[RocketsNW] It's quiet (GPS trackers)

[Greg Deputy]

On the losing lock at boost issue, it depends on the flight profile, and how
you mount the electronics.  You'll have much MUCH better luck maintaining
lock if the unit is wrapped in bubble wrap or other padding and allowed to
float somewhat vs hard mounting the unit to the airframe like you would a
regular altimeter.  GPS engines don't like high G loads and REALLY don't
like vibration.  

> -----Original Message-----
> From: rockets-bounces at rocketsnw.com [mailto:rockets-bounces at rocketsnw.com]
On
> Behalf Of Keith Packard
> Sent: Sunday, April 19, 2009 2:35 PM
> To: Eugene Samsonov
> Cc: rockets at rocketsnw.com
> Subject: Re: [RocketsNW] It's quiet (GPS trackers)
> 
> On Sun, 2009-04-19 at 13:12 -0700, Eugene Samsonov wrote:
> 
> > 1. Batteries pop out at shute ejection. Tracking is obviously lost
> > after that. Solution: duct tape (or PCB redesign with a solid steel
> > battery
> > holder)
> 
> Yeah, I've soldered 9V batteries into flight computers in the past.
> Works OK, but changing batteries on the rail is a challenge. Greg's
hardware,
> and my own TeleMetrum (http://altusmetrum.org) use 3.7V rechargeable LiPo
> cells that can be more-or-less permanently affixed to the rocket and
recharged
> before flight. Couple that with a USB charging circuit, and you can
recharge
> almost anywhere.
> 
> > 2. GPS unit looses tracking at launch. This is a hard one, where I
> > suspect it all depends on the particular type of the GPS unit used.
> > Most of those are likely consumer-grade, and probably not designed to
> > be used in a high-g / high-velocity mode. It is not the hardware that
> > glitches, but most likely the math. Solution: trying out a bunch of
> different models...
> 
> The PSAS (http://psas.pdx.edu) GPS unit tracked all the way up and back
down
> on one flight; perhaps having an external cylindrical patch antenna on the
> outside of the rocket and an amplifier helped? A previous version, with a
> simple internal patch antenna, lost lock (and went nuts) after chute
ejection.
> It re-acquired the signal closer to the ground, so recovery of the rocket
was
> fairly easy.
> 
> TeleMetrum provides a 3.3V serial interface, and I've got a SiRF III board
> plugged in now, but I haven't flown that yet. Will be very interesting to
see
> if it works.
> 
> > 2a. GPS unit does not provide altitude data. When at launch pad,
> > altitude numbers look fine. But after take off, they become mostly
random...
> 
> Altitude numbers from GPS are never very good; having a pressure altimeter
in
> addition to the GPS makes it possible to use both as a check. And, yeah,
> solving GPS is easier when you limit what velocity and acceleration you
> consider 'reasonable'.
> 
> > 3. Antenna size, orientation, max distance. I was never able to get a
> > signal much beyond couple miles at best, though the stated distance is
> > said to be around 20 miles for the 1 watt transmitter used with the
> > "full-size" antenna shown
> 
> TeleMetrum runs down at 70cm (435MHz). At 10dBm I am concerned about
range.
> We'll use a yagi for the receiver end, instead of a simple vertical, so
> that'll give us quite a bit of additional range. Experience with Greg's
> cc1050-driven BeeLine seems to indicate that we'll have plenty of range
> though.
> 
> > 4. The lack of hand-held receiver. The receiver box is connected to
> > either a laptop or to a blue-toooth adaper talking to a PocketPC. With
> > either of these one is stuck within 20 feet of one's vehicle (where the
> power is).
> > Solution: invest a few hundred thousand bucks into a custom hand-held
> > device
> > :)
> 
> We're building another board, TeleTerra, that will hold a 4-line LCD
screen to
> report position from the TeleMetrum. It will have a cable running to a
> (partially populated) TeleMetrum board which will continue to have the
main
> processor and battery.
> 
> > Considering the amount of effort it would require to solve the issues
> > above and the tiny niche market for the device, I decided to keep the
> > project at its "propotype" stage :)
> 
> Yeah, making things in commercial volume is way too much work. I'm really
glad
> Greg is willing to build useful stuff. TeleMetrum is just a hobby project,
> although as the hardware and software are all freely licensed, anyone
could
> take the design and build commercial versions without paying any royalty
to
> us.
> 
> --
> keith.packard at intel.com