GPS plays role in black hole image

Tracy Cozzens

On April 10, the world looked in awe at the first image of a black hole. The image was captured by a world-spanning network of radio telescopes that together, using Orolia atomic-clock technology, create the Event Horizon Telescope.

It zeroed in on the supermassive monster — 6.5 billion times the mass of the sun — in Galaxy M87 to create the image.

As Innovation Editor Richard Langley explains, the technique used to capture the image — very long baseline interferometry (VLBI) — relies on GPS. (VLBI was the topic of Langley’s Ph.D. thesis.)

VLBI links two or more radio telescopes that can be many kilometers apart, or even on different continents. VLBI is used in both geodesy and astronomy. There is also a practical GPS link to the Event Horizon Telescope. From the second of six simultaneously published open-access papers on the result: “All timing is locked to a 10-MHz [hydrogen] maser reference and synchronized with a pulse-per-second (PPS) Global Positioning System (GPS) signal…”

“[T]he long-term drift of the maser [is] compared to GPS, measured by differencing [and plotting] the 1 PPS ticks from the maser and local GPS receiver. The vertical width of the trace is due to variable ionospheric and tropospheric delays of the GPS signal, while the long-term trend represents the frequency error of the maser. The drift measured from this plot, and its effects on the fringe visibility, are removed during VLBI correlation.”

Image: Event Horizon Telescope Collaboration

From the third paper: “In order to reconstruct the brightness distribution of an observed source, VLBI requires cross-correlation between the individual signals recorded independently at each station, brought to a common time reference using local atomic clocks paired with the Global Positioning System (GPS) for coarse synchronization.”

Read more about the image and GPS.