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Geodetic receivers are able to decode the so-called ionosphere-free code
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(P3), which is transmitted over the L1 and L2 carriers. Notice that L1
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and L2 are two different frequencies. As ionospheric delay changes
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affect differently L1 and L2, it is possible to calculate an additional
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correction to the ionospheric delay model. Additionally the common view
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technique helps reducing the impact of ionospheric delays on baselines
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at the 800 km level, by selecting satellites which are visible
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simultaneously by both base stations. On top of that, the use a cesium
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atomic clock allows for additional filtering of daily delay
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oscillations. Once all these techniques are applied the overall impact
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of ionospheric delays is inferior to 1ns. See [USE OF GEODETIC RECEIVERS
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FOR TAI](http://tycho.usno.navy.mil/ptti/ptti2001/paper35.pdf) by P.
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affect differently L1 and L2, it is possible to calculate an ionospheric
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delay approximation. Additionally the common view technique helps
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reducing the impact of ionospheric delays on baselines at the 800 km
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level, by selecting satellites which are visible simultaneously by both
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base stations. On top of that, the use a cesium atomic clock allows for
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additional filtering of daily delay oscillations. Once all these
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techniques are applied the overall impact of ionospheric delays is
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inferior to 1ns. See [USE OF GEODETIC RECEIVERS FOR
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TAI](http://tycho.usno.navy.mil/ptti/ptti2001/paper35.pdf) by P.
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Defraigne, G. Petit and C. Bruyninx.
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If needed, the time transfer could still be improved by adding a
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