GPS

  Bangle.setLCDMode('120x120');
  g.clear();
  g.setFont("6x8");

  const Width  = g.getWidth();
  const Height = g.getHeight();

/**** show screen ****/

  g.drawString('GPS', 5,5);

  g.drawString('lat:',   10,25);
  g.drawString('long:',  10,40);
  g.drawString('alt:',   10,55);
  g.drawString('speed:', 10,70);
  g.drawString('course:',10,85);

  g.drawString('Tap to start', 5,105);

/**** GPS Monitoring ****/

  function showGPSValues (ValueSet) {
    g.clearRect(45,25, Width,105);

    showGPSValue(ValueSet.lat,   55,25);
    showGPSValue(ValueSet.lon,   55,40);
    showGPSValue(ValueSet.alt,   55,55);
    showGPSValue(ValueSet.speed, 55,70);
    showGPSValue(ValueSet.course,55,85);

    g.flip();
  }

  function showGPSValue (Value, x,y) {
    if (isNaN(Value)) {
      g.drawString('n/a', x,y);
    } else {
      g.drawString('' + Value, x,y);
    }
  }

/**** install touch event handler ****/

  let monitoringGPS = false;

  function toggleGPS () {
    g.clearRect(5,105, Width,Height);

    if (monitoringGPS) {
      Bangle.removeAllListeners('GPS');
      Bangle.setGPSPower(false);
      g.drawString('Tap to start', 5,105);
    } else {
      Bangle.setGPSPower(true);
      Bangle.on('GPS',showGPSValues);
      g.drawString('Tap to stop', 5,105);
    }
    g.flip();

    monitoringGPS = ! monitoringGPS;
  }

  setWatch(toggleGPS, BTN4, {edge:"rising", debounce:50, repeat:true});
  setWatch(toggleGPS, BTN5, {edge:"rising", debounce:50, repeat:true});

  g.flip();