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What is the origin of the cap, gown, hood, sash, and tassel I'll be wearing? And when did it become a tradition for professors to wear their alma mater's gowns to commencements? 18 * 2 comments How true is the meme that revolutions tend to happen or only happen when some critical mass of the population is literally starving, versus other explanations? 97 * 7 comments How could knights be effective as heavy cavalry if their horses were largely unarmored? Welcome to Reddit, the front page of the internet. Become a Redditor and join one of thousands of communities. x 1759 1760 1761 Why is GPS free? Why is GPS free? (self.AskHistorians) submitted 1 day ago by zakelijke[silver_48] As far as I can remember, I never needed a paid data bundle to use GPS on my phone and old car navigation devices didn't require a subscription to get a good GPS signal. This seems odd to me since a lot of money had to be spent on sattelites when GPS was created. Why did the creators of GPS decide not to charge any money for it? * 229 comments * share * save * hide * report top 200 commentsshow all 229 sorted by: best topnewcontroversialoldrandomq&alive (beta) [ ] Want to add to the discussion? Post a comment! Create an account 2 [-]AutoModerator[M] [score hidden] 1 day ago stickied commentlocked comment (0 children) Welcome to /r/AskHistorians. Please Read Our Rules before you comment in this community. Understand that rule breaking comments get removed . Please consider Clicking Here for RemindMeBot as it takes time for an answer to be written. Additionally, for weekly content summaries, Click Here to Subscribe to our Weekly Roundup. We thank you for your interest in this question, and your patience in waiting for an in-depth and comprehensive answer to show up. In addition to RemindMeBot, consider using our Browser Extension, or getting the Weekly Roundup. In the meantime our Twitter, Facebook, and Sunday Digest feature excellent content that has already been written! I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns. * permalink * embed * save * report * give award [-]Conrolder 26810Answer Link2680 points2681 points2682 points 1 day ago*[silver_48][gold_48][Illuminati]2[Starry_48] (198 children) Oh man! A question I can answer! I'm a GPS engineer. I'll answer this in a sort of roundabout way by explaining the history of GPS and how it works - then get into why it's used for civil application and why you don't have to pay for it. GPS was originally a US AirForce program called Navstar. Navstar started in 1973. It was a spiritual follower of other navigation-based programs such as Loran (a 2D positioning system for ships on water), and Decca (a hyperbolic radio navigator based on calculating one's position based on the intersection time of radio signals). These hyperbolic navigation systems were originally started in WW2 to assist bomber runs. The idea of a space-based version of a navigation system is said to have started with the Soviet launch of Sputnik-1. A group of DoD funded engineers at APL were tasked with figuring out where Sputnik-1 was, and because Sputnik-1 transmitted a continuous waveform, it experienced a measurable doppler shift (if it traveled towards you, it sounded higher pitched - when it passed overhead and continued on, it had a lower pitch). In this way, a group of scientists at APL were able to figure out where Sputnik-1 was! [1] The US DoD then began to investigate new methods for navigating off of radio signals from space specifically, eventually leading to Navstar. Navstar as a program was born near the end of Vietnam. During Vietnam, if the US wanted to destroy a bridge, they had to fly sorties over that bridge and drop bombs in the hope that one of those bombs would hit. They had a very high miss rate, caused immense collateral damage, and costed a lot of money because the accuracy of bomb drops was so low (I won't pull a reference for this, but the Thanh Hoa bridge is a great example of this problem). Thus, the Navstar program which would become GPS was implemented to try to resolve the massive challenges associated with target accuracy and navigation. The Navstar program spent 25 years getting from program inception to final delivery of a full GPS constellation (you need around 30 to navigate, because they're medium-earth orbit globally orbiting satellites, and you need four overhead at any given time to work - it took them a while to get all of those up!) GPS works by resolving the GPS pseudorange equation through trilateration. That is, the satellites transmit two things (broadly): 1) their own precise position, monitored by a group of surveilled ground control monitoring stations around the world, and 2) the precise atomic reference time at which their signals are transmitted using on-board clocks occasionally updated/corrected from the ground. A receiver on the ground has a bad clock and doesn't know where it is, so it resolves a nonlinear equation with four unknowns (it's position in 3 dimensions and its clock error) from the GPS satellites. It's hard to explain without getting into the math, but just know that in this way, all GPS receivers receive very precise timing, as well as their position, by calculating the intersection of four spheres (a great depiction of this is here: https://ciechanow.ski/gps/). During the Navstar program, there was a big push for GPS to be provided as a civil service. For starters, it gave near-atomic clock quality time for next to nothing in cost (you get the benefit of the GPS satellite clocks on your handheld receiver), as well as instantaneous position globally. The timing in particular was a really big deal to the US here - the power grid requires precise timing, the stock market does, etc. The GPS program made all of those things cheaper, better, and easier. So the DoD was always considering some version of a civil service for GPS. And then in September, 1983, Korean Airlines Flight 007 accidentally flew through restricted soviet airspace and was shot down, killing 269 people. This was the final incentive that the US needed to publicly provide a GPS civil service. Another reason that the civil service was allowed was technological. The GPS satellites, which were AirForce assets, transmit a signal called P(Y)-code, which is a military GPS signal with an encrypted code (only military receivers can use it). At the inception of GPS, it could not be directly acquired (doing so required that you knew pretty well where you are), so the Navstar team developed something called "Coarse Acquisition", which was another, worse signal that could be navigated off of in order to get 'good enough navigation' to get to P(Y)-code. This signal was already being transmitted for military use, and by providing it for civil use, civilian users got a worse version of GPS through C/A. In other words, providing civil use didn't negatively interfere with military use, made stock market and power grid work cheaper (and many other things like public infrastructure development, surveying, etc.). When they first provided 'free to all' GPS, the AirForce created Selective Availability - a scrambling code on the C/A signal that made it worse than it normally would be (by about 10x). This made C/A GPS 'good enough to navigate off of' but not good enough for military application, as the US was worried about adversaries using it. In 2000, the US formally turned off Selective Availability, allowing civil use (/u/abbot_x gives a great answer as to why in the comments below). Today, the GPS program is one of the only military programs where civil services (the Department of Transportation, I believe) sits on the stakeholder committee for the branch that runs it out of AFRL, and they use it for everything. And a lot of other countries have navigation satellite constellations too now (the EU, Russia, China, Japan, and India). TL;DR: US taxes paid for GPS, but you really get access to it because it helps the US government substantially in aviation, civil, infrastructure, economic, and military sectors, and the version of GPS that you're using is still substantially worse than the one the military uses. There's some legacy effect here too - the US originally only let civil users use an acquisition code that was never meant for navigation, whereas now they have dedicated civil use signals (mostly due to the intense peer pressure of continued civil reliance). [1] https://web.archive.org/web/20120512002742/http://www.jhuapl.edu/ techdigest/td/td1901/guier.pdf Recommending a few books that talk about these topics and history in the historical chapters: 1. Kaplan and Hegarty, Understanding GPS/GNSS: Principles and Applications, Third edition 2. Misra and Enge, Global Positioning System: Signals, Measurements, and Performance Also a good online resource for all things GPS is Navipedia, produced by the European Space Agency but broadly maintained as a wiki (if you want to take a look at more of the math). Edit: u/victorfencer pointed out that Loran pre-dated Sputnik-1, and I've gone back and checked my textbooks and fixed this. My apologies! Edit 2: /u/chteme pointed out I should have said surveying, not surveilling (though you know, it's probably applicable to a lot of stuff). Edit 3: I've gotten a good number of questions about why they turned off SA, and /u/abbot_x gives a great answer below, much better than I would have given, if you want to know more! Edit 4: Very incredibly kind of all of you. I've got several updates here. First, (and I've fixed the post above with this), the GPS trilateration equation is nonlinear, and you can see a great visual of it here: https://ciechanow.ski/gps/ (somebody posted this and it's very cool and I think their comment got deleted). Second, I commented on some major differences between the different constellations here: https://www.reddit.com/r/bestof/comments/13ypf9i /comment/jmql9g2/?utm_source=share&utm_medium=web2x&context=3. Third, there are a lot of comments regarding time dilation. Fun history fact - the first space-based precursor to GPS was called Transit, and was the first technology that had to actively account for time dilation or stop working, and it assisted in proving Einstein's Theory of Relativity (or perhaps more aptly, continued to prove it). GPS does the same thing! Today it still accounts for time dilation through regular updates to the timing on-board satellites. Fourth, just as a note to really try to hammer home WHY GPS is free, GPS is estimated to produce $1.4 trillion per year in economic gains for private-sector businesses (https://www.nist.gov/news-events/news/ 2019/10/ economic-benefits-global-positioning-system-us-private-sector-study). This is in addition to all of the governmental gains in infrastructure, transportation, aviation, power grids, stock markets, good ol' timing, etc. I think part of the trick here is that the US knew this would have impact that extended way beyond the already massive military application, and events like Korean Airlines 007 were a straw that broke the camel's back on that discussion. But making it 'free' already saves the US a ton of money (both for private and public use) and that more than any other reason is why it's free! * permalink * embed * save * report * give award * reply [-]Joker042 345 points346 points347 points 1 day ago* (71 children) Thanks, that's a great answer, with a lot of context you don't normally see. Potentially stupid question: If I know I'm on the surface of the earth, do I really need four satellites to determine my position, or can I do it with three and the assumption that of the two potential solutions, I'm closer to the earth's core? * permalink * embed * save * parent * report * give award * reply [-]Conrolder 478 points479 points480 points 1 day ago* (60 children) Not stupid at all! The traditional GPS trilateration equation would be underdetermined with fewer than four satellites, so if you only have GPS you can't normally resolve it without four. However, there are lots of ways to fix that, one of which you mentioned! That's called a nonholonomic constraint. You constrain the possible positions and motions of your vehicle/position such that it reduces the number of possible solutions to the math problem. Ultimately, someone would have to do math to know if that constraint in particular would be enough. Another great way to need only 3 satellites is to just have an atomic clock with you! If you don't have to resolve your clock error, you can solve the equation easier. Finally, most navigators nowadays use an inertial measurement unit (IMU) to navigate, and just aid it with GPS. There are a lot of reasons for that (IMUs measure attitude, they have high update rates, but they drift wildly and GPS fixes that drift). But if you fuze the data between GPS and IMUs in a specific way, you can always get some information from even one GPS satellite (basically, you resolve how far away from that satellite you are, and that helps constrain IMU drift only in that direction). So having fewer than four satellites is not necessarily a dealbreaker. Fun (related) history fact: GPS satellite signals are extraordinarily weak and can't pass through buildings. If you try to use GPS in New York City, you'll often get lost very quickly because of this. To solve this, Japan built the coolest thing ever--their satellite constellation, QZSS, is designed with a really wonky orbit to align to have a great number of satellites overhead (near-zenith), so that you can always get at least four combined QZSS/GPS satellites even when you're in Tokyo. So even though GPS doesn't work in New York, it does in Tokyo! Edit: /u/GregHall44 corrected my poor phrasing in reference to Tokyo's grid pattern, and I've fixed that little bit of misinformation in my previous reply. * permalink * embed * save * parent * report * give award * reply [-]Numpostrophe 61 points62 points63 points 1 day ago (33 children) Why is that, in a plane, my GPS only works like 2% of the time? Is it true that it's disabled at certain altitudes for civilian use? * permalink * embed * save * parent * report * give award * reply [+][deleted] 1 day ago (8 children) [removed] [+][deleted] 1 day ago (7 children) [removed] [+][deleted] 1 day ago (2 children) [removed] [+][deleted] 1 day ago (1 child) [removed] [+][deleted] 1 day ago (1 child) [removed] [-]rocketsocks 88 points89 points90 points 1 day ago[silver_48] (3 children) There are a few reasons for this depending on the details of the situation. First, you are inside of a metal tube which is good at blocking outside radio signals. There will still be some signal through the windows but it may or may not be enough to get a lock on the satellites. The second is that there are different ways to use GPS and most of the time when you use GPS with a smartphone you "cheat" to make it work faster. In order for GPS to work you need data about the satellite locations, and this data needs to be updated over time so it can't just be stored once forever, it needs to be updated regularly. This data (the almanac and ephemeris data) needs to be downloaded in order for the GPS handset to be able to get a location fix. Fortunately, the GPS satellite signals broadcast this data, but they do so only at a very low bitrate and only periodically (along with the time code that is the core of the positioning system). During a "cold start" where you have no data and no fix your handset has to wait until it acquires signal from satellites, which might take a while, and then you have to wait until all of the necessary data is downloaded. This typically takes several minutes. So if you are using a smartphone with no access to the internet (perhaps in "airplane mode") and you are trying to get a GPS fix it will usually take several minutes, during which time you might give up and decide "it's not working". This workflow might be fine if you understand the limitations and are using a dedicated GPS handset specifically for a location fix and you are in a circumstance where it's an acceptable tradeoff (it could still be faster, and more accurate, than busting out a map and compass). Especially since after the first cold start subsequent "warm" starts or hot starts will take much less than a minute (or just a few seconds) to acquire a fix. However, if you're trying to use GPS as a convenience feature in day to day life this workflow is not ideal, which is where assisted GPS or A-GPS comes in. If your GPS handset (or GPS functionality integrated into a computing device like a smartphone) has the ability to connect to the internet then it can simply download the necessary data out of band, at high data rates and low latency. This is what basically all smartphones do when they use GPS. They download A-GPS data over wifi or the cell data network (4G/5G) in a fraction of a second and then use that to get a GPS fix in mere seconds. They can also use lower resolution location tracking (such as via cellphone tower triangulation) to jump directly to a "hot fix" very quickly. Many smartphone map applications (like Google maps) are just not well designed to work in fully offline mode so they may be heavily dependent on the A-GPS workflow. However, you can get GPS only apps which you can use on planes though you will typically have to wait several minutes for them to go through the cold start process, assuming that you can receive enough GPS signal within the plane. tl;dr: Plane bodies block radio signals and GPS relies on data that has to be downloaded. Phones download that data separately over an internet connection, and without it you will have to wait several minutes for a fix, but the app you're using may not be designed for a fully offline workflow even so. * permalink * embed * save * parent * report * give award * reply load more comments (1 reply) [-]Sharlinator 28 points29 points30 points 1 day ago* (0 children) What the others said (a plane is a Faraday cage!) and also the fact that phones cheat like hell to obtain a fix quicker and to get a fix even without a direct LoS to at least four satellites. This is called A-GPS or Assisted GPS. When they're connected to a cell tower, which is almost all the time, they already know roughly where they are thanks to a database of cell tower locations, which helps with the calculations. They can also use a database of wifi hotspots to get an even tighter approximate location if there happens to be a known hotspot close enough. (It's not usually easy, if possible at all, to get a GPS fix indoors except maybe near a window. Your phone still probably gives you a precise location - unless you turn on flight mode!) It also helps a lot that the software can assume that if the last fix was an hour ago, the device almost certainly hasn't moved too far from the last known position. But that doesn't hold in an airliner traveling at 900 kph either! If you turn off your phone, drive a couple hundred km to wilderness where there's no cell signal, and turn the phone on again, you'll likely have to wait for a few minutes for it to figure out where it is. * permalink * embed * save * parent * report * give award * reply [+][deleted] 1 day ago (1 child) [removed] [+][deleted] 1 day ago (1 child) [removed] [+][deleted] 1 day ago (1 child) [removed] [+][deleted] 1 day ago (7 children) [removed] [+][deleted] 1 day ago (6 children) [removed] [+][deleted] 1 day ago (3 children) [removed] [+][deleted] 1 day ago (1 child) [removed] load more comments (4 replies) [+][deleted] 1 day ago (2 children) [removed] [+][deleted] 1 day ago (1 child) [removed] [-]postmodest 12 points13 points14 points 1 day ago (13 children) Is there some international cooperation for navigation systems? Like, is there some minimum standard for "using whoever's satellites you can see"? Or at least, agreeing globally about "What time it is in orbit"? (corollary: what time is it in orbit? How do the ground transmitters that update the clocks account for time dilation when setting multiple clocks?) * permalink * embed * save * parent * report * give award * reply [-]Conrolder 39 points40 points41 points 1 day ago (3 children) There is some international cooperation (especially between allies, like NATO has led to some collaboration between Galileo and GPS), but the biggest part of international regulation for these signals is frequency allocation (which is a big deal for all spectrum transmission content globally). All of these signals are fully passive - no one has to coordinate what a user does with it, just makes sure the signal structure aligns. Ultimately, if a receiver wants to listen to any of these signals, it has to know the answers to some questions about that constellation like: 1) the signal structure, 2) codes for the signal matched-filter tracks, 3) position ephemerides for the satellites, 4) message structure used by that satellite (to include timing information about how that constellation's clock works). This really gets into the technical challenges with using these constellations, but I would say, the countries tend to build their own standards, and GPS receiver companies figure out how to handle those standards. * permalink * embed * save * parent * report * give award * reply [-]Nong_Chul 7 points8 points9 points 1 day ago (2 children) So if you're using a phone in Japan are you more likely to rely on the Japanese positioning satellites, or do civilians all over the world use a preferred satellite group (US or some other)? I guess what I'm trying to ask is how the device determines which satellites you use, is it just whatever the vendor for your device decided to program? * permalink * embed * save * parent * report * give award * reply [-]Conrolder 22 points23 points24 points 23 hours ago (1 child) This is an interesting question with a somewhat complicated answer, and someone from a GPS receiver company may give a better answer, but I'll share what I know. Let's say you have a receiver that can listen to every constellation (if it can only listen to GPS, it'll only listen to GPS signals, obviously). If it can listen to all of them, usually what it does is try to minimize something called geometric dilution of precision (DOP). A receiver has a set number of correlators in it (if a receiver has N available correlators, it can track N signals. Someone will inevitably comment on this and say that with SDRs/new receivers, there may be a dynamic correlator spinup, and that's true - but most receivers will allow up to N signals to be tracked, where N depends upon the receiver). Most receivers will identify signals that can be tracked, and check their health (how stable the peak is, that the timing makes sense, data on it looks good, that the power is clear enough that it's navigable). If all these heuristics look great, the receiver will then take as many signals as it can reasonably track and pull them into the solution (with some caveats - usually a receiver will leave some correlators open to go look for other signals, perform security checks, etc.). A receiver will almost always try to use as many signals as possible (from ANY constellation available) because the more signals you have, the more accurately you can navigate in a least-squares sense (prob and stats 101, translates to 'you are a little more accurate with more signals'). if there are more available signals than correlators , the receiver has to downselect. To do that, it will pick satellites with the most varied geometries that are healthy by whatever metric it decides means 'healthy', because the greater the geometric diversity, the better the accuracy of the GPS solution. TL;DR a receiver that can track multiple constellations usually tries to maximize the geometric diversity of satellites it's listening to, rather than which government built the satellite, because that's what gives it the most accurate solution. there are lots of caveats to that in the form of signal health though. * permalink * embed * save * parent * report * give award * reply load more comments (1 reply) [-]Conrolder 27 points28 points29 points 1 day ago (7 children) With regards to your question about time dilation, the GPS user standard references a note that the satellites compute for relativity for their velocity referenced to a specific point on the surface of the earth relative to them at any given time. The accumulated doppler the receiver tracks is then part of the nav message picked up by the user that they can use to navigate (specifically, doppler is a function of relative velocity between the satellite and user receiver, so you can back out your velocity from it). Time dilation is fascinating here - the satellites DO experience time dilation. Every 4-6 hours, Schriever AirForce base in Colorado Springs updates satellite ephemerides and resets the time according to the international standard for GPS reference time, which is LUDICROUSLY set to the number of seconds which have passed since September 1, 1983 (I think - it might be a different day). THAT's the time reference used by satellites. And every 4-6 hours they try to fix miniscule errors to keep that time standard. With drifting time dilation, every great once in a while the AirForce (now SpaceForce, actually) adds a 'leap second' to GPS clock time, and satellites adjust for that. If a receiver doesn't realize the time has changed, and gets the time wrong by a second, it would instantaneously be wrong in position on the order of 1s * c (or, about 300,000km). Therefore, it's very important that receivers know there is a leapsecond and can fix it, and that's part of the message transmitted by satellites. * permalink * embed * save * parent * report * give award * reply [-]cosmitz 16 points17 points18 points 1 day ago (3 children) So what you're saying is that during a zombie apocalypse where all infrastructure stops being maintained, GPS will very quickly become useless? It's fascinating to me to realise how many things quickly go down the drain the moment we stop caring for it. * permalink * embed * save * parent * report * give award * reply load more comments (3 replies) [-]bluegreencurtains99 2 points3 points4 points 20 hours ago (0 children) Thanks so much for all your detailed answers. I never realised the history of GPS technology was so interesting! * permalink * embed * save * parent * report * give award * reply [-]silverappleyardModerator | FAQ Finder 2 points3 points4 points 10 hours ago* (0 children) With drifting time dilation Just a small correction - time dilation doesn't drift, but the precise speed of Earth's rotation does. As a result they have had to add leap seconds to keep UTC time in line with Earth Solar time. The whole thing was disruptive to industries that use GPS for precise timing. The drift between these two times has been slowing and, based on the trend, in the future they'd need negative leap seconds - even more disruptive because now you could have identical timestamps for two non-simultaneous events. So last I heard the assumption was that leap seconds wouldn't be applied any more. * permalink * embed * save * parent * report * give award * reply [-]kc2syk 1 point2 points3 points 19 hours ago (0 children) They stopped making leap seconds a couple years ago. No more planned for the foreseeable future. * permalink * embed * save * parent * report * give award * reply load more comments (1 reply) [+][deleted] 1 day ago (5 children) [removed] [+][deleted] 1 day ago (1 child) [removed] load more comments (1 reply) [+][deleted] 1 day ago (3 children) [removed] load more comments (2 replies) [+][deleted] 23 hours ago* (1 child) [removed] [-]abbot_x 130 points131 points132 points 1 day ago* (5 children) I want to augment this answer to address the end of Selective Availability (SA), which drastically increased the utility of GPS. First let's talk about SA: When they first provided 'free to all' GPS, the AirForce created Selective Availability - a scrambling code on the C/A signal that made it worse than it normally would be (by about 10x). This made C/A GPS 'good enough to navigate off of' but not good enough for military application, as the US was worried about adversaries using it. Specifically, SA worked by introducing deliberate errors into the time signal sent by the satellite and providing incomplete information about the satellite's position. This was done because without it C/A would have been too precise for the military's comfort--it was arguably more accurate than had been intended. In the 1980s, civilian receivers just using C/A had been able to get approximately 20m accuracy in horizontal position. SA, implemented in March 1990, made that more like 100m. You could stand in a field with a civilian GPS receiver and watch your position bounce around--more on this later. So under the SA regime, GPS could not be used for some of the precise navigation tasks we now take for granted. My favorite example of this is that the hobby of geocaching is an entirely post-SA phenomenon, with the first geocachers being attested just after SA was turned off in the first seconds of May 2, 2000. In addition, car navigation was pretty terrible under SA: getting within 100m of a particular address might be acceptable, but the stupid GPS not knowing if you're on this road or some parallel road makes it useless. You simply couldn't conduct street-level navigation. And of more significance, both commercial aviation and maritime shipping didn't find SA-corrupted GPS sufficiently precise so had to continue to use other navigation systems that cost a lot to maintain. This was frustrating for stakeholders in those industries including the government agencies that promoted, facilitated, and regulated them. So here's the end of SA: In 2000, the US formally turned off Selective Availability, allowing civil use (I am omitting the reasoning I have always heard for this because I cannot cite a source). Officially, President Clinton claimed the benefits of zeroing out SA and allowing everyone to use accurate signals were great while there would be "minimal impact on national security." What I've heard may or may not be what other people such as u/ Conrolder heard: basically, SA was in the process of being defeated by technologies that were being not just developed but actually fielded by the Department of Defense's adversaries in . . . other agencies of the federal government. (You thought it would be the Russians.) And I'll point to the National Academy of Sciences' (NAS) 1995 report The Global Positioning System: A Shared National Asset ( download) as a source for this though I'll admit that was not the way I first became aware of this issue. Remember that guy standing still in a field in the mid-90s watching his position bounce around? Advocates for civilian use of GPS had been doing similar experiments since the inception of GPS and realized that some of the errors in position are systematic. The could be corrected in real-time if you had a ground station send information about the discrepancy between its fluctuating GPS-indicated position and unchanging actual position. SA-introduced errors were very similar for users who were close to each other and relying on the same satellites. The basic concept of correcting GPS based on fixed ground stations is called Differential GPS (DGPS). DGPS was actually being implemented by U.S. government agencies in the 1990s. Most notably: * The U.S. Coast Guard (USCG) created a system simply called DGPS that broadcast correction signals on marine longwave frequencies, prioritizing maritime users. DGPS was generally available to maritime users who had compatible units starting in 1994; prior to that, USCG and several other agencies (including the U.S. Army Corps of Engineers, National Oceanographic and Atmospheric Administration, and National Air and Space Administration) had been using DGPS for internal purposes such as precise siting of navigational and surveying aids, cartography, and scientific research. * The FAA and U.S. Department of Transportation (USDOT) was aiming for a more ambitious system known as Wide Area Augmentation System (WAAS). The DGPS system from the ground stations would be relayed to satellites over the United States, who would send it back down to primarily aviation users in the same frequency band as GPS. WAAS wasn't operational till 2003 but it was already under discussion in the 1990s. These systems actually improved the positions that could be derived from the C/A to better than had been available in the pre-SA days, by not just rendering SA a non-factor but also counteracting some of the environmentally-induced imprecisions. There are actually a bunch more DGPS and other GPS enhancements. Your smartphone probably uses some of them, but I'll leave that aside for now. Note USCG DGPS was discontinued in 2020 because it was redundant to WAAS. If you read the NAS report I referenced, it contains strong recommendations that SA be discontinued in part because the various DGPS systems had or would soon render it obsolete. The war between SA and DGPS was being waged by the U.S. government on both sides. But there was no reason other actors couldn't also come up with their own DGPS systems. And--again--GPS had substantial potential that SA was blocking. I'd go so far as to say you probably wouldn't have GPS in your smartphone if SA were still running and DGPS had somehow been banned. EDITED for clarity. * permalink * embed * save * parent * report * give award * reply [-]cairdazar 38 points39 points40 points 1 day ago (1 child) During the same time frame, Civilian departments in several European country's where also developing/deploying DGPS systems, some using the mobile phone network to send corrections to GPS units in the field. Many of those systems are still in use today, enabling sub-centimeter accuracy, without need for post-possessing, in professional equipment. * permalink * embed * save * parent * report * give award * reply [-]abbot_x 24 points25 points26 points 1 day ago (0 children) True, a lot of people were working on SA workarounds. But U.S. non-defense agencies were the furthest along and were throwing the most resources at this problem. DGPS and WAAS were massive undertakings. The non-defense agencies were also able to make themselves heard within the U.S. government. And one of the arguments they could make was that if we can do this so can somebody else, so SA is ultimately a waste. * permalink * embed * save * parent * report * give award * reply [-]victorfencer 22 points23 points24 points 1 day ago (2 children) I really appreciate your joy and technical expertise! One quick question though: I was under the impression that loran predated Sputnik, is that a misstatement/mistake or do you have some other info about it? I'd like to learn more . A Quick Google search brings up world war II information. And quite frankly, the technology is quite simpler since it is a ground-based system. Fun fact, lots of old hands in the commercial fishing and whale watch world of New England will still use Loran numbers for navigation and communicating positions to each other. They often have a GPS system that can convert the lat/long into the old numbers that they have memorized. It's like a tiny microcosm of why it's so hard to change measurement systems in the US from imperial to metric * permalink * embed * save * parent * report * give award * reply [-]Conrolder 15 points16 points17 points 1 day ago (1 child) Oh man, you're right! I just checked a textbook and I was wrong. I'll fix that. Thank you! * permalink * embed * save * parent * report * give award * reply [-]abbot_x 10 points11 points12 points 1 day ago (0 children) It really depends on to what degree you distinguish WWII-era LORAN from the Loran-C system that debuted in 1957. They are both hyperbolic navigation systems but Loran-C has some technical differences that make them different systems. In particular, Loran-C required affordable phase-locked loops, which weren't available till the 1950s. This allowed use of lower frequencies, thus longer ranges. * permalink * embed * save * parent * report * give award * reply [+][deleted] 1 day ago (4 children) [removed] [+][deleted] 1 day ago* (1 child) [removed] [-]Spare-Competition-91 16 points17 points18 points 1 day ago (23 children) This makes me wonder how it is for military. That must be wild to have that kinda tech. * permalink * embed * save * parent * report * give award * reply [-]Conrolder 66 points67 points68 points 1 day ago (21 children) I can tell you! The GPS satellites transmit P(Y)-code and M-code. P(Y)-code is a legacy military system, but it is more accurate than C/A-code. Normal civil C/A code provides ~3m-accurate navigation. P(Y)-code quotes around 1m-accurate navigation (in my personal experience, it's better than C/A but not always this good, but I assume it will vary by receiver like C/A code does). M-code wasn't really built to beat out the accuracy of P(Y)-code, it was built for security and resilience to jamming. Because the GPS signal is so quiet, it is eminently 'jammable'. As once was quoted to me (so take this with a grain of salt), the power of GPS is about the same as the light output of a yankee stadium lightbulb observed in Denver, CO (it's 10^-16W received power on the ground). That means it's really vulnerable to jammer signals, where someone tries to intentionally produce noise over that band to prevent GPS. The M-code signal structure was designed to improve anti-jam resiliency (and anti-spoof resiliency, there are security requirements for M-code that allow it to self-check against false signals that pretend to be GPS). TL;DR - about 3x as good, but also some other neat security features! Interestingly, there's a lot of active work to improve accuracy even better. There are methods to get sub-centimeter accuracy (but they're REALLY hard so not commonly used), and also there's some on-going efforts to provide augmentation to satellite constellations to provide decimeter-accurate navigation by putting up THOUSANDS of Low Earth Orbit (LEO) satellites which transmit GPS-like signals. China's BeiDou-3 constellation is the main one currently planning LEO augmentation. * permalink * embed * save * parent * report * give award * reply [-]Xanimede 6 points7 points8 points 23 hours ago (3 children) Do you know what would the benefits of sub-centimetre accuracy be? What kind of things can be achieved in that but not with meter accuracy? * permalink * embed * save * parent * report * give award * reply [-]Southforwinter 12 points13 points14 points 22 hours ago (0 children) That kind of accuracy is good for surveying, especially construction surveying. Telling someone that their boundary is in this spot +- 50 centimeters is seldom acceptable and it's even more important if you're laying out building plans. * permalink * embed * save * parent * report * give award * reply [-]Conrolder 9 points10 points11 points 23 hours ago (0 children) There's a lot of things, but the most common civil thing I hear about is augmented reality! You need to know exactly where something is to put a hologram on it * permalink * embed * save * parent * report * give award * reply load more comments (1 reply) [+][deleted] 1 day ago (1 child) [removed] [+][deleted] 1 day ago (6 children) [removed] [+][deleted] 1 day ago (2 children) [removed] [+][deleted] 1 day ago (1 child) [removed] load more comments (1 reply) load more comments (6 replies) load more comments (1 reply) [-]SkipsH 13 points14 points15 points 1 day ago (7 children) Can I ask what the uncited reason for turning off selective ability was that you've heard? * permalink * embed * save * parent * report * give award * reply [-]Conrolder 28 points29 points30 points 1 day ago (1 child) /u/abbot_x gave the answer I had heard, which was DGPS competition. He found citations too! I'd reference this as the main response to this question: https://www.reddit.com/r/AskHistorians/comments/ 13y7ee7/comment/jmmqcyy/?utm_source=share&utm_medium=web2x&context=3 I had also heard that some military users were buying civil receivers from walmart when they ran out, and that was causing problems for some military application at the time - but that's much less substantiated haha. * permalink * embed * save * parent * report * give award * reply [-]abbot_x 25 points26 points27 points 1 day ago* (0 children) Thanks! Your answer is so good. I'm just glad I had access to a source that could corroborate what I was pretty sure you were trying to get at. I had also heard that some military users were buying civil receivers from walmart when they ran out, and that was causing problems for some military application at the time - but that's much less substantiated haha. I'd consider that substantiated! It's discussed in the NAS report I linked. Basically, during Desert Storm, the U.S. and Coalition military need for GPS was much greater than anticipated in peacetime procurements, just because it is so useful for everybody to know exactly where they are. The ground commanders realized it would be preferable to have a GPS on every single vehicle, if possible every person. So they bought up tons of civilian units, like those old Garmins some of us may still have lying around. Because so much of the force was using civilian units that were subject to SA, they just turned SA off. It's said, I believe in the U.S. Army official history of that war, that it was the first war in which a commander knew with a high degree of certainty where all his own forces were. Now if you think about it, this is kind of crazy. GPS was supposed to give the American military an advantage in combat. SA was introduced to prevent the enemy gaining a similar advantage. But when it came time to fight, we had to turn SA off! So this was one of the cracks in the justifications for SA. * permalink * embed * save * parent * report * give award * reply [-]GBFelClassical Militaries 2 points3 points4 points 16 hours ago (0 children) The one oft repeated reason in military circles is simply that it benefits us to have our adversaries using our signals. GPS is laughably easy to jam but if the adversary is using it themselves they're less likely to jam it for fear of mucking up their own operations. The proliferation of other GNSS constellations would seem to indicate that such an argument was for naught, however, the prevalence of GPS devices found among Russian forces in Ukraine (violates 20-year, I know, but it's a very publicized example of a trend that has been going on for a looong time) shows that our system is still employed by those who should have another option. I also cannot cite this beyond hearing it quite frequently from some /very/ knowledgeable people on the subject, to include in the AF's now-retired course for training operators and planners on the GPS system. * permalink * embed * save * parent * report * give award * reply [+][deleted] 1 day ago (1 child) [removed] [-]snowemann 13 points14 points15 points 1 day ago (6 children) Erm, thought the Thanh Hoa bridge was finally bombed by the first generation of laser-guided smart bombs, nothing to do with radio. The inventor put fins on a classic munition, and a 4-quadrant nose sensor aimed them toward a stronger laser signal. * permalink * embed * save * parent * report * give award * reply [-]Conrolder 25 points26 points27 points 1 day ago (5 children) Yeah, sorry - My point was that they missed it a LOT because they didn't have good navigation accuracy in the Vietnam war, and GPS allowed them to go from that miss rate to a single IAD launch persecuting multiple targets using GPS guidance - not that the Thanh Hoa bridge specifically was destroyed with GPS. I don't at all want to give the impression that GPS solved all of the navigation problems, nor that there aren't other solutions for military application! * permalink * embed * save * parent * report * give award * reply [-]JtheNinja 8 points9 points10 points 1 day ago (2 children) When did actual GPS-guided munitions become a thing? Wasn't it more recent? I recall that during the first gulf war the oil well smoke blocking laser guided bombs became an issue. During the run up to the Iraq invasion in 2003, I recall it mentioned that wouldn't work anymore because the US now had GPS-guided munitions and cruise missiles that would simply steer themselves to a particular coordinate and blow up there. * permalink * embed * save * parent * report * give award * reply [+][deleted] 1 day ago (1 child) [removed] [-]abbot_x 15 points16 points17 points 1 day ago* (0 children) You start to see GPS as a navigation instrument in standoff munitions in the late 1980s. I believe the first such munition actually fielded and used was the AGM-84E SLAM (Standoff Land Attack Missile). That's an air-launched missile for attacking targets on the ground derived from the Harpoon, which could really only attack ships. They made the SLAM by the warhead and guidance systems of the Harpoon with components borrowed from other missiles, the big change being replacement of the Harpoon's radar (good for locking onto and hitting a ship) with infrared (which could see some kinds of ground targets). For reasons I don't understand they also threw in a GPS receiver to supplement the Harpoon's inertial navigation system. SLAM saw very limited use during Desert Storm. But keep in mind SLAM's GPS wasn't intended to get the missile to the target, just get it close enough to where the IR homer could lock on to the target. You see full GPS systems like the JDAM (Joint Direct Attack Munition), a guided bomb, maturing in 1997-99, with the first combat use being Operation Allied Force (bombing of Serbia during Kosovo War) in 1999. These systems replaced laser-guided bombs. One other point to make is GPS made all kinds of artillery much more accurate. The gun could instantly know exactly where it was. The observer could also instantly know his own location. Just knowing those things makes hitting the target much easier. * permalink * embed * save * parent * report * give award * reply [-]thumbnailmoss 22 points23 points24 points 1 day ago (2 children) continued civil reliance). Indeed. The development of constellations by other countries and international bodies to reduce reliance on the US. * permalink * embed * save * parent * report * give award * reply [-]Conrolder 42 points43 points44 points 1 day ago (0 children) Yeah I even think one of the explicit EU missions with Galileo (their version of GPS) was to reduce reliance on US support. Ultimately, the redundancy of constellations is a good thing for everyone since we all rely so heavily on it now! * permalink * embed * save * parent * report * give award * reply load more comments (1 reply) [-]citationstillneeded 9 points10 points11 points 1 day ago (4 children) Hi! Thanks for the great answer. I'm an arborist and I use an Arrow GPS with a silly antenna on my backpack to (semi) precisely locate trees and plot their location in ArcGIS (or similar). I'm curious what's allowing my dedicated setup to achieve +/- 1 metre accuracy that my phone can't do on its own, is it just the antenna? Basically the arrow is a magic box to me and I want to understand how it works. Clearly I am an arborist and not an engineer. * permalink * embed * save * parent * report * give award * reply [-]Conrolder 6 points7 points8 points 1 day ago (3 children) I don't know precisely, but my guess is it's using differential GPS with multiple frequencies and trying to actively remote atmospheric interference (ionospheric and tropospheric errors). Interestingly, GPS under tree canopies can get pretty tricky because of multipath (the signal will bounce off the trees and make the solution worse, or in dense cover you'll lose it altogether). So a lot of these systems are especially augmented with either vector tracking with an IMU, or other alternative sensor choices, to try to navigate better through these environments. So it's very possible your system has some wacky GPS signal processing techniques to try to get around the problem. If you pay a monthly service for the Arrow GPS, it's probably using RTK or WAAS (some sort of augmentation service that improves the accuracy). And finally, if you have a camera on it to locate the trees, it's probably fuzing the camera data with GPS/IMU to improve the accuracy further! Not sure how many of these are applicable, but hopefully one of them is haha * permalink * embed * save * parent * report * give award * reply [-]citationstillneeded 6 points7 points8 points 23 hours ago (2 children) It definitely does use DGPS bouncing off of nearby phone towers. There's a little light that lights up when the signal is being corrected. I'm not sure about a subscription to those other services. I can always instantly tell when my connection to the arrow gets broken (it's bluetooth) because my phones on board GPS is remarkably worse. Regarding accuracy loss under canopies, I was actually trained to walk to the trunk and quickly plot the location without waiting for the signal to degrade. I take photos of the trees separately so there's no interaction there. We're not surveyors so I think it's the case that DGPS is 'good enough'. When I need maps to be position precise (construction inpact on tree root zones for example), I align my trees with a provided feature survey. Thanks!! * permalink * embed * save * parent * report * give award * reply load more comments (2 replies) [-]chteme 5 points6 points7 points 1 day ago (1 child) In other words, providing civil use didn't negatively interfere with military use, made stock market and power grid work cheaper (and many other things like public infrastructure development, surveilling, etc.). Emphasis mine. Do you mean surveying? Although both are probably true... * permalink * embed * save * parent * report * give award * reply [-]Conrolder 5 points6 points7 points 1 day ago (0 children) Oh gosh I did haha sorry! Surveying. * permalink * embed * save * parent * report * give award * reply [-]angelicism 9 points10 points11 points 1 day ago (4 children) And a lot of other countries have navigation satellite constellations too now (the EU, Russia, China, Japan, and India). Does this mean I'm connecting to different satellites when I am in Europe than when I am in the US? How does my phone know which satellites to talk to? * permalink * embed * save * parent * report * give award * reply [-]Conrolder 34 points35 points36 points 1 day ago (0 children) Your phone only passively receives signals, it doesn't have to tell the satellites anything - and 4 of 6 of them are global. GPS (US), Galileo (EU), GLONASS (Russia), and BeiDou-3 (China) are all global. NAVIC (India) and QZSS (Japan) are regional over those countries and surrounding areas (you can get QZSS in northern Australia I believe, for example). The signals are all band-limited and specific, so as long as your phone/watch/whatever can see those signals and is designed to listen to them, it will! There's a great image of all the different frequencies here: https://gssc.esa.int/navipedia/images/c/cf/ GNSS_All_Signals.png. You can picture it thusly: Your phone has to have an antenna designed specifically for whatever space along the X-axis here it wants to track, and code designed to handle any unique-looking bump on this chart. Otherwise, it can do it all! It's still somewhat rare to use all GNSS constellations (I know Apple products do because they, somewhat weirdly, advertise it). I don't know about many other products. But it's becoming increasingly common! (BeiDou-3 and Galileo are pretty new, like became available for general use in just the last 5-6 years, so there's still some catch-up being played on our phones). * permalink * embed * save * parent * report * give award * reply [-]GBFelClassical Militaries 1 point2 points3 points 16 hours ago (0 children) GPS satellites are in Medium Earth Orbit (MEO) along with most other GNSS satellites with the exception of some outliers such as QZSS and some components of Beidou and GLONASS. The satellites don't just hang out over North America since that would be impossible at anything but a Geosynchronous orbit, they cover the entire globe though the geometry gets a little sus at extreme north and south latitudes. If your receiver is looking for only GPS signals, it will only find GPS signals. Some modern phones like newer iPhones do receive signals from multiple constellations (which as stated are all zipping around in MEO), which is really a matter of programming more than hardware differences since most GNSS signals are in L-band and can be received on the same antenna. * permalink * embed * save * parent * report * give award * reply load more comments (2 replies) [+][deleted] 1 day ago (2 children) [removed] [+][deleted] 1 day ago (1 child) [removed] [+][deleted] 1 day ago (1 child) [removed] [-]simonjp 2 points3 points4 points 1 day ago (1 child) Thanks so much, that was fascinating! Also, Navstar is a great name and we should go back to that. * permalink * embed * save * parent * report * give award * reply [-]sierrafoxtrotwhiskey 3 points4 points5 points 21 hours ago (0 children) Wonderful answer! Thanks for the detailed context, that was fascinating. I just want to throw on a little extra here: For starters, it gave near-atomic clock quality time for next to nothing in cost (you get the benefit of the GPS satellite clocks on your handheld receiver) This is true in more ways than one. I work for a cell phone provider. Accurate timing is a critical aspect of cellular service. This is even more important with 5G. All of our cell towers get timing from GNSS in some way. Some are connected to our stratum-1 time servers that have gnss receivers. Most of the 5G towers have their own GNSS receivers to reduce latency (milliseconds matter). * permalink * embed * save * parent * report * give award * reply [-]NotYourLawyer2001 3 points4 points5 points 18 hours ago (0 children) Brilliant answer that gave me a bit of nostalgia. My dad was a Marconi, chief radio engineer on transoceanic vessels of the Black See merchant fleet, he was the Morse code and teletype guy who worked 20 hour days during ocean crossings doing comms, weather reports, you came it, provably second or third most important guy in the ship. His last jobs in the late 90s I think was to install all the satellite equipment that made his job obsolete. It was an end of an era but he was always so excited to talk about Inmarsat and maritime navigation systems that were revolutionizing the shipping industry. * permalink * embed * save * parent * report * give award * reply [-]JohnnyMnemo 12 points13 points14 points 1 day ago (3 children) In 2000, the US formally turned off Selective Availability, allowing civil use (I am omitting the reasoning I have always heard for this because I cannot cite a source). Couple of footnotes: That was by order of Clinton, so you can thank a Democrat for liberating a military application for civil use; but, like you, I don't know exactly why he did. And unfortunately I think that's really OP's question: why were those assets liberated, for free? There's lots of other tax-funded projects that civilians don't get access to. The second footnote is that GPS satellites would be much more inaccurate if both principles of Relativity weren't factored in; so if you need a real world consequence of the application of Relativity, there you have it. iirc they time dilate due to their speed, but also time expand due to their orbit and distance from the gravity well of earth, so both calcs need to be made and factored against the atomic timing that parent post describes in detail. * permalink * embed * save * parent * report * give award * reply [-]abbot_x 10 points11 points12 points 23 hours ago (0 children) That was by order of Clinton, so you can thank a Democrat for liberating a military application for civil use; but, like you, I don't know exactly why he did. Remember it was a multistep process. Reagan wanted GPS to be available as a navigation aid for non-military use, and the particular application he had in mind (preventing KAL 007) implied use by non-Americans outside America; in other words, free for the whole world. So C/A (a less precise signal) was made available for civilian use. But there were security concerns, so from 1990 to 2000 C/A was made fuzzier with Selective Availability (SA). Clinton ended SA in 2000. But as I pointed out in another comment, this was because other government agencies who wanted GPS to be useful to themselves and to non-government users had come up with workarounds that defeated SA. And unfortunately I think that's really OP's question: why were those assets liberated, for free? There's lots of other tax-funded projects that civilians don't get access to. There are a couple reasons for this. First, GPS is a broadcast signal you receive. There's no easy way to charge for it. Second, the U.S. government owns and operates a lot of infrastructure, including navigational infrastructure, that is available for free (meaning without user fees; we should understand the taxpayer ultimately bears the cost). Think of maritime navigation: lighthouses, buoys and markers for channels, radionavigation systems like Loran-C, etc. Think of aerial navigation: visual and instrument landing aids, various radionavigation systems like VOR/DME, ADF/NDB (and a bunch of obsolete systems), and the assistance given by air traffic controllers. These are all paid for out of general funds without specific use fees. The prevalence of GPS has allowed many of these systems to be significantly curtailed. These are provided free (again meaning without user fees) because they are so vital to rapid and safe transportation of goods and people and thus to commerce in the broad sense. * permalink * embed * save * parent * report * give award * reply load more comments (2 replies) [-]binzoma 2 points3 points4 points 1 day ago (0 children) I'd always heard that the geopolitcal reason for GPS being free and widely available was to prevent other countries or private business' from going full star link and developing their own GPS array- therefore keeping the US with a monopoly in the tech. is that just urban legend? * permalink * embed * save * parent * report * give award * reply [-]giblim 1 point2 points3 points 1 day ago (0 children) Thanks so much for this in-depth answer! Can you say anything about expected speed/accuracy for civil vs. military versions? * permalink * embed * save * parent * report * give award * reply [+][deleted] 1 day ago (1 child) [removed] [-]silverappleyardModerator | FAQ Finder 1 point2 points3 points 20 hours ago (0 children) As I understand it, AFRL considered several different potential system architectures, including one that would have required accurate clocks in the receivers. Can you speak to how they decided on the system we have today? * permalink * embed * save * parent * report * give award * reply [-]Darthrevan1789 1 point2 points3 points 20 hours ago (0 children) This was amazing, thank you. * permalink * embed * save * parent * report * give award * reply [-]racist-crypto-bro 1 point2 points3 points 14 hours ago (0 children) They had a very high miss rate, caused immense collateral damage, and costed a lot of money because the accuracy of bomb drops was so low (I won't pull a reference for this, but the Thanh Hoa bridge is a great example of this problem). Also probably suffering greater aircraft losses from needing to fly more sorties on average to achieve destruction of a given target. Great answer, thank you! * permalink * embed * save * parent * report * give award * reply [-]discourseur 1 point2 points3 points 11 hours ago (0 children) Amazing writing! You really covered a lot of ground. * permalink * embed * save * parent * report * give award * reply [+][deleted] 1 day ago (1 child) [removed] load more comments (1 reply) [-]shamam 0 points1 point2 points 9 hours ago (3 children) Hey, I have an unrelated question for you. I live in NYC and GPS on my iPhone is very reliable here (and honestly most everywhere I've been in the US and Europe), it always locks on quickly and accurately. I was just in Tokyo and I noticed that GPS takes forever to get a lock there, w/ the dot indicating my position sometimes bouncing around for a minute or two before it could determine my location. This happened both in high density urban environments as well as the suburbs. This confused me as GPS sats are in the sky, so why should the country make a difference? This also happened to a friend who had a Japanese iPhone which I noticed supports a slightly different set of frequencies, so it doesn't appear to be caused by the assist phones get from cell towers. * permalink * embed * save * parent * report * give award * reply load more comments (3 replies) [-]The_Highlife 0 points1 point2 points 6 hours ago (0 children) Curious if you've heard of Xona Space Systems? They're a startup developing secure GPS to combat GPS spoofing and other GPS-related problems that I don't understand. * permalink * embed * save * parent * report * give award * reply [-]tgeliot 0 points1 point2 points 4 hours ago (3 children) I occasionally want to know my altitude. Does the GPS system provide that information? Is there a straightforward way to get it on my smartphone? I'd be on the ground, not up in the air. In principle, Google Maps in "terrain" mode can give this, but I'd need a microscope to read those contour line designations. * permalink * embed * save * parent * report * give award * reply load more comments (3 replies) [-][deleted] 1 day agolocked comment (10 children) [removed] [-]terminus-trantorModerator | Portuguese Empire 1400-1580[M] 10 points11 points12 points 1 day agolocked comment (0 children) Thank you for your response, however, we have had to remove it. A core tenet of the subreddit is that it is intended as a space not merely for an answer in and of itself, but one which provides a deeper level of explanation on the topic than is commonly found on other history subs. We expect that contributors are able to place core facts in a broader context, and use the answer to demonstrate their breadth of knowledge on the topic at hand. If you need guidance to better understand what we are looking for in our requirements, please consult this Rules Roundtable which discusses how we evaluate answers on the subreddit, or else reach out to us via modmail. Thank you for your understanding. * permalink * embed * save * parent * report * give award [+][deleted] 1 day ago (6 children) [removed] [+][deleted] 1 day ago (3 children) [removed] load more comments (3 replies) [+][deleted] 1 day ago (6 children) [removed] [+][deleted] 1 day ago* (3 children) [removed] [+][deleted] 1 day ago* (6 children) [removed] [+][deleted] 1 day ago (4 children) [removed] [+][deleted] 1 day ago (1 child) [removed] [-]throwawaygreenpaq 50Answer Link4 points5 points6 points 1 day ago (0 children) Loads of fine answers here. 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