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.
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