Nowadays, GPS units generally have 12 channels, which means they can receive signals from 12 satellites simultaneously. In earlier models, such as the GARMIN 45C, there were only 8 channels. A GPS receiver can output 2D (two-dimensional) data when it receives signals from 3 satellites; this provides only latitude and longitude, without altitude. If the receiver gets signals from 4 or more satellites, it outputs 3D data, which includes elevation. However, due to the Earth not being a perfect sphere, there are some errors in altitude data. Nowadays, some GPS receivers come with built-in barometers, like the etrex SUMMIT and VISTA. These devices combine height data from two sources to produce a final and more accurate altitude reading.