Another limitation of GPS systems, as pointed out by OPB, is the accuracy factor of +/- 300', or 600' total. Not close enough for multiple main tracks or even yard trackage.
OPB, thanks for the clear, concise answers to my elementary questions. Of course they lead to more questions:
1) With the initial cost always a factor to freight RR's, will the Feds ( of one agency or another ) have to force the issue, before such systems are implemented ???
2) With so many competing systems, will a set standard ever be agreed upon ??? After all, the Model RR industry was able to agree on common standards for DCC track controls, in spite of potential loss of sales to a competing Mfr. I suppose the Beta vs. VHS licensing argument might work here.