8 ms·

Why do some radio towers blink?

179 points by warrenm 11mo ago

rollulus 11mo ago
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?
- asdefghyk 11mo ago
  One option - Maybe the blinking time is set to a INDEPENDENT? accurate time piece - ie Blink on the change of a second
  - anyfoo 11mo ago
    That's what DCF77 is. Or GPS.
- winrid 11mo ago
  It's probably just a side effect of them being powered on at the same time or not?
  - anyfoo 11mo ago
    If they don't sync to a common clock source, they won't stay in sync for long. Probably not even for a few minutes or so. You'd get the same phenomenon that you see when operating turn signals in traffic. They seem to weave "in and out" of sync. The frequency at which that happens is the beat frequency, i.e. the difference between the two blinking frequencies.
    - jrockway 11mo ago
      There's got to be some rubidium frequency standard that's a drop-in replacement for a 555 timer ;)
      - deleted 11mo ago
        [deleted]
      - scblock 11mo ago
        GNSS (GPS) is the typical standard used.
      - georgefrowny 11mo ago
        A chip scale atomic clock (with a handy 1Hz output) can be yours for barely over five thousand euros and half that if you buy 250 of them. https://www.microchip.com/en-us/product/csac-sa65 https://www.microchip.com/en-us/product/csac-sa65 It's a bit larger than a 555, but it should keep you within a small handful of microseconds per day until the aging effects start to add up which make getting more than a millisecond a year dicey, even if the thing doesn't die on you: https://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resources/Gardner%202016.pdf https://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resource...
    - georgefrowny 11mo ago
      A cheapo 20ppm quartz watch crystal definitely doesn't drift by a second every few minutes, but it will over about about half a day at maximum error. A 5ppm one (still sub one Euro) will keep a second about every two days. A 0.5ppm TCXO can be had for about 15 Euros from Mouser and that gets you two weeks. If you have shared line power you can just use that and everything will be locked in sync forever. If you don't want to use that or radio, and you are outside, you could try to be really clever am sync your flash phases to a specific position of the sun. This is what the Long Now clock does. It'll be a different time each day, but it'll be the same for all units, within a small tolerance.
      - anyfoo 11mo ago
        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, you'll need to sync them at some point, and you do that by using a common clock source. So either you shell out some effort for a real solution (power line is nice, and also qualifies as a common clock source as I've predicated), or you don't. And if you don't, there's no point in using a precise-ish clock at all, and you'll likely end up with very quick desyncing.
        georgefrowny 11mo ago
        Well yes, obviously, but I'm just being a internet pedantic about "a few minutes".
        anyfoo 11mo ago
        And you are technically correct, which is the best kind of correct anyway.
    - lxgr 11mo ago
      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.
      - anyfoo 11mo ago
        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.)
        georgefrowny 11mo ago
        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.
        anyfoo 11mo ago
        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<n*m;i++));do b=$((l&1));l=$(((l>>1)^(b*t)));c=$(((c<<1|b)&m));[ $((i%n)) -eq 0 ] &&{xs=($c $xs[1,4]);y=$((((xs[4]-xs[0])<<30-0x3fffd60f*ys[2]+0x7d32617c*ys[1])>>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<<1|b)&m)) The hand-made filter is this part: xs=($c $xs[1,4]);y=$((((xs[4]-xs[0])<<30-0x3fffd60f*ys[2]+0x7d32617c*ys[1])>>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.*
- scblock 11mo ago
  When it's on purpose it's typically done through GPS driven clocks. This is how wind farms manage it, where all towers are required to blink together.
  - floatrock 11mo ago
    Yeah, I've seen it with windfarms. Always wondered why do they need to blink at the same time. The scale of the blink is pretty jarring at night (but also awe-some, in the same way any big enough infrastructure project inspires a kind of awe). Wind farms have a certain amount of nimbyism because they "spoil the natural landscape." (So do regular farms -- nothing natural about grain silos or row crops, but that's a side topic...) Anyways, having that many towers blink in unison across that big a landscape is a weird effect when you first see it. I think there's an argument that if they blinked independently it would feel more natural in a way. But since the blinking is all FAA requirements, I assume it's to help identify all the individual towers from the air. I suppose if they were all blinking independently, it would be a predator-trying-to-focus-on-a-single-zebra-in-the-herd problem, except in this case the predator is a pilot trying not to crash into a turbine. Sure would emit more subtle 'part of the landscape' vibes though. (Which I guess is exactly what you don't want when you're flying above them. Sigh.)
    - evertedsphere 11mo ago
      > Always wondered why do they need to blink at the same time. presumably this makes it more striking, and thus easier to notice and avoid
    - scblock 11mo ago
      It's so pilots see the entire wind farm as a single entity and can interpret what they see and understand the extent of the wind farm easily. There is a pretty good study you can read on this: https://www.airporttech.tc.faa.gov/DesktopModules/EasyDNNNews/DocumentDownload.ashx?portalid=0&moduleid=3682&articleid=88&documentid=115 https://www.airporttech.tc.faa.gov/DesktopModules/EasyDNNNew... As to community impact, radar-activated lighting is an approach that is being used in places this is a concern. It allows the lights to remain off unless there is a plane within the envelope that requires the lights to activate. It's expensive though.
- daemonologist 11mo ago
  I believe it's usually GPS/GNS (they all receive the time via GPS independently, and flash at predetermined times). The FAA requires synchronization for many classes of obstruction because it makes it clear that you're looking at obstruction lights rather than e.g. brake lights or traffic lights on the ground.
  - geerlingguy 11mo ago
    Could they also use power grid sync? Not sure as I haven't talked to anyone in wind power, but grid sync would be pretty close to 1 Hz at least in the US. Building a product that would sync at 1 Hz via GPS that works in the US and other countries with 50 Hz power would be a little easier than syncing to grid phase though.
    - scblock 11mo ago
      Definitely GPS. Other methods have been used in the past--I remember reading operating reports from a wind farm nearly 20 years ago that slowly brought all its lights in line with each other over several months--but these days you can buy mainstream lighting with the GNSS receiver built in from a number of suppliers. They make it easy. For wind farm use most also have an external input for ADLS triggers, though that usually also requires a separate controller and communications connection to manage the ADLS signals. The flashing red lights are L-864 type. The requirements are 20 to 40 flashes per minute (FPM), and typically 30 FPM is used.
      - geerlingguy 11mo ago
        Hmm... maybe I could build a 1pps GPSDO based on light flashes from nearby towers, then. No need for my own GPS antenna!
        mjevans 11mo ago
        You'd probably have better timekeeping from a reasonably connected Pi with a common NTP daemon (take your pick, some are easier to configure / query), and a realtime-ish thread to emit your PPS signal on a GPIO or similar. Probably more robust than line of sight, and able to pool with other NTP servers in your home-lab (and beyond).
- askvictor 11mo ago
  My observation is not that they are sometimes synchronised, but some subset of the towers are synchronised (this was my observation in Melbourne AU). Upon asking reddit, it appears that it is the FAA-preferred option that all lights are synchronised: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_70_7460-1L_-_Obstuction_Marking_and_Lighting_-_Change_2.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...
- rts_cts 11mo ago
  Driving through a massive wind farm at night is a trip since they all blink in unison. Having them all independently would look interesting but could rapidly descend into madness: https://archive.org/details/bigclive_20230516 https://archive.org/details/bigclive_20230516
- SoftTalker 11mo ago
  I’ve driven through wind farms where the blinking tower lights are synchronized. Highly distracting.
  - bluGill 11mo ago
    If you look close - almost impossible in the dark - you will see that most wind turbines don't have any lights at all. Too many lights is a distracion but all the synchronized lights are an easy to understand 'stay away from this whole area'. There are FAA rules on this.
- deleted 11mo ago
  [deleted]
ck2 11mo ago
muuuuch better article, lol https://www.jeffgeerling.com/blog/2024/what-happens-when-you-touch-pickle-am-radio-tower https://www.jeffgeerling.com/blog/2024/what-happens-when-you...
barbazoo 11mo ago
These beacons are also great for navigation. Aeronautical charts usually show the color/pattern of the light. You can use those as points to triangulate your position.
- gorgoiler 11mo ago
  You have brought back to me memories from 30+ years ago, playing Microsoft flight simulator, trying to triangulate my position using VOR beacons as quickly as possible before my aircraft had moved so far on that I was no longer anywhere near my triangulated position, hah!
  - barbazoo 11mo ago
    I never appreciated how realistic FS was until I started studying for the navigation part of my PPL. I flew a 172 too so the instruments felt very similar.
- dylan604 11mo ago
  IIRC, light houses are marked on charts similarly. The lights have different patterns, maybe color too?? Maybe not on aeronautical charts though.
  - clort 11mo ago
    Lighthouses are usually white. They used to rotate (sweeping the beam) but tend to flash now I think. Nautical charts show the elevation of the light, the visibility ranges and the pattern of on/off (eg Fl (2) 10s 41m 17NM). Some lights are sector lights, so red/white/green depending on your position, they indicate if you are in the correct approach channel. It can be really confusing approaching a port at night. Never mind the city lights behind, and the cars driving about. 20+ lights flashing at different rates in the dark, some of them disappearing for seconds at a time. Oh, and if you turn your light on to refer to the chart, there goes your night vision! Boat masts above 25m also have blinking red lights for the low flying aircraft (I guess its a bragging point)
skinwill 11mo ago
I worked at a television station years back that was designed in such a way that the lights going up the tower were powered by the separate phases of three phase AC with the one at the top powered from all three combined. This was pretty normal but what the engineer had done was rotate them at every level so that if a phase was dropped you could count the lights and quickly see from a distance that the power wasn't right. 4 lights was good, 3 meant you dropped a phase, and so on. I thought it was a pretty clever way of keeping light on all sides of the tower while being able to tell from a distance that a phase was out.
- butlike 11mo ago
  Fascinating
  - dotancohen 11mo ago
    Phascinating I couldn't help myself, downvote at will.
    - deleted 11mo ago
      [deleted]
- xenadu02 11mo ago
  This is best practice for anyone who uses three phase power. A machine shop should connect 1/3 of their lights to each phase so it is immediately obvious if a phase gets dropped. Lots of equipment will suffer on two of three phases but with lower performance or even damage.
  - quickthrowman 11mo ago
    > This is best practice for anyone who uses three phase power. No, it’s not. It’s a neat trick that visually reveals when the utility drops a phase, but there are better ways to handle avoiding equipment damage. Best practice is to use phase monitoring relays that can de-energize a motor when a phase is dropped/reversed to prevent damage. The trip time is adjustable and it’s more reliable than manually hitting an e-stop. It also won’t let a motor with incorrect phasing start up either. You see phase loss relays on a lot of compressor motors and other large motors. Here’s a flyer for an Eaton product: https://www.eaton.com/content/dam/eaton/products/industrialcontrols-drives-automation-sensors/overload-relays/d65-monitoring-relays-pa049001en.pdf https://www.eaton.com/content/dam/eaton/products/industrialc...
ortusdux 11mo ago
Blinking the lights also helps prevent bird deaths - https://en.wikipedia.org/wiki/Towerkill https://en.wikipedia.org/wiki/Towerkill
Terr_ 11mo ago
> Joe: [...] So whenever there's a project on the tower, it's not unusual to see the guys in some kind of a, what do they call those? > Jeff: A full ghillie suit? Or I don't know what they're called. If you see someone up in a tall tower wearing a ghillie suit [0]... that sounds like time to call emergency services while avoiding their line-of-sight. :p (Perhaps they meant "Hazmat" [1]) [0] https://en.wikipedia.org/wiki/Ghillie_suit https://en.wikipedia.org/wiki/Ghillie_suit [1] https://en.wikipedia.org/wiki/Hazmat_suit https://en.wikipedia.org/wiki/Hazmat_suit
- mr_toad 11mo ago
  I can’t imagine why they’d need a hazmat suit either. It’s probably just protection from the cold.
  - fipar 11mo ago
    Seems to be needed only for old towers: > Tower painting has changed a lot over the years. The older towers have lead in them. So whenever there's a project on the tower, it's not unusual to see the guys in some kind of a, what do they call those?
- geerlingguy 11mo ago
  Haha, yes. For some reason, ghillie suit was the only thing I could think of at the time. I think they wear basic tyvek suits while doing paint work, not sure if they need full hazmat (maybe if the tower has lead paint?).
- kcplate 11mo ago
  > If you see someone up in a tall tower wearing a ghillie suit My first thought would be “that might be the dumbest sniper I have ever seen”…while I was taking cover, because even if they are dumb, they might still be a capable marksman.
  - MisterTea 11mo ago
    I imagine that a properly constructed ghillie suit covered with chain link fencing, barbed wire, no trespassing signs and a blinking red light might go undetected...
hinkley 11mo ago
Why do some radio towers NOT blink? Is the better question.
- dylan604 11mo ago
  Their height. Below a certain height, lights are not required
ofalkaed 11mo ago
In the past year or two they have also added a quick periodic flash of white light for when visibility is low; like a camera flash that happens every few seconds. I think it was added this spring but don't quite remember.
loph 11mo ago
It's not just radio towers. I've seen aircraft warning lights on tall buildings, particularly those near airports. see https://www.ecfr.gov/current/title-14/chapter-I/subchapter-E/part-77/subpart-C/section-77.17 https://www.ecfr.gov/current/title-14/chapter-I/subchapter-E...
- emmelaich 11mo ago
  mentioned in the article
lxgr 11mo ago
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)!
- scblock 11mo ago
  Radar, called Aircraft Detection Lighting Systems (ADLS). The requirements are summarized in the FAA Advisory Circular covering aviation marking and lighting.
  - lxgr 11mo ago
    As far as I understand, non-US systems don’t always use (primary) radar.
Razengan 11mo ago
To clean their eyes?
punnerud 11mo ago
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?
- deleted 11mo ago
  [deleted]
- SoftTalker 11mo ago
  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”
- teslatree 11mo ago
  In Norway this is regulated by Luftfartstilsynets BSL E 2-1, and the blinking white lights on our towers are called "hinderlys", for example category "Høyintensitet, type B". They are not uncommon in Norway. If you go to one of our major airports you will see one on the tower. The blinking lights also sit on wind turbines and TV masts, and anything taller than 15 meters in rural areas or 30 meters in populated areas will have some kind of light on it, sometimes blinking, either red or white.
WalterBright 11mo ago
Blinking uses less power.
- dylan604 11mo ago
  Blinking attracts attention which is the real purpose. I'd assume based on how we detect motion more easily, these add a bit of "motion" to attract our attention
  - WalterBright 11mo ago
    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.
    - dylan604 11mo ago
      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 blink incredibly fast, fast enough even for some slo-mo to not strobe.
      - WalterBright 11mo ago
        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.
      - CableNinja 11mo ago
        You only need about 40hz to make it seem on all the time, TVs (before 2020, maybe earlier) only did 25hz-ish. You can actually play pretty ridiculous tricks on the brain if you know this. For example, in VR, in a large enough room, you can slightly alter the angle the player sees inbetween frames. This alteration makes the brain think youre slightly offset from where it thinks you should actually be. This allows you to cause a person to walk in a circle irl, while they think they are walking in a straight line. Unfortunately only works up to a threshold of change, before the brain starts getting confused and causss the user motion sickness.
ikkun 11mo ago
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.
spc476 11mo ago
Changing a radio tower light bulb is not for the weak of heart: https://www.youtube.com/watch?v=9b9LahaBJIk https://www.youtube.com/watch?v=9b9LahaBJIk
not4uffin 11mo ago
Figured it was something to do with aircraft communications. I live near an airfield and the runway has flashing white lights at night to help guide the aircraft's.
alex_young 11mo ago
I love the narrative storytelling here, but the takeaways seem somewhat obvious. Lights on towers mean stuff, especially to airplanes. Lights are required for tall towers, and get this, towers next to airports. You can guess how tall a tower is by looking at the lights.
- themafia 11mo ago
  "My license to operate the tower is contingent upon my ability to appropriately light it."
gchokov 11mo ago
that article is incredibly hard to read. Luckily, an AI can summarise it in seconds.
LargoLasskhyfv 11mo ago
Flugwarnbefeuerung
wil421 11mo ago
At my work we are working on automating NOTAMs when our tower lights go out.
comrade1234 11mo ago
[flagged]
- deleted 11mo ago
  [deleted]
- satisfice 11mo ago
  Here is one nursing its young. https://www.howitworksdaily.com/wp-content/uploads/2016/09/Mining-Heavy-Lift-1024x680.jpg https://www.howitworksdaily.com/wp-content/uploads/2016/09/M...
- andrewflnr 11mo ago
  That's some crazy sexual dimorphism.
scblock 11mo ago
FAA details the marking and lighting requirements here: https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.current/documentnumber/70_7460-1 https://www.faa.gov/regulations_policies/advisory_circulars/...
- Johnny555 11mo ago
  One interesting fact I learned in a different discussion is that when LED lights are used for obstruction lighting, the FAA has standards that require infrared emitters to make them visible to night vision goggles, since unlike incandescent bulbs, some LED's can be invisible to NVG's. https://www.faa.gov/sites/faa.gov/files/airports/engineering/engineering_briefs/eb-98-NVG.pdf https://www.faa.gov/sites/faa.gov/files/airports/engineering...
  - wolrah 11mo ago
    That is one of those wonderful facts that's both interesting and in hindsight completely obvious.
  - ranger207 11mo ago
    Speaking of, I was recently looking into what exactly makes lights NVG compatible, and found this informative PDF: https://www.consolite.co.uk/wp-content/uploads/2017/05/Basics-of-NVG-Lighting.pdf https://www.consolite.co.uk/wp-content/uploads/2017/05/Basic... tl;dr NVGs are sensitive to near IR so you want lights that are dim in those wavelengths (but not completely out since you still want to see them with NVGs) while still being bright in visible light. There's a neat picture of the flying controll tower on one of the UK's Queen Elizabeth-class aircraft carriers that shows two rooms with similar illumination levels in visible light, but under NVGs one room which isn't NVG compatible is massively brighter than the other. In the NVG-compatible room there were still a handful of panel indicator lights that were not compatible and lit up the whole space
huflungdung 11mo ago
[dead]
pizzalife 11mo ago
This blog post has a really verbose format. TLDR; White lights are used during the daytime, red lights at night (less annoying), towers under 200 feet don't need blinking lights.
- jamesbelchamber 11mo ago
  It's a transcript of the video at the top.
  - skrebbel 11mo ago
    Wow that wasn't clear to me either, thanks for pointing it out
  - pizzalife 11mo ago
    Yes, reading transcripts is a terrible way of ingesting information in my opinion.
    - geerlingguy 11mo ago
      But at least you can scan and read through it instead of having to sit through an entire video :)
      - dylan604 11mo ago
        and thank you for doing it, at least! next, you'll be expected to turn that into an outline, index cards, and then a full term paper lest you be ridiculed for your work on the internet!
    - mikestew 11mo ago
      Then go watch the video? What are you asking for here?
      - pizzalife 11mo ago
        I'm not asking for anything. Just commenting that transcripts of videos make for bad blog posts. I'm not interested in watching the video.

179 points by warrenm 11mo ago

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?

asdefghyk 11mo ago
One option - Maybe the blinking time is set to a INDEPENDENT? accurate time piece - ie Blink on the change of a second
- anyfoo 11mo ago
  That's what DCF77 is. Or GPS.
winrid 11mo ago
It's probably just a side effect of them being powered on at the same time or not?
- anyfoo 11mo ago
  If they don't sync to a common clock source, they won't stay in sync for long. Probably not even for a few minutes or so. You'd get the same phenomenon that you see when operating turn signals in traffic. They seem to weave "in and out" of sync. The frequency at which that happens is the beat frequency, i.e. the difference between the two blinking frequencies.
  - jrockway 11mo ago
    There's got to be some rubidium frequency standard that's a drop-in replacement for a 555 timer ;)
    - deleted 11mo ago
      [deleted]
    - scblock 11mo ago
      GNSS (GPS) is the typical standard used.
    - georgefrowny 11mo ago
      A chip scale atomic clock (with a handy 1Hz output) can be yours for barely over five thousand euros and half that if you buy 250 of them. https://www.microchip.com/en-us/product/csac-sa65 https://www.microchip.com/en-us/product/csac-sa65 It's a bit larger than a 555, but it should keep you within a small handful of microseconds per day until the aging effects start to add up which make getting more than a millisecond a year dicey, even if the thing doesn't die on you: https://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resources/Gardner%202016.pdf https://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resource...
  - georgefrowny 11mo ago
    A cheapo 20ppm quartz watch crystal definitely doesn't drift by a second every few minutes, but it will over about about half a day at maximum error. A 5ppm one (still sub one Euro) will keep a second about every two days. A 0.5ppm TCXO can be had for about 15 Euros from Mouser and that gets you two weeks. If you have shared line power you can just use that and everything will be locked in sync forever. If you don't want to use that or radio, and you are outside, you could try to be really clever am sync your flash phases to a specific position of the sun. This is what the Long Now clock does. It'll be a different time each day, but it'll be the same for all units, within a small tolerance.
    - anyfoo 11mo ago
      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, you'll need to sync them at some point, and you do that by using a common clock source. So either you shell out some effort for a real solution (power line is nice, and also qualifies as a common clock source as I've predicated), or you don't. And if you don't, there's no point in using a precise-ish clock at all, and you'll likely end up with very quick desyncing.
      - georgefrowny 11mo ago
        Well yes, obviously, but I'm just being a internet pedantic about "a few minutes".
        anyfoo 11mo ago
        And you are technically correct, which is the best kind of correct anyway.
  - lxgr 11mo ago
    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.
    - anyfoo 11mo ago
      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.)
      - georgefrowny 11mo ago
        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.
        anyfoo 11mo ago
        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<n*m;i++));do b=$((l&1));l=$(((l>>1)^(b*t)));c=$(((c<<1|b)&m));[ $((i%n)) -eq 0 ] &&{xs=($c $xs[1,4]);y=$((((xs[4]-xs[0])<<30-0x3fffd60f*ys[2]+0x7d32617c*ys[1])>>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<<1|b)&m)) The hand-made filter is this part: xs=($c $xs[1,4]);y=$((((xs[4]-xs[0])<<30-0x3fffd60f*ys[2]+0x7d32617c*ys[1])>>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.*
scblock 11mo ago
When it's on purpose it's typically done through GPS driven clocks. This is how wind farms manage it, where all towers are required to blink together.
- floatrock 11mo ago
  Yeah, I've seen it with windfarms. Always wondered why do they need to blink at the same time. The scale of the blink is pretty jarring at night (but also awe-some, in the same way any big enough infrastructure project inspires a kind of awe). Wind farms have a certain amount of nimbyism because they "spoil the natural landscape." (So do regular farms -- nothing natural about grain silos or row crops, but that's a side topic...) Anyways, having that many towers blink in unison across that big a landscape is a weird effect when you first see it. I think there's an argument that if they blinked independently it would feel more natural in a way. But since the blinking is all FAA requirements, I assume it's to help identify all the individual towers from the air. I suppose if they were all blinking independently, it would be a predator-trying-to-focus-on-a-single-zebra-in-the-herd problem, except in this case the predator is a pilot trying not to crash into a turbine. Sure would emit more subtle 'part of the landscape' vibes though. (Which I guess is exactly what you don't want when you're flying above them. Sigh.)
  - evertedsphere 11mo ago
    > Always wondered why do they need to blink at the same time. presumably this makes it more striking, and thus easier to notice and avoid
  - scblock 11mo ago
    It's so pilots see the entire wind farm as a single entity and can interpret what they see and understand the extent of the wind farm easily. There is a pretty good study you can read on this: https://www.airporttech.tc.faa.gov/DesktopModules/EasyDNNNews/DocumentDownload.ashx?portalid=0&moduleid=3682&articleid=88&documentid=115 https://www.airporttech.tc.faa.gov/DesktopModules/EasyDNNNew... As to community impact, radar-activated lighting is an approach that is being used in places this is a concern. It allows the lights to remain off unless there is a plane within the envelope that requires the lights to activate. It's expensive though.
daemonologist 11mo ago
I believe it's usually GPS/GNS (they all receive the time via GPS independently, and flash at predetermined times). The FAA requires synchronization for many classes of obstruction because it makes it clear that you're looking at obstruction lights rather than e.g. brake lights or traffic lights on the ground.
- geerlingguy 11mo ago
  Could they also use power grid sync? Not sure as I haven't talked to anyone in wind power, but grid sync would be pretty close to 1 Hz at least in the US. Building a product that would sync at 1 Hz via GPS that works in the US and other countries with 50 Hz power would be a little easier than syncing to grid phase though.
  - scblock 11mo ago
    Definitely GPS. Other methods have been used in the past--I remember reading operating reports from a wind farm nearly 20 years ago that slowly brought all its lights in line with each other over several months--but these days you can buy mainstream lighting with the GNSS receiver built in from a number of suppliers. They make it easy. For wind farm use most also have an external input for ADLS triggers, though that usually also requires a separate controller and communications connection to manage the ADLS signals. The flashing red lights are L-864 type. The requirements are 20 to 40 flashes per minute (FPM), and typically 30 FPM is used.
    - geerlingguy 11mo ago
      Hmm... maybe I could build a 1pps GPSDO based on light flashes from nearby towers, then. No need for my own GPS antenna!
      - mjevans 11mo ago
        You'd probably have better timekeeping from a reasonably connected Pi with a common NTP daemon (take your pick, some are easier to configure / query), and a realtime-ish thread to emit your PPS signal on a GPIO or similar. Probably more robust than line of sight, and able to pool with other NTP servers in your home-lab (and beyond).
askvictor 11mo ago
My observation is not that they are sometimes synchronised, but some subset of the towers are synchronised (this was my observation in Melbourne AU). Upon asking reddit, it appears that it is the FAA-preferred option that all lights are synchronised: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_70_7460-1L_-_Obstuction_Marking_and_Lighting_-_Change_2.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...
rts_cts 11mo ago
Driving through a massive wind farm at night is a trip since they all blink in unison. Having them all independently would look interesting but could rapidly descend into madness: https://archive.org/details/bigclive_20230516 https://archive.org/details/bigclive_20230516
SoftTalker 11mo ago
I’ve driven through wind farms where the blinking tower lights are synchronized. Highly distracting.
- bluGill 11mo ago
  If you look close - almost impossible in the dark - you will see that most wind turbines don't have any lights at all. Too many lights is a distracion but all the synchronized lights are an easy to understand 'stay away from this whole area'. There are FAA rules on this.
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ck2 11mo ago

muuuuch better article, lol https://www.jeffgeerling.com/blog/2024/what-happens-when-you-touch-pickle-am-radio-tower https://www.jeffgeerling.com/blog/2024/what-happens-when-you...

barbazoo 11mo ago

These beacons are also great for navigation. Aeronautical charts usually show the color/pattern of the light. You can use those as points to triangulate your position.

gorgoiler 11mo ago
You have brought back to me memories from 30+ years ago, playing Microsoft flight simulator, trying to triangulate my position using VOR beacons as quickly as possible before my aircraft had moved so far on that I was no longer anywhere near my triangulated position, hah!
- barbazoo 11mo ago
  I never appreciated how realistic FS was until I started studying for the navigation part of my PPL. I flew a 172 too so the instruments felt very similar.
dylan604 11mo ago
IIRC, light houses are marked on charts similarly. The lights have different patterns, maybe color too?? Maybe not on aeronautical charts though.
- clort 11mo ago
  Lighthouses are usually white. They used to rotate (sweeping the beam) but tend to flash now I think. Nautical charts show the elevation of the light, the visibility ranges and the pattern of on/off (eg Fl (2) 10s 41m 17NM). Some lights are sector lights, so red/white/green depending on your position, they indicate if you are in the correct approach channel. It can be really confusing approaching a port at night. Never mind the city lights behind, and the cars driving about. 20+ lights flashing at different rates in the dark, some of them disappearing for seconds at a time. Oh, and if you turn your light on to refer to the chart, there goes your night vision! Boat masts above 25m also have blinking red lights for the low flying aircraft (I guess its a bragging point)

skinwill 11mo ago

I worked at a television station years back that was designed in such a way that the lights going up the tower were powered by the separate phases of three phase AC with the one at the top powered from all three combined. This was pretty normal but what the engineer had done was rotate them at every level so that if a phase was dropped you could count the lights and quickly see from a distance that the power wasn't right. 4 lights was good, 3 meant you dropped a phase, and so on. I thought it was a pretty clever way of keeping light on all sides of the tower while being able to tell from a distance that a phase was out.

butlike 11mo ago
Fascinating
- dotancohen 11mo ago
  Phascinating I couldn't help myself, downvote at will.
  - deleted 11mo ago
    [deleted]
xenadu02 11mo ago
This is best practice for anyone who uses three phase power. A machine shop should connect 1/3 of their lights to each phase so it is immediately obvious if a phase gets dropped. Lots of equipment will suffer on two of three phases but with lower performance or even damage.
- quickthrowman 11mo ago
  > This is best practice for anyone who uses three phase power. No, it’s not. It’s a neat trick that visually reveals when the utility drops a phase, but there are better ways to handle avoiding equipment damage. Best practice is to use phase monitoring relays that can de-energize a motor when a phase is dropped/reversed to prevent damage. The trip time is adjustable and it’s more reliable than manually hitting an e-stop. It also won’t let a motor with incorrect phasing start up either. You see phase loss relays on a lot of compressor motors and other large motors. Here’s a flyer for an Eaton product: https://www.eaton.com/content/dam/eaton/products/industrialcontrols-drives-automation-sensors/overload-relays/d65-monitoring-relays-pa049001en.pdf https://www.eaton.com/content/dam/eaton/products/industrialc...

ortusdux 11mo ago

Blinking the lights also helps prevent bird deaths - https://en.wikipedia.org/wiki/Towerkill https://en.wikipedia.org/wiki/Towerkill

Terr_ 11mo ago

> Joe: [...] So whenever there's a project on the tower, it's not unusual to see the guys in some kind of a, what do they call those? > Jeff: A full ghillie suit? Or I don't know what they're called. If you see someone up in a tall tower wearing a ghillie suit [0]... that sounds like time to call emergency services while avoiding their line-of-sight. :p (Perhaps they meant "Hazmat" [1]) [0] https://en.wikipedia.org/wiki/Ghillie_suit https://en.wikipedia.org/wiki/Ghillie_suit [1] https://en.wikipedia.org/wiki/Hazmat_suit https://en.wikipedia.org/wiki/Hazmat_suit

mr_toad 11mo ago
I can’t imagine why they’d need a hazmat suit either. It’s probably just protection from the cold.
- fipar 11mo ago
  Seems to be needed only for old towers: > Tower painting has changed a lot over the years. The older towers have lead in them. So whenever there's a project on the tower, it's not unusual to see the guys in some kind of a, what do they call those?
geerlingguy 11mo ago
Haha, yes. For some reason, ghillie suit was the only thing I could think of at the time. I think they wear basic tyvek suits while doing paint work, not sure if they need full hazmat (maybe if the tower has lead paint?).
kcplate 11mo ago
> If you see someone up in a tall tower wearing a ghillie suit My first thought would be “that might be the dumbest sniper I have ever seen”…while I was taking cover, because even if they are dumb, they might still be a capable marksman.
- MisterTea 11mo ago
  I imagine that a properly constructed ghillie suit covered with chain link fencing, barbed wire, no trespassing signs and a blinking red light might go undetected...

hinkley 11mo ago

Why do some radio towers NOT blink? Is the better question.

dylan604 11mo ago
Their height. Below a certain height, lights are not required

ofalkaed 11mo ago

In the past year or two they have also added a quick periodic flash of white light for when visibility is low; like a camera flash that happens every few seconds. I think it was added this spring but don't quite remember.

loph 11mo ago

It's not just radio towers. I've seen aircraft warning lights on tall buildings, particularly those near airports. see https://www.ecfr.gov/current/title-14/chapter-I/subchapter-E/part-77/subpart-C/section-77.17 https://www.ecfr.gov/current/title-14/chapter-I/subchapter-E...

emmelaich 11mo ago
mentioned in the article

lxgr 11mo ago

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)!

scblock 11mo ago
Radar, called Aircraft Detection Lighting Systems (ADLS). The requirements are summarized in the FAA Advisory Circular covering aviation marking and lighting.
- lxgr 11mo ago
  As far as I understand, non-US systems don’t always use (primary) radar.

Razengan 11mo ago

To clean their eyes?

punnerud 11mo ago

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?

deleted 11mo ago
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SoftTalker 11mo ago
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”
teslatree 11mo ago
In Norway this is regulated by Luftfartstilsynets BSL E 2-1, and the blinking white lights on our towers are called "hinderlys", for example category "Høyintensitet, type B". They are not uncommon in Norway. If you go to one of our major airports you will see one on the tower. The blinking lights also sit on wind turbines and TV masts, and anything taller than 15 meters in rural areas or 30 meters in populated areas will have some kind of light on it, sometimes blinking, either red or white.

WalterBright 11mo ago

Blinking uses less power.

dylan604 11mo ago
Blinking attracts attention which is the real purpose. I'd assume based on how we detect motion more easily, these add a bit of "motion" to attract our attention
- WalterBright 11mo ago
  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.
  - dylan604 11mo ago
    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 blink incredibly fast, fast enough even for some slo-mo to not strobe.
    - WalterBright 11mo ago
      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.
    - CableNinja 11mo ago
      You only need about 40hz to make it seem on all the time, TVs (before 2020, maybe earlier) only did 25hz-ish. You can actually play pretty ridiculous tricks on the brain if you know this. For example, in VR, in a large enough room, you can slightly alter the angle the player sees inbetween frames. This alteration makes the brain think youre slightly offset from where it thinks you should actually be. This allows you to cause a person to walk in a circle irl, while they think they are walking in a straight line. Unfortunately only works up to a threshold of change, before the brain starts getting confused and causss the user motion sickness.

ikkun 11mo ago

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.

spc476 11mo ago

Changing a radio tower light bulb is not for the weak of heart: https://www.youtube.com/watch?v=9b9LahaBJIk https://www.youtube.com/watch?v=9b9LahaBJIk

not4uffin 11mo ago

Figured it was something to do with aircraft communications. I live near an airfield and the runway has flashing white lights at night to help guide the aircraft's.

alex_young 11mo ago

I love the narrative storytelling here, but the takeaways seem somewhat obvious. Lights on towers mean stuff, especially to airplanes. Lights are required for tall towers, and get this, towers next to airports. You can guess how tall a tower is by looking at the lights.

themafia 11mo ago
"My license to operate the tower is contingent upon my ability to appropriately light it."

gchokov 11mo ago

that article is incredibly hard to read. Luckily, an AI can summarise it in seconds.

LargoLasskhyfv 11mo ago

Flugwarnbefeuerung

wil421 11mo ago

At my work we are working on automating NOTAMs when our tower lights go out.

comrade1234 11mo ago

[flagged]

deleted 11mo ago
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satisfice 11mo ago
Here is one nursing its young. https://www.howitworksdaily.com/wp-content/uploads/2016/09/Mining-Heavy-Lift-1024x680.jpg https://www.howitworksdaily.com/wp-content/uploads/2016/09/M...
andrewflnr 11mo ago
That's some crazy sexual dimorphism.

scblock 11mo ago

FAA details the marking and lighting requirements here: https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.current/documentnumber/70_7460-1 https://www.faa.gov/regulations_policies/advisory_circulars/...

Johnny555 11mo ago
One interesting fact I learned in a different discussion is that when LED lights are used for obstruction lighting, the FAA has standards that require infrared emitters to make them visible to night vision goggles, since unlike incandescent bulbs, some LED's can be invisible to NVG's. https://www.faa.gov/sites/faa.gov/files/airports/engineering/engineering_briefs/eb-98-NVG.pdf https://www.faa.gov/sites/faa.gov/files/airports/engineering...
- wolrah 11mo ago
  That is one of those wonderful facts that's both interesting and in hindsight completely obvious.
- ranger207 11mo ago
  Speaking of, I was recently looking into what exactly makes lights NVG compatible, and found this informative PDF: https://www.consolite.co.uk/wp-content/uploads/2017/05/Basics-of-NVG-Lighting.pdf https://www.consolite.co.uk/wp-content/uploads/2017/05/Basic... tl;dr NVGs are sensitive to near IR so you want lights that are dim in those wavelengths (but not completely out since you still want to see them with NVGs) while still being bright in visible light. There's a neat picture of the flying controll tower on one of the UK's Queen Elizabeth-class aircraft carriers that shows two rooms with similar illumination levels in visible light, but under NVGs one room which isn't NVG compatible is massively brighter than the other. In the NVG-compatible room there were still a handful of panel indicator lights that were not compatible and lit up the whole space

huflungdung 11mo ago

[dead]

pizzalife 11mo ago

This blog post has a really verbose format. TLDR; White lights are used during the daytime, red lights at night (less annoying), towers under 200 feet don't need blinking lights.

jamesbelchamber 11mo ago
It's a transcript of the video at the top.
- skrebbel 11mo ago
  Wow that wasn't clear to me either, thanks for pointing it out
- pizzalife 11mo ago
  Yes, reading transcripts is a terrible way of ingesting information in my opinion.
  - geerlingguy 11mo ago
    But at least you can scan and read through it instead of having to sit through an entire video :)
    - dylan604 11mo ago
      and thank you for doing it, at least! next, you'll be expected to turn that into an outline, index cards, and then a full term paper lest you be ridiculed for your work on the internet!
  - mikestew 11mo ago
    Then go watch the video? What are you asking for here?
    - pizzalife 11mo ago
      I'm not asking for anything. Just commenting that transcripts of videos make for bad blog posts. I'm not interested in watching the video.