In days gone by, captains sailed their vessels over the oceans by aid of the stars. Provided that the heavens weren’t clouded over, this was a reliable and accurate mode of navigation. Today, this has been mostly replaced by GPS. But the stars may soon make a comeback with so-called pulsar navigation.
We first discussed the possibility of pulsar navigation in 2013. To quickly review, pulsars are fast spinning neutron stars that emit electromagnetic radiation from their magnetic poles. Since the magnetic poles are not lined up with the axis of rotation, the bundles sweep through space with every rotation. About one thousand known pulsars hit the earth while doing so. This creates very stable pulses with periods in between 1.4 milliseconds to 8.5 seconds. According to a October 2014 feasibility study led by Henk Hesselink of the National Aerospace Laboratory (NAL) in Amsterdam, the signals received from pulsars and their known positions can be used to create an alternative to GPS. Researchers have called it pulsar navigation and are primarily developing it for aircraft.
Since no satellites are needed, pulsar navigation is reliable and relatively inexpensive. The thousand known pulsars are distributed evenly in the sky. The pulsars’ signals can be received at all times and are not significantly influenced by the weather. Although the signals are influenced by manmade noise, intentionally disrupting the signals is difficult. Pulsars use a much broader frequency range than GPS satellites, whereas the few frequencies that the GPS satellites use can be more easily targeted and disrupted.
This technology would be the first directly practical application of radio telescopes. Designing the antennae for receiving the signals is a challenge, though. Pulsar signals are more than a million times weaker than GPS signals. The former are easily lost in the background noise, or influenced by our own telecommunication and electromagnetic devices, such as microwaves. Until recently, only large radio telescopes can distinguish between the pulses and the noise. These telescopes use multiple phased array antennae to monitor multiple frequencies at once in a certain direction.
To solve this problem, NAL has brought together a consortium of researchers from universities and research laboratories in Europe, with diverse backgrounds covering areas such as aerospace, electronics, antennae, and signal processing. Their goal is to receive the pulsar signals with phased array antennae smaller than a penny. They have already achieved trustworthy localization after improving the antennae’s structure, signal processing, and algorithms. Using the pulsars with millisecond periods, the consortium was able to take about a thousand samples per second — enabling them to distinguish the pulsar reliably from the background noise.
The researchers hope to soon show that navigation with pulsar signal will yield better results than using GPS signal. The research project is part of the European FP7 – Aeronautics and Air Transport (AAT) Work Program.
Back in March, DARPA announced it’s developing a form of GPS navigation without the use of satellites as well, though it was less clear on the details of the project.