Our products use two different sources as their timing reference - either GPS or CDMA (indirect GPS).  Our CDMA-based products make installation easy.  But some customers prefer traditional GPS-based products.  Following are advantages and disadvantages of both methods...

Global Positioning System (GPS) is a 32-satellite constellation owned and operated by the United States Government as a national resource.
CDMA cellular mobile telecommunications network via countless CDMA system base stations scattered throughout the world.  Maintained by commercial cell phone system providers.  CDMA system timing accuracy is guaranteed to be within 10 microseconds in order for the cell phone system to work.
Worldwide coverage.
Localized coverage: USA, China, Japan, South Korea and elsewhere. 
Unit requires view-of-the-sky antenna. Sometimes not a problem, sometimes just a nuisance, sometimes impossible. Expensive installation and maintenance costs are often required.
Unit works deep inside buildings with small, integrated antenna.
Absolute timing accuracy of unit is under 30 nanoseconds.  (<10 ns for Meridian II.  <25 ns for Tycho II.)
Absolute timing accuracy of unit is typically under 10 microseconds.
Network timing accuracy 1/2 - 2 milliseconds, typical (for NTP products).
Network timing accuracy is same as GPS.
Frequency accuracy of unit is <10-13 at 1 day.
Frequency accuracy of unit is <10-11 at 1 day.

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