A ground-based transmitter could be destroyed in an attack, so having the clocks on orbit meant they were protected and would still be operational for a retaliatory ICBM launch.<p>Edit: So far as latency - the on-board clocks get resynced today every now and again against a ground-based reference time as they pass overhead of a USAF transmitter (primary + backup).  If they used only moderately-accurate clocks as a cost-saving measure (or were at the limit of late 1970&#x27;s technology) drift would be more severe and the tail-end of each orbit would be less correct than at the start of the orbit.  So additional ground stations would be required in parts of the planet not always so friendly to the US, and they would have to coordinate their time sources, allowing for the light-speed delay travelling around the earth.  Much more expensive&#x2F;harder solution from an operational aspect.<p>Edit Edit: Don&#x27;t forget that the GPS system was designed around military requirements. All the satellites (even today) have nuclear weapon detonation detectors on board and are a primary input to NORAD.<p>&gt; The 2nd SOPS has three missions: global navigation, time transfer and nuclear detection.<p><a href="http:&#x2F;&#x2F;www.schriever.af.mil&#x2F;GPS&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.schriever.af.mil&#x2F;GPS&#x2F;</a>