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YOU’RE whizzing through space, and you’ve just taken a wrong turn en route to a distant planet. From where you are sitting at the helm of your ship, your chances of getting back on course seem remote. GPS, with its network of Earth-orbiting satellites, is no use this far from home. Even the North Star can only tell you which way you are pointing, not where you are. What you need is some form of celestial satnav that can help you find your place among the stars.

Fortunately, we now have an orientation system that might be able to do the job, but constructing it hasn’t been easy. To chart our place in the universe, astronomers have looked billions of light years away, to some of the most extraordinary objects in the cosmos: quasars. These intense beacons of light surrounding black holes in distant galaxies are being used to fix physical positions back here in the solar system. And not only will they help guide our travels to distant worlds, they will also help us learn more about our own.

Earth has its own familiar coordinate system, a global grid of latitude and longitude that can pinpoint the location of anything on our planet’s surface relative to the Greenwich meridian and the equator. By using distance from Earth’s core as a third coordinate, we can leave the surface to pinpoint aircraft, clouds and satellites.

Extending this further out into space is possible, but there is a problem. Because the coordinate system is spinning along with Earth, every planet and star changes its latitude and longitude rapidly over the course of a day. If you work out how fast they appear to move in this coordinate system, the numbers soon become silly. Even the nearest star to our solar system, Proxima Centauri, seems to be flying along at several thousand times the speed of light – a spurious velocity obscuring any genuine motion it may have.

Clearly, we need a coordinate system that stays still. Until the end of the 20th century, astronomers hung their reference frame on the stars. As our measurements of stars’ positions improved, a series of catalogues were drawn up to give their locations as reference points. Looking out from Earth, an observer can measure the angle between, say, a comet and a reference star, to give the comet’s celestial coordinates. The fifth fundamental catalogue, or FK5, used more than 1500 stars to mark out the heavens. But even this had shortcomings. The stars aren’t fixed, they are fickle. They move, albeit slowly, across the sky. Although this motion can be allowed for, it still leads to inaccuracies, with angles uncertain by several millionths of a degree. While this was hardly catastrophic, it was ripe for a radical overhaul.

So in the 1990s, astronomers took a giant leap. Rather than relying on stars mere hundreds of light years away, they decided to look billions of light years away instead. Objects that distant don’t shift their position in the sky we see very fast, which made them ideal candidates as reference points. But to be clearly visible from so far away, they have to be bright, and the brightest beacons we know are quasars: the sites where supermassive black holes suck matter in and fire radiation out. A side benefit of using such heavy markers is that they don’t get pushed around easily. Being billions of times the mass of the sun, supermassive black holes tend to stay put at the centre of their galaxies.

While quasars shine bright in visible light, they can be pinpointed even more accurately from the radio waves they emit. That is thanks to a technique called very long baseline interferometry (VLBI), in which radio telescopes across the globe focus on one source. They all see the same radio signal, but with a time delay that depends on the angle of the source. “We can measure the time delay down to maybe 10 picoseconds,” says Patrick Charlot at the University of Bordeaux, France. This can give the angular location of a quasar with stunning accuracy.

Astronomical accuracy

In 1998, Charlot and his team issued their new reference grid, the International Celestial Reference Frame (ICRF-1). It went through an overhaul in 2009 (ICRF-2), and this year it has been upgraded again. The newly refurbished ICRF-3 is fixed in place by 303 quasars, up from 212 in 1998, chosen from among the most compact and stable sources available. What’s more, it includes observations at higher radio frequencies than before, which helps to zero in on each source. The upshot is that the angular location of the quasers is, on average, accurate to within about 30 microarcseconds, or eight billionths of a degree. That is equivalent to making out individual bacteria on the wings of a passing aeroplane. “This frame is now the basis for every position measurement in astronomy,” says Charlot.

To make it as convenient as possible for local use, the centre of ICRF-3’s coordinate system is set at the centre of mass of our solar system, the point that the planets and moons – and even the sun – orbit. The system’s north-south axis was chosen to lie parallel to Earth’s polar axis, specifically, the direction the pole was pointing on 1 January 2000.

As well as the 303 quasars that define the latest system, ICRF-3 includes about 4000 other quasars distributed around the sky that can be used as reference points for navigating in all directions. As well as helping astronomers find their way around the sky, these can guide space probes around the solar system. Interplanetary missions need very accurate route finding. “When you navigate to Saturn, say, you can calculate the trajectory using the laws of dynamics, but nothing is perfect: you have to implement corrections,” says Charlot.

Engineers detect any slight deviations from the planned course by observing the angular position of their spacecraft in relation to a handy quasar. ICRF-3 includes higher-frequency radio observations than previous versions, which makes it more useful for locating certain types of space flight. It allows comparison with high-frequency deep space navigation signals that cut through the ionised gas of the solar wind. “For missions near the sun this is a big deal,” says Christopher Jacobs at the Jet Propulsion Laboratory in California, who co-chaired the team that developed ICRF-3.

Keeping time

Perhaps more surprisingly, the ICRF frame is also being put to work here on Earth. “One thing we can use it to measure very precisely is the orientation of Earth in space,” says Johannes Böhm at the Vienna University of Technology in Austria. “Earth does not uniformly rotate in exactly 24 hours, sometimes it’s a bit faster, sometimes a bit slower.” Keeping track of these variations in rotation speed, by measuring how fast the planet turns relative to the ICRF, allows us to know when leap seconds must be inserted into Coordinated Universal Time to keep our clocks in sync with the planet.

Accurately gauging changes in Earth’s spin can also tell us about powerful planetary forces at work. The atmosphere pushes and pulls at the planet’s surface, changing the speed of its rotation over hours and years. So ICRF-based measurements can be used to validate weather models, says Böhm, especially the behaviour of the jet streams, high-altitude winds in Earth’s atmosphere.

Drifts in rotation rate over years and decades may be connected to Earth’s churning liquid outer core, and the way this drags unpredictably on the mantle above. Geophysicists have even suggested that a recent slowdown in rotation could herald an .

Changes in Earth’s rotation can affect GPS and other satellite navigation systems too. These systems are based on atomic clocks, which regulate time signals sent out by the satellites. As atomic time is independent of the vagaries of the weather, while Earth’s rotation is not, GPS can get slightly out of step with the true longitude lines on Earth. “After a few days, the error would be at the level of a few centimetres,” says Böhm. This degree of precision doesn’t matter for your average driver or hiker, but it can for surveyors and earth scientists, so satnav systems are recalibrated from time to time using VLBI measurements to check Earth’s true orientation in the celestial reference frame.

“As well as helping map the sky, these systems can guide interplanetary missions”

ICRF is also helping us measure the slow movement of tectonic plates. The time delay between receiving quasar signals at two telescopes on different continents will change as they drift together or apart. The telescope positions are also used to triangulate the exact altitude of Earth-observing satellites, which is needed in measurements of sea level – a gauge of climate change and local flooding hazard.

While ICRF-2 did a good job of making all these measurements, the switch to ICRF-3 will improve many of them. The biggest gain is seen in the southern hemisphere, which was sparsely covered in the frame’s previous incarnation. “It was not always easy to find a good quasar seen from many telescopes,” says Böhm. For ICRF-3, the sources have been selected to be evenly scattered over the whole sky.

Of course, more precision is always desirable. The next big reference frame changer will be a move back to visible light. “Up to now, VLBI was our only technique to measure extragalactic sources with such accuracy,” says Charlot. But in April, the Gaia mission published a catalogue of those same distant sources in optical wavelengths, letting astronomers compare two different kinds of measurement. That could tell them if the radio waves are coming from a source close to the black hole, or instead from some peripheral event such as a blob of plasma thrown away from the quasar’s core, which would be a less reliable reference point.

This could, in turn, be used to further refine the ICRF. Not that the new reference frame is shaky, of course. Thanks to those 303 super-bright successors to the North Star, our place in the universe is assured.

This article appeared in print under the headline “Celestial satnav”