Here’s a photographic step-by-step showing the build of a small, lightweight (16g) tracker from off-the-shelf modules.
First, the modules, from left to right the BMP085 pressure and temperature sensor, Arduino Mini Pro 3.3V 8MHz, RFM22B radio transceiver, and a Falcom FSA03 GPS receiver:
First step was to solder on wires and header pins to carry power and data. All the modules are held to each other using those headers, making a reasonably solid assembly. I chose the physical layout very carefully so that as many header pins as possible could be used to directly connect from board to board. In most cases those pins are used for power or data, but in some cases they are used for support only. Some of those link together unused lines but in a couple of cases the pins only are soldered at one end only and are cut short before the reach the adjacent module. These are the prepared modules, BMP085 then RFM22B then Arduino (GPS needs no preparation):
And here all of the modules on top of a scribbled circuit diagram:
Next is the RFM22B. This one is a lot more tricky as it has several wires going between it and the Arduino. Also the connector pitch at 2mm is different to the Arduino at 2.54mm, and the pin headers have to be splayed out for the extra width of the board.
Finally the GPS module and a DS18B20 for external temperature measurement:
That’s the complete tracker, needing just power (from 3 Lithium Energizer cells) and an aerial for the radio transmitter.