A method for navigating spacecraft autonomously using pulsars has been developed by a group of researchers in Germany. Although the idea of using pulsars for stellar navigation was first proposed in the 1970s, the team has, for the first time, discussed the type of pulsars best suited for such measurements and has determined which type of detector – a radio or an X-ray detector – would currently be most feasible and practical to use on-board satellites and other spacecraft.

Today, satellites and spacecraft that navigate through our solar system use a combination of technologies to triangulate their location at any time – these include a pre-determined orbit, radio signals sent from the craft to Earth-based tracking stations, as well as optical information from on-board cameras that look at the local environment. While the radio measurements made at the ground stations are highly accurate in terms of the distance and the radial velocity of the craft, it is the angular resolution of those measurements where large errors come in. This happens as a result of the very low angular resolution of the radio antennas from their seat on Earth such that accurate measurements are only available for a craft moving in a straight line away from the Earth. Unfortunately, as this is hardly ever the case, errors creep in. Indeed, the accuracy of such measurements also drops as the craft's distance from the Earth increases – the farther away it is the longer it takes for a signal to be sent and received. A one-way signal to Voyager currently takes about 17 hours.

Pulsar pathway

To overcome these problems, astronomers are keen to develop an "autonomous navigation system" that would sit on-board the spacecraft in question, thereby avoiding the disadvantages of any Earth-bound communications, increasing distance uncertainties or signal weakening. The possibility of using pulsars – highly magnetized, rapidly rotating neutron stars that emit "pulses" of broadband electromagnetic radiation at very regular intervals – was first suggested in 1974 for radio pulsars. At the time, however, the technology had not developed and there were not enough pulsars identified to make the method feasible.

In this latest research, a team led by Werner Becker at the Max Planck Institute for Extraterrestrial Physics in Germany has described an approach that could lead to viable pulsar navigation. The team's paper looks at the three basic types of pulsar – accretion-powered pulsars, magnetars and rotation-powered pulsars – and determined that only the rotation-powered are suitable for navigation. The rotation-powered pulsars are themselves divided into two types, namely field pulsars that have periods between 10 ms and several seconds, and millisecond pulsars that, as their name suggests, have periods of less than 20 ms. According to Becker and colleagues, it is the millisecond variety that is best suited for navigation purposes, thanks to their highly stable timing. Millisecond pulsars were first discovered in 1982 and initially studied only in the radio band, until Becker himself detected the first X-ray emission from a millisecond pulsar in 1993.

Exact position

"Only in very recent years have we developed the next generation of X-ray satellites and X-ray mirrors that are lightweight and highly compact, and these are the key to pulsar navigation," Becker told physicsworld.com. Becker's paper goes on to discuss how a "pulse" from a pulsar would be used as a navigational aid. The system is simple. As the initial position and velocity of the spacecraft is known thanks to planned orbital parameters, the process begins with a first observation of the arrival of individual photons from a pulsar pulse that are recorded. These are then corrected for the velocity of the moving craft to an inertial reference frame.

Becker explains that the best fixed point for this would be the solar system barycentre – the centre of mass for our entire planetary system. "In fact, even if future missions leave the solar system, we could still use the barycentre as it still would work as a reference point," says Becker, though it would be necessary to correct for the sun's motion in the galaxy . These measurements would then be used to build up a pulse profile that represents the timing signature of the pulsar, ultimately giving the pulse arrival time. Principally, a 3D position-fix could be derived from observations from three different pulsars located in different parts of the sky, but in reality more would be required. "I would say that about 10 millisecond pulsars would be best to get a good measurement, as you would want to check for errors, so some redundant information would actually be welcome," says Becker.

X-ray versus radio

The study looks at one of the essential practical aspects of this navigation technique: whether radio antennas would be used to pick up radio emissions from the pulsars, or X-ray detectors would be preferred. They found that a phased array of radio antennas could easily be used, but they would cover an area of at least 150 m2 and weigh a minimum of 170 kg. The X-ray telescopes using the mirrors would be much more lightweight and compact, but the detectors run the risk of being destroyed if exposed to an exceedingly bright X-ray source. The researchers estimated an accuracy of ±5 km/s in the solar system and beyond using their method – a huge improvement on current techniques.

Becker tells physicsworld.com that choosing a technique would depend on the specifications of each mission. "If you have a huge starship in the future using this method, then the radio array is preferable as we have more knowledge and accuracy when it comes to radio signals and the size and weight would not make a real difference," he said. "But on a current mission, where that is a priority, a compact X-ray detector would be much better, so you would have to check your mission details such as the size, orbit and power consumption before deciding." To help future missions do just this, the team has drawn up a flowchart (right) that determines the "navigator performance as a function of the technology parameters".

The researchers also discuss current detector technologies such as silicon pore optics, silicon drift detectors and "active pixel sensors" that might be used in missions that would adopt the pulsar navigation. While Becker believes that the method will not be used in practice any earlier than 30 years from now, the need for an autonomous navigation system and a pulsar's natural time-keeping abilities will undoubtedly lead to pulsar navigation in the future.

A pre-print of the work is available on arXiv.