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faq.md

@@ -91,6 +91,20 @@ METAS](https://www.ohwr.org/project/cngs-time-transfer/wikis/Documents/METAS-cal
 traveling GPS to our installations ([PTB calibration at CERN and
 LNGS](https://www.ohwr.org/project/cngs-time-transfer/wikis/Documents/PTB-calibration-report)).
 
+### What do you think of the paper [Time-of-flight between a Source and a Detector observed from a Satellite](http://arxiv.org/abs/1110.2685) by Ronald A.J. van Elburg?
+
+The author of that paper assumes time is being measured using clocks
+which are moving with respect to the detectors. This is not the case.
+GPS Disciplined Oscillators (GPSDOs) indeed use signals coming from GPS
+satellites to produce a clock signal, but both general and special
+relativity effects are taken into account to manufacture this clock
+signal. If these effects were not compensated, normal GPS applications
+such as positioning of hand-held receivers would not work. Common-view
+time transfer is a very well known and tested technique in the metrology
+world. It is used in the production of Universal Coordinated Time (UTC)
+and an effect such as that claimed in the paper would have made that
+task impossible.
+
 ## Fiber length calibration
 
 ### Why haven't you used fiber reflectometry to measure fiber lengths?