Copter: ensure we log both GPS at the time we receive a message

ArduCopter/ArduCopter.pde

@@ -1183,21 +1183,28 @@ static void update_optical_flow(void)
 // called at 50hz
 static void update_GPS(void)
 {
-    static uint32_t last_gps_reading;           // time of last gps message
+    static uint32_t last_gps_reading[GPS_MAX_INSTANCES];   // time of last gps message
     static uint8_t ground_start_count = 10;     // counter used to grab at least 10 reads before commiting the Home location
     bool report_gps_glitch;
+    bool gps_updated = false;
 
     gps.update();
 
     // logging and glitch protection run after every gps message
-    if (gps.last_message_time_ms() != last_gps_reading) {
-        last_gps_reading = gps.last_message_time_ms();
+    for (uint8_t i=0; i<gps.num_sensors(); i++) {
+        if (gps.last_message_time_ms(i) != last_gps_reading[i]) {
+            last_gps_reading[i] = gps.last_message_time_ms(i);
 
-        // log GPS message
-        if (g.log_bitmask & MASK_LOG_GPS) {
-            DataFlash.Log_Write_GPS(gps, current_loc.alt);
+            // log GPS message
+            if (g.log_bitmask & MASK_LOG_GPS) {
+                DataFlash.Log_Write_GPS(gps, i, current_loc.alt);
+            }
+
+            gps_updated = true;
         }
+    }
 
+    if (gps_updated) {
         // run glitch protection and update AP_Notify if home has been initialised
         if (ap.home_is_set) {
             gps_glitch.check_position();