In article 
<c1d8b4f1-c9a6-4801-bd52-339ca6564ad4@i38g2000yqd.googlegroups.com>,
 Cephalobus_alienus@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?

-- 
Michael Press