[talk-au] new toy found by son

b.schulz.10 at scu.edu.au b.schulz.10 at scu.edu.au
Thu Oct 1 13:14:58 BST 2009


There's only so much gain you can get out of a GPS antenna. Basically the best you can do to cover the whole sky (ie:180 degrees x 360 degrees) is 3dBi (double the power from a perfect isotropic antenna). Certainly you could design an antenna which had a reduced coverage, such as a cone, but it will always be a compromise between satellite count and gain.

But saying that most GPS antennas are pretty poor performers, particularly the small patch antennas which are surrounded by metal shielding/PCB traces etc in the unit. 

For reference, given that the GPS signals are ~1.3GHz (Wiki says they span from 1176.45MHz to 1575MHz) an ideal dipole antenna would be 11.5cm long. A Dipole has a peak gain of 2.7dBi (I think, it's about that).

Anyway. I wonder how much use accelerometers would be on a MTB? You experience lots of impulse forces (forces which act over time scales much shorter than the sampling rate of the accelerometer, ie: beyond it's nyquist limit) and so the integration would be seriously inaccurate.

Accelerometers would be an excellent solution for mapping tunnels though. Especially since you're only talking about a short-ish time span and you can potentially have 2 accurate calibration points (one at each end of the tunnel). I guess if the accelerometer could be mounted in some form of suspended cage to dampen high frequency noise it would be quite accurate.

</ramble>

-Brent

----- Original Message -----
From: John Smith <deltafoxtrot256 at gmail.com>
Date: Thursday, October 1, 2009 9:58 pm
Subject: Re: [talk-au] new toy found by son
To: Liz <edodd at billiau.net>
Cc: talk-au at openstreetmap.org

> 2009/10/1 Liz <edodd at billiau.net>:
> > he was hoping that gps+accelerometers+gyros would provide 
> accurate cheap
> > mapping of mtb trails
> 
> Why not just build/buy a better antenna for the GPS?
> 
> The higher the gain the better the accuracy etc, most GPS 
> antenna are
> tiny so if you make a massive one you should get good readings :)
> 
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