324 Chapter 17 be found. By adjusting the distance figures so that all the lines between satellites and receiver meet at a point, the time of the quartz clock can be adjusted so that it is synchronized to the atomic clocks, and the single point is the position of the receiver. Originally, the system was intended only for military use, and its precision was 22 m horizontally and 27.7 m vertically. A civilian version was limited to 100 m horizontal and 156 m vertical. More recently, the higher precision has become available on civilian GPS receivers. Though handheld GPS devices are still available, the main GPS civilian use is for car satellite navigation (satnav). On the whole, this works well, but there are still anomalies that lead drivers to river beds, deep fords, and roads that are too narrow for their vehicles. The combination of satnav and an old-fashioned map is still by far the best way of making an unfamiliar journey. The satnav really comes into its own when you are trying to find a city address on a dark night. Remote Controls Remote controls at one time were provided only for the more expensive brands of television receivers, but are now almost universal for television, radio, DVD players or recorders, set-top boxes, and such other purposes as garage doors, cameras, car door locks, electric and gas heater controls, and such hobby items as model aircraft. It is also possible to buy socket adapters that can be placed between a mains socket and an appliance plug to allow any electrical appliance to be switched on and off remotely. This latter use is very appealing to anyone who likes to believe that a vast amount will be saved by switching off appliances rather then leaving them on standby. Early types of television remote control used ultrasonic signals, but these are obsolete now. The remote controls for television, radio, DVD, and electric or gas heaters are of the infrared (IR) variety, but the others (for use outside the house) operate using radio transmitter and receiver technology. These latter types are liable to problems of interference, and there are well-documented examples of such problems (such as the street where all parked cars unlocked when a shop till was activated). Radio remotes can offer much greater range (typically 0.8e3.2 km; 0.5e2 miles) and are not seriously blocked by buildings in the line of sight. Taking the television type of remote control first, these operate by using a control system to send digital codes along an infrared beam from a light-emitting diode (LED). The binary code is carried using a brief burst of IR followed by a `dead' time (no transmission) for each bit. The precise coding depends on the manufacturer of the equipment, but the different systems are converging, so that it is possible to adapt a remote intended for a DVD player to control the television as well, and there are also `universal' IR controllers that can be programmed to any action that can be provided by IR. The snag with these universal remotes is that you have to specify what unit is being controlled before you send the command. The effect of this is that