May 18, 2004

Finding rockets with GPS

There's been a misunderstanding about how I intend to use GPS when tracking and recovering my rockets. I'll talk a little bit about what's available now, the excellent suggestions given, and then explain the technique that I'll actually use.

(in the extended entry)

There are non-GPS low-power transmitters that can be put into a rocket, including a system developed by Walston. The club that we occasionally fly with in Whitakers, North Carolina has the Walston system. Each rocketeer buys a transmitter on a different frequency, and they split the cost of the receiving unit. The nice thing about the Walston unit (as I understand it) is that you don't need a ham radio license from the FCC, because the unit is extremely low-powered. You have to use a directional YAGI antenna, and there's an art to the technique of tracking down your rocket once it touches down. This explanation on using the Walston Tracking System is the best I've ever seen. The author, Sue McMurray, is a wonderful lady who was head of motor testing and certifications for the national high-power rocket organization for a time. She also offered her assistance when a local girl scout troop leader decided that "rockets aren't something that girls do". The lady can flat-out write, but more importantly she builds and flies some impressive rockets.

Back to tracking. It's also possible to use a higher-powered transmitter, but in that case you'll need to obtain your ham radio license. From what I've heard, it's not difficult to become a ham, especially since you no longer need to know Morse code as a prerequisite.

These systems are costly, and the rockets have to be designed to contain the transmitter antenna. They really do work, both out west where they tend to much higher altitudes (it helps to fly on the desert), but their recovery area is exponentially greater, and here in the east where we are more limited on altitude but the recovery areas tend to be cropland and woods. Trust me, wading through high cotton, tobacco or corn is no way to spend a summer day searching for a rocket.

Neither of those options are GPS though, they're just simple beacon transmitters, and you triangulate on the signal to locate the rocket. It's possible to lose the signal behind obstructions, which is where the art of the search applies. Picking up a blocked signal is made more likely by understanding the way everything works and how to take advantage of it.

Putting a GPS into a hobby rocket introduces new problems. You'll still need the transmitter, but instead of a simple beacon signal it needs to transmit its coordinates. It also has to be able to maintain the GPS signal on the way down and on the ground, regardless of how and where it lands. There is also more government regulation on transmitting signal strength and frequency.

My intended method is simpler and doesn't rely on having a signal transmitted from the rocket at all. In fact, it's just using the GPS to refine the search area. It does require seeing the rocket come down, otherwise I'm back to covering the general area in a search pattern until I get lucky or give up.

Before the actual launch of our rockets, if I've got extra eyes (Mookie) to help track I'll send her out a ways in the general direction of expected drift. If I'm at the club launch alone I'll enlist a friend to track from the pad and I'll head out a ways to get a different angle on the flight. Once it comes down, I make note of a landmark on the line of sight and estimate how far away the rocket landed. Often it's by judging whether it came down in front of or behind treelines, which means it can be very imprecise (the field behind that treeline might be half a mile wide). Then I head along that sight line, while Mookie (if she's available) does the exact same thing from where she was watching. In a perfect world, where we come together is where the rocket landed. In reality, one of us didn't track the rocket all the way down, or we have to scramble around and over obstacles which throws off the line of sight, or many other gotcha's that keeps you from walking a straight line in nature.

And this is where the GPS comes in. Some models allow you to shoot an azimuth with a compass, orient your unit to it, then enter 'waypoints'. By doing this, the GPS tells you how far off your line of sight you've wandered as you head towards the rocket. Entering a second set of waypoints - from where Mookie is standing - increases the accuracy.

It's not the most efficient use for GPS, but it's definitely an improvement over the guess-and-by-golly method we use now.

Posted by Ted at May 18, 2004 11:24 AM
Category: Rocketry Rocketry Resources

now i am seeing what you mean.

by the way, i just rode from blacksburg to chincoteague and back, without my gps (don't have an attachment for my new bike yet). i was never lost, but as daniel boone once said, "i was a might bewildered sometimes" (paraphrase)

Posted by: chris at May 20, 2004 04:08 PM
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