Thanks Kurt, good info...
Your comment "once the main chute" comes out... Are you suggesting the EggFinder may be no help in finding rocket (or its "crater") in a ballistic "recovery" scenario?
Um. No and No. A tumbling rocket at high speed leads to different positions of the GPS antenna and the tracker transmit antenna. The signal coming off the 900Mhz tracking antenna is "polarized" depending on the
antenna geometry. I'll ignore "circularly polarized" antennas to keep this simple but it boils down to this: For optimal receiving of a signal, if the little straight vertical antenna on your tracking transmitter
is in a "vertical" position ie. up and down, that's the position you want your receive antenna to be in. If the antenna is horizontal ie. sideways that's the position one would like their receive antenna to be in
for optimal reception.
Now with a rocket flight, the rocket may be flopping around after apogee deployment of a dual deploy. The antenna orientation may be rapidly changing and if the rocket is far away, the ability to pick up the signal is degraded
due to the power output of the tracker and the less than optimal position of the receive/transmit antennas. Packet reception may be intermittent especially with the 900Mhz trackers. Once the main chute is deployed with a nosecone mounted tracker
the antenna may be in the up/down or down/up position which is pretty optimal for reception with a vertically oriented receiver antenna. If the antenna is pointing down, the reception would technically be lousy right underneath the rocket but that is likely not going to happen. The rocket is going to still be up pretty high with a slower descent rate, in a good geometry for reception and increased chances of position decoding. A few hundred feet in the air
improves your chances for receiving. Do a ground test with the EggFinder lying horizontally on the ground and you'll notice that the ground footprint is not as good as the "in air" footprint. Hence I generally use a higher main
deployment so I can to improve my chances of getting positions for developing a trend line for drift.
Now a few other variables are the 1000 knot speed limit on GPS reception as all the commercially available chipsets will lock out if that is exceeded. Decoding returns once the speed drops back down plus the fact that the doppler effect at high speeds
may degrade the performance of the onboard GPS chipset. There's a 60,000 foot altitude limit too but some GPS chipsets will work above that altitude as long as the speed limit isn't exceeded. The efficiency of the antenna system on the receiving end can
have a big effect. You may see people with multi-element antennas (Yagis) pointing them skyward and rotating them in different positions. They're trying to optimize positioning of the antenna systems to see if they can increase the ability to get reception from a sight unseen rocket. A Yagi is a directional antenna with increased sensitivity in the direction they are pointed hence they can improve decoding ability. Only problem with 900Mhz is the width of the beam is narrow which makes pointing a Yagi on that band at a sight unseen rocket very hard to achieve reliably. At 144Mhz, 220Mhz and 420Mhz ranges the beamwidth of the Yagi is wide enough so that antenna is workable on those bands. That's what those folks are using when you see them tracking. (As an aside, I've used a multi-element Yagi antenna on the 900Mhz band to go after the rocket once it is on the ground. The orientation is not likely changing that quickly and I've proven the ground footprint is improved substantially. This might be helpful with a project that lands several miles away from you. Not necessary for most sport fliers. Ummmm, there is an antenna type
that is called a patch antenna that can increase the reception range a bit and has a wide enough beamwidth to be workable on 900Mhz. I've tried one and it works. I use a live map so I was able to keep it pointed
in the direction of the sight unseen rocket) Example:
A patch antenna has polarity too and is usually marked on the back!
Lastly, more horsepower trumps everything. A high powered transmitter is decodeable from longer ranges no matter what the antenna positioning is. That's a no brainer of course.
With hobby rockety, there's a limit to tracker size and battery capacity so everything is a tradeoff.
Ballistic flight? I had an Eggfinder lead me to a rocket with the fincan sticking out of the ground. Only received two positions but it was just before the rocket hit ballistic. It was an H motor in a fiberglass rocket that was totally sight unseen. I wouldn't have known where to look but walked out to the position on the map and there it was. Sure if the rocket gets knocked side ways and goes off ballistic at a 45 degree angle away from you, you
probably won't receive anything before it hits but if it's closer, relatively speaking you stand a chance to find the crash site with the EggFinder as I did. The tracker gave up its life but I got a flyable rocket back after replacing the nosecone I cracked trying to get it out of the ground. If it was RDF tracking, one might not be able to get a reliable bearing before the rocket hits because the tracker usually dies in that instance.
That's GPS tracking in a nutshell. Kurt