AP_GPS: Set unknown DOP's to UINT16_MAX, rather then 9999

This fixes the MAVLink reporting for unknown dops, and avoids the situation where a GPS driver could report a worse DOP then we could handle.

Also corrects an apparent error in the HIL_GPS MAVLink message, where we would always select the unknown dop value rather then provided DOP.

libraries/AP_GPS/AP_GPS.cpp

@@ -446,8 +446,8 @@ void AP_GPS::detect_instance(uint8_t instance)
 
     state[instance].instance = instance;
     state[instance].status = NO_GPS;
-    state[instance].hdop = 9999;
-    state[instance].vdop = 9999;
+    state[instance].hdop = GPS_UNKNOWN_DOP;
+    state[instance].vdop = GPS_UNKNOWN_DOP;
 
     switch (_type[instance]) {
     // by default the sbf/trimble gps outputs no data on its port, until configured.
@@ -578,8 +578,8 @@ void AP_GPS::update_instance(uint8_t instance)
     if (_type[instance] == GPS_TYPE_NONE) {
         // not enabled
         state[instance].status = NO_GPS;
-        state[instance].hdop = 9999;
-        state[instance].vdop = 9999;
+        state[instance].hdop = GPS_UNKNOWN_DOP;
+        state[instance].vdop = GPS_UNKNOWN_DOP;
         return;
     }
     if (locked_ports & (1U<<instance)) {
@@ -614,8 +614,8 @@ void AP_GPS::update_instance(uint8_t instance)
             memset(&state[instance], 0, sizeof(state[instance]));
             state[instance].instance = instance;
             state[instance].status = NO_GPS;
-            state[instance].hdop = 9999;
-            state[instance].vdop = 9999;
+            state[instance].hdop = GPS_UNKNOWN_DOP;
+            state[instance].vdop = GPS_UNKNOWN_DOP;
             timing[instance].last_message_time_ms = tnow;
         }
     } else {
@@ -1255,8 +1255,8 @@ void AP_GPS::calc_blended_state(void)
     state[GPS_BLENDED_INSTANCE].time_week = 0;
     state[GPS_BLENDED_INSTANCE].ground_speed = 0.0f;
     state[GPS_BLENDED_INSTANCE].ground_course = 0.0f;
-    state[GPS_BLENDED_INSTANCE].hdop = 9999;
-    state[GPS_BLENDED_INSTANCE].vdop = 9999;
+    state[GPS_BLENDED_INSTANCE].hdop = GPS_UNKNOWN_DOP;
+    state[GPS_BLENDED_INSTANCE].vdop = GPS_UNKNOWN_DOP;
     state[GPS_BLENDED_INSTANCE].num_sats = 0;
     state[GPS_BLENDED_INSTANCE].velocity.zero();
     state[GPS_BLENDED_INSTANCE].speed_accuracy = 1e6f;

libraries/AP_GPS/AP_GPS.h

@@ -32,6 +32,7 @@
 #define GPS_BLENDED_INSTANCE GPS_MAX_RECEIVERS  // the virtual blended GPS is always the highest instance (2)
 #define GPS_RTK_INJECT_TO_ALL 127
 #define GPS_MAX_RATE_MS 200 // maximum value of rate_ms (i.e. slowest update rate) is 5hz or 200ms
+#define GPS_UNKNOWN_DOP UINT16_MAX // set unknown DOP's to maximum value, which is also correct for MAVLink
 
 // the number of GPS leap seconds
 #define GPS_LEAPSECONDS_MILLIS 18000ULL

libraries/AP_GPS/AP_GPS_MAV.cpp

@@ -123,8 +123,8 @@ void AP_GPS_MAV::handle_msg(const mavlink_message_t *msg)
             loc.alt = packet.alt;
             state.location = loc;
             state.location.options = 0;
-            state.hdop = MAX(packet.eph, 9999);
-            state.vdop = MAX(packet.epv, 9999);
+            state.hdop = MIN(packet.eph, GPS_UNKNOWN_DOP);
+            state.vdop = MIN(packet.epv, GPS_UNKNOWN_DOP);
             if (packet.vel < 65535) {
                 state.ground_speed = packet.vel / 100.0f;
             }