all cases landmarks should be as close to the diver as possible and spread over a large arc for best accuracy. Equipment Various pieces of equipment are available to assist divers navigating underwater. Magnetic compasses, set either in a diver's console or wrist mounted. Various forms exist. Scuba sextant, or Nav-finder, to enable a diver to plot an ongoing course during a dive. Compass boards Hand-held sonar Cave and wreck reels and line Jump spools Periodically reports are issued suggesting the development of underwater GPS technology, but no system is currently available on market. It is generally thought that the difficulty of locating satellite by signals from underwater at present is not capable of being overcome by existing technology. Diving compasses Construction The typical diving compass is made from a card with graduation in degrees, mounted on a pivot in a transparent housing filled with fluid which damps the movement and prevents pressure collapse of the housing. It may be wrist mounted, console mounted or carried some other way. It is desirable that the compass can operate accurately at significant tilt angles without sticking. On the card there is a magnet which will interact with the ambient magnetic field so as to align itself and the card with the field provided it is free to rotate. There will be other marks on the housing which are intended to be aligned with the direction of travel of the user, so the offset of the card to the housing will indicate the direction of the magnetic field and the orientation of the user. Important features of a diving compass are that it can easily be read in dim light, the card or needle does not easily jam if the housing is tilted slightly, and that it can be securely attached to the diver's arm or equipment and does not get lost. It is useful for any straps to be adjustable while wearing gloves, and any clips that may attached should be non-magnetic. The strap should be long enough to go round the diver's wrist over the diving suit glove, and if it is slightly elastic it will stay in place when the suit compresses. There may be a movable bezel which can be set to record a course and to help set a reciprocal course. There are also electronic compasses which can provide a digital or analogue display These are based on