From ae65f2e6ab58c4f8189c02c27445ab69b1c0bc3f Mon Sep 17 00:00:00 2001
From: bugobliterator <siddharthbharatpurohit@gmail.com>
Date: Thu, 30 Sep 2021 10:46:55 +0530
Subject: [PATCH] AP_GPS: fix build and working with single GPS Receiver config

---
 libraries/AP_GPS/AP_GPS.cpp |  9 ++++++---
 libraries/AP_GPS/AP_GPS.h   | 13 +++++++++++--
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/libraries/AP_GPS/AP_GPS.cpp b/libraries/AP_GPS/AP_GPS.cpp
index aa2f58ceaa..4d445fc59c 100644
--- a/libraries/AP_GPS/AP_GPS.cpp
+++ b/libraries/AP_GPS/AP_GPS.cpp
@@ -347,7 +347,7 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
     AP_GROUPINFO("_PRIMARY", 27, AP_GPS, _primary, 0),
 #endif
 
-#if GPS_MAX_RECEIVERS > 1 && HAL_ENABLE_LIBUAVCAN_DRIVERS
+#if HAL_ENABLE_LIBUAVCAN_DRIVERS
     // @Param: _CAN_NODEID1
     // @DisplayName: GPS Node ID 1
     // @Description: GPS Node id for discovered first.
@@ -355,25 +355,28 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("_CAN_NODEID1", 28, AP_GPS, _node_id[0], 0),
 
+#if GPS_MAX_RECEIVERS > 1
     // @Param: _CAN_NODEID2
     // @DisplayName: GPS Node ID 2
     // @Description: GPS Node id for discovered second.
     // @ReadOnly: True
     // @User: Advanced
     AP_GROUPINFO("_CAN_NODEID2", 29, AP_GPS, _node_id[1], 0),
-
+#endif // GPS_MAX_RECEIVERS > 1
     // @Param: 1_CAN_OVRIDE
     // @DisplayName: First UAVCAN GPS NODE ID
     // @Description: GPS Node id for first GPS. If 0 the gps will be automatically selected on first come basis.
     // @User: Advanced
     AP_GROUPINFO("1_CAN_OVRIDE", 30, AP_GPS, _override_node_id[0], 0),
 
+#if GPS_MAX_RECEIVERS > 1
     // @Param: 2_CAN_OVRIDE
     // @DisplayName: Second UAVCAN GPS NODE ID
     // @Description: GPS Node id for second GPS. If 0 the gps will be automatically selected on first come basis.
     // @User: Advanced
     AP_GROUPINFO("2_CAN_OVRIDE", 31, AP_GPS, _override_node_id[1], 0),
-#endif
+#endif // GPS_MAX_RECEIVERS > 1
+#endif // HAL_ENABLE_LIBUAVCAN_DRIVERS
 
     AP_GROUPEND
 };
diff --git a/libraries/AP_GPS/AP_GPS.h b/libraries/AP_GPS/AP_GPS.h
index 294fa36e41..f1ce8a576d 100644
--- a/libraries/AP_GPS/AP_GPS.h
+++ b/libraries/AP_GPS/AP_GPS.h
@@ -31,9 +31,18 @@
 #ifndef GPS_MAX_RECEIVERS
 #define GPS_MAX_RECEIVERS 2 // maximum number of physical GPS sensors allowed - does not include virtual GPS created by blending receiver data
 #endif
-#ifndef GPS_MAX_INSTANCES
+#if !defined(GPS_MAX_INSTANCES)
+#if GPS_MAX_RECEIVERS > 1
 #define GPS_MAX_INSTANCES  (GPS_MAX_RECEIVERS + 1) // maximum number of GPS instances including the 'virtual' GPS created by blending receiver data
+#else
+#define GPS_MAX_INSTANCES 1
+#endif // GPS_MAX_RECEIVERS > 1
+#endif // GPS_MAX_INSTANCES
+
+#if GPS_MAX_RECEIVERS <= 1 && GPS_MAX_INSTANCES > 1
+#error "GPS_MAX_INSTANCES should be 1 for GPS_MAX_RECEIVERS <= 1"
 #endif
+
 #if GPS_MAX_INSTANCES > GPS_MAX_RECEIVERS
 #define GPS_BLENDED_INSTANCE GPS_MAX_RECEIVERS  // the virtual blended GPS is always the highest instance (2)
 #endif
@@ -559,7 +568,7 @@ class AP_GPS
     AP_Float _blend_tc;
     AP_Int16 _driver_options;
     AP_Int8 _primary;
-#if GPS_MAX_RECEIVERS > 1 && HAL_ENABLE_LIBUAVCAN_DRIVERS
+#if HAL_ENABLE_LIBUAVCAN_DRIVERS
     AP_Int32 _node_id[GPS_MAX_RECEIVERS];
     AP_Int32 _override_node_id[GPS_MAX_RECEIVERS];
 #endif