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Comment by jrochkind1 | Hacker News Reader

My memory says that US government GPS at one point intentionally introduced reduction in accuracy/resolution as well, but they stopped, which was part of what led to the commercial GPS revolution (along with cheaper tech of course).

Let's see... Wikipedia seems to confirm:

> During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability"; this was discontinued on May 1, 2000 by a law signed by President Bill Clinton.

https://en.wikipedia.org/wiki/Global_Positioning_System

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This is different but related. China has no ability to influence GPS accuracy within its borders. What they do is manipulate all of the authoritative maps available so that GPS coordinates won’t map cleanly to the digital waypoints in the map. The GPS locations are very precise, they are just off by as much as a quarter mile in varying directions depending on where in the county you are.

Can OpenStreetMap or someone make non-authoritative maps, or is this impossible without the cooperation of people on the ground who cannot operate safely within the borders of China?

It's "impossible".

Private, independent geographical surveying is illegal in China. [0] So technically you can't do it even in the local coordinate system.

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

But I guess to OPs point, you could make a non-authoritative map based on satellite imagery, it just wouldn’t be legal to use in China.

OSM has an extensive article about mapping in China. Generally it's not allowed. Still, in the bigger OSM is quite accurate. :>

Anything that can be seen from a satellite is quite accurate, that's true, but oftentimes things like number of lanes, type or direction of the roads is unknown, footpaths in rural areas are very spotty. Basically anything that requires local human verification (addresses, business location, etc) is off limits and accuracy in those points comes from pre-ban times (or illegal activity).

At one point we spent some time reverse engineering the Chinese coordinate system, it’s actually quite fascinating. So terrible though.

But OSM actually doesn’t use that system! They use the normal coordinate system, which makes them unique across mapping services (and also a good tool to use to RE geojson away from chinas system).

> This is different but related. China has no ability to influence GPS accuracy within its borders.

FYI: And even far outside of them too

I don't think so Tim.

https://spacenews.com/pentagon-report-china-amassing-arsenal...

At this point, destroying the positioning network/s would be mutually assured destruction. Pretty much every country relies on those networks equally for controlling their own vehicles and weaponry.

Some of those systems do. Not all.

" It is an Inertial Guidance System with an additional Star-Sighting system (this combination is known as astro-inertial guidance), which is used to correct small position and velocity errors that result from launch condition uncertainties due to errors in the submarine navigation system and errors that may have accumulated in the guidance system during the flight due to imperfect instrument calibration. GPS has been used on some test flights but is assumed not to be available for a real mission. "

https://en.wikipedia.org/wiki/Trident_(missile)

Tomahawk missiles are also fed a navigation package before launch which allows them to navigate without additional signals (I'm sure accuracy goes up if they do have GPS available). I believe it's something akin to terrain maps that it can use to navigate to it's target.

I would argue that any nation that is reliant on those public networks to be effective are going to lose within minutes of an actual conflict.

I believe it's already established that the Russians are capable of completely blocking out GPS signals.

https://www.nbcnews.com/news/vladimir-putin/russia-spoofing-...

So no, I don't think we'd be facing mutually assured destruction.

Now the impact on civilian life if some nation decided to start the space wars? Catastrophic. We'd basically block off space for the next 100 years because of deadly debris in orbit :/

And the debris created from a kinetic weapon would indiscriminately destroy other satellites at those altitudes. See https://en.wikipedia.org/wiki/Kessler_syndrome

The current ASAT weapons can only hit targets in low earth orbit, and the GPS sats are way above that. Anything that high up that is military run also likely has capability to maneuver to avoid something shot from the surface (which will take a lot of time to get up that high even if it has enough energy).

"China already has operational ground-based missiles that can hit satellites in low-Earth orbit and “probably intends to pursue additional ASAT weapons capable of destroying satellites up to geosynchronous Earth orbit,” says the Defense Department’s annual report to Congress on China’s military capabilities."

The problem with defending a satellite is the missile really only needs to get kinda close. And yeah, relativistic speeds and distances in space are huge, but I would find it hard to believe we could defend our satellites in any meaningful way against a nation-state level threat.

Interesting to note that the removal of selective availability enabled the creation of Geocaching ( https://en.wikipedia.org/wiki/Geocaching#History ).

It also enabled the modern mobile navigation industry. I was working on automotive navigation systems in the early 90s and SA was a killer for the product, options were various dead-reckoning and inertial sensors or differential GPS, both of which ended up being cost-prohibitive. But you can't do usable route guidance with a 100m CEP in an urban area.

With modern high powered CPU's, more detailed maps, and particle filters (which require all that cpu), dead reckoning has become much more viable.

I suspect you could go hours driving round a city with the GPS and WiFi location turned off before losing your position - simple wheel speed, gyro and compass is sufficient for most stuff.

Hmmm. What about the sensors on my phone? For a while now I've wanted to know my realtime speed in the underground sections of the local rail/metro network.

I suspect the most accurate measurements would be done using wideband SDR, iff I were able to acquire absolute position references for the signals I was seeing. Not likely, especially for something I might like to let others play with and/or generalize.

Everywhere I read about the subject the general consensus is that using accelerometer and gyro data from the average phone is a fool's errand. I have zero experience with the field so I wouldn't know if failure was incorrect signal processing or just had sensors.

Without wheel speed sensors, distance and speed errors quickly accumulate.

With a particle filter and enough computation, that error can be eliminated after the fact. Ie. At the time you won't know where you are, but after you get out of the metro and the particle filter reconverged you'll know where you were with more accuracy at some point in the past.

Ah. I see. Thanks very much for that insight - so that's why it's possible, but not generally interesting and/or widely pursued.

You'd also need to account for slope.

I’ll be overjoyed the day that the compass in the vehicle can tell my phone what direction the vehicle is pointed to avoid making a left instead of a right out of a driveway. Yes, there are solutions to these problems in theory, but in real-world application today they’re still pretty lame / inaccurate, and the solutions still seem far away (try getting a few auto mfgs to agree to a standard way of communicating a compass heading to a phone over CarPlay or Android Auto)

I've used a phone that didn't have a built in compass and one that does. The one that has an actual compass knows which direction I'm facing even without moving. The one that didn't would usually have no idea until I started moving.

They make external GPS units that plug into your car and connect to your phone wirelessly for example Garmin GLO that are supposed to improve accuracy although I haven't used one myself and thus can't vouch for it.

The built in compass in a phone is pretty inaccurate for driving because it can't compensate for the unknown shape of the body shell of the car it's in.

When turn-by-turn directions specify traveling 1 mile down a very steep grade, do they mean 1 mile along the slope, or 1 mile along a flattened map?

Only the former would match the car's odometer, of course.

There were already workarounds in place before they switched off SA - the introduced error was consistent within a given area, so provided you had a fixed location that broadcast its coordinates, you could correct for the error. I believe there were products and possibly even standards that did all of this; would have been even easier in today's world of mobile internet. I have suspicions that this was a large reason for disabling SA - your enemy can work around it, so it's not much use, but if you get your enemy hooked on it without the work-around, you can turn SA back on in a war situation.

Also, WAAS https://www8.garmin.com/aboutGPS/waas.html

When I was a civil engineer last century gps accuracy was an issue because people wanted to use gps for surveying. They came up with a system that would use 2 receivers and a radio between them to get much higher accuracies.

https://www.e-education.psu.edu/natureofgeoinfo/book/export/...

I think the US Government can also shut it off at a moments notice.

There are two frequencies (sometimes three) transmitted by the satellites. Using two frequencies allows certain atmospheric delays to be accurately estimated. Trouble is only one code is available for civilian applications. The trick with two receivers is to solve for the phase of secret signal (rather than decode it) by solving an integer least squares problem. This allows accuracy of the order of +/- 5cm

Mobile phones use dual frequencies now. Some use the Broadcom BCM47755 chip, but the most common ones are various Qualcomm Snapdragon SoCs.

https://www.broadcom.com/products/wireless/gnss-gps-socs/bcm...

https://www.euspa.europa.eu/newsroom/news/qualcomm-launches-...

https://www.gpsworld.com/qualcomm-launches-3-dual-frequency-...

There are multiple civilian frequencies available now, and cheap receivers for 'em. I have a pair of F9P's running at present, and just received some GT-U12's for testing.

They could, but multiple governments now publicly broadcast from similar constellations, so it wouldn't have nearly as much benefit for them to do so.

Modern dirt cheap receivers can pick up European, Chinese, Russian, etc. constellations for location just as well as the American one.

Always wondered about this, can overlapping systems be used simultaneously for higher reliability or precision?

Surprisingly the answer is most often no, however in poor signal areas like in a city or under canopy the answer is yes as you are more likely to get a signal from the remaining bits of sky view.

The other benefit is when doing PPP the convergence time is dramatically shorter with multi constellation.

When doing RTK most receivers will use only GPS, as it is generally the most accurate, but it can and will use GLONASS occasionally. I have never seen one use beidu or Galileo

Those two radio systems are still used. Just that many states have CORS sites that post correction information.

What is the accuracy of GPS surveying?

I briefely looked through the pdf, and probally missed it?

I live in the Bay Area, and homeowners are concerned over a few inches.

(There's a huge need for cheap surveys. Until GPS gets it to under a inch, traditional surveys will off pipes, and landmarks, is here to stay? Or, am I wrong?)

Civil and mining engineers have been making use of centimeter-accurate GNSS configurations for years now. The equipment necessary costs a few thousand dollars, more if you need even more precision. Most major civil and mining engineering projects use self-guided earth-moving equipment, not feasible without this technology.

The problem for homeowners is that surveys aren't enough. The vast majority of land titles are not registered in precise coordinates; they're registered in terms of landmarks, benchmarks, and so forth. This is changing at a glacial pace, but for now, the tech isn't the problem.

RTK has an accuracy of around 1cm. Its widely used by surveyors (and other things like agriculture, construction machine control, etc).

I've heard that just before Operation Desert Storm began in 1991 the reduced accuracy that affected civil GPS was temporarily turned off. This was a result of not being able to procure enough military grade GPS devices for army vehicles etc. If this is true it may also have had an effect on the decision to completely turn it off.

Circa 1999, I was a member of a search and rescue team through the explorer scouts. We got to carry milspec GPS devices on a hike once, because the forestry service wanted accurate maps of some trails. We were under strict orders not to deviate from the trail or tamper with the devices. Very fun cloak&dagger atmosphere for what was otherwise a lovely walk in a park. Hilarious that the need for such missions was obviated a few months later

I love walking around with purpose. Like when my car didn't have a tire iron in it, so I walked home and walked back in ratty clothes with a tire iron through a couple miles of nice neighbourhood.

That's really cool. So your tracked tracks (so to speak) helped inform maps of the trails you walked?

Yes, when it first came out, it was a boon for cruising sailors such as myself who were using radio-based Loran up to that time. If I remember correctly, the civilian resolution was originally 50 meters, then lowered to 10 meters. I believe it is 1 meter today.

As I recall, it was due to plane crash and US h̶a̶d̶ decided to make precise GPS available to public

After KAL007 was shot down by the Soviet Union in 1983 Reagan announced the fuzzy GPS signal would be publicly available. The US government didn't turn off Selective Availability until the late 90s.

That's weird though as a reason. KAL007 was so far off course that the distortion of SA wouldn't have mattered. It only distorted location by a couple hundred metres at most.

I read the GP as saying that SA GPS was originally made available as a result of the KAL007 crash, so presumably they didn’t have access to GPS at all on the flight? Then much later the SA restriction was removed, unrelated to a plane crash.

Though KAL007 deviated from course not because of some INS fault but just because autopilot didn't switch into INS following mode at all. Most likely crew just forgot to flip the switch, so GPS availability wouldn't have helped anyway.

Being out in the middle of the ocean with only INS supposedly keeping you on track is different than a GPS receiver that you can look at and see that you are not where you should be. Obviously the same thing would happen if no one was paying any mind to the cockpit instruments but at least a GPS receiver offers a solid reference to compare against. I really can't imagine how nerve wracking it would be trying to fly a plane without INS or GPS over an ocean. Your navigator would do their best to keep track of location but you'd probably end up way off course by the time you saw land.

Exactly.

IIRC civil GPS chips won't work above a specific altitude or when moving above some speed limit. I think the idea was to prevent people from guiding missiles using those chips.

Seems this is an ITAR restriction and only applies to chips exported from the US: https://en.wikipedia.org/wiki/Global_Positioning_System#Rest...

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