Atmel ATR0640 basebone GPS processor (14 channel correlator from Thales Navigation, ARM7TDMI thumb processor core) is the main part of the receiver.
There are two different voltage supplies: at the left-top corner 5V comes from the external power supply (AC power plug); at the righ-bottom corner 5V comes from the USB bus (when Explorist is connected to PC with link cable). When only the left-top 5V applied; Explorist shows "battery charging". When only the right-bottom 5V applied; the Explorist shows "Ext. power"
Two GND pads are internally connected
Tx is an output (e.g. NMEA); Rx is an input.
USB connector of Explorist's link cable is shown below. 5V comes from PC.