ping360: update automatic transmit duration calculation

Author: jaxxzer

src/sensor/ping360.h

@@ -383,7 +383,19 @@ class Ping360 : public PingSensor
     void adjustTransmitDuration()
     {
         if (_autoTransmitDuration) {
-            int autoDuration = round(5.1 + 2.833*(range() - 2));
+            /*
+             * Per firmware engineer:
+             * 1. Starting point is TxPulse in usec = ((one-way range in metres) * 4000) / (Velocity of sound in metres per second)
+             * 2. Then check that TxPulse is wide enough for currently selected sample interval in usec, i.e.,
+             *    if TxPulse < (1.25 * sample interval) then TxPulse = (1.25 * sample interval)
+             * 3. Perform limit checking
+             */
+
+            // 1
+            int autoDuration = round(4000*range()/_speed_of_sound);
+            // 2 (transmit duration is microseconds, samplePeriod() is nanoseconds)
+            autoDuration = std::max(static_cast<int>(1.25*samplePeriod()/1000), autoDuration);
+            // 3
             _transmit_duration = std::max(static_cast<int>(_firmwareMinTransmitDuration),
                                           std::min(transmitDurationMax(), autoDuration));
             emit transmitDurationChanged();