[doc] architecture: add Autocalibration section

TODO.t2t

@@ -8,7 +8,6 @@ Work to do in OwlPS
 - Doxygen: solve problem with structs (member fields are not documented).
 
 - Topics to write on:
-  - Architecture
   - General user manual (common command-line options, etc.)
   - Per module user manual (manpages)
 

doc/owlps-architecture.t2t

@@ -1,6 +1,6 @@
 The Owl Positioning System's Architecture
 OwlPS {{OWLPS_VERSION}}
-11 January 2013
+June 2013
 
 %%%
 % Man title & section:
@@ -67,6 +67,25 @@ Each step of this protocol corresponds to a different module:
 
 
 
++ Autocalibration +
+
+With the self-calibration mechanism on, the capture points will play the
+role of pseudo-clients and send so-called autocalibration requests,
+which are very similar to the positioning requests sent by the mobile
+terminals. The transmission of the autocalibration requests is scheduled
+by the Aggregator, which sends an //autocalibration order// to each
+capture point, in a round-robin. When it receives such an order, the
+Listener transmits an autocalibration request. For this to work, the
+Listener transmits a //hello// packet from time to time to the
+Aggregator, which allows the latter to maintain a list of currently
+active capture points. If no //hello// packet is received from a given
+capture point for some time, it is deleted from the list and the
+Aggregator doesn't send it any more autocalibration orders; this delay
+is an option of the Aggregator, cf. owlps-aggregator.t2t.
+
+
+
+
 + Coordinate system +
 
 OwlPS uses a coordinate system relative to the deployment area. The