Millisecond pulsars offer the most reliable timing standard known to us. Hence it appears logical to investigate if navigation in deepspace can benefit from using this timing information. The possibility of using pulsar signals for the positioning of spacecraft was reviewed in the Ariadna study Feasibility study for a spacecraft navigation system relying on pulsar timing information conducted by the Universitat Politècnica de Catalunya and Universitat de Barcelona.

Earth-based navigation of deepspace satellites has several drawbacks: The navigational accuracy is high only along the line-of-sight (this is still true even if ΔVLBI is taken into consideration). During Solar conjunctions no precision navigation is possible. A remedy could come from two possible approaches. The first one would be a positioning system based on a network of deepspace satellites. This solution would be extremely costly. The second possibility would be autonomous navigation. In order to enable high-precision autonomous navigation the spacecraft position needs to be tied to a precisely known natural reference frame. The best currently known reference frame is set up by the positions of remote millisecond pulsars.

While exhibiting an extremely high timing stability the pulsar signals are very weak compared to a conventional radio-tracking signal. Hence the question arises if pulsar signals can effectively be employed for navigation when the constraints that a deepspace mission poses to the mass and power consumption of the navigation hardware are taken into account. The study treated convincingly the characterisation of suitable astrophysical sources and derived from this the minimal hardware requirements for a positioning system. This was done for both, radio and x-ray signals. The results are however not clear cut: A positioning accuracy of 1,000 km is attainable with a moderate antenna size (10 m²) and a signal integration time of the order of an hour. However simultaneous observation of several pulsars with separate antennas seems desirable for a reduced integration time or an enhanced accuracy. However, this seems to require prohibitively heavy hardware.

Literature

The study report can be downloaded here:
J. Sala, A. Urruela, X. Villares, R. Estalella and J. Paredes: Feasibility study for a spacecraft navigation system relying on pulsar timing information.