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Comment by madaxe_again | Hacker News Reader

I’ve been collecting GPS tracks of all the commercial flights I’ve been on for the past several years, using a little pocket receiver.

It’s been pretty interesting data, from seeing how ATC works differently from country to country (China is nuts - a flight from Beijing to Hong Kong will have hundreds of course changes to minimise overflight of populated areas), to how airports stack traffic, to where GPS jammers are being operated - many large airports seem to have a curtain of interference, presumably to stop drone intrusions. It’s also interesting seeing things like routes dog-legging around Ukraine, or ascending to a very high level to overfly Afghanistan - apparently pot-shots by armed groups are of not insignificant concern.

So far it’s only a few hundred flights, but it’s still pretty neat.

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What countries have you seen GPS jamming in? And if in the US, which airports?

If in North America, I suspect there’s another explanation for what you’ve seen given the degree to which civilian air traffic relies on GPS for navigation, including for precision approaches at many airports.

Jamming - Brazil, Argentina, Russia, China, and there’s a small dead zone in the UK over the Brecon Beacons.

Intriguing, I wonder what could be the reason for and source of jamming over the Brecon Beacons?

One possible explanation is that your GPS receiver front-end was overloaded by transmissions from Woofferton transmitting station[0].

[0] https://en.wikipedia.org/wiki/Woofferton_transmitting_statio...

Brecon beacons are a training ground for UK special forces soldiers - maybe either testing their ability to cope with jamming, or testing their offensive jamming equipment?

Allegedly, airports' coordinates are hard coded into consumer drones' firmware, at least in Russia. That's the reason why gps devices at Moscow Kremlin walls think, that they are at VKO airport. So jamming gps signal in the airports seems counterproductive. I would think that they'd want drones to know they are in a no-flight zone.

Is the in-flight map often fictional data then?

Not necessarily. Even without GPS, commercial airliners have a good idea of where they are using their INSes (inertial navigation systems). That technology well predates GPS receivers being common in aircraft. Most of the Boeing and Airbus aircraft you fly, for example, have 3 INSes and probably 2 GPS receivers. All of the data feeds into the computers to average to the position the plane thinks it's actually at. When the GPS signal is reliable, that may be used as the actual position and can be used to keep the INS drift minimized, so if the GPS signal is lost (or the receivers malfunction), the INS takes over as the primary source of position data. This is all further supplemented and verified with radio navigation aids -- as the planes fly along their routes, the airplane computers automatically tune in to various radio navigation aids. In short, even for the newest big commercial jets, functional GPS is very much optional and positional data is quite good without it.

As for the moving maps on the seat-back displays: I guess it all depends on the data source they use. If they have their own GPS receiver for the plane's "entertainment" system, that doesn't get the computed current position from the real navigation systems, then the map would presumably stop working or make up (e.g. interpolate) data from when it last had good position data.

But now that I think about it... I've only ever seen those maps draw the route as the great-circle path from the departure airport to the destination airport. So they're fictional in that regard -- whether they're interpolating the position on the great-circle path as a percentage of the actual track distance flown and remaining from the real navigation data, or doing something even more fictional, I don't know.

It varies between airlines - off the top of my head I couldn’t tell you which does what, but some are completely accurate, others are essentially a great circle progress bar - and they always turn it off for the interesting bits around takeoff and landing.

> But now that I think about it... I've only ever seen those maps draw the route as the great-circle path from the departure airport to the destination airport. So they're fictional in that regard -- whether they're interpolating the position on the great-circle path as a percentage of the actual track distance flown and remaining from the real navigation data, or doing something even more fictional, I don't know.

Very interesting. Thanks for the detailed answer. Next time I'll make sure to record the position information independently as well. For some reason I had just taken their great-circle path at its face value and never questioned it. In hindsight, it was all too perfect. :)

A few hundred flights in several years? I'm jealous! What is your line of work if I may ask?

Not OP, but my busiest year was 2014 was 119 flights. 6 were in business, all of those were upgrades (once on miles, the others were upgraded at the gate due to overselling down the back)

Fortunately most of the long haul flights were in premium economy, about 26,000 miles were in economy.

2013 was only 112 flights, but longer than 2014, 163k miles. I was luckier that year with upgrades for work, and did a few leisure trips which were in business/first.

By 2015 I was getting tired of hotels and jet planes. Still knocked up 57 flights and 121k miles. 80% of those miles were in business or first though, which made it more bearable.

There's a certain type of person who sustains hundreds of flights a year, year after year (which IME means half the time you're staying in a hotel somewhere), most of those that I know are either under 30, or they're divorced.

Jealous? I unfortunately have about 50-60 flights in a couple of years. And that is too much. The airport and air travel experience is really not very good. Add to it the emissions. I personally would prefer less flights and more effective work. Where are the great AR conference facilities?

High level business/tech consultancy these days - although 90% of the travel is me picking cheap and weird flights in order to get to cheap and weird corners of the world while I figure out where to start the next business, and because I have a hunch that it may not be so easy to travel in the not-too-distant future. For instance, I just did a nine day trip for the sake of it, flew from UK to Amsterdam, a night there, flew to Porto Alegre (Brazil), spent two nights there, flew to Montevideo, spent three nights there, flew to BA, spent a night there, flew to Lisbon, two nights there, then home. £450 airfare, total - for two of us, as we booked it as a series of long-layover low-demand flights. Now I’m in Poland. Latvia this morning, Istanbul last week, Istanbul again in a few weeks after a bit of Belarus (London to Riga via Istanbul isn’t very direct, but it was dirt cheap) - and then I’ll hibernate for a month as as fun as seeing new places is, air travel is pretty miserable in and of itself.

It’s cheaper than staying at home - Airbnb the apartment out while I’m not there, and it usually covers the travel costs.

What do you use to book those long layover flights?

Skyscanner, google flights - I’ll usually have an ultimate destination in mind (e.g. Montevideo), and will then either find an acceptably cheap series of flights by turning off most of the filters, or setting up price alerts. Sometimes I’ll book months in advance, sometimes it’ll be “pack your bags, we’re going to Vladivostok... today!”

Also, my wife scours sites like https://www.holidaypirates.com/ which occasionally have some steals of deals.

Frankly the hardest bit has been organising catsitting.

    I have a hunch that it may not be so easy to travel in the not-too-distant future

That's an interesting hunch. Can I get you to elaborate?

Neat indeed; you seem to have discovered lots of interesting tidbits.

Of all you mention, two are extremely surprising to me, and I'm not even sure which is more - China course corrections and GPS jamming. The former sounds straight out of Cold War counterespionage playbook. In this day and age, why would they bother? The latter... again, are drones really that big of an issue to justify the jammers (and, presumably, legal clearance for jamming GPS)?

I'm not sure that it's fair to infer that China's course corrections are to avoid espionage. Google Maps satellite view will show you far more, for far less money. And military areas are of far more concern for espionage than cities.

I suspect the reroutes are more for noise reasons, though I am not certain.

Could it be concerns about planes crashing into populated areas?

That is so rare you don't really plan for it.

Most Chinese airspace is managed by the Chinese military, and military flights take priority. So that may be a reason why as well. https://www.economist.com/graphic-detail/2017/10/30/why-chin...

> The latter... again, are drones really that big of an issue to justify the jammers (and, presumably, legal clearance for jamming GPS)?

If birds are that big of an issue, would drones not be, too? https://en.wikipedia.org/wiki/Bird_strike

Which device did you use? I always thought consumer devices were supposed to disable themselves above certain altitudes and/or speeds to prevent them being used in cruise missiles (https://en.wikipedia.org/wiki/Coordinating_Committee_for_Mul...)

Airliners don't go over 1,200MPH or above 59,000 feet...

Airplanes broadcast their location using ADS-B. Using an SDR hooked up to a raspberry pi, you can get in on the action.

www.flightradar24.com

I would recommend something like adsbexchange instead, which ignores any filtering lists.... Information wants to be free?

Just a Garmin eTrex 20 - OSM maps, runs off (rechargeable) AAs - happily lasts 18 or more hours, although I do have to have a window seat on non-composite airliners.

An iphone will capture GPS on a plane just fine, although don't wedge it in the window shade - it will soon overhear.

Interesting, I encountered that kind of strangeness aboard a Paris-Moscow, where my phone GPS tracker pointed me earth level in Pulkovo airport (Saint-Petersburg, Russia). I estimated I was over Belarus at that time.

I never got to the why of this behavior.

I did a screenshot of that event here https://ibb.co/eCrV9y

What device do you use?

I don't see why any of the off the shelf GPS trackers for things like hiking wouldn't work just fine. One of the hundreds of cellphone apps that do this would probably suffice too.

Garmin make a ton of cheapish GPS trackers in lines like eTrex etc as well, but given cellphone battery life now being so good the need for such a thing is much less than it once was.

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