SITL: Improved Sub simulation

-Constrain z axis movement between -100m (seafloor) and 0m (water
surface)
-More accurate thruster modelling

libraries/SITL/SIM_Aircraft.h

@@ -174,7 +174,7 @@ protected:
     const float FEET_TO_METERS = 0.3048f;
     const float KNOTS_TO_METERS_PER_SECOND = 0.51444f;
 
-    bool on_ground() const;
+    virtual bool on_ground() const;
 
     // returns height above ground level in metres
     float hagl() const;  // metres

libraries/SITL/SIM_Submarine.cpp

@@ -20,6 +20,8 @@
 #include <AP_Motors/AP_Motors.h>
 #include "Frame_Vectored.h"
 
+#include <stdio.h>
+
 using namespace SITL;
 
 static Thruster vectored_thrusters[] =
@@ -37,7 +39,7 @@ Submarine::Submarine(const char *home_str, const char *frame_str) :
     Aircraft(home_str, frame_str),
     frame(NULL)
 {
-    frame_height = 0.1;
+    frame_height = 0.0;
     ground_behavior = GROUND_BEHAVIOR_NONE;
 }
 
@@ -45,19 +47,34 @@ Submarine::Submarine(const char *home_str, const char *frame_str) :
 void Submarine::calculate_forces(const struct sitl_input &input, Vector3f &rot_accel, Vector3f &body_accel)
 {
     rot_accel = Vector3f(0,0,0);
-    body_accel = Vector3f(0,0,0);
+
+    // slight positive buoyancy
+    body_accel = Vector3f(0,0,-GRAVITY_MSS * 1.1);
+
     for (int i = 0; i < 6; i++) {
         Thruster t = vectored_thrusters[i];
         int16_t pwm = input.servos[t.servo];
         float output = 0;
         if (pwm < 2000 && pwm > 1000) {
-         output = (pwm - 1500) / 400.0f; // range -1~1
+         output = (pwm - 1500) / 400.0; // range -1~1
         }
 
-        body_accel += t.linear * output * 1.2 * GRAVITY_MSS;
+        // 2.5 scalar for approximate real-life performance of T200 thruster
+        body_accel += t.linear * output * 2.5;
+
         rot_accel += t.rotational * output;
     }
 
+    // Limit movement at the surface of the water
+    if (position.z < 0 && body_accel.z < 0) {
+    	body_accel.z = GRAVITY_MSS;
+    }
+
+    // Limit movement at the sea floor
+    if (position.z > 100 && body_accel.z > -GRAVITY_MSS) {
+    	body_accel.z = -GRAVITY_MSS;
+    }
+
     float terminal_rotation_rate = 10.0;
     if (terminal_rotation_rate > 0) {
         // rotational air resistance
@@ -94,3 +111,11 @@ void Submarine::update(const struct sitl_input &input)
     // update magnetic field
     update_mag_field_bf();
 }
+
+/*
+   return true if we are on the ground
+*/
+bool Submarine::on_ground() const
+{
+	return false;
+}

libraries/SITL/SIM_Submarine.h

@@ -41,7 +41,11 @@ public:
         return new Submarine(home_str, frame_str);
     }
 
+
 protected:
+
+    bool on_ground() const override;
+
     // calculate rotational and linear accelerations
     void calculate_forces(const struct sitl_input &input, Vector3f &rot_accel, Vector3f &body_accel);
     Frame *frame;