[TowerTalk] TIC-Ring Question

[Lux, Jim]

On 11/11/22 12:24 PM, Mike Lyon wrote:
> If I was a coding geek, it would be cool to take 2x, highly accurate,
> GPS receivers, place one GPS antenna on the front and back booms of
> the beam and then diff the two coordinates to come up with a heading.
> Don't think it would be that hard to do. but again, I couldn't code
> myself out of a paper bag.
>
> The two things I would be concerned about is (A) RF field around the
> beam getting into the GPS system and frying it and (B), switching PS
> noise coming out of the GPS system and getting into the beam and
> making really annoying QRM.
>
> Kind of the same idea as one of these that we use when aligning
> microwave dishes:
> https://www.gmesupply.com/smartaligner-antenna-alignment-tool-with-case-by-multiwave?searched=true
>
> -Mike Lyon, KE6MRE
>

I'd not worry about switching noise.. decent filtering solves that, and 
what I'd do is have the GPS downlink their outputs on WiFi or LoRa to 
the shack. Then all you're feeding is DC power.

The GPS receiver is tiny, so the field from the HF on the *boom* is 
fairly small, across the receiver.  You'd want to test, but off hand, I 
don't think that would be a problem.

As for calculating orientation.  This is trickier than it seems, because 
although both receivers will be subject to common errors that would 
theoretically cancel out, in reality, what you get out of each receiver 
is a "noisy" position estimate with biases that are somewhat position 
dependent.  That is, if you put one up, and swung the beam around the 
circle, the plot of (averaged) positions probably won't be a circle.   
I've got a bunch of little receivers and when I've tried position 
averaging and then just subtracting to get position, it hasn't worked 
very well (maybe 1-2 meter random uncertainties) - So on a behemoth 160m 
3 element Yagi with a 80 meter long boom, yeah, works great.  Not so hot 
on a 2 meter long VHF antenna. YMMV - there is a LOT of difference among 
models of receivers.


They are cheap enough (~$20-30 from adafruit, for example) so you could 
just buy a couple and try it in the backyard.   What you might do is 
look for one that says "RTK" (real time kinematic)


If you got the right receiver that can do differential corrections, and 
used one as a base station, and the other receiving differential 
corrections from the first, then you've got a good chance of it 
working.  That kind of setup, with cheap receivers and no special 
measures can easily get 10cm relative position, 5 m from the pivot 
point, you're looking at an angular uncertainty of about a degree or two.