I was wondering about this... don't ask me why. GPS began in 1978. Were the specifications the same as today? Would a 1978 GPS device work today... and would a modern one have worked then?
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19$\begingroup$ I'm almost certain the answer is 'no' since it's a cellphone. If you had a modern, military GPS unit it might work since it wouldn't be restricted like civilians', but that's being very hopeful about military security backwards compatibility. $\endgroup$– RangerSep 7, 2016 at 17:36
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11$\begingroup$ Should point out that GPS on phones isn't always GPS either. Your phone provider access points and your wifi information is frequently used to determine your location on your smart phone.I was running some reporting on phone usage by region. Either a large number of people are standing on top of a cell tower or your GPS is defaulting to the cell tower location since it can't fully pinpoint you. Another no answer here, cell towers in 1978 were different beasts. $\endgroup$– TwelfthSep 7, 2016 at 17:47
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5$\begingroup$ You might need to wait until 1980. Although some vehicles were in space in 1978, the system wasn't really stable then. The GPS epoch was reset to zero in Jan 1980, and I can't find references to how it behaved prior to that date. $\endgroup$– BowlOfRedSep 7, 2016 at 20:15
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14$\begingroup$ @Twelfth, while cell signals can assist with aquisition, all the GPS chipsets can work without such assistance. $\endgroup$– BowlOfRedSep 7, 2016 at 20:18
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16$\begingroup$ Hi, Ne Mo. Can you clarify a little on how this question relates to worldbuilding? $\endgroup$– HDE 226868 ♦Sep 7, 2016 at 23:47
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7 Answers
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Your limiting factor is selective availability. Prior to the year 2000, the GPS system was considered an instrument of national security, so the signals were intentionally degraded for other than U.S. Military use. Any device could listen to the signals, but only those with the proper codes could use them effectively.
There was a joke going around at the time of a grandfather and grandson going hiking in the Appalachian region. Grandfather had a topo map and a compass. Grandson had a GPS. During a break, they each took a moment to use their respective tools to fix their position. After a few minutes, the grandson announced, "According to this, we're on top of that mountain, right over there."
Modern GPS accuracy can be as good as 6 ft (2 meters) and is seldom worse than 40 ft. (12 meters). Before 2000 and without the security code to SA, it was usually no better than several hundred feet/ meters.
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8$\begingroup$ Even with SA one could get much better accuracy if one had a live link to a GPS receiver at a known nearby location, since the same errors would be received by the receiver at the fixed location as by the unit of interest. When I was working with such systems in 1994, the equipment could have accurate GPS while one was willing to pay the per-minute cost of a cell modem connection. $\endgroup$– supercatSep 7, 2016 at 21:20
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3$\begingroup$ You know the McGuffin in Tomorrow Never Dies? You'd probably need one of those. $\endgroup$ Sep 8, 2016 at 11:01
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$\begingroup$ I love it when my IPhone GPS places me in the center of Pacific Ocean. $\endgroup$– SkyeSep 8, 2016 at 12:45
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1$\begingroup$ This answer seems to be addressing if GPS would work well. It seems the OP is asking if it would receive and interpret any signal at all regardless of accuracy. $\endgroup$ Sep 9, 2016 at 12:59
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$\begingroup$ When you say “Any device could listen to the signals, but only those with the proper codes could use them effectively”, it obviously raises the question whether these codes are still a secret. Since selective availability is history, there is no reason to keep them secret. But the bigger question is whether the modern receiver will work at all. $\endgroup$– HolgerSep 9, 2016 at 13:08
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In 1979 GPS network was "in diapers" even for military use. There were only four satellites in orbit and that didn't give enough coverage or accuracy.
By then the satellite navigation system in use was TRANSIT, but you had to wait about an hour for a position or several satellite passes to increase accuracy.
In those years we used radio positioning systems such as Decca Navigator or Loran-C.
GPS began as a global positioning system with wide coverage (not global yet) and accurate positioning (for military) with enough satellites (10) in the mid-80s
But even then, for precise positioning you needed radio positioning systems as Syledis or Trisponder (local and specific use, and you needed to set up your own network)
So to answer your question:
Your actual cell phone would be useless as a GPS receiver in 1979, the existing GPS network wasn't enough for accurate position everywhere. Even with military-grade GPS receivers, was of not much use then.
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It's likely that you would have to wait just a few more years. As of 1979, there were only four GPS satellites in orbit. Since a 3D position fix requires four satellites in view at once, the times and places where you could get a GPS lock in 1979 would be quite limited. By the end of 1980 there were six satellites; by the end of 1983, seven; by the end of 1984, nine; and by the end of 1985, ten. In 2016 there are 31 operational satellites. I'm not good enough with the orbital mechanics to say exactly at what point there would have been four satellites above the horizon more often than not, but it seems unlikely that it would have been possible with fewer than eight satellites in orbit, a milestone that was reached in 1984. Nonetheless, geodetic receivers (meant for non-moving applications where you can collect data for a long period of time and process it afterwards) were available as early 1982.
Compatibility shouldn't be a concern; the signals broadcast by the very first GPS satellites are still in use today and decoded by receivers. In the earliest years the system was a test platform, and its actual reliability unknown to me, but by 1982, with universities and government departments using it for geodetic work, you can expect that it would be somewhat reliable.
You should expect lower accuracy (due to Selective Availability, fewer satellites available to compute a fix, and lack of WAAS or internet augmentation), a slower time-to-first-fix (due to lack of internet augmentation), worse indoor performance (the first flight of satellites had a lower broadcast power), and worse "urban canyon" performance (due to lack of favorable geometry with fewer satellites). Also, map applications that rely on the internet for map data would obviously be out of commission. But given favorable conditions, I would expect your time-travelling smartphone to be usable for navigation from about 1982 onwards.
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$\begingroup$ A 3d fix requires four satellites in view, but being able to see two satellites would allow one to know the difference in distance between them. I would expect that if one wasn't moving too much, equipment that was designed to process partial-fixes received at different times might be able to get an approximate location more quickly than equipment that couldn't do anything without four satellites at once. Newer phones probably wouldn't be prepared to do that. I also suspect that a GPS receiver would need information about satellites' locations in 1979, and modern receivers... $\endgroup$– supercatSep 8, 2016 at 15:37
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1$\begingroup$ ...would have omitted such data from their almanacs. $\endgroup$– supercatSep 8, 2016 at 15:38
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3$\begingroup$ @supercat the satellites transit that information, that's the "ephemeris". $\endgroup$– hobbsSep 8, 2016 at 19:45
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1$\begingroup$ Please note that only 3 satellites are needed if you know you altitude (you may use the center of the earth as #4). In addition to this, having 3 satellites (or 2 + center of earth) yields a handful of discreet positions you can eliminate all but one of because they don't occur on the same continent. $\endgroup$– JoshuaSep 8, 2016 at 21:43
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$\begingroup$ Correction to previous comment: "transmit" not "transit". $\endgroup$– hobbsSep 10, 2016 at 4:47
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L2C only became operational in 2014 so all current receivers are likely to be C/A code compatible. As well as selective availability there is a possibility that modern receivers would get confused when operating prior to the 1999 GPS Time roll-over (overflow). Poorly designed receivers might also get confused by leap seconds.
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1$\begingroup$ Comments are not for extended discussion; this conversation about GPS time roll-over, Y2k, etc has been moved to chat. $\endgroup$ Sep 9, 2016 at 2:30
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The current standard civil signal, L2C, is only broadcast from satellites launched in and after 2005. The answer is no; GPS was extant, but the signal used was restricted.
EDIT: A bit of research tells me commercial products use a combination of the classic L1 signal and the civil L2C signal. Either alone would not be as accurate, however I don't know if a commercial product can use the L1 alone.
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$\begingroup$ What were the sources for your research? Those might be interesting/telling. $\endgroup$– ruffinSep 9, 2016 at 13:21
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$\begingroup$ @ruffin by research, I mean, of course, the first result of a google search :) gpsworld.com/… $\endgroup$– RossSep 9, 2016 at 13:46
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GPS reports your position (and the time) by calculating distances from at least 4 satellites. This crucially relies on knowing the positions of the satellites. As I understand it, part of the problem of a cell phone not being able to report its position when there is no network available is that it is attempting to download the latest "ephemeris", or satellite-positions information, and unable to do so.
I doubt it would be possible to get the ephemeris for the satellites of the period, but if you somehow could manage it you would then have to force the phone to use that arbitrary ephemeris. (Since even if your phone could connect to the towers using whatever technology was current, and you could establish an account with a provider of the time, that provider would not yet have protocols for furnishing the needed data.)
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6$\begingroup$ Almanac and ephemeris data are sent from the satellites. Even with zero information on the constellation, the receiver can search for a vehicle. When it finds one, it can gather the entire almanac. The almanac and ephemeris format on L1 has not changed. $\endgroup$ Sep 8, 2016 at 19:12
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1$\begingroup$ Note that downloading the ephemeris from the satellite takes approximately 15 minutes of continuous GPS reception - hence the huge advantage of downloading it over the cell network. $\endgroup$– LeBleuSep 8, 2016 at 20:55
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No it wouldn't work simply because the communication and data protocols have changed many times since GPS has been introduced. You wouldn't get any signal at all because your device and satellite wouldn't be speaking the same 'language'.
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2$\begingroup$ Wikipedia says that the protocol has been enhanced, maintaining backwards compatibility with older devices. Purely based on logical inference it is possible that newer devices, relying on the enhancements, are not able to process the older protocols, although the page doesn't explicitly say so. (Akin to a USB mouse which doesn't fit in a PS/2 connector of an old computer, although the modern computers stayed backwards-compatible for a while by providing both ports.) $\endgroup$ Sep 9, 2016 at 9:59