Personal Digital Assistants or PDAs are truly useful devices, they are hand held devices which remind you of appointments, store your business and personal records and can even double as a phone. By putting a GPS system on your PDA it can even give you direction and location information.Just How Accurate Do You Have To Be?Before investing in a GPS system for your PDA, ask how accurately it should operate. All units use data from the GPS satellite network, but the accuracy of the information relies on the PDA clock hardware.The satellites are fitted with expensive atomic clocks which have great precision. PDA GPS units have less accurate clocks. Unless you have to know your position with detailed accuracy, say around 12 inches, you can choose a PDA GPS unit with the less expensive time piece and still find it extremely useful.The GPS device transmits signals to the GPS satellite network, transmitting a timestamp, and the available satellites compare that with the timestamp from the signal which was previously received. The satellite then figures in how long it took to receive the signal, comparing this time calculation and the relative location of the other satellites. This is how it triangulates your earth based position. Information from a minimum of three satellites is required to collate all of the information required to advise your location. Another satellite adds enough accuracy to calculate altitude.How Will You Use Your GPS?Decide how you need to use a PDA GPS and get the features you need.GPS systems designed to be used while driving a car, come with spoken instructions advising which direction to turn, how far from destination and other pertinent information. The safety aspects of spoken instructions rather than glancing at a device continually are obvious.If you only need a guide while walking about, the less expensive and advanced alternatives may be suitable.Generally, a the most advanced PDA GPS systems come equipped with maps which continually update your location on a updating map, also giving verbal instructions such when and where to turn next. Less pricey options let you transfer non updating maps via your PC or Mac and compare your position relative to your destination.