Distance may also be determined using angular measurement. The spacecraft's position in the sky is expressed using angular quantities. While the angles at which the DSN antennas point are monitored with an accuracy of thousandths of a degree, they are not precise enough to be used in determining a distant interplanetary spacecraft's position in the sky for navigation. Except when the spacecraft is close to Earth after launch, antenna angles are useful only for pointing the antenna to the predicts given for acquiring the spacecraft. While precision ranging provides a distance measurement accurate within meters at the distance of Jupiter, it also provides precise angular location.
Triangulation then pinpoints the spacecraft.
VLBI Extremely precise angular measurements can be provided by an independent process, VLBI, Very Long Baseline Interferometry . High-rate recordings are made of the downlink's wave fronts by each station, together with precise timing data. After a few minutes, and while still recording, both DSN antennas slew directly to the position of a quasar, which is an extragalactic object whose position is known to a high precision. Then they both slew back to the spacecraft, and end recording a few minutes later. Correlation and analysis of the recorded wavefronts yields a very precise triangulation from which both angular position and radial distance may be determined.