AP_GPS: Expose COM port and Output Rate in header

* This removes magic numbers of hard coding the hardware port and output
  rate
* This also fixes configuring the incorrect hardware port
* Now, COM2 (TTL) is configured for GSOF output
* The data rate remains the same as before

Signed-off-by: Ryan Friedman <ryanfriedman5410+github@gmail.com>

libraries/AP_GPS/AP_GPS_GSOF.cpp

@@ -17,6 +17,11 @@
 //  Trimble GPS driver for ArduPilot.
 //  Code by Michael Oborne
 //
+//  Usage in SITL with hardware for debugging: 
+//    sim_vehicle.py -v Plane -A "--serial3=uart:/dev/ttyUSB0" --console --map -DG
+//    param set GPS_TYPE 11 // GSOF
+//    param set SERIAL3_PROTOCOL 5 // GPS
+//
 
 #define ALLOW_DOUBLE_MATH_FUNCTIONS
 
@@ -70,8 +75,7 @@ AP_GPS_GSOF::read(void)
 
     if (gsofmsgreq_index < (sizeof(gsofmsgreq))) {
         if (now > gsofmsg_time) {
-            requestGSOF(gsofmsgreq[gsofmsgreq_index], 0);
+            requestGSOF(gsofmsgreq[gsofmsgreq_index], HW_Port::COM2, Output_Rate::FREQ_10_HZ);
-            requestGSOF(gsofmsgreq[gsofmsgreq_index], 3);
             gsofmsg_time = now + 110;
             gsofmsgreq_index++;
         }
@@ -167,16 +171,15 @@ AP_GPS_GSOF::requestBaud(const uint8_t portindex)
 }
 
 void
-AP_GPS_GSOF::requestGSOF(const uint8_t messagetype, const uint8_t portindex)
+AP_GPS_GSOF::requestGSOF(const uint8_t messageType, const HW_Port portIndex, const Output_Rate rateHz)
 {
     uint8_t buffer[21] = {0x02,0x00,0x64,0x0f,0x00,0x00,0x00, // application file record
                           0x03,0x00,0x01,0x00, // file control information block
-                          0x07,0x06,0x0a,portindex,0x01,0x00,0x01,0x00, // output message record
+                          0x07,0x06,0x0a,static_cast<uint8_t>(portIndex),static_cast<uint8_t>(rateHz),0x00,messageType,0x00, // output message record
                           0x00,0x03
                          }; // checksum
 
     buffer[4] = packetcount++;
-    buffer[17] = messagetype;
 
     uint8_t checksum = 0;
     for (uint8_t a = 1; a < (sizeof(buffer) - 1); a++) {

libraries/AP_GPS/AP_GPS_GSOF.h

@@ -40,15 +40,39 @@ public:
 
 private:
 
+    // A subset of the port identifiers in the GSOF protocol that are used for serial.
+    // Ethernet, USB, etc are not supported by the GPS driver at this time so they are omitted.
+    // These values are not documented in the API.
+    enum class HW_Port {
+        COM1 = 3, // RS232 serial
+        COM2 = 1, // TTL serial
+    };
+
+    // A subset of the data frequencies in the GSOF protocol that are used for serial.
+    // These values are not documented in the API.
+    enum class Output_Rate {
+        FREQ_10_HZ = 1,
+        FREQ_50_HZ = 15,
+        FREQ_100_HZ = 16,
+    };
+
     bool parse(const uint8_t temp) WARN_IF_UNUSED;
     bool process_message() WARN_IF_UNUSED;
     void requestBaud(const uint8_t portindex);
-    void requestGSOF(const uint8_t messagetype, const uint8_t portindex);
+
+    // Send a request to the GPS to enable a message type on the port at the specified rate.
+    // Note - these request functions currently ignore the ACK from the device.
+    // If the device is already sending serial traffic, there is no mechanism to prevent conflict.
+    // According to the manufacturer, the best approach is to switch to ethernet.
+    void requestGSOF(const uint8_t messageType, const HW_Port portIndex, const Output_Rate rateHz);
+
     double SwapDouble(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
     float SwapFloat(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
     uint32_t SwapUint32(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
     uint16_t SwapUint16(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
 
+    bool validate_baud(const uint8_t baud) const WARN_IF_UNUSED;
+
     struct Msg_Parser
     {