AP_GPS: add an AP_GPS_FixType header

useful when you need nice symbols for the numbers, but are not compiling the GPS headers in

libraries/AP_GPS/AP_GPS.cpp

@@ -53,6 +53,7 @@
 
 #include <AP_AHRS/AP_AHRS.h>
 #include <AP_Logger/AP_Logger.h>
+#include "AP_GPS_FixType.h"
 
 #define GPS_RTK_INJECT_TO_ALL 127
 #ifndef GPS_MAX_RATE_MS
@@ -111,6 +112,16 @@ static_assert((uint32_t)AP_GPS::GPS_Status::GPS_OK_FIX_3D_RTK_FLOAT == (uint32_t
 static_assert((uint32_t)AP_GPS::GPS_Status::GPS_OK_FIX_3D_RTK_FIXED == (uint32_t)GPS_FIX_TYPE_RTK_FIXED, "FIX_RTK_FIXED incorrect");
 #endif
 
+// ensure that our own enum-class status is equivalent to the
+// ArduPilot-scoped AP_GPS_FixType enumeration:
+static_assert((uint32_t)AP_GPS::GPS_Status::NO_GPS == (uint8_t)AP_GPS_FixType::NO_GPS, "NO_GPS incorrect");
+static_assert((uint32_t)AP_GPS::GPS_Status::NO_FIX == (uint8_t)AP_GPS_FixType::NONE, "NO_FIX incorrect");
+static_assert((uint32_t)AP_GPS::GPS_Status::GPS_OK_FIX_2D == (uint8_t)AP_GPS_FixType::FIX_2D, "FIX_2D incorrect");
+static_assert((uint32_t)AP_GPS::GPS_Status::GPS_OK_FIX_3D == (uint8_t)AP_GPS_FixType::FIX_3D, "FIX_3D incorrect");
+static_assert((uint32_t)AP_GPS::GPS_Status::GPS_OK_FIX_3D_DGPS == (uint8_t)AP_GPS_FixType::DGPS, "FIX_DGPS incorrect");
+static_assert((uint32_t)AP_GPS::GPS_Status::GPS_OK_FIX_3D_RTK_FLOAT == (uint8_t)AP_GPS_FixType::RTK_FLOAT, "FIX_RTK_FLOAT incorrect");
+static_assert((uint32_t)AP_GPS::GPS_Status::GPS_OK_FIX_3D_RTK_FIXED == (uint8_t)AP_GPS_FixType::RTK_FIXED, "FIX_RTK_FIXED incorrect");
+
 AP_GPS *AP_GPS::_singleton;
 
 // table of user settable parameters

libraries/AP_GPS/AP_GPS_FixType.h

@@ -0,0 +1,18 @@
+#pragma once
+
+// a AP_GPS-library-independent enumeration which lists the commonly
+// accepted set of GPS Fix Types which GPSs report.  This header can
+// be used even if AP_GPS is not compiled in.
+
+// this is not enum-class as many places in the code want to check for
+// "a fix at least this good" using "<".
+
+enum AP_GPS_FixType {
+    NO_GPS = 0,           ///< No GPS connected/detected
+    NONE = 1,             ///< Receiving valid GPS messages but no lock
+    FIX_2D = 2,           ///< Receiving valid messages and 2D lock
+    FIX_3D = 3,           ///< Receiving valid messages and 3D lock
+    DGPS = 4,             ///< Receiving valid messages and 3D lock with differential improvements
+    RTK_FLOAT = 5,        ///< Receiving valid messages and 3D RTK Float
+    RTK_FIXED = 6,        ///< Receiving valid messages and 3D RTK Fixed
+};