[HN Gopher] Why do some radio towers blink?
___________________________________________________________________
Why do some radio towers blink?
Author : warrenm
Score : 81 points
Date : 2025-10-28 19:38 UTC (3 hours ago)
(HTM) web link (www.jeffgeerling.com)
(TXT) w3m dump (www.jeffgeerling.com)
| scblock wrote:
| FAA details the marking and lighting requirements here:
| https://www.faa.gov/regulations_policies/advisory_circulars/...
| Johnny555 wrote:
| 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...
| pizzalife wrote:
| 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 wrote:
| It's a transcript of the video at the top.
| skrebbel wrote:
| Wow that wasn't clear to me either, thanks for pointing it
| out
| pizzalife wrote:
| Yes, reading transcripts is a terrible way of ingesting
| information in my opinion.
| geerlingguy wrote:
| But at least you can scan and read through it instead of
| having to sit through an entire video :)
| dylan604 wrote:
| 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!
| rollulus wrote:
| 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 wrote:
| One option - Maybe the blinking time is set to a INDEPENDENT?
| accurate time piece - ie Blink on the change of a second
| anyfoo wrote:
| That's what DCF77 is. Or GPS.
| winrid wrote:
| It's probably just a side effect of them being powered on at
| the same time or not?
| anyfoo wrote:
| 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 wrote:
| There's got to be some rubidium frequency standard that's a
| drop-in replacement for a 555 timer ;)
| scblock wrote:
| GNSS (GPS) is the typical standard used.
| georgefrowny wrote:
| 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
|
| 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_Eu
| roOBS_workshop/Resource...
| georgefrowny wrote:
| 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 wrote:
| 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 wrote:
| Well yes, obviously, but I'm just being a internet
| pedantic about "a few minutes".
| anyfoo wrote:
| And you are technically correct, which is the best kind
| of correct anyway.
| lxgr wrote:
| 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 wrote:
| 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.)
| scblock wrote:
| 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 wrote:
| 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 wrote:
| > 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 wrote:
| 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/EasyDNNNe
| w...
|
| 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.
| mulmen wrote:
| Does it have to be radar? Can it use ADS-B?
| scblock wrote:
| The FAA document says "sensor-based" but every
| installation I have seen in the US uses radar.
| daemonologist wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| Hmm... maybe I could build a 1pps GPSDO based on light
| flashes from nearby towers, then. No need for my own GPS
| antenna!
| askvictor wrote:
| 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/...
| rts_cts wrote:
| 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
| ck2 wrote:
| muuuuch better article, lol
|
| https://www.jeffgeerling.com/blog/2024/what-happens-when-you...
| barbazoo wrote:
| 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 wrote:
| 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!
| dylan604 wrote:
| IIRC, light houses are marked on charts similarly. The lights
| have different patterns, maybe color too?? Maybe not on
| aeronautical charts though.
| skinwill wrote:
| 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 wrote:
| Fascinating
| dotancohen wrote:
| Phascinating
|
| I couldn't help myself, downvote at will.
| xenadu02 wrote:
| 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 wrote:
| > 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/co
| ntent/dam/eaton/products/industrialc...
| geerlingguy wrote:
| Clever! I know I talked to the folks at Masterclock in St.
| Louis recently about one of their clock displays; they
| intentionally default the separators to flash if the clock is
| not synced to NTP, and then they go solid once the connection
| is established.
|
| It's a quick way to know if something is down, using context
| clues that are already there to begin with!
| ortusdux wrote:
| Blinking the lights also helps prevent bird deaths -
| https://en.wikipedia.org/wiki/Towerkill
| Terr_ wrote:
| > 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
|
| [1] https://en.wikipedia.org/wiki/Hazmat_suit
| mr_toad wrote:
| I can't imagine why they'd need a hazmat suit either. It's
| probably just protection from the cold.
| geerlingguy wrote:
| 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?).
| hinkley wrote:
| Why do some radio towers NOT blink? Is the better question.
| dylan604 wrote:
| Their height. Below a certain height, lights are not required
| ofalkaed wrote:
| 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 wrote:
| 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...
| lxgr wrote:
| 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 wrote:
| Radar, called Aircraft Detection Lighting Systems (ADLS). The
| requirements are summarized in the FAA Advisory Circular
| covering aviation marking and lighting.
| Razengan wrote:
| To clean their eyes?
| punnerud wrote:
| 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?
| WalterBright wrote:
| Blinking uses less power.
| dylan604 wrote:
| 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 wrote:
| 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 wrote:
| 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.
___________________________________________________________________
(page generated 2025-10-28 23:00 UTC)