To correctly calculate your position your GPS receiver needs to 'see' at least 4 satellites. Most modern GPS receivers can use up to 12 satellites at a time. Satellite 'visibility' can be affected by atmospheric and topographical conditions including tall buildings, steep crevices and dense foliage. If Memory-Map displays the 'No Signal' or 'No Position' message this means your GPS is not receiving a strong enough signal - try moving slightly or re-positioning your receiver.