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<p>signals are slowed in the <a href="page.php?w=ionosphere">ionosphere</a>, which is affected relatively unpredictably by <a href="page.php?w=space_weather">space weather</a>. The ionosphere is <a href="page.php?w=Dispersion_%28optics%29">dispersive</a>, meaning that signals of different frequency are slowed by different amounts. By measuring the difference in the delays between signals of different frequencies, the receiver software (or later post-processing) can model and remove the delay at any frequency. This process is only approximate, and</p><p>
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