How does a Global Navigation Satellite know where it is to tell you where you are - State of the Map 2013

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*** Presented by Martin Wass at State of the Map 2013 …

*** Presented by Martin Wass at State of the Map 2013
*** For the video of this presentation please see http://lanyrd.com/2013/sotm/scpktb/
*** Full schedule available at http://wiki.openstreetmap.org/wiki/State_Of_The_Map_2013

The satellites in Global Navigation Satellite Systems get their position data regularly updated from ground stations. But how do ground stations 'know' where they are, and relative to what? The Airy transit circle at Greenwich once defined the Prime Meridian and the spinning Earth the Equator. We now know the tectonic plate Greenwich sits on is moving and the Earth wobbles... Any defined datum causes difficulties when moving away from the vicinity, say to Mars. Using several different datums raises other problems. When everything is sliding around, how do we define and use a co-ordinate system that works?

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  • Credit:
    Prime Meridian, Greenwich From Wikipedia ChrisO https://en.wikipedia.org/wiki/User:ChrisO
  • Credit:
    Newlyn_Tidal_observatory_Map_1946 Ordnance Survey Public Domain
    Newlyn photo http:⁄⁄www.openstreetmap.org⁄user⁄davidearl⁄diary
  • Credit:
    Global_plate_motion_2008-04-17 Plate motion based on GPS satellite data from NASA JPL vectors show direction and magnitude of motion
  • Credit:
    GRACE gravity Europe ggm01_euro2_full
  • Credit:
    NASA
  • Credit:
    Pioneer10-plaque credit NASA-ARC Sagan Salzman Sagan Drake
    Photo NASA
  • Among the 295 selected defining sources of the ICRF2, only 97 are also
    defining sources of the ICRF1. Most of them are in the northern hemi-
    sphere, making the sample badly distributed for a reliable estimation of
    rotation angles. To remedy, 41 ICRF2 defining sources (but not defining
    sources of the ICRF1) preferably taken in the southern hemisphere were
    added, resulting in 138 common objects for comparison which have been
    used for the link between the gsf008a catalogue and ICRF1-Ext2. The
    defining sources, the linking sources and the common to both ICRF1
    and ICRF2 are displayed in Figure 40. The status in ICRF1-Ext.2 of
    the 41 additional sources is: 24 candidate sources, 16 other, and 1 new.
    Figure 41 displays the distribution of formal errors of the various subsets
    of sources before inflation, after inflation (see next paragraph), and of
    the corresponding errors in the ICRF1-Ext.2.

Transcript

  • 1. How do Global Positioning Satellites know where they are, to tell you where you are?
  • 2. In the old days you could touch a datum... Credit:: Prime Meridian, Greenwich From Wikipedia User ChrisO; https://en.wikipedia.org/wiki/User:Prioryman
  • 3. Credit: Newlyn_Tidal_observatory_Map_1946 Ordnance Survey Public Domain Newlyn photo http:⁄⁄www.openstreetmap.org⁄user⁄davidearl⁄diary
  • 4. But the Earth slides around Credit: Global_plate_motion_2008-04-17 Plate motion based on GPS satellite data from NASA JPL vectors show direction and magnitude of motion
  • 5. Rotational speed and shape change with time Credit: Gravity Recovery and Climate Experiment (GRACE) gravity Europe ggm01_euro2_ful
  • 6. It gets hard when moving off-world Credit: NASA Crater Airy-0, Prime Meridian, Mars
  • 7. But Pulsars move too! We need to go further out to minimise movement. To the ends of the observable Universe.
  • 8. The International Celestial Reference Frame (mark 2) Fig 40, The Second Realization of the International Celestial Reference Frame by Very Long Baseline Interferometry. International Earth Rotation and Reference System Technical Note 35
  • 9. Thank you