The speed of your device determines the speed of the SD card you should use. Satellite navigation systems, like the Global Positioning System used in the United States, require that maps be stored in memory. You can augment the internal memory of your unit with an SD memory card, if the unit can support it. The speed of the card determines how quickly information can be written to and read from it.
SD Card Recommendations -The speed of the SD card that you use in your GPS unit is dependent on the requirements of the device. According to Garmin, any speed of card will work. TomTom, however, recommends that you purchase the fastest SD card available. Magellan supports the use of SD cards, but makes no recommendation for the speed you should use. Ultimately, the speed of the SD card will only affect reading and writing data to that card. A memory card that is faster or slower than the specifications of the device will still store data reliably.
SD Card Speed -The Secure Digital memory card specification uses speed classes to indicate how quickly the card can read and write data. The classes are ranked from class two to class ten and the differentiation is given in the ability to transmit data. A class two SD card can read and write at 2MB per second, class four runs at 4MB per second and so on. In 2009, a new speed class was established, known as "Ultra High Speed," which starts at UHS class one at 10MB per second.
SD Card Size -In addition to the speed of the card, you should also consider the size of the card for your GPS device. Garmin recommends a 4GB card and TomTom states that it has successfully tested 2GB cards. If the software of the GPS device isn't able to read the larger sized cards, you may not be able to use them. For example, Garmin's software will only recognize 3,000 map segments, regardless of the size of the data, so even if you have a much larger SD card, you will only be able to store 3,000 map segments worth of data.
GPS In the United States, the government owns and operates the Sat-Nav system. It uses 24 low-orbit satellites to provide location data to receivers on Earth. The satellites orbit the Earth at the equator, which puts them in the southern sky for residents of North America, in a geosynchronous orbit. This orbit matches the rotational speed of the earth so that the satellite remains in a fixed position in the sky, relative to the ground, providing a consistent point of reference. GPS receivers only receive coordinates from the orbiting satellites, they must apply those coordinates to a map, which is stored internally. No map data is transmitted from the satellites and GPS receivers cannot send information to the satellites.