The shield hardware is based around an Atmel ATMega328 microcontroller, an MCP6L04T-E/SLCT rail-to-rail op-amp, a Ublox MAX-M8Q GPS module, and optionally a RadioMetrix HX1 VHF transmitter.
The ArduinoTrack shield is currently in version 1.11, which includes the switch to the Ublox GPS. The decision to switch to Ublox from the Telit Jupiter SL869 v002 was primarily due to a lack of support from the manufacturer as Digikey stopped sourcing the part and the HAB-compatible models were changing too often. The SL869 v002 has been proven to work in near-space though. Also, the Linear Technology's LT1679 op-amp was replaced with a cheaper Microchip MCP6L04T rail-to-rail op-amp.
As you can see from the schematic, the heart of the ArduinoTrack shield is U1, the ATMega328. This microcontroller is running at 16MHz, and new to the v1.10 version, all of the standard Arduino I/O pins are either connected to the shield headers, or they are used internally to the shield, thus giving much greater I/O possibilities over prior versions.
If you are interested in building an ArduinoTrack Shield v1.11, the Gerber files are available as well as the Bill of Materials (BOM). Nearly all of the components can be sourced from Digikey and those part numbers are included in the BOM. The Ublox GPS engines can be sourced from the UK from HAB Supplies and the optional RadioMetrix HX-1 transmitter come from Lemos International.
The latest hardware files including the EagleCad drawings can always be downloaded from the Custom Digital Services GitHub Repository, along with the firmware files.