If we can use GPS satellites to find out where we are on Earth, why not turn to the same principle for navigation in space? The idea has a certain currency — I remember running into it in John Mauldin’s mammoth (and hard to find) Prospects for Interstellar Travel (AIAA/Univelt, 1992) some years back. But it was only a note in Mauldin’s ‘astrogation’ chapter, which also discussed ‘marker’ stars like Rigel (Beta Orionis) and Antares (Alpha Scorpii) and detailed the problems deep space navigators would face.
The European Space Agency’s Ariadna initiative studied pulsar navigation relying on millisecond pulsars, rotating neutron stars that spin faster than 40 revolutions per second. The pitch here is that pulsars that fit this description are old and thus quite regular in their rotation. Their pulses, in other words, can be used as exquisitely accurate timing mechanisms. You can have a look at ESA’s “Feasibility study for a spacecraft navigation system relying on pulsar timing information” here (download at bottom of page).
Pulsars have huge advantages. A deep space satellite network to fix position is a costly option — it doesn’t scale well as we expand deeper into the Solar System and beyond it. Autonomous navigation is clearly preferable, tying the navigation system to a natural reference frame like pulsars. The down side: Pulsar signals are quite weak and thus put demands upon spacecraft constrained by mass and power consumption concerns. So there’s no easy solution to this.
But several readers (thanks especially to Frank Smith and Adam Crowl) have pointed out a recent paper by Bartolome Coll (Observatoire de Paris) and Albert Tarantola (Institut de Physique du Globe de Paris) that speculates on a system based on four millisecond pulsars: 0751+1807 (3.5 ms), 2322+2057 (4.8 ms), 0711-6830 (5.5 ms) and 1518+0205B (7.9 ms). The origin of the space-time coordinates the authors use is defined as January 1, 2001 at the focal point of the Cambridge radiotelescope where pulsars were discovered in 1967. Thus, the paper continues:
…any other space-time event, on Earth, on the Moon, anywhere in the Solar system or in the solar systems in this part of the Galaxy, has its own coordinates attributed. With present-day technology, this locates any event with an accuracy of the order of 4 ns, i.e., of the order of one meter. This is not an extremely precise coordinate system, but it is extremely stable and has a great domain of validity.
If these numbers are correct, they represent quite a jump over the ESA study cited above, which worked out the minimal hardware requirements for a pulsar navigation system and arrived at a positioning accuracy of no better than 1000 kilometers. ESA is working within near-term hardware constraints and discusses ways of enhancing accuracy, but the report does point out the huge and perhaps prohibitive weight demands these solutions will make upon designers.
The paper is Coll and Tarantola, “Using pulsars to define space-time coordinates,” available online.