Index: trunk/BNC/bnchelp.html
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--- trunk/BNC/bnchelp.html	(revision 2213)
+++ trunk/BNC/bnchelp.html	(revision 2214)
@@ -993,5 +993,12 @@
 <p><a name="ppptropo"><h4>3.11.2.3 Estimate Tropo - optional</h4></p>
 <p>
-By default BNC does not estimate troposphere parameters. Tick 'Estimate tropo' to estimate troposphere parameters together with the coordinates.
+BNC estimates the tropospheric delay according to equation
+<pre>
+T(z) = T_apr(z) + dT / cos(z)
+</pre>
+where T_apr is the a-priori tropospheric delay derived from Saastamoinen model.
+</p>
+<p>
+By default BNC does not estimate troposphere parameters. Tick 'Estimate tropo' to estimate troposphere parameters together with the coordinates and save dT in BNC's log file.
 </p>