[HN Gopher] No GPS required: our app can now locate underground ...
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No GPS required: our app can now locate underground trains
Author : dotcoma
Score : 892 points
Date : 2024-11-13 01:46 UTC (21 hours ago)
(HTM) web link (blog.transitapp.com)
(TXT) w3m dump (blog.transitapp.com)
| mdergosits wrote:
| Some cities will put BLE beacons in tunnels that transmit the
| location and then you can find your location via the strongest
| beacon signal. This seems like a nice way to figure that out
| without installing hardware!
| IshKebab wrote:
| Some trains even have modern technology called "screens" that
| display their current location, next stop, route and so on!
|
| (Although in fairness they so often get this laughably wrong,
| and cycle through useless "remember your luggage" style
| messages so you have to wait like 20s to see the information
| you want - critically bad on a train.)
| the_mitsuhiko wrote:
| I really don't understand why not every subway shows the next
| station on the screen instead of warning messages and other
| stuff. (And a marker for side the doors will open)
| dietr1ch wrote:
| I don't get why ads are prioritised over travel
| information.
| eythian wrote:
| For at least months in my city, the screens in buses have
| been showing a "broken image" icon as part of their ad
| cycle. I'd much rather know about the next stop.
| kdmccormick wrote:
| It's very simple: Transit in many places is underfunded.
| Travel info screens cost money to install and maintain.
| Ads, on the other hand, make money.
| bluGill wrote:
| Ads make money, but not that much. I'm not convinced they
| are enough to be worth the bother. To have ads you need
| to pay people in sales to sell them, people to install
| them (now that this is electronic it is easier than the
| old paper days, but you need a more expensive tech person
| to keep them working). At best they are 5% of your gross
| budget, so not very significant and they often hard
| harmful to your riders.
| rtkwe wrote:
| Transit in the US seems to be constantly on a starvation
| diet budgetarily speaking so I'm not shocked small sums
| of money get chosen over options that would be better for
| their users.
| rangestransform wrote:
| Starvation diet? You should see how much NYC MTA spends,
| it's a feast compared to even the rest of the anglosphere
| rtkwe wrote:
| The MTA is a huge outlier but still gets less money for
| upgrades and upkeep than they need. They limped along for
| years on extremely outdated train tracking infrastructure
| and are just getting around to doing updates that needed
| to be started two decades ago.
| rangestransform wrote:
| They get way more than they need, it's just burnt on
| unions, pensions, and consultants
|
| I will literally volunteer my time to canvass for anyone
| who runs on busting NYC public sector unions
| akadruid1 wrote:
| 5% of a transit authority's budget would be a lot of
| money! Advertising made PS158 million for Transport for
| London in 2019, much less than 5% of their budget.
| bigfatkitten wrote:
| What often happen is a a company comes into install and
| maintain the displays for "free", on the proviso that
| they can display ads and keep most/all of the ad money.
| dietr1ch wrote:
| Ads just make money out of people, why not cut off the
| middleman?
| IshKebab wrote:
| I assume it's because the person deciding what goes on the
| screens is some manager somewhere. They probably just don't
| really thing about it.
| IncreasePosts wrote:
| Also doesn't help travelers who aren't too familiar with
| English - I always wonder how many NY-bound tourists have
| stepped off the train at "Newark Penn Station".
| SoftTalker wrote:
| Even better, they have a thing called a "window" which is
| transparent and you can look out on the platform and see the
| sign posting the station name.
| spaceywilly wrote:
| I take the train every day in NYC. Around half the time the
| train is brand new and has nice displays that even show you
| where the stairs are located in each station as you pull up.
| The other half the train is from the 1960s and has no
| information whatsoever. So yeah this app will definitely be
| an upgrade over peering out the window to read the station
| names. Even the trains with displays are occasionally
| wrong/not working.
| scottbez1 wrote:
| This is super cool! As a regular BART commuter I always thought
| it'd be fun to try and build a location classifier based on the
| screeches in different tunnel locations, but using accelerometer
| data is probably much more practical.
| sitkack wrote:
| Tune the screeches so they make a different chord for each
| stop.
| Lance_ET_Compte wrote:
| I have no hearing in any of the frequencies BART trains emit.
| Burned away...
|
| I do love this "Transit" app though.
| sitkack wrote:
| I am sorry for your loss, but does this explain dubstep?
| ant6n wrote:
| Using the microphone as a sensor is a bit of a no-go a transit
| app. You could just record the noise of the train rolling to
| find out the movement, but your users will suspect you are
| listening to them.
| isaacfrond wrote:
| Anybody know what model architectures they used?
|
| The motion detection might be a convolutional network or an svm.
| The mixer model perhaps a classic neural network.
| iAkashPaul wrote:
| Could be that or an RNN/LSTM as well
| mjamesaustin wrote:
| This is one of the best apps I've ever used, and I'm excited to
| see it's only getting better!
| fullofstack wrote:
| Is GPS spoofing an option, underground? Would instantly work on
| all phones.
| dietr1ch wrote:
| Is it spoofing if you are telling the truth? Just pretend to be
| a GPS satellite during its morning commute.
| szszrk wrote:
| Well, you are not a GPS satellite, so you are indeed spoofing
| military equipment.
|
| On a serious note, I recall that smartphone location in metro
| in my city started to "just work" as soon as all stations and
| tunnels had indoor cell towers. Suddenly all apps worked fine
| and I forgot that problem ever existed.
|
| Modern location detection is as scary as it is amazing.
| bluGill wrote:
| Cell towers also transmit location data
| _flux wrote:
| You'd need to spoof at least three satellites to get a fix,
| right? And then you'd need to spoof different signals for
| different regions of the subway, because your signal sources
| aren't really in the orbit.
|
| Sounds to me this could be very complicated and expensive. I
| wonder if it would even be possible because you'd need to
| have the same signal spoof the correct positions to everyone
| who hears that signal.
| dietr1ch wrote:
| No, you need 3 satellites because they are far enough that
| their position and time gives you too much uncertainty.
|
| If the entire sphere where I could be fits within 5m I
| guess I don't need other satellites and their time to start
| intersecting spheres.
| _flux wrote:
| I don't understand this. As far as I know, the satellite
| basically sends its identifier and precise time, and from
| this information (combined with the information in GPS
| calendar that tells the locations of the satellites) the
| recipient can determines its own location.
|
| How could it be possible to determine the location from a
| single timestamp and information about which satellite it
| belongs to? I suppose if the recipient already has a fix,
| then it could perhaps survive with less than 3 satellites
| by making some assumptions, but I imagine this will
| result in lower quality location information.
|
| Were you proposing to assume the location of a 5m sphere
| the recipient is in?
| bluGill wrote:
| The satellite also sends information about all the other
| satellites that a receiver should be able to see. This is
| what makes adding other transmitters hard/impossible I
| guess - no way to tell the overhead satellites that your
| ground station also exists (there would presumably be
| thousands of them - most not in range of your receiver
| which would be even more problems.)
| dietr1ch wrote:
| > I don't understand this. As far as I know, the
| satellite basically sends its identifier and precise
| time, and from this information (combined with the
| information in GPS calendar that tells the locations of
| the satellites) the recipient can determines its own
| location.
|
| I'd assumed the GPS calendar was somehow broadcasted by
| the GPS network too, which kind of means that they also
| share their location.
|
| > Were you proposing to assume the location of a 5m
| sphere the recipient is in?
|
| I guess the proposal was to change the problem from
| pinpoint a single point in space, to figure out roughly
| where I am, in which case being anywhere in a tiny sphere
| is pretty much the same as being in a point after you
| account for errors.
| jillesvangurp wrote:
| No need; phones use sensor fusion these days. Wifi, ble, 4G/5G
| base stations, signals, etc. it's all taken into account. Which
| is why you can get accurate position in a lot of places where
| you definitely don't have any line of sight to any GPS
| satellites. Some of the more recent wifi standards also have
| some positioning features.
| cryptoz wrote:
| Early Android dev had a guide on how devs could pick the best
| location from various sources individually, and it was
| massive pain with dubious results.
| qwertox wrote:
| Having the train use Bluetooth Low Energy advertising to
| broadcast its location and some other data, like time-to-next
| station, next station name, would be a nice feature.
|
| GPS spoofing should not be done in my opinion until the
| negative side-effects are well understood.
| flemhans wrote:
| Ona similar note I always thought it would be cool if there
| was a standard allowing (trusted) WiFi access points to relay
| location data, so that in-flight WiFi could pass on the
| plane's GPS feed.
| h1fra wrote:
| This is the case in some stations in Paris. To retrofit
| ancient trains with station announcements, they have
| installed Bluetooth broadcasters on each station so the train
| can detect when it enters/leaves and announce the next
| station for visually impaired people. Smart and simple imo.
|
| https://www.leparisien.fr/info-paris-ile-de-france-
| oise/tran...
| immibis wrote:
| Not balises?
| gield wrote:
| Waze Beacons [1] do this for car tunnels. Some underground
| systems, e.g. in Barcelona IIRC, also use BLE beacons to help
| phones position themselves.
|
| [1] https://support.google.com/waze/partners/answer/9416071
| RobotToaster wrote:
| Would it be necessary? I thought there was a standard for
| ground based augmentation of GPS?
| dietr1ch wrote:
| This is really cool.
|
| Sort of related, what I find kind of silly, is that my car
| probably knows really well where I am, but can't help my phone
| figure out where my car is pointing to despite being connected to
| it.
| MaxikCZ wrote:
| Thats still surprisingly unsolved problem. I was daydreaming
| about pulling my laptop out somewhere without known wifi and it
| automatically using my phone as a hotspot, without going trough
| manual process of enabling said hotspot. Somehow we connected
| the world but forgot to connect out devices..
| vineyardmike wrote:
| This has almost been a feature for Macs+iPhones for like a
| decade. You need to manually "pick a wifi network" but it'll
| turn on your phone's hotspot if you pick it from the list.
| pjerem wrote:
| Never tried on my Mac, but on iPad, it's totally automatic
| : the iPad automatically tells the iPhone to enable the
| hotspot (over Bluetooth I presume) then connects to it.
| wisenull wrote:
| Is the hotspot truly off? I am asking because it was really
| common to see Whoever's iPhone whenever you were picking a
| wifi network.
| astrange wrote:
| The hotspot uses a lot of power, so it really is off when
| it's not being used.
| klausa wrote:
| My understanding of CarPlay is that it does offload GPS to the
| car (when available, not all units/manufacturers might support
| it).
| fragmede wrote:
| It does! Unfortunately for me, I rented this one Ford
| Explorer where the car's GPS was off by a mile, so that was
| the opposite of helpful, and I didn't find a way to turn that
| off in the short time I had it.
| longtimelistnr wrote:
| This is actually a problem for my Volkswagen Tiguan, the car
| GPS data is wildly inaccurate probably ~30% of the time. The
| only fix is force it to use wired CarPlay or turn of CarPlay
| and let my map grab my phone's data before reconnecting.
| porphyra wrote:
| This is one thing that is nice about using the Tesla in-car
| navigation. It always knows which way the car is pointed.
| __MatrixMan__ wrote:
| Oh thank goodness. I knew I buried a train _somewhere_ around
| here but I just can 't find it.
| aziaziazi wrote:
| Lets be the bad guy today:
|
| https://news.ycombinator.com/item?id=7695956
| ThePowerOfFuet wrote:
| That says "Please don't post substanceless one-liners here."
|
| The joke was _two_ sentences, and I, for one, appreciated it.
|
| If you're going to quote regulations, make sure to bring a
| gun to a gun fight!
| aziaziazi wrote:
| Well I think "one-liner" here adds more weight to
| "substanceless" and was not indented to be taken literally:
|
| - it has no useful meaning in a media you can resize.
|
| - adding dots (.) does not noticeably change the meaning,
| you could replace with a comma : "Oh thanks goodness, I
| new...", allowing _cheating_ on the _fight rules_. I don't
| think that's desirable way we want to interact as a HN-
| user-community (personal opinion)
|
| - taking the literal read anyway, I see one line and two
| sentences right now. It's a one liner. (not English native,
| may I missed a secondary meaning?)
|
| Dang clarify it bellow the post in linked and cite scott_s,
| none of which talking about the length of the joke but both
| referring to noise.
|
| I personally also found it funny BUT I also saw like 10
| substanceless not-so-funny jokes today on HN. INHM problem
| is not the joke itself but the emptiness of the post if you
| take the joke aside.
| blitzar wrote:
| Be kind. Don't be snarky.
|
| Please respond to the strongest plausible interpretation of
| what someone says, not a weaker one that's easier to
| criticize. Assume good faith.
|
| Please don't post shallow dismissals, especially of other
| people's work. A good critical comment teaches us something.
|
| Please don't complain about tangential annoyances--e.g.
| article or website formats, name collisions, or back-button
| breakage. They're too common to be interesting.
|
| https://news.ycombinator.com/newsguidelines.html
| aziaziazi wrote:
| Thanks for sharing those other guidelines.
|
| Some could feel my comment escape them from it's tone and
| it will be wonderful if you or someone else share a better
| way to say what I said. I'm doing my best to not hurt
| others, which sometimes is seen as coldness. Which I admit
| is not kind.
| blitzar wrote:
| I can offer no better words for you, however can offer
| advice which I myself shall no heed.
|
| Vote accordingly and proceed onward. If collectively a
| negligible fraction also do the same then the results
| might just surprise. Ostracism is a powerful tool, while
| not always used for good, it does serve a purpose in
| society.
| aziaziazi wrote:
| Thanks, that is an idea I didn't have and worth
| considering. Exclusion traumatized me in the past and
| that still haunts everyday. I didn't thought it can be
| used for good.
| aeternum wrote:
| Why does the acceleration graph not take direction into account?
| Seems like you could get much cleaner data if you looked only at
| the directions orthogonal to gravity.
| kaonwarb wrote:
| They're using Fourier transforms to focus on frequencies of
| acceleration associated with subway travel; gravity should
| mostly fall out as an independent factor.
| nickmooney wrote:
| Is there always a great reference for which way down is, given
| that the phone can be in various orientations? You might have
| to do some other heuristics / build some other model for that
| to figure out which axis to subtract your 9.8 m/s^2 from -
| maybe it's easier not to worry about it.
| zokier wrote:
| Isn't merging gyro data with accelerometer data pretty
| standard approach?
| grav wrote:
| Very impressive. Also interesting that the motion type prediction
| ("is this a moving train?") generalizes to underground trains in
| other cities.
|
| I see they support the the largest cities in Sweden and Norway,
| wonder if there are any plans for Copenhagen, Denmark?
| dotcoma wrote:
| Let them know you'd like CPH !
|
| Especially if you know someone at the transit agency or can
| help them even in a very small way.
| O-stevns wrote:
| While I understand your question is about the Transit app in
| general, I'd just like to mention in correlation to the article
| that my team and I worked with one of the public transport
| operators in Denmark, to utilize the motion predictability
| feature found in Android and iOS SDK's, so I can enlighten you
| with some details regarding that.
|
| Our conclusion was that the feature didn't work in the danish
| metros for reasons we never got to deep dive into. It's most
| likely related to the fact that many of the metro stations are
| built in concrete, as such there's no GPS data in most of them
| unless you're very close to or on the surface and no motion
| data.
|
| I'd be surprised if they got this particular feature working
| but who knows... maybe if we had looked into the raw sensor
| output we might have been able to work something out.
|
| In the end we made a solution to help determine when you're
| moving or not by utilizing beacons.
| modeless wrote:
| Seems like you could do a better job of tracking progress between
| stations by detecting the specific acceleration signature of each
| segment of track. At least for tracks that are not completely
| straight and level. Somewhat similar to the first primitive in-
| car navigation systems before GPS, that worked on dead reckoning
| with drift correction by matching the shape of the measured path
| to map data.
| fire_lake wrote:
| Shazam for train tracks
| modeless wrote:
| Now accepting funding offers at $10m post-money valuation cap
| blitzar wrote:
| Ai Powered Shazam for train tracks.
|
| Now accepting funding offers at $500m post-money valuation
| cap.
| rvnx wrote:
| Then in 5 years: "Turns out this was marketing for
| investors and that our classifier was using 99% of the
| time last location and itinerary as primary factor to
| determine location"
|
| The magic formula: "if is_train_moving, countdown to next
| station"
| nuccy wrote:
| Acceleration signature depends on:
|
| - human driver
|
| - train capacity and current load
|
| - train model (or any powertrain variations)
|
| More reliable might be to check rail track features from
| accelerometer: tilt, turns, bumps, or a combinations of
| everything. Even sounds on turns, changes of backround during
| merging tunnels, etc. Integrated acceleration gives train
| speed, which is also useful along with other inputs.
| modeless wrote:
| Yes, tilt, turns, and bumps are what I meant by
| accelerations, not just the train's forward acceleration.
| sakjur wrote:
| If you're not putting the phone on the floor, wouldn't the
| person holding it introduce a randomness factor that would
| make it much harder to reliably measure those? And then
| there's sun curves, packed leaves, ice and the cars'
| suspension.
|
| I think I'd struggle with finding a signal through all that
| noise.
| Normal_gaussian wrote:
| Ive seen two seperate demos of doing this for the london
| underground based purely on the accelerometer.
|
| Accuracy is poor when blind, but if you have any info
| about where the user last had a location fix it became
| very good.
|
| Neither company found sufficient interest to deploy the
| sensor.
| TeMPOraL wrote:
| Contribution from the phone being held should be easy to
| cancel out for the most part, as high frequency noise. It
| of course gets much easier if you have acceleration data
| from two or more devices held by different people.
| sofixa wrote:
| > human driver
|
| Not in any even remotely modern system that will have a high
| degree of automation and if there's a driver, they're only
| closing and opening doors, and telling the train to get
| going.
| AndrewSwift wrote:
| I just took a one day initiation with the SNCF for driving
| trains in France.
|
| I assumed that everything would be automated but in fact
| nothing is.
|
| They explained that they regularly face situations that
| require human intervention (several times a day), and that
| even a little automation would reduce the level of
| attention on the part of the conductor.
|
| The conductor is required to interact with the controls at
| least every 30 seconds to avoid setting off alarms.
| maeln wrote:
| The SNCF is for regional and national train (with the
| exception of the RER) where the possibility of having a
| cow coming on the rail is non-negligible ... or a hot air
| ballon landing on the rail and stopping the Nantes-Paris
| lines for a whole morning :D .
|
| So I think it is a much, much harder environment to
| automate. Paris do have some metro line that are fully
| automatic (line 1 at least) and both Rennes metro lines
| are fully automatic. It is much easier to control the
| environment around a metro and ensure that nothing can go
| on the rails, and have surveillance system to check if,
| if it does happen, it is detected ASAP.
| rossant wrote:
| Paris metro lines 1, 4, and 14 are automatic at the
| moment.
| sofixa wrote:
| > The SNCF is for regional and national train (with the
| exception of the RER) where the possibility of having a
| cow coming on the rail is non-negligible ... or a hot air
| ballon landing on the rail and stopping the Nantes-Paris
| lines for a whole morning :D .
|
| Or camels: https://www.francetvinfo.fr/animaux/un-
| dromadaire-apercu-aux...
|
| > So I think it is a much, much harder environment to
| automate. Paris do have some metro line that are fully
| automatic (line 1 at least) and both Rennes metro lines
| are fully automatic. It is much easier to control the
| environment around a metro and ensure that nothing can go
| on the rails, and have surveillance system to check if,
| if it does happen, it is detected ASAP.
|
| Even the non-fully automatic lines use heavy automation
| (e.g. lines such as 2, 3, 5, 7, 8, 9, 11, 12, 13).
| Lance_ET_Compte wrote:
| "Leaves on the line."
|
| https://www.networkrail.co.uk/running-the-
| railway/looking-af...
| lpribis wrote:
| Not true for a lot of (widely used) systems. Both the NY
| subway and London Underground are manually driven for the
| majority of lines.
| ThePowerOfFuet wrote:
| The comment to which you replied specifically said "Not
| in any even remotely modern system"; neither of those are
| remotely modern, _especially_ not NYC. Only two or three
| lines of the famed Paris system would qualify.
|
| Barcelona, on the other hand, would qualify for
| everything except line 4.
| lmm wrote:
| Every London Underground line built after 1906 is
| automatically driven - the human driver just pushes a
| button and the train drives itself to the next station.
| iggldiggl wrote:
| That makes it sound as if they'd been automated right
| from 1907, which most definitely isn't the case - on most
| line automation only happened very recently. (The
| Victoria Line was automated right from its opening in
| 1968, then the Central line was automated in the 90s, the
| Jubilee and Northern lines in the 2010s and the
| subsurface network is currently under way.)
|
| Also if we want to quibble, at least a few sections of
| the Bakerloo and Piccadilly lines (which are still
| completely manually driven) were opened _after_ 1906.
| sofixa wrote:
| London Undeground has high degree of automation (driver
| presses a button, train drives itself optimally to the
| next station).
|
| Paris has the same, and also a number of lines (1, 4, 14,
| and soon 15) which have no driver at all.
|
| NYC Subway is an outlier in how obsolete everything is.
| rangestransform wrote:
| The NYC train drivers union continue to fight against one
| person train operation and automation, to them the subway
| is more of a jobs program than transportation
| sofixa wrote:
| I saw that, I saw trains with more than one MTA person
| onboard, and saw some construction work which had 30
| workers sitting on their phones and 2 working (passed by
| multiple times that day, the ration never changed).
|
| Grand Paris Express' president also talked about this,
| and compared the 2nd avenue subway to the Grand Paris
| Express (well, one cost $4.45 billion for 1.8 mi / 2.9 km
| and 4 stations; the first line of GPE, 15 south is 33km,
| 16 stations, and costs ~8 billion euros so it's a really
| bad comparison), and has said that if he had to do things
| the way the MTA do it, GPE would have gone nowhere.
| rangestransform wrote:
| I will literally volunteer to canvass for anyone who runs
| on busting NYC public sector unions
| porphyra wrote:
| We should automate the trains and repurpose the train
| staff to be conductors patrolling the trains to assist
| individuals in need and help in an emergency. That way,
| the jobs are saved, and the trains get more pleasant to
| ride!
| bluGill wrote:
| Most train systems around the world are not automated. Even
| though electronics can do better than a human driver, most
| trains have a human driver. (in subway systems you should
| be able to keep all dangers off the track so the safety
| problem is much easier than roads where kids might be
| playing in the street. But this requires platform screen
| doors which is also rare)
|
| The real advantage of automation though is without a human
| driver it becomes cheap to run a lot of trains that are not
| very full and that makes your system nearly as convenient
| as driving in the suburbs.
| sofixa wrote:
| > Most train systems around the world are not automated
|
| All modern ones (in an urban/suburban context, which is
| what the post and thread are about; interurban is a bit
| more mixed, but every high speed line is highly automated
| too, because at those speeds there simply isn't the time
| for a human to react) are.
|
| > The real advantage of automation though is without a
| human driver it becomes cheap to run a lot of trains that
| are not very full
|
| And you can also have a very high frequency (60-90s
| intervals), which is impossible with manual operations.
| This increases capacity, on top of the convenience.
| KeplerBoy wrote:
| On my local tram network you could even do that just by
| listening to the noises made by the inverters, which drive the
| motors.
|
| They have a very distinct hum that matches the rpm of the
| wheels. I once built a crude speedometer using SFTs, peak
| detection and kalman filtering.
| dariosalvi78 wrote:
| I believe that their intention is to have something that works
| on any track, without signatures. Collecting signatures would
| be cool, but a massive work if you want to have good coverage
| of cities AND phones. Maybe companies like Google or Apple have
| the data and the capacity to do so, smaller companies less
| likely.
| bornfreddy wrote:
| I was surprised to read they don't collect any data. That
| would be a great way to gather samples to train the new
| generations of detection NNs on. If it is properly anonymized
| it shouldn't be a problem, imho (and I'm pretty paranoid wrt.
| privacy).
| DrNosferatu wrote:
| Nice! dead reckoning to another level (Y)
|
| ML level - for some reason, made me think of ITER / fusion
| research trying to predict plasma behavior with ML also. Any
| specific connections people in the know care to point out?
| eps wrote:
| There are metro systems where trains sometimes stop in the
| tunnels or go at slower speeds.
|
| I'm guessing this app won't work that well there. In fact it
| would probably generate false positives when labeling stations...
| ?
| wffurr wrote:
| I was just wondering how well this would do on a system in poor
| repair with mixed generation trains that stop mid tunnel, i.e.
| Boston.
| daghamm wrote:
| Correct me if I am wrong, but this looks like something that
| Google would buy for 3.14B and quietly roll out to anything
| including your smart toaster.
|
| My point is, I see only one real business case for this.
| froddd wrote:
| It's a strong business case for the founders
| araes wrote:
| Similar thought, and Google acquire does seem like a plausible
| route. However, checking over on Play [1] they've apparently
| got In-App purchasing at $3-50 and 300+ cities and 1000+
| transits that let you buy fare and passes. Seems like they're
| already making money anyway. Teams pretty extensive though, so
| it does look like a lot of annual money's probably necessary to
| float. [2]
|
| [1]
| https://play.google.com/store/apps/details?id=com.thetransit...
|
| [2] https://transitapp.com/team
| xav0989 wrote:
| They also partner with transit agencies where the transit
| agencies pays them to upgrade everyone in the region/system
| to the premium offering, and the transit app becomes the
| official app for that transit agency.
| RobotToaster wrote:
| Yeah, I downloaded the app out of curiosity, but uninstalled
| it once I saw it had the usual freemium subscription
| nonsense.
| kccqzy wrote:
| I never paid a single cent in this app and yet the app is
| perfectly enjoyable. I've used it since 2019. What's wrong
| with freemium? I don't need extra features and the free
| features are good enough for me.
| procflora wrote:
| The thing I specifically paid for (that Google Maps
| doesn't seem to offer) is answering the question "what
| transit lines/stops are nearby and how soon will a
| service arrive at the stop?" Last I looked they limit the
| number of lines shown in this way for freemium users.
|
| If you're just putting in a destination and getting a
| fastest route (much like you'd do in Google Maps), then I
| didn't see much benefit to the premium version.
| exabyte wrote:
| https://youtu.be/bFM9HHB9JXI
|
| I found this to be a really well-done video on using quantum
| physics to track location integrating upon acceleration
| mattlondon wrote:
| Just tried installing it on my commute in London. Didn't work -
| in fact it thought I was at the stop _before_ the one I got on at
| (when I did have GPS) even when I was 5 or 6 stations further
| along on my journey. Then it spent the whole rest of the journey
| spamming me with notifications about the next departure time at
| the station I didn 't get on at.
|
| Wildly inaccurate even with GPS it seems.
|
| Instant uninstall. Sorry.
| wtk wrote:
| Their website doesn't include London in the UK section
| (https://TRANSITAPP.COM/REGION).
| andylynch wrote:
| Pretty sure that's a mistake, they do elsewhere
| (https://transitapp.com/en/region/london/tfl ), and I'm using
| it ,right now.
|
| Not doing so would be astonishing given how many people use
| public transport here, and that TFL's data is really, really
| open + easy to use.
| wtk wrote:
| I checked twice in disbelief, but amongst many many places
| - London (UK) not being listed was surprising to me. I
| thought it's maybe some extra challenge of the oldest
| subway system.
| itsgrimetime wrote:
| That's a bummer, I use it daily in SF and the tracker it has
| for upcoming buses/trains has always been super accurate, and
| the stop countdown is always dead on. It even tells me when I
| need to speed up my walking to make it to the next bus in case
| it's running a little early.
| beejiu wrote:
| CityMapper is probably the best app for London, although I
| can't recall how it behaves in deep tunnels. I think it may
| just use a timer to alert you when to get off.
| wffurr wrote:
| You could write to them and ask them to support London. Maybe
| they'll fly Etienne over and have him ride the whole system!
| rossant wrote:
| Maybe ten years ago, I read a blog post by, I think, a French
| company called snips, that explained how their app used the
| pressure sensor to detect when the train entered or leaved a
| station. It turned out there was a very clear signal due to the
| sudden pressure increase or decrease when the train entered or
| left the tunnel between the stations.
|
| Edit: found it. https://medium.com/snips-ai/underground-location-
| tracking-3e...
| LukaszWiktor wrote:
| Very few Android phones have a pressure sensor.
| rossant wrote:
| Maybe premium Android smartphones do? My Galaxy S24+ seems to
| have one.
|
| Edit: at least the excellent Physics Toolbox Sensor Suite
| gives me a barometer signal indicating 102570 Pa right now.
| LukaszWiktor wrote:
| True. Flagship smartphones, and some rugged models do have
| a pressure sensor.
| plantain wrote:
| Very many do -
| https://m.gsmarena.com/results.php3?chkBarometer=selected
| LukaszWiktor wrote:
| > Your search returned 653 results.
|
| When you remove the filter, it returns 12177 results. So
| only ~5% of phone models include a barometer.
| yccs27 wrote:
| That's all phones ever, not current/popular models. When
| you restrict it to the last 10 years, you get about 12%,
| and of the 70 most popular models almost a quarter are
| equipped with a barometer.
| antonkochubey wrote:
| Huh? I restricted it to 2023-now, and got 146 results. 80
| for 2024 only.
| jrvieira wrote:
| 146 in how many? 80 in how many?
| nielsole wrote:
| 80/557
|
| 146/1109
| jrvieira wrote:
| that's above 12% on both.
|
| now correct it for number of sold phones each (estimated
| by looking at the 70 most popular models) and we'll get
| why they said 1/4.
| lolinder wrote:
| For an app as popular as Transit this is a purely
| academic exercise--25% of the most popular models is
| _way_ too low to be worth building their detection
| around, even if they could assume that all of their
| customers use the most popular models (which they
| obviously can 't).
| stavros wrote:
| 25% of popular models doesn't mean 25% of phones
| currently in circulation. If the iPhone has a barometer
| (which it does), that's already a huge share of phones
| out there.
| hluska wrote:
| The conversation was about Android.
| azinman2 wrote:
| But the point stands, it's about the distribution of
| phones not how many models support it.
| lolinder wrote:
| I'm in favor of Transit trying to reach more than 25% of
| the most popular phone models even if doing so would get
| them 75% of the population.
| michaelmior wrote:
| It may be too low, but I'd imagine the app could also
| look for other Transit users nearby and as long as one of
| them has the appropriate sensor, this might be
| sufficient. Probably still not something you want to rely
| on since it would be pretty annoying to have a feature
| not work when you're alone on a train.
| cbhl wrote:
| > the app could also look for other Transit users nearby
|
| That seems unlikely in an environment with no cellular /
| wifi signal. Theoretically possible, but expensive for
| battery and probably disallowed by the OS.
| thruway516 wrote:
| Does the current version of their app run on 25% of the
| most popular models? This update runs a classifier on
| your phone. I imagine that would be no less restrictive
| than the barometer requirement.
| lolinder wrote:
| Most classifiers are nowhere near as heavy as the LLMs
| that are trending right now. The article doesn't specify,
| but I would be surprised if the resource requirements are
| especially onerous.
| fragmede wrote:
| you wouldn't need all of them to have it, just enough so that
| wifi beacons in-between train stations could get added to the
| bssid - gps coordinate map. Though if the vendors wanted work
| with you, you could just tell them wifi bssid and the gps
| coordinates of them and the rest of it would just work.
| Normal_gaussian wrote:
| There are unlikely to be wifi basestations detectable in
| between stations, because these are underground trains.
| fragmede wrote:
| unless installed by the system operator, which is why
| they'd know the gps coordinates for them
|
| I'd have to have missed the title of the post, not read
| the post itself, not read GP's comment, not thought about
| why there'd be a pressure change, to have missed that
| particular detail. I appreciate you trying to be helpful
| though. :)
| _kb wrote:
| You don't need fixed base stations. Just infra on the
| train that rolls bassid based on location data the train
| already has. This would silently hook into native
| location services already on devices without additional
| sensing or models trained on other sensor data.
| bluGill wrote:
| There should be - everyone on the train wants
| connectivity. Even if there is wifi on the train, that
| wifi needs something to connect to.
| oniony wrote:
| I believe many do because it's used to fix GPS data when at
| different elevations, if I remember correctly.
|
| https://nextnav.com/why-phones-have-barometers/
| telgareith wrote:
| Maybe it'll help achieve a lock faster- but theres nothing
| else needed for GPS besides GPS. The military really
| doesn't like dependencies.
| traverseda wrote:
| Not a faster lock, air pressure is actually more accurate
| on the z axis for the most part. GPS is not great at
| height, and including an extremely accurate barometer is
| a big help.
| umeshunni wrote:
| Why is GPS not great at height? If you know your location
| is x,y and you have an elevation map, you can map x,y to
| an altitude z, right?
| Wowfunhappy wrote:
| I know nothing about this, but I imagine two people at
| the same latitude and longitude could be at different
| altitudes.
|
| A trivial example would be people on different floors of
| a skyscraper--although I suppose gps works poorly indoors
| anyway. Still, even outdoors there are peaks and
| crevices, and on a steep slope a very trivial change in
| lat/lon could lead to a major change in altitude.
| ScottEvtuch wrote:
| I would imagine it's because your distance to the
| satellites changes more when you move along the ground
| than when you move up and down the same amount.
| avidiax wrote:
| This is the basic reason.
|
| https://www.ncesc.com/geographic-pedia/why-is-my-gps-
| elevati...
| ianburrell wrote:
| One problem is that using position to get altitude would
| require a detailed elevation map and that would use a lot
| of storage space or require internet.
|
| But elevation maps are not detailed enough and position
| is not accurate enough to get accurate elevation. Think
| about standing on trail along steep slope. The position
| not being that accurate is fine since you know you are on
| trail. But altitude could vary wildly going up or down
| slope, or even up or down trail. It is probably similar
| to GPS vertical accuracy, but were going for more
| accuracy that barometer provides.
| bobmcnamara wrote:
| > Why is GPS not great at height?
|
| Because the SATs only give you a pseudorange distance
| between you and the sat, so each say is most accurate
| solving for distance to/from that sat, and much less
| helpful resolving angle to the sat.
|
| With a clear SkyView, around just under half of the sats
| are hidden by the earth.
|
| This means that you get a full 360 degrees of data that
| can be near the horizon helping resolve lat/long.
|
| But only about half of that sky is helpful for altitude.
| The birds you can't hear directly below the earth would
| be the most helpful for improving the altitude fix if you
| could hear them.
|
| Baro is handy because you can take the absolute altitude
| from GPS as a low frequency baseline and use the baro for
| high frequency changes. Then when GPS says we teleported
| +200 feet when a new sat comes into view, we can temper
| that that with baro information.
| sgarman wrote:
| https://ciechanow.ski/gps/
| dorfsmay wrote:
| Do you then use the internet to find the local pressure,
| or only use it to calculate changes in altitude?
| traverseda wrote:
| It's one factor in estimating altitude you feed into an
| extended kalman filter. If the GPS altitude is holding
| steady but there's a sudden jump in pressure that's
| probably a storm front not indicative of motion.
|
| You also probably want an accelerometer.
| CamperBob2 wrote:
| GPS itself is one _hell_ of a dependency. What the
| military (and pretty much everyone else) wants right now
| is redundancy.
| rightbyte wrote:
| I did an IMU once. You get way better and faster Z position
| with barometer than with only GPS.
| BurningFrog wrote:
| GPS already gives you altitude in addition to longitude and
| latitude, so I don't understand what the air pressure adds.
| tqi wrote:
| Vertical error for GPS is about 2X to 3X more than
| horizontal error[1], so the barometer helps correct that.
|
| [1] https://www.nstb.tc.faa.gov/reports/2020_Q4_SPS_PAN_v
| 2.0.pdf...
| maven29 wrote:
| Just like the heart rate monitor, this was also moved into
| smartwatches for optimizing revenue from those who truly need
| it.
| resoluteteeth wrote:
| Are you saying that smartphones used to have heart rate
| monitors but they were removed to force people to buy smart
| watches?
| astrange wrote:
| Any phone that can record 120fps video has a heart rate
| monitor, if you can calibrate it.
| connicpu wrote:
| I forget which generation it was now, but many years ago
| I had a Samsung Galaxy phone that had a sensor that could
| measure your heart rate if you put your finger over it
| superhuzza wrote:
| The Galaxy S7 definitely had this feature.
|
| The problem is, it was basically useless. The main use
| case for heart rate monitoring is continuously throughout
| the day/night, or during exercise. A watch is very good
| at this. An optical sensor on the back of your phone is
| not.
|
| Periodically checking your heart rate by holding your
| phone in a specific way is not a useful feature for that
| many people.
| 6510 wrote:
| For me, just having something that you can check 1 time
| per year is much better than nothing. The anecdote: I was
| working two heavy manual labor jobs 7 days per week and
| felt absolutely fantastic, glowing with power. Turned out
| the heart rate at rest was 180 lmao. Like a rabbit. Took
| a few days off, it dropped below 70.
| idiot900 wrote:
| If your heart rate at rest is truly 180, you have an
| arrhythmia and need to see a doctor.
| 6510 wrote:
| It should decline some 60 sec after physical activity. I
| think I measured after 10-15 minutes. Fatigue,
| dehydration, lack of sleep, a diet with lots of coffee. I
| just added those to the todo list. I went home to sleep
| (and confiscated the heart rate meter), the next morning
| it was around 100 bpm which is still terrible shape. Over
| the day it sunk to 80, over the 2nd day to 70, 3rd a bit
| lower. Back at work it would barely elevate. The moral of
| the story: Don't work out 11 hours per day for 6 months
| straight.
| chucksmash wrote:
| +1. If a phone has a stopwatch, you can get your bpm at a
| given moment with a finger and a multiplication (or
| patience). Given the limited real estate on a mobile
| phone it's crazy to devote any space for something so
| trivial.
| telgareith wrote:
| It's not a separate sensor- 30fps on an iPhone was more
| than enough.
|
| Turns out, when you have a known luminance, white
| balance, and frame-rate... the DSP to grab heart rate
| from a finger is trivial.
| adastra22 wrote:
| I'm pretty sure every Android phone does. It's called a
| microphone. It doesn't give absolute pressure values, but
| that is not needed for this use case.
| coumbaya wrote:
| Another tidbit: on high speed trains (maybe on regular trains
| too idk) the 'door flange' which is actually a hollow pneumatic
| tube (it can detect spike in pressure that's how the train know
| it's trying to close the door on somebody's hand) get's
| pressurized higher before entering the tunnel, to mitigate the
| "slam" effect due to the difference in pressure.
| the-rc wrote:
| Even before that, there was a Google Maps experiment using the
| compass (magnetometer) to infer train movement, but IIRC the
| accuracy of that depended on the network's current (DC vs AC)
| and even the vintage of the cars.
| mapt wrote:
| Thing about training sensor fusion algorithms - more sensors,
| more better. Often adding a terribly inaccurate sensor whose
| failures are uncorrelated with the ones you're currently
| using, because it operates on an unrelated principle, is the
| thing that makes the solution highly accurate.
| 30blay wrote:
| Hi! Having worked on this project, I can provide some details
| on why we didn't end up using the pressure sensor.
|
| As others have stated, not all phones have pressure sensors,
| and the quality of the readings also varies a lot between
| different models. For example, we had one device where the
| readings would spike when squeezing the phone.
|
| Transit also doesn't have permission to read the pressure
| sensor, and our use case wouldn't justify asking for it.
| voidUpdate wrote:
| I'm still a little confused as to how this shows where a train
| actually is, it seems like this is only a solution to know when
| you're on a train or not. Does it just guess based on what trains
| should be moving at that time?
| rdsubhas wrote:
| I'd assume it's the length (in time) of the vibration since the
| last stop. There may be edge cases where if the train is moving
| slowly, but overall it's a smart technique.
| voidUpdate wrote:
| If there's a delay that will really confuse it though, either
| if your train isn't going at the speed the app expects or it
| stops at a point outside of a station
| bluGill wrote:
| It probably would, but if someone has the app running, then
| the app knows what train they are trying to get on, and so
| if the data is all consistent with being on a train they
| are probably on that train. The app also knows where the
| train is, so it knows where you are as well. It won't know
| which train car you are on, but that normally doesn't
| matter.
|
| This probably is wrong if trains for both directions stop
| at the same time from opposite sides of the same platform
| and you get on the wrong train. However this is a rare
| enough case that we can ignore it - but very annoying if
| you are that person (this case happens to most transit
| riders once or twice in their life). Even when this happens
| there often is other location data scattered around so you
| would likely only ride a stop or two before the app
| realizes you are on the wrong train and can reroute you.
| svag wrote:
| This is a very interesting article. I suppose something similar
| can be made for vehicles entering a tunnel, where eventually will
| lose the GNSS signal.
|
| I have noticed that a year ago or so, Google Maps app would lose
| the GNSS signal and stop updating the position while there was no
| signal. But now I have noticed that the position is updated,
| although is not accurate. I wonder if something similar has been
| implemented...
| michaelt wrote:
| 15 years ago, sat nav systems built into cars would sometimes
| be fitted with a rate gyro and get a wheel speed signal from
| the car - to make them work better in tunnels and other areas
| with poor GPS signals.
|
| (Of course, that one area of superior performance couldn't make
| up for the awful GUIs, the awful resistive touchscreens, the
| $100 map updates, and suchlike)
| Cthulhu_ wrote:
| I can't see inside Maps, but it does look like they use
| accellerometer data or just assumptions to continue animating
| your position on the map. Assumption being for example that you
| will follow the route through the tunnel at your current
| average speed. Close and good enough for most tunnels I
| suppose.
| sitkack wrote:
| Apple maps does this.
| maxehmookau wrote:
| This is so freaking cool. What a fantastic idea and a great
| write-up.
| h1fra wrote:
| Oh they so going to get bought by Google
| tdiff wrote:
| I believe Yandex does smth. similar in their navigation in
| Moscow, because GPS is heavily jammed in some areas.
| elromulous wrote:
| Citymapper has been able to tell you what stop you're at for many
| years (at least on the NYC subway). How does their solution work?
| f1shy wrote:
| No idea, but think about fingerprinting through available Wi-Fi
| ssid and/or BT devices?
| agos wrote:
| not in Milan though, even though we have full cellular coverage
| underground. I wonder why the difference
| aitchnyu wrote:
| Tangential, why dont phones use accelerometer+gravity to figure
| out the path travelled and the "plane" its on when gps is out?
| Somehow my running app knows my stride length but may refuse to
| count the distance travelled when gps was out.
| killingtime74 wrote:
| (guessing) Stride length is a scalar, path is a vector?
| Inertial navigation systems suffer from a build up of the
| error. Perhaps it's not practical even if it's possible to
| calculate position based on those sensors alone
| sorenjan wrote:
| To get position from accelerometer data you have to integrate
| it twice. Accelerometer data is noisy, and integrating noise
| quickly leads to large deviations. That's why you need some
| kind of absolute position reference, like GPS, to not get
| bigger and bigger errors.
|
| Here, they seem to estimate which stations they are at or
| between, and use that as absolute references.
|
| Your running app is using accelerometer data to recognize when
| you're taking a step, and combining that with GPS data to
| measure how far you're running. So if it measures 100 meter
| using GPS and counts 100 steps during that time, your step
| length is 1 m. Garmin watches uses this to measure your running
| distance on treadmills, but they want you to calibrate it by
| running with GPS on a flat surface outdoor first. I don't know
| if any watches or apps use this in combination with GPS to make
| better position estimates when the GPS reception is poor, my
| guess is no because it's really only useful for distance, not
| position, and generally when running outdoors you're not
| running in completely straight lines. But it should be possible
| to estimate distance and combining it with their routing
| software to guess which way you took for short parts of the
| course.
| Woeps wrote:
| Uhm, Don't most underground trains already show you where you
| are? Or at worst, the sign of the station shows you your
| location?
| yaky wrote:
| I think it's one of the "pointless but cool" projects, but yes,
| I don't get it either. Any decent underground system would
| already have audio and visual announcements; I don't think I
| have ever cared to know where between stations the train is.
| makeworld wrote:
| Supporting it in the app allows connection info (bus, other
| train, etc) to be updated and to warn the user if they'll be
| late.
| aucisson_masque wrote:
| Side note, that's why I don't like that the Google plays services
| in Android have a permanent access to Physical Activity that
| can't be disabled.
|
| Even without gps data, just having access to your phone
| accelerometer is enough to give a lot of data about your life.
| Cumulated with Google insanely big amount of data about wifi
| access point location, it means that they know where you are even
| without gps activated and how you got there.
| sourcepluck wrote:
| Any good links for reading more about Play Services access to
| the phone's accelerometer?
| alkonaut wrote:
| I visited London this weekend and found even many underground
| _stations_ were without mobile service, 15 years after I got used
| to having spotless end-to-end mobile coverage even in deepest
| tunnels. I don't know why the London Underground lags that of
| other cities?
|
| Knowing the next station is usually a solved problem that doesn't
| need a smartphone, because that's displayed in the train itself
| and called out on speakers. But once you are on the platform and
| you need to ask the route planner what the fastest route is to a
| specific station (It could be walking to the surface and taking a
| bus, so it's not as simple as looking at the subway map) - then
| you are out of luck if platforms don't have 4/5G coverage!
| vinay427 wrote:
| The usual answer that will be thrown around to justify this is
| that London has the oldest underground metro system, whether
| that's relevant or not. It doesn't seem very relevant when even
| the deepest stations in Prague (which are newer but much deeper
| than most of London) have later added 5G throughout tunnels
| with download speeds of ~200 Mbps between stations.
|
| I suspect the UK just doesn't have modern standards for mobile
| internet service, as it isn't limited to TfL services in
| London.
|
| There isn't even reliable mobile coverage on stretches of the
| West Coast Main Line rail corridor in England, which is
| outdoors and connects the UK's most populous cities.
|
| Much of London isn't particularly reliable either on most
| mobile networks, in my experience. Meanwhile, in cities like
| Prague or Washington I can often get mobile download speeds
| outdoors of 400 Mbps - 1-2 Gbps.
| beejiu wrote:
| Almost all tunnel sections now have 4G/5G and everything should
| be done by the end of this year according to Tfl.
| lol768 wrote:
| They've given "targets" and deadlines before and failed to
| meet them. I would take whatever they say with a large
| portion of salt.
| DrBazza wrote:
| Half the LU tunnels are barely larger than the train, the other
| half are cut'n'cover. And they're rarely straight. And most
| lines run from 5am to midnight. Some now run almost 24 hrs.
|
| That probably influences the decisions on roll-out and cost.
|
| TFL has been ahead-of-the-curve with the rollout of some tech
| (Oyster cards, tap'n'go), but not so much with others when they
| get hacked.
|
| https://www.bbc.co.uk/news/articles/c4gqg2elkj4o
|
| https://en.wikipedia.org/wiki/Oyster_card#Precursor
| dmazin wrote:
| This is SO cool.
|
| I am actually currently working on a project to record the sound
| of the London Underground passing under me.
|
| We can very clearly hear the Northern Line under us. It's < 30
| meters below us.
|
| I have become obsessed with getting high-quality, low frequency
| recordings of it passing under us.
|
| Why? I don't know. I just can't take my mind off it.
|
| For example, there are two tunnels (north and south bound). By
| correlating it with actual TfL data, can I figure out the sound
| signature of each?
|
| More intriguingly, I know that there are maintenance vehicles
| that operate under us in off hours. Can I "catch" them?
|
| I'm not sure what else I might do with this project, but the idea
| of capturing the sound of this semi-ephemeral creature that
| operates below me has captivated me.
| dmos62 wrote:
| That's awesome. What sensors have you considered?
|
| I'm interested in extremely weak, high-frequency vibrations of
| everyday things while in resting state (which is sort of the
| opposite of what you're after, as I understand), but have not
| gotten far in acquiring the sensors. I'd love to get a laser
| doppler vibrometer, but they're pricey.
| dmazin wrote:
| Well, I was just going to put a contact mic in my cellar. My
| understanding is that this is the best way to pick up low-
| frequency vibrations.
|
| I know very little about audio engineering. I wonder what
| else I might be able to use to pick up the vibration
| signature?
|
| I completely understand your drive to pick up those high-
| frequency vibrations! There's a whole secret world of
| vibrations out there, and we can analyze it!
| jbl0ndie wrote:
| You might struggle using microphones if it's very low
| frequency because most human audio microphones
| intentionally filter frequencies that are below (and above)
| human hearing range. Perhaps fixing to he contact mic to
| the right resonant object might serve to translate the low
| frequency to a higher frequency?
|
| Could you use the accelerometer on a smart phone to 'hear'
| the vibrations?
| dmazin wrote:
| I don't know if a contact mic really has any filtering
| though; it's just a piezo element that I plug into a pre-
| amp.
|
| However, what you're saying is completely legit. I read
| that I might want to lay a large, sturdy thing on the
| floor and put the mic on that, for the same reason that
| you gave. I used a large old pane of glass that I found
| in the cellar.
| cruffle_duffle wrote:
| All that came to my mind is making sure to physically
| bond it to the floor. But then you still need to have
| something that moves relative to the magnet and then I
| got confused again.
|
| What do seismographs do? People also use acoustic pings
| to measure soil and rock density underground. What are
| those devices using for their "microphone"?
|
| And yeah, would an accelerometer work? I don't know what
| the temporal resolution is on your standard Amazon /
| AliExpress accelerometer is but it's probably pretty
| decent?
| myself248 wrote:
| "Geophone" is the device you seek.
|
| Amplifying and digitizing the signal is straightforward for
| an experienced EE, but you might look into acquiring some
| Raspberry Shake hardware which has it all already done.
| dmazin wrote:
| Oh, snap. Thank you. Guess I'm about to be $350 poorer!
| pavel_lishin wrote:
| Stupid question, but how far can you get with a laser, a
| mirror, and a light sensor?
| richjdsmith wrote:
| That's actually a really cool idea. I am not sure what
| variance there would be on a light sensor though.
| pavel_lishin wrote:
| I remember building a toy interferometer in high school -
| a laser, splitter and a mirror. You could definitely see
| the interference patterns when you'd press on a wall with
| a finger.
|
| Maybe something like that could be turned into a
| reasonable microphone?
| comboy wrote:
| My kind of people. Good luck!
| meitham wrote:
| Ha! If you happen to live in the battle area of South Wimbledon
| then the noise from the northern line in the early hours of the
| morning can wake you up! It's roughly 30 meters under too!
| dmazin wrote:
| What I learned is that the Northern Line has the most noise
| complaints (from people who live near it) of any TfL line!
| immibis wrote:
| These are electric trains. I'd be interested to know if a big
| coil of wire picks up a signature as well.
| dmazin wrote:
| Woah. A bit of reading shows that the signal would be
| incredibly weak, especially because of shielding.
|
| I wonder what other ways there might be to detect it.
|
| Someone else mentioned literal vibration, but I figure a
| contact mic would already pick that up - I mean, sound is
| already vibration...
| Symbiote wrote:
| The oldest deep London Underground lines have iron tunnel
| segments, which might make this method particularly
| difficult.
|
| https://www.ltmuseum.co.uk/collections/collections-
| online/in...
| toomuchtodo wrote:
| Could you use a ground mount seismic sensor?
| frenchman_in_ny wrote:
| I was at an Open House at the Lamont-Doherty Earth Observatory
| [0] a few weeks ago, and one of the researchers was doing
| something adjacent to this -- recording the sound of
| earthquakes for analysis. Will try to find who it was.
|
| [0] https://lamont.columbia.edu/
| IIAOPSW wrote:
| I am sure that your work in discriminating what's happening
| deep underground with nothing more than passively recording
| surreptitiously available signals at the surface level on non-
| specialist hardware is of interest to a few governmental
| offices.
| yzydserd wrote:
| Have you looked into whether a Raspberry Shake might be
| adequate?
| kjkjadksj wrote:
| I used to live about 300 yards from where the subway went and I
| felt like I could hear it. I'm not sure if it was residual
| sounds from the sidewalk emergency exits or from the earth
| ringing like a bell.
| snodnipper wrote:
| I worked for signalbox.io and they had this for the London
| Underground ~8 years ago. There was some impressive maths /
| approaches behind all of the "magic".
| bambax wrote:
| Dead reckoning is... hard.
|
| https://en.wikipedia.org/wiki/Dead_reckoning
|
| The first car navigator, the Etak, came out in 1985 and used dead
| reckoning and quantization to tell where the car was on the map;
| see this excellent article from 2017:
|
| https://www.fastcompany.com/3047828/who-needs-gps-the-forgot...
|
| Today dead reckoning is used in aerial navigation, and commercial
| planes (and others) are equipped with Inertial Navigation systems
| to supplement GPS information; they are getting more and more
| precise but can still go wrong and need frequent re-calibration.
|
| The next step is "Quantum Positioning System" that promises to
| detect infinitely small movements and produce perfect dead
| reckoning at all times, with a precision of the order of one
| centimeter. It has already been tested successfully. For now the
| machines are heavy and extremely expensive, but it's imaginable
| that in some not-so-distant future the technology will be much
| more available.
|
| https://newatlas.com/aircraft/quantum-navigation-infleqtion-...
| jackdh wrote:
| There is a very good set of podcasts[0] by Cautionary Tales
| about the German V2 rockets in WW2 which tried to use this. It
| was so hard the Allies allowed them to continue wasting money
| on it during the war. Well worth a listen to if you're into
| that.
|
| [0] https://timharford.com/2023/07/cautionary-tales-
| the-v2-trilo...
| zokier wrote:
| General dead reckoning is hard.
|
| But dead reckoning for train travel should be massively easier.
| Train movement is constrained to tracks so you only need to
| resolve how far along track train has moved + possible
| junctions, which should already make the problem much simpler
| than e.g. airplane ins where you are resolving full 3d
| position. To make things even more easier, you only need to
| reckon between individual stops which prevents error
| accumulation over the whole trip. Lastly you have schedule
| information, so you know roughly where the train should be at
| any given moment.
| DrBazza wrote:
| Is this the same sort of technique that Shazam uses?
| barbegal wrote:
| With this sort of tech it only achieves good usability if it is
| accurate a high percentage of the time. Anything less than 90%
| accuracy on a whole trip and I likely won't trust it. So that
| means getting to >99% accurate on each underground train stop.
| I'm still not sure if this tech reaches that sort of threshold or
| if this is one of those things that looks like a good idea on
| paper but falls apart in real world use cases.
| sitkack wrote:
| Once enough people have the app, there will be a whole time
| varying point cloud of data both on and off the train.
| datameta wrote:
| Is that their stated goal? Are they not saying they will not
| use user virbation data?
| sitkack wrote:
| I'd assume they would have some opt in, or allowance for
| either federated learning or sharing of anonymized sensor
| data.
|
| Google is a huge advantage here, they have both the spatio-
| temporal intent of the user as well as the physical flow
| feed. There is so much position data flowing off android
| phones, that they are able to see the whole topology.
| wffurr wrote:
| "Classifier" how quaint. I am impressed that they wrote the whole
| article without mentioning "AI" one time.
| worewood wrote:
| Good. Shows that engineering is on the driver seat, not
| marketing.
| devit wrote:
| Wouldn't dead reckoning with the accelerometer and gyroscope in
| addition to their machine learning improve this significantly?
| (constraining motion along the known tunnel path when in a moving
| train, constraining the user to be within the train stopping
| rectangle when detecting a train starting to move)
|
| Or is the hardware in smartphones too inaccurate even with the
| extra information?
| pbmonster wrote:
| > Wouldn't dead reckoning with the accelerometer and gyroscope
| in addition to their machine learning improve this
| significantly?
|
| The thing that should help the most would probably be the hall
| sensor/magnetometer/compass. That should output decent dead
| reckoning.
|
| Doing this just with the gyroscope will work very well for
| short movements, but it will be close to useless on long,
| gentle curves. Unfortunately, those MEMS gyroscopes drift quite
| a lot over tens of seconds. Not a problem if you can do sensor
| fusion with the magnetometer (reckoning) and the accelerometer
| (where is "down"), but the latter can't be used on a fast
| train, acceleration/deceleration of the vehicle and forces in
| curves make finding gravity challenging. No idea how well a
| compass works inside a subway tunnel.
|
| But maybe I'm wrong, I just have experience trying an
| "artificial horizon" app on an aircraft - and here, the
| accelerometer is completely useless for "down". A single
| maneuver with some Gs and the horizon has no idea what the
| pitch angle is. Noisy magnetic environment, GPS off? It also
| doesn't know where it's going.
| porphyra wrote:
| I wonder how well magnetometers would work inside metal
| trains full of electric motors.
| bluGill wrote:
| That is what they are doing. Dead reckoning is generally very
| inaccurate though, so by recognizing when they are on a train
| they can greatly increase accuracy because they know exactly
| where the train is.
| crazygringo wrote:
| Is it? The article doesn't say so.
|
| It says they use the accelerometer for detecting the user's
| current mode of transportation, not the distance.
|
| It says they use the train schedules to figure out the
| current location of the train, based on when it started
| moving and how long it's been moving.
|
| They don't mention anything like dead reckoning anywhere,
| unless I missed it?
| KTibow wrote:
| Yeah while the solution they implemented is very interesting it
| doesn't actually "predict your location in a subway tunnel
| using your phone's vibration signature"
| porphyra wrote:
| Using the IMU seems hard as true signal of the train's gradual
| acceleration is likely drowned out by the noise of vibrations
| and the person's natural movements. I guess every time the
| train is stopped, you can calibrate the bias of the IMU if the
| user is standing/sitting sufficiently still, but even then the
| dead reckoning would drift a lot.
| gregoriol wrote:
| This is way too heavy computing, battery consumption, ... while
| fun engineering, it's very much inefficient to perform
| 0xFEE1DEAD wrote:
| @OP how can I get my city added to the app?
|
| I was excited to try it out but bummed to see my city isn't
| listed. It's even more disappointing considering Ulm Germany
| (with around 100k people) is there, but Cologne Germany (with a
| population of about 1.1mil) isn't.
|
| There are lots of potential users here, especially since the
| official apps are aweful.
| starlite-5008 wrote:
| Innovative
| ripe wrote:
| Can we take a moment to appreciate the engaging, conversational
| tone of the writing? The article was a pleasure to read, even
| when it gets into some weeds explaining the frequency charts,
| etc. (I read the English-language version).
|
| Whoever wrote this did a fantastic job.
| justinko wrote:
| Yes, the lovely tone of a human being (not AI).
| lallysingh wrote:
| I donno which humans you read, but most people I know
| defecate their thoughts onto a page. Actually good writing
| takes a lot of effort most people don't do. AI's got a decent
| baseline.
| vaughnegut wrote:
| Their product release posts and their engineering posts are
| usually really well written. Transit App is a gem
| yonatan8070 wrote:
| This is the first time I'm hearing about it I downloaded it
| pretty much as soon as I finished reading the blog post, only
| to discover it's not available in my region :-(
| tgtweak wrote:
| I think you can "spoof" gps satellite timestamp transmissions
| with single transmitter inside the train. This would require on-
| train equipment but typically the trains know where they are and
| could translate that into the required gps satellites and
| timestamp signatures to allow any devices onboard to acquire a
| "gps" signal and decode the location using timestamp variation.
|
| I don't believe any of those are encrypted and transmit
| ~1000-10000 time per second on 1500-1600mhz spectrum which is
| fairly simple to reproduce using even a cheap SDR kit.
|
| It could also work with 0 input from the train's telemetry - in
| much the same way as the app - the device would get a reference
| gps signal (or wifi/BLE when it knows it's in the station), then
| with a built-in accelerometer (which it has the luxury of
| direction + stability if it's mounted in the train car) it can
| determine with greater accuracy where the train is and how far
| it's moved.
| Aurornis wrote:
| > I think you can "spoof" gps satellite timestamp transmissions
| with single transmitter inside the train.
|
| Spoofing GPS as a commercial operation is a quick way to get
| your company crushed by regulatory agencies.
| bluGill wrote:
| I think there should be regulations that allow sending real
| GPS from low power ground based system. Sending incorrect GPS
| is a major problem, but a lot of location issues would become
| much easier if the owners of buildings would just install
| cheap GPS transmitters.
| acc_297 wrote:
| It seems like this functionality could be tacked on to
| "MSA" which I just learned is an FCC mandatory technology
| for most cell phones
|
| https://en.wikipedia.org/wiki/Assisted_GNSS
| cruffle_duffle wrote:
| I mean obviously you'd set it up through the proper channels.
| I'm sure there is a regulatory framework for doing exactly
| this sort of thing already.
| acc_297 wrote:
| I wonder if you could just create a map of the cellular
| antennas along the metro/subway tunnels. Maybe I misunderstand
| how connectivity is delivered in underground systems it may
| pretend to be a "single cohesive base station" from a phones
| perspective but if there are thousands of antennas throughout
| the system and the phone switches from one to the next
| regularly you can decently estimate the location.
| ingen0s wrote:
| This is why I open this site everyday several times a day
| zephyreon wrote:
| As a user of Transit (thank you for such a great app!) this was
| always one of my biggest annoyances. Not just transit though --
| with any app that supports navigation on public transit (looking
| at you Apple Maps).
|
| I figured someone was working on this but it's so refreshing to
| read about the thought and level of detail that went into your
| design. What an effort too! Congratulations transit team, you all
| should be so proud for solving what I'd imagine is one of
| transit's largest small gripes.
| bsimpson wrote:
| I live between stations. One is a nicer walk; the other is a
| shorter walk. I check the next train when I leave to see which
| station to walk to.
|
| Would be useful if I could teach this to your app.
| mateobelanger wrote:
| Test
| 333c wrote:
| I used this feature in the New York subway last week. I didn't
| know it was new. It's a cool idea, but it didn't work for me. The
| app said that the train was several stops behind its actual
| location.
| yonran wrote:
| > It now makes the right location prediction 90% of the time.
|
| I'm curious about the failure cases. Are they caused by
| exceptional circumstances, such as the train moving more slowly
| than normal or skipping a station? Or when you unexpectedly catch
| an express train or go the opposite direction? Does the algorithm
| know that it doesn't know where you are, or does it confidently
| tell you the wrong station until the GPS is acquired?
| Lance_ET_Compte wrote:
| For the express vs. local, sometimes it will ASK me what train
| I am on. When there are delays that cause trains to be closer
| together, it does this. I presume they are crowd-sourcing some
| data instead of trying to only use an algorithm.
|
| I do love this "Transit" app though.
| 30blay wrote:
| Great question! I'm one of the developers behind this project
| and your guesses are good.
|
| Other difficult cases include trains stopping between two
| stations (doesn't happen everywhere, but it's frequent in NYC),
| or a user walking fast onboard a moving train, which can be
| mistaken for the user having gotten off the train.
|
| Taking a train in the opposite directions will break the
| assumptions we make and we won't know until the next GPS
| location
| clarkmoody wrote:
| Alternative title idea: "Instead of Reaching for AI, we used Good
| Old-Fashioned Engineering to Solve a Tough Problem"
| alanbernstein wrote:
| Is machine learning old-fashioned already?
| ant6n wrote:
| Machine learning is part of AI.
| riffic wrote:
| would love to use collective sensor data to shame operators who
| accelerate or brake too harshly. that's kind of my transit
| annoyance these days.
| admax88qqq wrote:
| I find the bus tracking of this app in my city is pretty poor
| unfortunately. Probably not the apps fault probably an issue of
| the municipality but still annoying.
|
| That being said, if this app could convince cities to also be
| used for payment that would be a game changer. Uber for public
| transit would really remove so much friction from using transit.
| 43920 wrote:
| They do support payment in some cities (mine included), and it
| works pretty well.
|
| A bunch of the larger / better-funded systems are also moving
| to just accepting credit cards directly on the readers, which
| is even easier.
| jer0me wrote:
| Some cities let you pay with contactless credit cards (at least
| London and New York have it; Boston and San Francisco are
| working on it) which seems like the most elegant solution to
| this, at least from a commuter's point of view.
| ladams wrote:
| You can pay in the app on the Denver RTD system!
| admax88qqq wrote:
| Awesome!
| lacoolj wrote:
| The timing of this post is nuts. I was thinking about a post-
| apocalyptic future last night (as one does) and wondered if using
| the gyro on your phone would suffice if the initial location and
| full map was already available.
|
| I think the answer is still unclear since they are using pre-
| determined routes (easier to track east -> west or east ->
| southwest than it is east -> north -> south -> north -> east
| again). But this is very cool that they have so much of the work
| done already. Maybe even all of it? I don't have the code to look
| at so -\\_(tsu)_/-
|
| Either way, still freaked out they read my mind lol
| showerst wrote:
| This is called inertial navigation, and it works over short
| distances but then breaks down due to cumulative error.
|
| This is a pretty cool use case though since as you said it
| would be much easier over a fixed route.
| nycerrrrrrrrrr wrote:
| Great article, I'll have to try it out next time I'm on the
| subway.
|
| One correction though - there is no subway line that goes across
| the Queensboro.
| sailfast wrote:
| I appreciate that this is not a mass surveillance play and all of
| the data stays on your phone. Well done!
| ipython wrote:
| So true. This is the sort of thing that is so exciting from a
| usability and engineering standpoint. Until some marketer gets
| their hands on it and decide that they can increase the ROI on
| microtargeting advertisements by 0.1% if they use this data.
| Suddenly, we continue to slide into the advertising and
| propaganda driven attention economy dystopia.
|
| Here's to the future!
| jacooper wrote:
| Google maps(wuth google location service) is alao crazy accurate
| underground somehow.
| armada651 wrote:
| Why all the complicated detection? The user already knows whether
| or not they are on a train, you don't need to tell them.
|
| Just show the predicted location of the train they should be on
| separately from their last known GPS position. Of course it would
| be difficult to market that alone as a novel innovative AI
| feature.
| patrickmcnamara wrote:
| Trains at my U-Bahn station are as frequent as every two/three
| mins sometimes. There is no "the train they should be on".
| IIAOPSW wrote:
| A few thoughts come to mind. Some said others not.
|
| 1. Seems a bit weird to be looking at the accelerometer data yet
| miss the obvious approach of summing up the acceleration to get
| the velocity and then summing that up to get position. Yes I know
| about drift but even then I'd assume the fairly constant several-
| second g force of pulling in or out of a station or taking a
| curve would be a strong signal easy to distinguish from short
| lived jostling.
|
| 2. The "train moving" frequency you discovered via fourier
| analysis was most likely the hunting oscillation. This has to do
| with how the wheels of the train are designed to force it to turn
| opposite to any deviation from the direction of the track. Thus
| there is a back and forth "hunting" for the center that is
| completely determined by the geometry of the track and the
| wheels, and therefore the length of track per complete back-and-
| forth cycle (aka the wavelength) is constant. The frequency of
| the oscillation (aka back-and-forth cycles per second) is just
| this constant length divided by the velocity of the train. This
| fact could be leveraged to estimate the actual train speed rather
| than just moving/not moving.
|
| 3. Combining 1 and 2, a combination of integrating acceleration
| and confirming / correcting estimated velocity with the expected
| hunting oscillation would likely be the most powerful / reliable
| model.
|
| 4. Using a classifier seems overkill here. But I'm sure at some
| point it was easier to just raw-data it than work out a theory
| driven model which accounted for all the practical confounding
| factors.
| ezfe wrote:
| You still need to know when they are on a train, because you
| need to know what time they got on the train and started
| moving.
|
| You also have to assume that GPS has already been lost at this
| point, so you have to do it from the departure location of the
| train at the right time.
| cenamus wrote:
| That's just an inertial nav systems, quite useful for missiles
| before/without usable GNSS, but also probably quite inaccurate
| with the MEMS accelerometers in phones.
|
| https://en.m.wikipedia.org/wiki/Inertial_navigation_system
|
| Semi-related but just as fascinating:
| https://en.m.wikipedia.org/wiki/Terrain-following_radar
| simsla wrote:
| I was thinking in a different direction. Maybe we can
| fingerprint the turns or other jerks/shakes along a route.
| Accelerometer Shazam.
| 30blay wrote:
| I'm one of the developers behind this project, so I want to
| jump in because we did explore some of these solutions.
|
| You are correct in saying the low frequency acceleration from
| starts, stops and turns can be distinguished from the higher
| frequency noise.
|
| One big challenge was with orientation. Acceleration can look
| the same as deceleration and turns from the sensor's
| perspective, if you turn the phone around. Taking the integral
| of the gyro reading, the error would grow quadratically, and we
| found magnetometer readings unreliable depending on the
| vehicles.
|
| Your point about the hunting oscillation is interesting and I
| agree, estimating the speed would be a great improvement.
| CarVac wrote:
| I'm trying this out literally right now and it got fooled by an
| unscheduled stop due to train traffic. It thought we were already
| at the next stop and took a bit to get its bearings again.
|
| At least in NYC it should listen for door beeps.
| ekzy wrote:
| Could you also pick up the audio announcements? Could be weird
| for the app to request microphone permission though, but I think
| lots of lines have them and should be doable to transcribe them
| locally on the phone. I think your approach is better and
| simpler, but as I was reading the edge cases (like someone
| changing train and going back in the other direction), the audio
| could maybe help getting from 90% accuracy to 98%. Sounds a bit
| more involved to implement though.
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