My first tracker is built from a Garminx Etrex Summit, Alinco DJ-C5 transmitter, and a TinyTrak-II.

The TinyTrak-II is the guts of the operation. This one chip wonder receives the NMEA data stream from the GPS receiver, plucks out the position, formats it into an APRS packet, and then outputs the packet as a FSK sinewave using a resistor ladder. All of that for only $32 for the kit. A bargain at twice the price. :-)

One of the nicer features of the TinyTrak is that the rate at which positions are transmitted is programmable. you could set it to a fixed rate but using the SmartBeaconing features is preferable. Give it a slow speed and beacon rate plus a fast speed and beacon rate. Then while the rocket is on the pad (or after it lands) it will transmit at the slow rate. But when it starts moving it will transmit at a faster rate. Very neat.

Here is the assembled tracker. The parts are mounted to a 38mm body tube which goes into the payload bay of my 6" dia. Assault Breaker. In this picture you can see the DJ-C5.
This view shows the Garmin GPS unit.
In this last view, the TinyTrak is visiable at the top and its 9V battery in the center.

This is not very pretty but it does work.