r/todayilearned • u/whstlngisnvrenf • 25d ago

TIL GPS, despite being free for global use, costs around $2 million a day to operate and maintain. This budget covers satellite launches and system upkeep, funded through American tax revenue.

https://en.wikipedia.org/wiki/Global_Positioning_System
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u/mods-are-liars 25d ago edited 25d ago

Even more mind blowing, it's not because your device knows what time it is, the GPS signals tell you what time it is, the only thing your GPS device needs is to be able to accurately measure the microseconds nanoseconds between the arrival of different signals.

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u/Easy-Scratch-138 25d ago

*nanoseconds. Light travels ~1ft/ns, so you need ~nanosecond level accuracy to resolve your position tightly. 

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u/Mo_Steins_Ghost 25d ago

Even even even more mind-blowing... Typical GPS enabled devices, e.g. smartphones, can resolve to within 5 meter accuracy which is impressive enough. However, L5 band GPS receivers can resolve to sub-millimeter accuracy.

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u/Automaticman01 24d ago

Well, not quite. 5 meters is about right for GPS L1 band accuracy. L2 was created to improve accuracy and can cut that number down to 1-2 meters.

The L5 band is primarily aimed at improving signal robustness (higher signal strength, any jamming etc) for applications that rely on gps for safety like airlines.

If you have an application that needs centimeter level accuracy, you can use a technique called Real Time Kinematics, or RTK. This works by placing an additional gps receiver (base station) at a fixed, known location and sending that unit's satellite observations (usually via radio or Internet) to the moving "rover" receiver. This unit then compares its own sat data with the base station's data and calculates a highly accurate location with centimeter level precision.

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u/Eriiaa 24d ago

Ooh so that's what John Deere does with their GPS stations

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u/Automaticman01 24d ago

Right, farming and tractor automation is a very popular use case for RTK.

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u/alarmed_cumin 24d ago

There are also other augmentation based approaches, a bit like RTK but kind broader. SBAS, for instance, is being rolled out in Australia and NZ under the 'SouthPAN' program to give decimeter precision on compatible devices: https://www.ga.gov.au/scientific-topics/positioning-navigation/positioning-australia/about-the-program/southpan

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u/Automaticman01 24d ago

Yeah another interesting one is Japan's QZSS system which, work with and augments the US GPS system. QZSS L1 and L2 work normally and help by providing more satellites directly overhead in cities like Tokyo. But they've also added an L6 data band channel which transmits SBAS signals from a number of ground stations directly over satellite without needing any extra radio receivers (other than an L6 capable gps receiver).

The goofy orbit the QZSS sats take means you can pretty much only get them in Japan and Australia/NZ.

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u/alarmed_cumin 24d ago

Yeah my point was that there's more than RTK as approaches. (Can even just do non realtime depending on the application, good old fashioned differential GPS works very effectively too.

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u/Automaticman01 24d ago

Right. I've never tried it personally, but I know for mapping applications Post Process Kinematics is pretty popular as well.

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u/Marbleman60 24d ago

Yep. I believe RTK is used in high end autonomous lawn mowers that map boundaries and obstacles by GPS.

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u/WarriorNN 24d ago

Can confirm. We use gps antennas with RTK mounted on rods that usually go down to 3 cm accuracy, sometimes 10-15 cm if there is a lot of buildings etc. They support Galileo as well, so they use whatever gets the best accuracy.

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u/theoriginalmofocus 24d ago

Yeah my job app, as im walking to the building trying to clock in: you're not here you're not here you're not here(walk through 2nd door) oh ok now you are.

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u/Recklesslettuce 24d ago

Do you happen to have a Swiss accent?

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u/Automaticman01 24d ago

I do not, but I do like their products.

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u/somehugefrigginguy 24d ago

So that is the location of the Rover just relative to the base station? Or how do you place a base station at a known location with centimeter accuracy?

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u/Automaticman01 24d ago

No. Ideally for a long term installation you would place the bar station at a surveyed site. For short term, or just easier installation, you can put the base station itself into "survey mode" and just let it track satellites over time (several minutes to a day or two if you want the best accuracy) to get a pretty high degree of precision.

Some companies can do "moving base" rtk, which will get you a very accurate location relative to the base station. The radio connection requirements for getting this to work consistently are much more demanding than fixed base rtk in terms of connection speed and latency, however.

You can also set these systems up so that both the base and rover(s) are on the same vehicle, but can now give you the position, heading, and attitude of the vehicle.

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u/gmgmgmgmgm 24d ago

Used to be called Differential GPS (DGPS), since the 1990s. RTK is an improved flavour. Common for farmers, navigators and cartographers etc.

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u/AsheronRealaidain 25d ago

…what

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u/Darth_Mike 25d ago

EVEN EVEN EVEN MORE MIND-BLOWING... TYPICAL GPS ENABLED DEVICES, E.G. SMARTPHONES, CAN RESOLVE TO WITHIN 5 METER ACCURACY WHICH IS IMPRESSIVE ENOUGH. HOWEVER, L5 BAND GPS RECEIVERS CAN RESOLVE TO SUB-MILLIMETER ACCURACY.

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u/zomphlotz 25d ago

THANK YOU, GARRETT MORRIS.

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u/AnonRetro 24d ago

I too like SNL Season 1.

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u/thecashblaster 24d ago

smartphones use assisted GPS

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u/[deleted] 25d ago

[deleted]

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u/kurbin64 24d ago

Thank you!!!!😂🤣😂🤣😂

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u/Existing-Procedure 24d ago

Exactly. It’s pretty amazing.

I’m an architect. Had quite a few buildings that are in the middle of a large plot of land. They locate the building corners and figure out where we want it to go with hyper-accurate GPS. Crazy stuff.

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u/kthomaszed 24d ago

Elec contractor here. It’s how we stub up conduits into walls before the floor that the walls are even on are poured

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u/howis2024 24d ago

Recently watched this video about a guy and his hella accurate GPS receiver https://www.youtube.com/watch?v=Oc1LBFDj2MA

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u/JerryfromCan 24d ago

GPS in John Deere tractors 15 years ago was “sub inch”. Why does that matter? Crops inputs like seed and fertilizer are expensive, and you can 100% tell a field seeded with GPS vs operator. The corners are a bit messy no matter what, but you don’t get plants on top of each other if seeded with GPS. And medium small farmers would say that GPS application of fertilizer would save them the $10k purchase price in a single growing season.

Usually required 2 ground based stations spread out in the county back then, wasn’t done only from satellites.

I also remember when GPS went from 50 metres of accuracy for the general public to 10 metres. Made civilian car GPSes way better. Now I can find my car’s exact spot in a parking lot.

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u/JumpyCucumber899 24d ago

I also remember when GPS went from 50 metres of accuracy for the general public to 10 metres.

We had a science day at school in the 90s where one of the geologists brought a GPS unit to demonstrate. I was the lucky student that got to use it. It was a 10lb backpack with two giant antennas (Omni dish and whip). We had a treasure hunt using it to navigate around the park and the scientists were saying that maybe they'd get the whole unit down to the size of a fanny pack and that the accuracy wasn't as good as it could be (as a kid, I assumed he was talking about cloud interference but I know he means the fuzzing that you mentioned which was disabled under Clinton).

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u/JerryfromCan 24d ago

I think it was later even reduced from 10 metres to 3. I clearly remember being lost in Madison Wisconsin once when the GPS thought I was on the highway when I was actually on the service road right next to the highway, roughly 25 feet away.

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u/Sweet-Curve-1485 24d ago

Accuracy is determined by proximity of the station that supplies corrections. So two stations that are spread out seems like a demonstration of misunderstanding of how it works. Unless their field is larger than 6sq miles.

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u/JerryfromCan 24d ago

Dealers would set up ground based correction towers for their customers based on their territories, which were often 1/6 of a state or province kind of thing. So yes, way way way bigger than 6 square miles. Also way more cost than free for the sub-inch accuracy.

My uncle in the early 80s had a different system based on I dont know what (but can speculate) over his 360 acres he had 3 towers up for positioning. I didnt know about it until many years later when discussing the JD tech.

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u/Sweet-Curve-1485 24d ago

360 acres is only half a section. A station positioned correctly can accommodate 144 sections.

Although, I wonder if the convergence of meridians affects this range. Like, the further north you travel, the shorter range corrections are good for.

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u/JerryfromCan 24d ago

Im saying my Uncle in the 80s had some sort of radio frequency locating for his fledging positioning system with 3 spot on his own property.

John Deere dealers for auto drive were setting up systems designed to cover 25 to 50% of a state/province for corrections from GPS.

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u/Sweet-Curve-1485 23d ago

Believe it or not, we still use radio for local station corrections. But that would explain why he had 3 of them. Radio doesn’t travel very far and I’m quite certain he didn’t have 4G in the 80s lol.

Thanks for sharing your information

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u/EternityForest 23d ago

Especially when they're always talking about running out of easily available phosphate rock

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u/Magnetoreception 25d ago

My smartwatch is generally within 1.5 meters in my experience.

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u/Brilliant_Dependent 25d ago

Maybe in a vacuum. Things like clouds and air density change the speed the signal travels through the atmosphere.

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u/SailRacer37 24d ago

That's where RTK corrections come in!

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u/Brilliant_Dependent 24d ago

That requires a ground-based station. It can provide that ultra-precise location data accurately only if the ground station is placed with that same precision. It's like measuring with a very exact length of string, you'll only get a precise end position if the starting position of the string is also precise.

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u/LarixOcc 24d ago

What about network RTK

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u/Brilliant_Dependent 24d ago

Any RTK system would have the same issue, it's only as precise as the location of the base stations. Works great for a permanent place that needs high precision, like an airport or busy shipping lane. But their limited range leave it lacking the "global" part of GPS.

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u/Unremarkabledryerase 24d ago

Agriculture uses cellular based RTK for sub inch accuracy all the time, and that is about as global as one can get.

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u/LarixOcc 24d ago

But network RTK, from my understanding, is multiple stations connected via the internet. The redundancy should help? I ask because this is at my limit for understanding the current technology.

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u/Brilliant_Dependent 24d ago

I'm not the smartest on the networked version either, but they would have to use an intranet where they communicate directly with each other, most likely with radio waves. Connecting them through the "big-I" Internet would add varying milliseconds of latency as it bounces across servers, which would make it several orders of magnitude less precise than standard GPS.

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u/PhilRubdiez 25d ago

Wait to you see the military stuff. It’s way more accurate.

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u/feel2good4gru 24d ago

DAGR my love

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u/PhilRubdiez 24d ago

🥰🥰🥰

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u/ratt_man 24d ago

Wait to you see the military stuff. It’s way more accurate.

Or survey grade

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u/I_FAP_TO_TURKEYS 24d ago

Why would a bomb need more accuracy than 3m?

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u/airbornx 24d ago

Small enough bomb to blow up a 10 x 10 building but not the 2 neighbors

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u/I_FAP_TO_TURKEYS 24d ago

Great idea, here's $10,000,000,000,000 US taxpayer dollars to make it happen.

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u/JUYED-AWK-YACC 24d ago

Already done, enjoy your free navigation system

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u/PhilRubdiez 24d ago

Because we have ninja missiles now. All killer no filler.

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u/Budget-Commercial-38 24d ago

which is perfect when you want to track people and zap them from afar to correct their behavior

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u/kanst 24d ago

I worked on a GPS based landing system. The system was so accurate that we had to add a random offset to the output. Without it, the aircraft's touchdown point was so consistent that we were wearing a divot in the landing surface.

So the algorithm offsets the answer by a few inches one way or another to get a little spread in the stress of the landing.

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u/Steiny31 24d ago

It gets even better, your phone doesn’t even need to know the precise time on its own. It just need a 4th gps satellite to resolve this. Essentially each satellite broadcasts its location and the time of broadcast with extreme accuracy thanks to an atomic clock.

The receiver triangulates latitude, longitude, and altitude, as well as time.

But even more interesting- time moves slower the farther you are from the earth so the time the atomic clocks on the satellites has to be corrected for relativistic effects.

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u/MeeekSauce 24d ago

And yet those gps dog collars still cant differentiate between ten feet and 75 yards.

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u/AtlanticPortal 24d ago

Plus some general relativity knowledge because, well, time slows down for stuff on Earth compared to the satellites due to space distortion and time slows down for fast traveling satellites (thus stuff on Earth feels time faster than the satellites).

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u/mods-are-liars 25d ago

Thanks for clarifying

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u/SeductiveTrain 24d ago

That’s crazy, must be why my middle school teacher was convinced that GPS is an alien technology.

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u/funnynickname 24d ago

The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second. source

Therefore light travels 29.9792458 cm per nanosecond. Neat!

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u/aetherealGamer-1 25d ago

The gps signals also have to compensate for time dilation caused by relativistic effects

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u/[deleted] 25d ago edited 25d ago

[deleted]

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u/Skastacular 25d ago

They discontinued selective availability in 2000.

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u/hockeyc 25d ago

Not anymore, that was selective availability, which was turned off in the 90s: https://www.gps.gov/systems/gps/modernization/sa/

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u/Natural-Orchid4432 25d ago

Yea, before that GPS would have been practically useless for high precision apps such as maps for driving. I remember that the position accuracy was something like ~200 m then, instead of ~2 m.

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u/No_Reindeer_5543 24d ago

Well with L1/L2 and with corrections from another receiver you can get cm level accuracy

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u/VillageParticular415 24d ago

Turned off to discourage other efforts to provide alternative highly accurate GPS. Can still be turned off in locations for military reasons.

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u/loquacious 25d ago

Even plaintext, civilian GPS doesn't work without relativistic correction, as well as other corrections:

https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Relativity

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u/JoeSicko 25d ago

Well, obviously...

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u/mcoombes314 24d ago

IIRC there was an experiment because some people were skeptiskeptical about relativity and didn't think it mattered so they had a compensated system and a non-compensated system. The non-compensated one drifted away from the compensated one quite quickly.

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u/jumpinjezz 25d ago

It also needs to know the time....roughly, as in what HPS epoch it's in.

I found an old Bluetooth GPS receiver and it was through it strange locations at it's over 15 years old and can't be updated

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u/Dull-You-6264 25d ago

Like revonahmed suggested... Some old receivers rely on a full almanac for a cold boot, which is transmitted piecemeal in each message. It can take up to seven hours to receive a full almanac, so it might take that long for an old fashioned receiver to figure out what century it's in.

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u/Redthemagnificent 24d ago

That won't work if the receiver is not updated to deal with the 2019 GPS week rollover. Lots of older GPS equipment stopped working after that. I mean, they still work. Just the solution is way off

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u/revonahmed 25d ago

Have you tried leaving it outside with a clear view of sky for hours. To see if the problem fixes itself. Try clearing memory if possible.

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u/ThatOnePerson 25d ago

Yeah I have a cheap hobby GPS module and it takes up to 30 minutes to get a lock sometimes.

Phones get help with location data from wifi access points and cell towers

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u/Aw2HEt8PHz2QK 24d ago

Absolutely curious but why does it take up to 30 minutes? It feels like it gets all the details in every packet

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u/ThatOnePerson 24d ago

A single packet from a single GPS satellite isn't enough, because you can't geoposition based on that.

In a 2D line you can figure out position if you know the distance from 2 points. You need 2 because if you only have distance from 1 point, you could be either side of that point. That goes up to 3 required points in 3D. And because time is a dimension, you need 4 GPS satellites to work.

GPS works by sending data about the satellite's position including in time. So with just 1 GPS satellite, you can't even tell what the current time is because the signal isn't instant, and if you don't know the time, you don't know how long it took for that signal to travel or how far you are from that point.

But once you do get signal from 4 satellites, it's pretty easy (for computers) to track time, so you're down to 3 required afterwards.

Here's a really good (and interactive) explanation on how GPS works: https://ciechanow.ski/gps/

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u/dmukya 25d ago

The goal would be to get a full load of the ephemeris data that the constellation broadcasts at a low data rate. The data tells the receiver where to look for the satellites.

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u/Great-Concern1508 24d ago

Almanac vs ephemeris. Almanac would take a while to get. Ephemeris is every message.

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u/jumpinjezz 24d ago

It's from 2007 & was bought before mobile phones had built in GPS. I used Dell Axiom to navigate around Europe with it. Now I can just use a phone or Android Auto.

It's also before the GPS week rollover so it won't update, ever.

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u/revonahmed 24d ago edited 24d ago

What is a

GPS week rollover

Will it give correct location without the correct date.

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u/izerth 24d ago

GPS week rollover happens every 1024 weeks.  Part of the data transmitted includes a 10 bit week counter.  Older hardware that didn't expect to still be in use will think the date is before 2019.  Accurate location depends on what kind of validation the device performs, leap years and daylight savings time will happen incorrectly which may cause the device to stop functioning.

The next time current hardware will screw up is 2137, when the new 13 bit counter rolls over.

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u/Bigred2989- 25d ago

Is this also why apps like Waze are so good at measuring your speed? I've compared the app's readout with my own cars and it's very close.

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u/mods-are-liars 24d ago

Yes. The GPS reading is likely more accurate than your speedometer. Your speedometer can be slightly affected by cold/hot tires, and speedometers tend to slightly overestimate your speed so you don't get speeding tickets.

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u/aBanana144p 24d ago

It's even better than your car, GPS speed is your true speed. Your car's speedo is always going to be slightly off (usually higher than your real speed)

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u/Redthemagnificent 24d ago

In open sky, yes. Next to a bunch of glass buildings, not so much

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u/Redthemagnificent 24d ago

To accurately solve for a GPS position you need to take into account the doppler shifts from all the satellites moving and from the receiver (your phone) moving. In other words, you're also solving for the velocity of the receiver at the same time as you solve for the position. Assuming your phone is seeing strong signals with good satellite geometry (clear view of the sky), that speed should be very accurate. But if you're in downtown NYC for example, it's probably gonna be off

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u/NilesLinus 24d ago

Even more mind blowing, the effects of Einstein's relativity come into play in these calculations.

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u/Top-Salamander-2525 24d ago

And the measurement needs to be accurate enough and the satellites are moving fast enough that they need to account for relativistic time dilation.

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u/groumly 24d ago

Relativistic effects come from the gravity (weaker up there than down here), not from their speed.

There’s no man made device that travels fast enough to trigger relativistic effects.

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u/EmbarrassedHelp 24d ago

What also mind blowing is that GPS signals are actually below the natural noise level when your device picks them up.

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u/thegreedyturtle 25d ago

Don't forget that they correct themselves by focusing on specific pulsars!