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faq.md

@@ -36,6 +36,13 @@ UTC.
 
 ### Does this system suffer from any kind of Sagnac effect, related to the rotation of the Earth?
 
+Sagnac effect corrections and many other effects on the time transfer
+are properly treated by the RINEX to CGGTTS conversion software. See
+[USE OF GEODETIC RECEIVERS FOR
+TAI](http://tycho.usno.navy.mil/ptti/ptti2001/paper35.pdf) by P.
+Defraigne, G. Petit and C.
+Bruyninx.
+
 ### Was the PTB calibration done with a traveling cesium clock? If so, did you apply relativistic corrections to its time base?
 
 No, it wasn't. The [PTB calibration](https://www.ohwr.org/project/cngs-time-transfer/wikis/Documents/PTB-calibration-report)
@@ -48,6 +55,24 @@ level.
 
 ### What is the impact of the Ionosphere on the accuracy of the time transfer?
 
+The GPS system itself distributes a real time approximation of
+ionospheric delays. On top of that geodetic receivers are able to decode
+the so called ionosphere-free code (P3), which is transmitted over the
+L1 and L2 carriers. Notice that L1 and L2 are being transmitted over two
+different frequencies. As ionospheric delay changes affect differently
+to L1 and L2, it is possible to calculate an additional correction the
+ionospheric delay model. Additionally the common view technique helps
+reducing the impact of ionospheric delays on base lines at the 800Km
+level, by selecting satalites which are visible simultaneously by both
+base stations. On top of that, the use a cesium atomic clock allows for
+additional filtering of daily delay oscillations. Once all these
+techniques are applied the overall impact of ionospheric delays is
+inferior to 1ns.
+
+If needed, the time transfer could still be improved by adding a posteri
+knowledge of the satellite orbits, atmospheric delays and GPS carrier
+frequencies.
+
 ### Why the GPS receivers are installed on the surface?
 
 It is very convenient to keep an antenna cable length of ~50m, in such a