AP_GPS: fixed UAVCAN GPS status when disconnected

this fixes the status reported for a UAVCAN GPS when the cable is
unplugged. It should be "NO GPS", but was instead reporting "NO
FIX". The user had no way to distinguish between not finding a CAN GPS
at all and it not getting satellites

thanks to CUAV for reporting

libraries/AP_GPS/AP_GPS.cpp

@@ -635,7 +635,7 @@ void AP_GPS::update_instance(uint8_t instance)
         return;
     }
 
-    if (drivers[instance] == nullptr || state[instance].status == NO_GPS) {
+    if (drivers[instance] == nullptr) {
         // we don't yet know the GPS type of this one, or it has timed
         // out and needs to be re-initialised
         detect_instance(instance);
@@ -721,7 +721,7 @@ void AP_GPS::update(void)
 
     // calculate number of instances
     for (uint8_t i=0; i<GPS_MAX_RECEIVERS; i++) {
-        if (state[i].status != NO_GPS) {
+        if (drivers[i] != nullptr) {
             num_instances = i+1;
         }
     }

libraries/AP_GPS/AP_GPS_UAVCAN.cpp

@@ -229,6 +229,15 @@ void AP_GPS_UAVCAN::handle_fix_msg(const FixCb &cb)
     interim_state.last_gps_time_ms = AP_HAL::millis();
 
     _new_data = true;
+    if (!seen_message) {
+        if (interim_state.status == AP_GPS::GPS_Status::NO_GPS) {
+            // the first time we see a fix message we change from
+            // NO_GPS to NO_FIX, indicating to user that a UAVCAN GPS
+            // has been seen
+            interim_state.status = AP_GPS::GPS_Status::NO_FIX;
+        }
+        seen_message = true;
+    }
 }
 
 void AP_GPS_UAVCAN::handle_aux_msg(const AuxCb &cb)
@@ -275,6 +284,10 @@ bool AP_GPS_UAVCAN::read(void)
 
         return true;
     }
+    if (!seen_message) {
+        // start with NO_GPS until we get first packet
+        state.status = AP_GPS::GPS_Status::NO_GPS;
+    }
 
     return false;
 }

libraries/AP_GPS/AP_GPS_UAVCAN.h

@@ -57,6 +57,7 @@ private:
     HAL_Semaphore sem;
 
     uint8_t _detected_module;
+    bool seen_message;
 
     // Module Detection Registry
     static struct DetectedModules {