Q: What is the difference between UART port and USB port on Mediatek MT3329 based GlobalTop GPS modules?
There is no difference, they are actually the very same port.
The USB port of the module can be connected to the USB port of a host computer, provided that the GlobalTop USB driver is installed. The driver causes the USB device to appear as an additional COM port available to the host computer. Application software can access the USB device in the same way as it would access a standard COM port. The USB port is therefore more suitable for user applications running on Microsoft Windows operating systems. Interestingly enough, the USB port of the module is not a native one, but rather embeds a USB-to-UART bridge.
The UART port of the module can be connected to the serial port of a host computer, provided that signal level are adapted if necessary (e.g. in case the serial port is RS-232). If the host computer does not have a serial port, then a USB-to-UART bridge can do the job. The UART port is therefore more suitable for embedded applications running on microcontrollers.
Moreover, USB part and UART port can be used at the same time as they are mirrored in content. For example, if you send a PMTK command packet to the USB port, you can receive the acknowledge packed from both USB and UART ports. And vice versa. This parallel usage is not recommended though.