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Why do some radio towers blink?

jeffgeerling.com

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Re: Why do some radio towers blink?

#71

Post by anyfoo »

georgefrowny wrote: post #69
anyfoo wrote: post #58

Earlier quoted context omitted.

Well, yeah. I was sort of assuming that you won't use a TCXO or even just a cheapo quartz crystal, because it doesn't make a lot of sense: You've now thrown money at something that will relatively quickly desync anyway. I mean, sure, the TCXO will mean that you only start seeing a phase difference between the two after weeks instead of minutes, but what's the point of that? I you want them to be at the same phase, yo…

Well yes, obviously, but I'm just being a internet pedantic about "a few minutes".

And you are technically correct, which is the best kind of correct anyway.

Re: Why do some radio towers blink?

#72

Post by SoftTalker »

rollulus wrote: post #7

Slightly off topic: typically, lights of neighboring towers blink asynchronously. But sometimes they are synchronized. Very satisfying. Anyone knows how this works? My best guess is e.g. DCF77. Thoughts?

I’ve driven through wind farms where the blinking tower lights are synchronized. Highly distracting.

Re: Why do some radio towers blink?

#73

Post by SoftTalker »

punnerud wrote: post #48

Never seen a blinking light on a tower in Norway. Why the difference between countries, can’t be that huge difference risk if some don’t have them?

Strictly speaking they should be unnecessary because there are published minimum safe altitudes for every air space over land. But some aircraft must be able to “See and avoid”

Re: Why do some radio towers blink?

#74

Post by lxgr »

scblock wrote: post #59
lxgr wrote: post #46

Related: Some wind turbines apparently only turn on their position lights when there's any aviation traffic nearby (as detected by either local transponder interrogators (possibly ADS-B receivers?) or radar)!

Radar, called Aircraft Detection Lighting Systems (ADLS). The requirements are summarized in the FAA Advisory Circular covering aviation marking and lighting.

As far as I understand, non-US systems don’t always use (primary) radar.

Re: Why do some radio towers blink?

#75

Post by georgefrowny »

anyfoo wrote: post #61
lxgr wrote: post #55

Earlier quoted context omitted.

Wouldn't the mains frequency be a common clock source, if nothing else? But presumably these lights at least have battery backup, given the obvious risks in case all of them were to fail at the same time due to a grid issue.

Yeah, the mains frequency qualifies, if you explicitly use that. (Doesn't solve the problem if you want them to be in sync phase-wise, i.e. blink at the same time or similar, but at least they won't drift apart, which was what this is about.)

You could still sync with that signal because it's not perfect.

For example, say you have a scheme where a period longer than the last one is symbol A, about the same period is B and shorter is C. You will get a random-ish sequence of symbols.

If you have an algorithm that, say, resets the timer to zero whenever a certain symbol sequence is detected, you can eventually get back in sync. With some care you can make sure you only sync when the sequence happens and the light has only been off for a short period to avoid excessively long off periods or truncated on periods.

Then you just need to have a local oscillator good enough to do that timing analysis and that can maintain sync between these symbol occurrences.

You could do it on the tiniest micro. Once you've counted the zero crossing detector, these days you might save 3 to 5 whole dollars over a GPS receiver on your very expensive ICAO compliant lamp and also ruled out using DC into the bargain! And theoretically it desyncs when the grid is too stable for days on end (and you just get BBBBB or ABABAB for millions of cycles)!

In terms of what is actually used, they do often use GPS and many of them have MODBUS or similar data connections which presumably wire into the wind turbine's telemetry somehow for fault detection.

Re: Why do some radio towers blink?

#77

Post by WalterBright »

dylan604 wrote: post #70
WalterBright wrote: post #67

Earlier quoted context omitted.

I know. But when I was programming LEDs for my single board computer, I found that blinking the LEDs used a lot less power. If the blink rate was fast enough, your eye did not distinguish that from the LED being on 100% of the time. I don't know if battery powered devices generally use that trick or not.

all dimmable LEDs use blinking, but interesting case for your suggestion for an even more efficient use of a "steady" LED. what blink frequency did you use to keep it from being visible to the eye? did it not dim the overall brightness to a meter? early LEDs were atrocious with their slow blink rates that quickly scanning across them could see it, but even more noticeable if you pointed your camera at them. now, they…

Oh, I just hooked it up to a square wave generator and an oscilloscope. I just played with the square wave frequency. It was 50 years ago, I don't recall values and such.

Re: Why do some radio towers blink?

#78

Post by ikkun »

the towers in my area all switched to LED recently. the slow, glowing blink of the incandescent ones probably isn't as visible as the modern ones, but I do dearly miss seeing it out my window.

Re: Why do some radio towers blink?

#79

Post by anyfoo »

georgefrowny wrote: post #75
anyfoo wrote: post #61

Earlier quoted context omitted.

Yeah, the mains frequency qualifies, if you explicitly use that. (Doesn't solve the problem if you want them to be in sync phase-wise, i.e. blink at the same time or similar, but at least they won't drift apart, which was what this is about.)

You could still sync with that signal because it's not perfect. For example, say you have a scheme where a period longer than the last one is symbol A, about the same period is B and shorter is C. You will get a random-ish sequence of symbols. If you have an algorithm that, say, resets the timer to zero whenever a certain symbol sequence is detected, you can eventually get back in sync. With some care you can make su…

Maybe you'll get a kick out of this, I did it in a bored afternoon a while back:

    echo -ne '\e[8;32;90;t';n=20;t=524292;l=$((t-1));m=$((2**n-1));c=0;xs=(1);ys=(1);for ((i=0;i>1)^(b*t)));c=$(((c>30));ys=($y $ys[1]);yd=$((120+(y >> 24)));printf '\e[48;5;%um ' $yd};done
It is entirely self-contained (but needs zsh, not bash, for dumb reasons). Terminal at 90 columns works best.

It is just a very simple integer LFSR as a random number source, followed by a hand-made integer IIR filter (manually placing poles on the z-plane). All of this entirely with trivial integer operations only (effectively using 32 bit fixed point arithmetic)

So without any external input or tools at all, and not even using zsh's $RANDOM, it makes an "analog" weavy pattern.

The LFSR is this part:

    b=$((l&1));l=$(((l>>1)^(b*t)));c=$(((c
The hand-made filter is this part:

    xs=($c $xs[1,4]);y=$((((xs[4]-xs[0])>30));ys=($y $ys[1])
I specifically tuned the filter peak just slightly away from being an integer divisor of 90 columns, to give the pattern a slight "rolling" effect.*
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