USB GPS Receivers
New APRS users are frequently confused by the differences between serial and USB GPS receivers. Part of this confusion arises from the terminology; USB stands for Universal Serial Bus, which leads some to believe that it's more closely related to the traditional serial interface than it is. While there are some electrical similarities, the differences run much deeper.
Two types of serial interface are common for GPS receivers - RS-232 and TTL. These two varieties of serial interface differ in their voltage levels and polarities, but are otherwise identical and can be used interchangably with simple converter circuits. TTL signals are typically used within devices or over short cable runs. RS-232 signals can reliably be used over several meters of cable and are compatible with PC serial ports, though these are becoming increasingly rare in new computers.
Prior to the adoption of USB, most GPS receivers used one of these interfaces, usually at the relatively low speed of 4800 bits per second. The data carried over the serial interface can take several forms, though the NMEA 0183 standard is by far the most common. All APRS trackers on the market support NMEA 0183.
USB was introduced in 1996, and it offers a number of advantages for PC-connected devices - it provides electrical power, supports hot swapping, and allows devices to automatically identify themselves to the system. Electrically, it's still a serial protocol, but with much higher signalling rates, and while RS-232 simply defines how bits get from one device to another, the USB standard describes a far more complicated system.
Unfortunately, USB complicates things for small devices. It requires a relatively powerful 'host' - usually a full PC - to control communications. And while USB hosts are getting smaller, a bigger problem is that there's no standard defining how to carry GPS data over USB. Most USB GPS receivers require a matching driver to be loaded on the host, and it's the driver that knows how to communicate with the GPS receiver. Often, the drivers are provided by the manufacturer of the USB chip in the GPS receiver - the GPS receiver manufacturer may not even know the details.
RS-232 to USB converters are available, but they only provide an RS-232 serial port on a USB-equipped PC - they can't be used to connect a USB device to anything because of the differences described above.
Further confusing matters is the fact that some Garmin GPS receivers that appear to be USB-only devices have converter cables that provide an RS-232 interface. In fact, these models (like the Colorado 300) are able to sense the special cable and switch to a serial mode. The cable provides only voltage level conversion. Some automotive models use a similar scheme, but using a proprietary data format rather than NMEA 0183. The Argent Data Systems GTRANS converter cable translates between the Garmin and NMEA protocols using a microprocessor embedded in the cable, but it's not a USB host.
What This Means for APRS
Aside from the Garmin devices with 'hidden' serial interfaces, USB GPS receivers aren't usable with any current APRS trackers, and converters are no help. If you're looking for a GPS receiver to use with an APRS tracker, forget about the great deals on USB GPS receivers and spend a few dollars more for a GPS receiver with an RS-232 interface.