diff --git a/infrastructure-centred/owlps-aggregator/owlps-aggregator.h b/infrastructure-centred/owlps-aggregator/owlps-aggregator.h
index b0d8c6c..0bcf13c 100644
--- a/infrastructure-centred/owlps-aggregator/owlps-aggregator.h
+++ b/infrastructure-centred/owlps-aggregator/owlps-aggregator.h
@@ -15,11 +15,13 @@
 
 
 /* Arguments & program configuration */
-#define OPTIONS "Aa:c:f:hi:k:l:o:p:t:" // getopt string
+#define OPTIONS "Aa:c:C:f:hi:k:K:l:o:p:t:" // getopt string
 #define DEFAULT_CONFIG_FILE "/usr/local/etc/owlps/owlps-aggregator.conf"
 #define DEFAULT_AGGREGATE_TIMEOUT 1500 // milliseconds
 #define DEFAULT_KEEP_TIMEOUT 3000 // milliseconds
 #define DEFAULT_CHECK_INTERVAL 500000 // microseconds
+#define DEFAULT_AP_KEEP_TIMEOUT 600 // s
+#define DEFAULT_AP_CHECK_INTERVAL 1000 // ms
 #define POSITIONER_DEFAULT_IP "127.0.0.1"
 
 
diff --git a/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c b/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c
index aa59748..43a2b51 100644
--- a/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c
+++ b/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c
@@ -103,8 +103,12 @@ void parse_config_file(int argc, char **argv)
       // Autocalibration activated?
       CFG_BOOL("autocalibration", cfg_false, CFGF_NONE),
       // Port on which autocalibration data are exchanged:
-      CFG_INT("autocalbration_port", AUTOCALIBRATION_DEFAULT_PORT,
+      CFG_INT("autocalbration_port", DEFAULT_AUTOCALIBRATION_PORT,
               CFGF_NONE),
+      // Time we keep APs in the list (in seconds):
+      CFG_INT("ap_keep_timeout", DEFAULT_AP_KEEP_TIMEOUT, CFGF_NONE),
+      // Time between two checks of the AP list (in milliseconds):
+      CFG_INT("ap_check_interval", DEFAULT_AP_CHECK_INTERVAL, CFGF_NONE),
 
       CFG_END()
     } ;
@@ -168,6 +172,9 @@ void parse_command_line(int argc, char **argv)
 	case 'c' :
 	  cfg_setint(cfg, "check_interval", strtol(optarg, NULL, 0)) ;
 	  break ;
+        case 'C' :
+          cfg_setint(cfg, "ap_check_interval", strtol(optarg, NULL, 0)) ;
+          break ;
 	case 'f' : // Config file
 	  break ; // (already parsed)
         case 'h' :
@@ -179,6 +186,9 @@ void parse_command_line(int argc, char **argv)
 	case 'k' :
 	  cfg_setint(cfg, "keep_timeout", strtol(optarg, NULL, 0)) ;
 	  break ;
+        case 'K' :
+          cfg_setint(cfg, "ap_keep_timeout", strtol(optarg, NULL, 0)) ;
+          break ;
 	case 'l' :
 	  cfg_setint(cfg, "listening_port", strtol(optarg, NULL, 0)) ;
 	  break ;
@@ -247,6 +257,26 @@ void check_configuration()
 #endif // DEBUG
       cfg_setint(cfg, "check_interval", DEFAULT_CHECK_INTERVAL) ;
     }
+
+  // ap_keep_timeout //
+  if (cfg_getint(cfg, "ap_keep_timeout") < 0)
+    {
+#ifdef DEBUG
+      fprintf(stderr, "Warning! ap_keep_timeout cannot be negative:"
+              " failing back to default value.\n") ;
+#endif // DEBUG
+      cfg_setint(cfg, "ap_keep_timeout", DEFAULT_AP_KEEP_TIMEOUT) ;
+    }
+
+  // ap_check_interval //
+  if (cfg_getint(cfg, "ap_check_interval") < 0)
+    {
+#ifdef DEBUG
+      fprintf(stderr, "Warning! ap_check_interval cannot be negative:"
+              " failing back to default value.\n") ;
+#endif // DEBUG
+      cfg_setint(cfg, "ap_check_interval", DEFAULT_AP_CHECK_INTERVAL) ;
+    }
 }
 
 
@@ -763,7 +793,7 @@ void monitor_aps()
       if (nb_aps > 1)
         order_send(aps) ;
 
-      usleep(cfg_getint(cfg, "ap_check_interval")) ;
+      usleep(cfg_getint(cfg, "ap_check_interval") * 1000) ;
     }
 }
 
@@ -779,7 +809,7 @@ void delete_old_aps()
 
   while (aps != NULL)
     if (sub_date(aps->last_seen, now) >
-        (unsigned long) cfg_getint(cfg, "keep_ap_timeout"))
+        (unsigned long) cfg_getint(cfg, "ap_keep_timeout") * 1000)
       delete_ap(aps) ;
     else
       return ;
@@ -819,9 +849,25 @@ void unlink_ap(ap_list *ap)
 }
 
 
+/*
+ * Sends a send order to the given AP.
+ */
 void order_send(ap_list *ap)
 {
+  autocalibration_order message ;
+  struct sockaddr_in serv;
+  struct sockaddr_in client ;
+  socklen_t serv_len = sizeof(serv);
+  int sockfd =
+    create_udp_sending_socket(ap->ip_addr,
+                              cfg_getint(cfg, "autocalibration_port"),
+                              &serv, &client) ;
 
+  message.order = AUTOCALIBRATION_ORDER_SEND ;
+  sendto(sockfd, (void *)&message, sizeof(message), 0,
+         (struct sockaddr *)&serv, serv_len) ;
+
+  (void) close(sockfd) ;
 }
 
 
@@ -970,8 +1016,8 @@ void print_usage()
 
          "Aggregation options:\n"
          "\t-t aggregate_timeout\tRequests are stored during"
-         " 'aggregate_timeout' before to be grouped (default:"
-         " %d millisecondes).\n"
+         " 'aggregate_timeout' milliseconds before to be grouped"
+         " (default:  %d ms).\n"
          "\t-k keep_timeout\t\tAggregated requests are kept during"
          " 'keep_timeout' milliseconds (default: %d milliseconds).\n"
          "\t-c check_interval\tTime between two checks of the stored"
@@ -981,6 +1027,10 @@ void print_usage()
          "\t-A\tEnable autocalibration (default: disabled).\n"
          "\t-a port\tPort on which autocalibration data"
          " are exchanged with the listeners (default: %d).\n"
+         "\t-K ap_keep_timeout\tInactive APs are kept during"
+         " 'ap_keep_timeout' seconds (default: %d s).\n"
+         "\t-C ap_check_interval\tTime (in milliseconds) between two"
+         " checks of the stored APs (default: %d ms).\n"
          ,
 	 program_name,
 	 AGGREGATE_DEFAULT_PORT,
@@ -989,6 +1039,8 @@ void print_usage()
 	 DEFAULT_AGGREGATE_TIMEOUT,
 	 DEFAULT_KEEP_TIMEOUT,
          DEFAULT_CHECK_INTERVAL,
-         AUTOCALIBRATION_DEFAULT_PORT
+         DEFAULT_AUTOCALIBRATION_PORT,
+         DEFAULT_AP_KEEP_TIMEOUT,
+         DEFAULT_AP_CHECK_INTERVAL
 	 ) ;
 }
diff --git a/infrastructure-centred/owlps-listener/owlps-listener.h b/infrastructure-centred/owlps-listener/owlps-listener.h
index ef14216..79fef70 100644
--- a/infrastructure-centred/owlps-listener/owlps-listener.h
+++ b/infrastructure-centred/owlps-listener/owlps-listener.h
@@ -51,9 +51,9 @@
 #define OPTIONS "Aa:d:f:hH:kl:m:n:p:qr:t:vw:" // getopt string
 #define DEFAULT_CONFIG_FILE "/usr/local/etc/owlps/owlps-listener.conf"
 enum {MODE_ACTIVE = 'a', MODE_PASSIVE = 'p', MODE_MIXED = 'm'} ;
-#define AUTOCALIBRATION_DEFAULT_HELLO_DELAY 120 // seconds
-#define AUTOCALIBRATION_DEFAULT_DELAY 25000 // ms
-#define AUTOCALIBRATION_DEFAULT_NBPKT 20
+#define DEFAULT_AUTOCALIBRATION_HELLO_DELAY 120 // seconds
+#define DEFAULT_AUTOCALIBRATION_DELAY 25000 // ms
+#define DEFAULT_AUTOCALIBRATION_NBPKT 20
 
 
 /* Error codes */
diff --git a/infrastructure-centred/owlps-listener/owlps-listenerd.c b/infrastructure-centred/owlps-listener/owlps-listenerd.c
index bb79a7b..c580d29 100644
--- a/infrastructure-centred/owlps-listener/owlps-listenerd.c
+++ b/infrastructure-centred/owlps-listener/owlps-listenerd.c
@@ -53,10 +53,10 @@ struct {
   "",
 #ifdef USE_PTHREAD
   FALSE,
-  AUTOCALIBRATION_DEFAULT_PORT,
-  AUTOCALIBRATION_DEFAULT_HELLO_DELAY,
-  AUTOCALIBRATION_DEFAULT_DELAY,
-  AUTOCALIBRATION_DEFAULT_NBPKT,
+  DEFAULT_AUTOCALIBRATION_PORT,
+  DEFAULT_AUTOCALIBRATION_HELLO_DELAY,
+  DEFAULT_AUTOCALIBRATION_DELAY,
+  DEFAULT_AUTOCALIBRATION_NBPKT,
 #endif // USE_PTHREAD
   FALSE
 } ;
@@ -163,18 +163,18 @@ void parse_config_file(int argc, char **argv)
       // Autocalibration activated?
       CFG_BOOL("autocalibration", cfg_false, CFGF_NONE),
       // Port on which autocalibration data are exchanged:
-      CFG_INT("autocalibration_port", AUTOCALIBRATION_DEFAULT_PORT,
+      CFG_INT("autocalibration_port", DEFAULT_AUTOCALIBRATION_PORT,
               CFGF_NONE),
       // Delay between two hello messages:
       CFG_INT("autocalibration_hello_delay",
-              AUTOCALIBRATION_DEFAULT_HELLO_DELAY,
+              DEFAULT_AUTOCALIBRATION_HELLO_DELAY,
               CFGF_NONE),
       // Delay between two calibration packet transmission:
-      CFG_INT("autocalibration_delay", AUTOCALIBRATION_DEFAULT_DELAY,
+      CFG_INT("autocalibration_delay", DEFAULT_AUTOCALIBRATION_DELAY,
               CFGF_NONE),
       // Number of packets for a calibration request:
       CFG_INT("autocalibration_nb_packets",
-              AUTOCALIBRATION_DEFAULT_NBPKT, CFGF_NONE),
+              DEFAULT_AUTOCALIBRATION_NBPKT, CFGF_NONE),
 #endif // USE_PTHREAD
       // Display capture packets, or not:
       CFG_BOOL("verbose", cfg_false, CFGF_NONE),
@@ -1005,9 +1005,9 @@ void print_usage()
          DEFAULT_CONFIG_FILE,
          LOC_REQUEST_DEFAULT_PORT,
          AGGREGATE_DEFAULT_PORT,
-         AUTOCALIBRATION_DEFAULT_PORT,
-         AUTOCALIBRATION_DEFAULT_HELLO_DELAY,
-         AUTOCALIBRATION_DEFAULT_DELAY,
-         AUTOCALIBRATION_DEFAULT_NBPKT
+         DEFAULT_AUTOCALIBRATION_PORT,
+         DEFAULT_AUTOCALIBRATION_HELLO_DELAY,
+         DEFAULT_AUTOCALIBRATION_DELAY,
+         DEFAULT_AUTOCALIBRATION_NBPKT
          ) ;
 }
diff --git a/libowlps/owlps.h b/libowlps/owlps.h
index f9fe4d1..8c65de9 100644
--- a/libowlps/owlps.h
+++ b/libowlps/owlps.h
@@ -44,7 +44,7 @@
 #define POSITIONER_DEFAULT_PORT 9902
 // Port on which listener listens for order from aggregator:
 #define LISTENER_DEFAULT_PORT 9903
-#define AUTOCALIBRATION_DEFAULT_PORT 9904
+#define DEFAULT_AUTOCALIBRATION_PORT 9904
 // Port on which the mobile listens for its position:
 #define MOBILE_DEFAULT_PORT 9910