function changeImage(imagem){
			
	switch(imagem){
				
	
		case 1:
			document.getElementById('um').src  ="imgs/bt_terrestre.gif";
			
		break;
		
			case 2:
			document.getElementById('um').src ="imgs/bt_over_terrestre.gif";
			

		break;
		
		case 3:
			document.getElementById('dois').src  ="imgs/bt_maritimo.gif";
			
		break;
		
		case 4:
			document.getElementById('dois').src ="imgs/bt_over_maritimo.gif";
			

		break;
		
		case 5:
			document.getElementById('tres').src  ="imgs/bt_aereo.gif";
			
		break;
		
		
		case 6:
			document.getElementById('tres').src ="imgs/bt_over_aereo.gif";
			

		
	}
}