A new frame for the sky

Artist’s impression of the huge outflow ejected from the quasar SDSS J1106+1939Credit: ESO/L. Calçada

The sky gets a new reference frame. On 30 August the International Astronomical Union adopted the International Celestial Reference Frame 3 (ICRF-3) during their general assembly in Vienna, Austria. As of 1 January 2019 this reference frame has global validity. It serves for example for the orientation of GPS systems as well as the navigation of space probes.

For any kind of positioning and navigation on Earth or in space reference frames are needed. Just like the longitudes and latitudes on the Earth’s surface, the sky can be covered with a grid net. This reference frame allows for a precise positioning of objects in the sky in relation to Earth. For the establishment of this grid net “anchor points” are needed: For the Earth, there are about 50 radio telescopes and for the sky 4536 so called quasars, galaxies that circle around a black hole, serve as anchors of the grid.

Over the last 40 years, the radio telescopes, located on every continent, determined the positions of the 4536 quasars via Very Long Baseline Interferometry (VLBI), with significant contribution of the VLBI working group of GFZ section Space Geodetic Techniques. Robert Heinkelmann, head of the working group: “Based on all VLBI observations of the past 40 years we were able to calculate the exact locations of all extra galactic objects. We furthermore found a solution to compensate for the observation bias caused by the rotation of our galaxy, the Milky Way.”

The last reference frame (ICRF-2) was published in 2010. Compared to that the new system improves the precision on average by a factor of 1.5. Furthermore, about 30 percent more objects were included in the calculations, and the rotation of the Milky Way was included for the first time. In addition, the results are now available in three different radio band widths which makes them available for a wide user group.

Based on the new system, the position of objects in space can be determined with a precision of a hundredth of a millionth angular degree, equivalent to spotting a tennis ball on the surface of the Moon from the Earth.

Quasars permanently emit radio waves that can be detected via the radio telescopes on Earth. Since the quasars are extremely far away from Earth (about 100 million to 10 billion light years) they can be considered as stationary in relation to Earth, even if in permanent motion. This is why they are suitable as anchor points for the celestial reference frame.

Not only positioning systems like GPS or the European pendant Galileo depend on a reference system in the sky, also changes on the surface of the Earth like the movement of plates, volcanic eruptions, sea level changes, earthquakes or changes in the position of the Earth in space are based on it. For the surface of the Earth the International Terrestrial Reference Frame (ITRF) determines every position on the Earth in a coordinate system with the centre of the Earth as the centre of the system. To measure, for example, if sea level is rising or land is sinking the reference frame on the Earth’s surface needs another reference frame in the sky that it can be related to. The more precise the reference frame in the sky, the more precise an observation of changes on the surface of the Earth can be.