establish.

Rely on this page for

The specific claims that carry a cited passage, at the scope that passage states.

Working definition

VLBI measures the difference in arrival time of a radio wavefront from a distant quasar between two Earth-based antennas. NASA states the timing is precise to a few picoseconds, which yields antenna positions to a few millimetres and quasar positions to fractions of a milliarcsecond. From many such measurements the technique determines three things at once: the terrestrial reference frame of antenna locations, the celestial reference frame of quasar positions, and the orientation of the Earth in space. Its inertial anchor is an assumption with a stated reason, that extragalactic objects are far enough away for their motion across the sky to be undetectable.

What is measured

  • The arrival-time difference of a quasar radio wavefront between two Earth-based antennas.
  • That time difference is stated to be measured to a few picoseconds.
  • Many such differences are collected across a global antenna network and many quasars.

What is inferred

  • Antenna positions on the Earth, stated to a few millimetres.
  • Quasar positions on the sky, stated to fractions of a milliarcsecond.
  • The orientation of the Earth with respect to the celestial frame.

Fact-layer dependency

The explanatory layer cannot rewrite these fields.

Fact schema →
time.utcInstantobserver.positionreference.framesubject.identifiers

What is observed

The arrival-time difference of a quasar radio wavefront between two Earth-based antennas. That time difference is stated to be measured to a few picoseconds. Many such differences are collected across a global antenna network and many quasars.

Calibrated measurementRestates source[1]

VLBI measures the arrival-time difference of a quasar radio wavefront between two Earth-based antennas, to a stated few picoseconds.

Boundary: The stated precisions are capabilities of the technique rather than a result reported for a particular session.

How inference enters

Antenna positions on the Earth, stated to a few millimetres. Quasar positions on the sky, stated to fractions of a milliarcsecond. The orientation of the Earth with respect to the celestial frame.

Model-dependentRestates source[1]

Antenna positions to a few millimetres, quasar positions to fractions of a milliarcsecond, and Earth orientation are determined jointly from many such measurements.

Boundary: Extragalactic objects are treated as an inertial frame, which NASA justifies by their motion across the sky being undetectable at such distances.

Limits and unresolved questions

The stated precisions are capabilities of the technique rather than a result reported for a particular session. The source describes what the technique determines without deriving the relation between baseline length and angular resolution, so no resolution formula rests on it here. The inertial assumption is an approximation justified by distance, not an exact statement that the sources are motionless.

Open questionMaha inference[1]

The quoted precisions are stated capabilities of the technique, and the inertial reference assumption holds by distance rather than exactly.

Boundary: The inertial assumption is an approximation justified by distance, not an exact statement that the sources are motionless.

Sources

Each source states both what it establishes and where its authority ends. Access dates record the last public verification.

  1. [1]Very Long Baseline Interferometry · NASA Earthdata · accessed 2026-08-15

    Establishes: That VLBI measures the arrival-time difference of a quasar radio wavefront between two Earth-based antennas, with stated precisions of a few picoseconds in time, a few millimetres in antenna position and fractions of a milliarcsecond in quasar position, and that it determines the terrestrial frame, the celestial frame and Earth orientation.

    Boundary: An agency technique description. The quoted precisions are stated capabilities rather than a result for any particular observation, and the page describes what the technique determines without deriving the relation between baseline length and resolution.

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Foundations

Sky position

Observing

Astrometry

Direct answer

  • VLBI measures the difference in arrival time of a radio wavefront from a distant quasar between two Earth-based antennas. NASA states the timing is precise to a few picoseconds, which yields antenna positions to a few millimetres and quasar positions to fractions of a milliarcsecond. From many such measurements the technique determines three things at once: the terrestrial reference frame of antenna locations, the celestial reference frame of quasar positions, and the orientation of the Earth in space. Its inertial anchor is an assumption with a stated reason, that extragalactic objects are far enough away for their motion across the sky to be undetectable.

Mechanism and method

  • The arrival-time difference of a quasar radio wavefront between two Earth-based antennas.
  • That time difference is stated to be measured to a few picoseconds.
  • Many such differences are collected across a global antenna network and many quasars.
  • Antenna positions on the Earth, stated to a few millimetres.
  • Quasar positions on the sky, stated to fractions of a milliarcsecond.
  • The orientation of the Earth with respect to the celestial frame.

What is measured

  • The arrival-time difference of a quasar radio wavefront between two Earth-based antennas.
  • That time difference is stated to be measured to a few picoseconds.
  • Many such differences are collected across a global antenna network and many quasars.

Limitations

  • The stated precisions are capabilities of the technique rather than a result reported for a particular session.
  • The source describes what the technique determines without deriving the relation between baseline length and angular resolution, so no resolution formula rests on it here.
  • The inertial assumption is an approximation justified by distance, not an exact statement that the sources are motionless.

Boundaries declared by the cited sources

  • An agency technique description. The quoted precisions are stated capabilities rather than a result for any particular observation, and the page describes what the technique determines without deriving the relation between baseline length and resolution. (boundary declared by Very Long Baseline Interferometry)
  • An agency technique description. The precisions are stated capabilities rather than results for a particular observation. (boundary declared by Very Long Baseline Interferometry)
  • An agency technique description. The precisions are stated capabilities rather than results for a particular observation. (boundary declared by Very Long Baseline Interferometry)

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