MAVLink: don't waste 100 bytes of bandwidth sending 1 byte of information

the GPS_STATUS message is a massive waste of bandwidth, but it is the
only message that tells us the number of visible satellites. So only
send it if that information changes.

This makes MAVLink work better at low baud rates

ArduCopter/GCS.h

@@ -43,6 +43,7 @@ public:
 	void		init(FastSerial *port) {
         _port = port;
         initialised = true;
+        last_gps_satellites = 255;
     }
 
 	/// Update GCS state.
@@ -89,6 +90,9 @@ public:
     // set to true if this GCS link is active
     bool initialised;
 
+    // used to prevent wasting bandwidth with GPS_STATUS messages
+    uint8_t last_gps_satellites;
+
 protected:
 	/// The stream we are communicating over
 	FastSerial			*_port;

ArduCopter/GCS_Mavlink.pde

@@ -697,11 +697,18 @@ GCS_MAVLINK::data_stream_send(uint16_t freqMin, uint16_t freqMax)
 		if (freqLoopMatch(streamRateExtendedStatus, freqMin, freqMax)) {
 			send_message(MSG_EXTENDED_STATUS1);
 			send_message(MSG_EXTENDED_STATUS2);
-			send_message(MSG_GPS_STATUS);
 			send_message(MSG_CURRENT_WAYPOINT);
 			send_message(MSG_GPS_RAW);			// TODO - remove this message after location message is working
 			send_message(MSG_NAV_CONTROLLER_OUTPUT);
-			//Serial.printf("mav2 %d\n", (int)streamRateExtendedStatus.get());
+
+            if (last_gps_satellites != g_gps->num_sats) {
+                // this message is mostly a huge waste of bandwidth,
+                // except it is the only message that gives the number
+                // of visible satellites. So only send it when that
+                // changes.
+                send_message(MSG_GPS_STATUS);
+                last_gps_satellites = g_gps->num_sats;
+            }
 		}
 
 		if (freqLoopMatch(streamRatePosition, freqMin, freqMax)) {

ArduPlane/GCS.h

@@ -43,6 +43,7 @@ public:
 	void		init(FastSerial *port) {
         _port = port;
         initialised = true;
+        last_gps_satellites = 255;
     }
 
 	/// Update GCS state.
@@ -89,6 +90,9 @@ public:
     // set to true if this GCS link is active
     bool initialised;
 
+    // used to prevent wasting bandwidth with GPS_STATUS messages
+    uint8_t last_gps_satellites;
+
 protected:
 	/// The stream we are communicating over
 	FastSerial			*_port;

ArduPlane/GCS_Mavlink.pde

@@ -900,11 +900,19 @@ GCS_MAVLINK::data_stream_send(uint16_t freqMin, uint16_t freqMax)
 		if (freqLoopMatch(streamRateExtendedStatus, freqMin, freqMax)) {
 			send_message(MSG_EXTENDED_STATUS1);
 			send_message(MSG_EXTENDED_STATUS2);
-			send_message(MSG_GPS_STATUS);
 			send_message(MSG_CURRENT_WAYPOINT);
 			send_message(MSG_GPS_RAW);            // TODO - remove this message after location message is working
 			send_message(MSG_NAV_CONTROLLER_OUTPUT);
 			send_message(MSG_FENCE_STATUS);
+
+            if (last_gps_satellites != g_gps->num_sats) {
+                // this message is mostly a huge waste of bandwidth,
+                // except it is the only message that gives the number
+                // of visible satellites. So only send it when that
+                // changes.
+                send_message(MSG_GPS_STATUS);
+                last_gps_satellites = g_gps->num_sats;
+            }
 		}
 
 		if (freqLoopMatch(streamRatePosition, freqMin, freqMax)) {