Two sections near the end were added; one on Augmentation systems, and one on Global Navigation Systems. General updating: I have updated the text in quite a few places, such as eliminating the word “recently” which is no longer really recently. Also the Conclusion has been appropriately reworded. Added one reference . Living Reviews applied its current layout to this revision.
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|Corrected sign of the last term.|
|Added reference to Figure 4.|
|WGS-84 (837) changed to WGS-84 (873).|
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