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......@@ -56,29 +56,14 @@ Bruyninx.
No, it wasn't. The [PTB calibration](https://www.ohwr.org/project/cngs-time-transfer/wikis/Documents/PTB-calibration-report)
was done with a traveling GPS used for time comparison between metrology
labs. This unit consists of a GR50 GPS receiver, a SR60 Time interval
labs. This unit consists of a GTR50 GPS receiver, a SR60 Time interval
Counter (TIC), a GPS antenna, and ~50m of low tempco HELIAX foam coaxial
cable. This system has been traveling through several metrology labs
where it has been verified to be stable at the 1ns
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 two different frequencies.
As ionospheric delay changes affect differently L1 and L2, it is
possible to calculate an additional correction to the ionospheric delay
model. Additionally the common view technique helps reducing the impact
of ionospheric delays on baselines at the 800 km level, by selecting
satellites 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. See
[USE OF GEODETIC RECEIVERS FOR
TAI](http://tycho.usno.navy.mil/ptti/ptti2001/paper35.pdf) by P.
Defraigne, G. Petit and C. Bruyninx.
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 two different frequencies. As ionospheric delay changes affect differently L1 and L2, it is possible to calculate an additional correction to the ionospheric delay model. Additionally the common view technique helps reducing the impact of ionospheric delays on baselines at the 800 km level, by selecting satellites 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. See [USE OF GEODETIC RECEIVERS FOR TAI](http://tycho.usno.navy.mil/ptti/ptti2001/paper35.pdf) by P. Defraigne, G. Petit and C. Bruyninx.
If needed, the time transfer could still be improved by adding a
posteriori knowledge of the satellite orbits, atmospheric delays and GPS
......