GPS World, February 2018

Inertial Ranging eLoran Wi Fi Bluetooth X RAYS AND PULSARS Galactic Positioning System A team of engineers at the U S National Aeronautics and Space Administration NASA has demonstrated fully autonomous X ray navigation in space a capability that could enable robotic spacecraft to navigate beyond the edges of the solar system The experiment Station Explorer for X ray Timing and Navigation Technology SEXTANT showed that millisecond pulsars could be used to accurately determine the location of an object moving at thousands of miles per hour in space functioning in a way similar to GPS The system provides a new option for spacecraft to autonomously determine their locations outside Earth based global navigation networks because pulsars are accessible in virtually every conceivable flight regime from low Earth to deepest space The SEXTANT demonstration used the 52 X ray telescopes and silicon drift detectors that make up NASAs Neutronstar Interior Composition Explorer NICER an external attached payload on the International Space Station The size of a washing machine NICER studies neutron stars which emit radiation across the electromagnetic spectrum Incredibly dense one teaspoonful of neutron star matter would weigh a billion tons on Earth these objects would collapse into black holes if compressed any further Pulsars The SEXTANT experiment focuses on a particular type of neutron star pulsars highly magnetized rotating neutron stars Their electromagnetic radiation can be observed only when the beam of emission points toward Earth thus their pulsed appearance The short regular rotational period NeutroN star INterIor ComposItIoN explorer or NICER is an external attached payload on the International Space Station Image NASA Goddard Space Flight Center produces a precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar These predictable pulsations can provide highprecision timing information similar to the atomic clock signals supplied through GPS Demonstration A demonstration in November 2017 selected four mil l isecond pulsar targets J0218 4232 B1821 24 J0030 0451 and J0437 4715 and directed NICER to orient itself so it could detect X rays within their sweeping beams of light These millisecond pulsars are so stable that their pulse arrival times can be predicted to accuracies of microseconds for years into the future During the two day experiment the payload generated 78 measurements to get timing data which the SEXTANT experiment fed into its onboard algorithms to autonomously stitch together a navigational solution that revealed the location of NICER in its orbit around Earth The team compared that solution against location data 12 GPS WORLD WWW GPSWORLD COM FEBRUARY 2018 gathered by NICERs onboard GPS receiver For the onboard measurements to be meaningful we needed to develop a model that predicted the arrival times using ground based observations provided by our collaborators at radio telescopes around the world said Paul Ray a SEXTANT co investigator with the U S Naval Research Laboratory The difference between the measurement and the model prediction is what gives us our navigation information Three Mile Accuracy The goal was to demonstrate that the system could locate NICER within a 10 mile radius as the space station sped around Earth at slightly more than 17500 mph Within eight hours of starting the experiment on Nov 9 the system converged on a location within the targeted range of 10 miles and remained well below that threshold for the rest of the experiment In fact a good portion of the data showed positions that were accurate to within three miles See Galactic page 49