MHz bands. These advances have made it possible to create GPS-based rocket tracking system
for a fraction of what they would have cost 10 years ago.
Meet the Eggfinder
The Eggfinder is a GPS-based rocket tracking system that transmits using the license-free 900
MHz band. The radio modules used in the Eggfinder use the maximum legally-permissible
output power for this band, 100 mW. There are actually two pieces to the Eggfinder system,
the TX (transmitter) module and the RX (receiver) module. The Eggfinder TX board is about
3” long x 1” wide (+ 3” for the antenna), and weighs about 20 grams without the battery. It is
mounted in your rocket, preferably in the nose cone or payload bay area, and sends out a data
stream with GPS location data once per second. The Eggfinder RX board has a USB data
cable that plugs into your laptop or tablet, and receives the signal from the TX board in your
rocket. Optionally, you can connect a Bluetooth®-Serial board and a battery to the Eggfinder
RX board, so you can wirelessly connect it to a tablet.
Using readily-available (and mostly free!) software on your laptop or tablet, you can “read” the
transmitted GPS location data from your rocket and use that data to plot its location in real-
time. Once the rocket lands, you record the landing coordinates, and you can either draw a
track line (compass direction/distance) to the landing spot and follow that heading, or you can
plug the coordinates into a smartphone app and have your GPS-enabled smartphone draw a
track that takes you right to your rocket.
The Eggfinder is a no-frills system designed for one purpose: getting your rocket back. It does
not have any kind of built-in recording capability (although you can, and most likely will,
record the received data onto your laptop/tablet). The range is adequate for the vast majority
of sport rocketry flights; we have tested it to over 8,000’ range line-of-sight with good signal
strength. This does not necessarily mean that you will be able to maintain a GPS signal
throughout the entire flight; in fact, it’s fairly normal to lose the signal while the rocket is
under boost and through the initial coasting phase, because the GPS module isn’t going to be
able to keep a fix on the GPS satellites while it’s moving fast. Once the rocket starts slowing
down, you’ll see it pick up again, and you will probably keep a lock on it until it gets fairly
close to the ground (well under 100’). Even if you lose the GPS data at that altitude, it’s so
close to the ground by then that finding it is not going to be a problem.
If your rocket does go out of the range of the Eggfinder, once it starts to come down you will
most likely be able to pick up the signal again, unless your rocket has drifted about 2 miles
downrange. That’s not something that happens very often, because you’re already going to
know about how high your rocket is going to go before you launch, and you’re not going to fly
if there’s too much wind. Even with a tracking transmitter, it’s still possible to lose rockets…
if you try hard enough.
A few guidelines…:
• Fly sensibly: If you KNOW that you’re going to fly out of the Eggfinder’s range, use a
higher-gain receiver antenna, or position yourself so that you will be most likely to pick up the
signal as the rocket comes down.
