[gps] example of broadcasting GPS messages between aircrafts

This is used by the traffic info module to have position of surrounding
aircraft at higher rate and lower latency.
Based on PPRZLINK v2.

conf/modules/gps.xml

@@ -8,6 +8,7 @@
       Still requires at least one module providing the actual GPS implementation.
     </description>
     <configure name="GPS_LED" value="2" description="LED number to indicate fix or none"/>
+    <define name="GPS_POS_BROADCAST" value="FALSE|TRUE" description="Broadcast GPS position instead of sending it to the ground station only. This requires to use PPRZLINK version 2 and a capable modem"/>
   </doc>
 
   <settings>

sw/airborne/modules/multi/traffic_info.c

@@ -79,7 +79,11 @@ bool parse_acinfo_dl(void)
   uint8_t msg_id = IdOfPprzMsg(dl_buffer);
 
   /* handle telemetry message */
+#if PPRZLINK_DEFAULT_VER == 2
+  if (pprzlink_get_msg_class_id(dl_buffer) == DL_telemetry_CLASS_ID) {
+#else
   if (sender_id > 0) {
+#endif
     switch (msg_id) {
       case DL_GPS_SMALL: {
         uint32_t multiplex_speed = DL_GPS_SMALL_multiplex_speed(dl_buffer);

sw/airborne/subsystems/gps.c

@@ -134,7 +134,19 @@ static void send_gps(struct transport_tx *trans, struct link_device *dev)
   int16_t climb = -gps.ned_vel.z;
   int16_t course = (DegOfRad(gps.course) / ((int32_t)1e6));
   struct UtmCoor_i utm = utm_int_from_gps(&gps, 0);
-  pprz_msg_send_GPS(trans, dev, AC_ID, &gps.fix,
+#if PPRZLINK_DEFAULT_VER == 2 && GPS_POS_BROADCAST
+  // broadcast GPS message
+  struct pprzlink_msg msg;
+  msg.trans = trans;
+  msg.dev = dev;
+  msg.sender_id = AC_ID;
+  msg.receiver_id = PPRZLINK_MSG_BROADCAST;
+  msg.component_id = 0;
+  pprzlink_msg_send_GPS(&msg,
+#else
+  pprz_msg_send_GPS(trans, dev, AC_ID,
+#endif
+                    &gps.fix,
                     &utm.east, &utm.north,
                     &course, &gps.hmsl, &gps.gspeed, &climb,
                     &gps.week, &gps.tow, &utm.zone, &zero);
@@ -169,7 +181,18 @@ static void send_gps_rxmrtcm(struct transport_tx *trans, struct link_device *dev
 
 static void send_gps_int(struct transport_tx *trans, struct link_device *dev)
 {
+#if PPRZLINK_DEFAULT_VER == 2 && GPS_POS_BROADCAST
+  // broadcast GPS message
+  struct pprzlink_msg msg;
+  msg.trans = trans;
+  msg.dev = dev;
+  msg.sender_id = AC_ID;
+  msg.receiver_id = PPRZLINK_MSG_BROADCAST;
+  msg.component_id = 0;
+  pprzlink_msg_send_GPS_INT(&msg,
+#else
   pprz_msg_send_GPS_INT(trans, dev, AC_ID,
+#endif
                         &gps.ecef_pos.x, &gps.ecef_pos.y, &gps.ecef_pos.z,
                         &gps.lla_pos.lat, &gps.lla_pos.lon, &gps.lla_pos.alt,
                         &gps.hmsl,
@@ -189,7 +212,18 @@ static void send_gps_lla(struct transport_tx *trans, struct link_device *dev)
   uint8_t err = 0;
   int16_t climb = -gps.ned_vel.z;
   int16_t course = (DegOfRad(gps.course) / ((int32_t)1e6));
+#if PPRZLINK_DEFAULT_VER == 2 && GPS_POS_BROADCAST
+  // broadcast GPS message
+  struct pprzlink_msg msg;
+  msg.trans = trans;
+  msg.dev = dev;
+  msg.sender_id = AC_ID;
+  msg.receiver_id = PPRZLINK_MSG_BROADCAST;
+  msg.component_id = 0;
+  pprzlink_msg_send_GPS_LLA(&msg,
+#else
   pprz_msg_send_GPS_LLA(trans, dev, AC_ID,
+#endif
                         &gps.lla_pos.lat, &gps.lla_pos.lon, &gps.lla_pos.alt,
                         &gps.hmsl, &course, &gps.gspeed, &climb,
                         &gps.week, &gps.tow,