QUESTION:
What's the relationship between the NIST-F1 Cesium Fountain Atomic Clock
and GPS and LORAN-C/E-LORAN?
ANSWER:
The relationship is somewhat indirect. In all three cases the underlying time-base is based on the use of the Cesium clock(s)as the reference.
The traditional Cesium-beam tube (the work-horse) has some limitations, such as the consumption of the Cesium material and the time-duration that a Cesium atom (in the beam) is allowed to interact with the microwave energy (a process refrred to as "interrogation"). The longer the interrogation interval, the better the signal-to-noise ratio (in a general sense).
The Cesium fountain is a device that tries to alleviate these two "limitations" of the traditional beam tube. NIST uses several Cesium clocks in parallel to generate the (synthetic) "perfect" time base for the actual time standard. Other clocks, such as Hydrogen masers and Mercury-ion devices, are thrown into the average for different characteristics (short-term stability, long-term stability, etc., etc.).
The reference for the GPS constellation is provided by the US Naval Observatory (USNO). The USNO has a collection of Cesium clocks and Hydrogen masers to keep "DoD-time" which is continually checked against NIST (so, for all practical purposes, they are equivalent). Each GPS satellite has three atomic clocks on board (for redundancy) that are monitored, and controlled, by Ground Stations that are linked to the USNO. So, for all practical purposes, the time-base for GPS is equivalent to NIST.
LORAN-C is operated by the US Coast Guard. Each LORAN-C station has multiple (for redundancy) Cesium clocks that are routinely calibrated (though without the traceability to NIST). For LORAN-E, the idea is to continually calibrate the LORAN transmissions with respect to GPS and/or NIST.
So, in some sense they are equivalent.
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