How do Global Positioning Satellites know where they are,

to tell you where you are?

In the old days you could touch a datum...

Credit:: Prime Meridian, Greenwich From Wikipedia User ChrisO; https://en.wikipedia.org/wiki/User:Prioryman

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.orguserdavidearldiary

Credit:Newlyn_Tidal_observatory_Map_1946 Ordnance Survey Public Domain

Newlyn photo http:www.openstreetmap.orguserdavidearldiary

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

Credit: Global_plate_motion_2008-04-17 Plate motion based on GPS satellite data from NASA JPL vectors show direction and magnitude of motion

Rotational speed and shape change with time

Credit: Gravity Recovery and Climate Experiment (GRACE) gravity Europe ggm01_euro2_ful

Credit:GRACE gravity Europe ggm01_euro2_full

It gets hard when moving off-world

Credit: NASA Crater Airy-0, Prime Meridian, Mars

Credit:NASA

Credit:Pioneer10-plaque credit NASA-ARC Sagan Salzman Sagan Drake

Photo NASA

But Pulsars move too!

We need to go further out to minimise movement.

To the ends of the observable Universe.

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

Among the 295 selected defining sources of the ICRF2, only 97 are alsodefining sources of the ICRF1. Most of them are in the northern hemi-sphere, making the sample badly distributed for a reliable estimation ofrotation angles. To remedy, 41 ICRF2 defining sources (but not definingsources of the ICRF1) preferably taken in the southern hemisphere wereadded, resulting in 138 common objects for comparison which have beenused for the link between the gsf008a catalogue and ICRF1-Ext2. Thedefining sources, the linking sources and the common to both ICRF1and ICRF2 are displayed in Figure 40. The status in ICRF1-Ext.2 ofthe 41 additional sources is: 24 candidate sources, 16 other, and 1 new.Figure 41 displays the distribution of formal errors of the various subsetsof sources before inflation, after inflation (see next paragraph), and ofthe corresponding errors in the ICRF1-Ext.2.

Thank you