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faq.md

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 ### How do you know the GPS receivers in Gran Sasso and CERN are not introducing any systematic error?
 
+Metrology labs around the world use GPS time transfer techniques to  
+exchange observations of the delays of their local clocks with respect  
+to GPS clocks. These observations are then merged by the Bureau  
+International des Poids et Mesures (BIPM) in Paris to produce  
+Coordinated Universal Time (UTC). In addition to these techniques,  
+metrology labs also exchange observations using other independent
+means  
+such as Two-Way Satellite Time Transfer (TWSTT). In this setup, a  
+certain bandwidth is rented from a geostationary satellite operator to  
+establish a two-way link, which is completely independent of GPS.  
+Travelling atomic clocks (after accounting for relativity and other  
+effects) are a third way which is used to ascertain that GPS time  
+transfer is unbiased. The satellite receivers in Gran Sasso and CERN
+are  
+the same brand and model. So are the antennae and the antenna cables.  
+They have been characterized by two independent metrology labs (METAS  
+and PTB) using different methods, which agreed to within 2 ns even if  
+conducted with a 2-year separation in time.
+
 ### Have you taken the antenna cable delay into account?
 
+Calibrations conducted by METAS and PTB both included the final cables  
+and antennae. This is a very important concept: inclusive calibration.  
+It is also used to calibrate other things like fiber links. You have
+to  
+try to include as much as possible of your final setup in the  
+calibration procedure. So at METAS, for example, both GPS receivers
+were  
+set up with their final cables and antennae, and their PPS outputs
+were  
+measured against the METAS reference PPS. This provided, in addition
+to  
+the relative calibration of interest, an absolute calibration with  
+respect to
+UTC.
+
 ### Does this system suffer from any kind of Sagnac effect, related to the rotation of the Earth?
 
 ### What is the impact of the Ionosphere on the accuracy of the time transfer?
 
 ### Why the GPS receivers are installed on the surface?
 
-GPS antenna, cable and receiver are normally characterized as a single
-entity. It is very convenient to keep an antenna cable length of ~40m,
-in such a way that the ensemble can be dismounted and sent to
-calibration by another lab (calibration by METAS) or another lab can
-bring a traveling GPS to our installation (PTB calibration).
+It is very convenient to keep an antenna cable length of ~40m, in such a
+way that the ensemble can be dismounted and sent to calibration by
+another lab (PolarRX2e sent to METAS) or another lab can bring a
+traveling GPS to our installations (PTB calibration at CERN and LNGS).
 
 ## Fiber length calibration