A GPS receiver can be a stand-alone handheld unit, a car navigation unit or a module for use with a Palmtop or PC.

A sonar is used at sea as a sort of underwater radar.

The most common receiver type is the 12 parallel channel receiver which can collect data from up to 12 satellites at one time.

This feature lets you calculate and plan trips in advance, to see how long a certain path is or to make sure that you don’t miss any exits.

Some GPS units have maps and city databases already loaded into memory

What interface can be used to transmit data from your computer. Most common are serial and USB.

Filtering on "Yes" will show all products available with either IPX-7 or IPX-8 waterproof specifcations. The available standards are the following:

IPX-0 No special protection

IPX-1 Protected against falling water Equivalent to 3-5mm rainfall per minute for a duration of 10 minutes. Unit is placed in its normal operating position.

IPX-2 Protected against falling water when tilted up to 15 degrees - Same as IPX-1 but unit is tested in 4 fixed positions - tilted 15 degrees in each direction from normal operating position.

IPX-3 Protected against spraying water - Water spraying up to 60 degrees from vertical at 10 liters/min at a pressure of 80-100kN/m2 for 5 min.

IPX-4 Protected against splashing water - Same as IPX-3 but water is sprayed at all angles.

IPX-5 Protected against water jets - Water projected at all angles through a 6.3mm nozzle at a flow rate of 12.5 liters/min at a pressure of 30kN/m2 for 3 minutes from a distance of 3 meters.

IPX-6 Protected against heavy seas - Water projected at all angles through a 12.5mm nozzle at a flow rate of 100 liters/min at a pressure of 100kN/m2 for 3 minutes from a distance of 3 meters.

IPX-7 Protected against water immersion - Immersion for 30 minutes at a depth of 1 meter.

IPX-8 Protected against water submersion - The equipment is suitable for continual submersion in water under conditions which are identified by the manufacturer.

To improve the accuracy of the GPS system, transmitters has been placed on various places in Europe and America. These beacons already know their exact position, and when they receive the GPS signals from the satellites, they calculate the error and transmit a correction factor. This technology is referred to as Differential GPS.

Some GPS receivers support the use of memory cards which allow more data to be stored than the in-build memory. If you want to be able to load large area maps into your GPS it’s wise to look at a GPS unit that has expandable memory, or large inbuilt memory.

This is the time from when you turn on your GPS unit until it has received signals from a sufficient number of satellites and calculate its first position. If the GPS unit has recently been used, the first fixed location will be quite fast.