*** 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?

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

9. But Pulsars move too!
We need to go further out to minimise movement.
To the ends of the observable Universe.

10. 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

11. Thank you