improve the GPS reliability with a homemade high performance antenna
The GPS comes generaly with a small one inch patch antenna. While this antenna has a good sensitivity you probably noticed some positioning errors due to indirect reception path. The backfire helix antenna is quite easy to build, cost near nothing and definively solve this position error problem.
On the images above you can see a position monitoring, first image is with the helix antenna with +/- 3m error and on the right is the original patch, more than 20 meters errors were observed.
If you are really looking for a very high reliability I would recommend to use a NEO-6 GPS module instead of the now popular M8N, this old NEO-6 device is less subject to position glitches. If you want to use a magnetometer you can use a separate HMC5883l module.
I am not detailing the antenna construction because it has already been done in a lot of pages, I used this one as a guide : http://lea.hamradio.si/~s53mv/navsats/analog.html
In this case I had to add a 18pF capacitor because the original patch antenna has a built in preamplifier that is powered with 3.3V. To avoid a short circuit with the new antenna this capacitor is required, the exact value is not critical, just select one from 10pF to 47pF.
I also did the same modification with a M8N module that has a built in magnetometer. This module is able to catch more constellations but is more subject to jumping errors than the old NEO-6, so use it only if you want a compact design with an embedded magnetometer.
I did my tests with this light tricopter, the magnetometer module is mounted on a 20cm vertical mast while the GPS module and helix antenna is mounted on the tail boom. When I switch the hold position it looks like my tricopter is nailed in the sky, no more fly away or erratic moving, I highly recommend this antenna when flying in a urban zone where multiple path reflections are a problem for the GPS.
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