On Feb 23, 3:28 pm, Michael Press <rub...@pacbell.net> wrote:
> In article
> <c1d8b4f1-c9a6-4801-bd52-339ca6564...@i38g2000yqd.googlegroups.com>,
>
>  Cephalobus_alie...@comcast.net wrote:
> > Currently, only Block IIR satellites are capable of autonomous
> > position monitoring and synchronization independent of the ground
> > control segment. This capability allows Block IIR satellites to
> > mutually correct their clocks and ephemerides information for a
> > period of up to several months should the ground segment be
> > knocked out by, say, nuclear war. For optimum performance under
> > non-wartime conditions, "ground truth" comparisons are still
> > performed on a daily basis to correct the satellites for drift.
>
> The Block IIR satellites use what references?

After decades of operation, the ground station positions are
known to within millimeters. So accurately have their positions
been characterized, that the effects of tides on the Earth's
crust need to be taken into account.

Jerry