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However, I do like the role GPS plays as a plot device in all kind of stories, where it's an active device, with GPS-enabled devices giving away position or the

However, I do like the role GPS plays as a plot device in all kind of stories, where it's an active device, with GPS-enabled devices giving away position or there's no GPS in the wilderness, as mobile connections fail. So there are two versions of GPS, the popular plot device and the actual navigation device. (The more sinister version is that this has actually been planted as a cover-up for more realistic electronic intrusions, as this is also a trope in popular media and news.)

beerandt 5y ago
There are way more (real-world) versions than two, with quite a few arrangements that can cancel out some of the systematic error, both with and without inputs requiring a data connection, and with some augmented solutions passively broadcast as "one-way" data. The funny thing is that the James Bond/ Tomorrow Never Dies plot device has turned out to be the most realistic, but doesn't actually require the theft of some encoding device, with various record and delay replay attacks. Other underused plot device: lots is made out of over reliance on GPS and what would happen in the event of an attack on the system. But most ignores the fact that the US NAVSTAR GPS constellation is multipurpose, and is also a (confirmed) primary component of worldwide nuke detection, with some (afaik unconfirmed) claims of a missile launch detection capability, as well.
Rebelgecko 5y ago
The original study by Woodford and Nakamura (which laid the foundations for what eventually became NAVSTAR/GPS) has a really fascinating slide where they consider the tradeoffs of alternative configurations. What if GPS receivers had transmitters? What if the mathy computation was offloaded to a nearby ground station? What if every GPS receiver had an atomic clock? Or just a cheap quartz clock? How does that impact the quality of the signal and the number of satellites you need to get a fix? I think we're really fortunate that they made the choices they did. If they hadn't take the route that was the most complicated technically, GPS wouldn't have become as ubiquitous as it is today.
- jcrawfordor 5y ago
  There's good reason for them to have considered these questions too, as a lot of the satellite-based positioning systems prior to GPS, such as the Navy's TRANSIT, involved both a more active receiver and offloading parts of the work to ground stations. This was very practical at the time, as is TRANSIT fixes were so complex that GE had to design a special computer with a cylindrical chassis so that it would fit through the porthole for installation in submarines. This replaced the previous situation of the submarine having to send its TRANSIT observations to a ground station for fix calculation. You can still do this with GPS if you want. In the surveying community, it's not unusual to collect an extended period of raw GPS observations (e.g. 48 hours) and then submit them to NOAA's offline GPS computation service OPUS which will return a fix by email a while later. This can result in a more accurate fix but perhaps more importantly a more consistent fix, since OPUS will apply the exact same sophisticated solver used for other government geodetics like survey benchmarks. The tradeoff is that OPUS is slow enough that it tends to run on a queue. In any case modern GPS involves surprisingly active receivers since smartphones commonly use AGPS over IP to accelerate receiving ephemera.
FabHK 5y ago
Related: When I fly, I like to cache a map of the region I fly over on my phone, particularly the airport region at the destination. Then, you can hold your phone to the window for a few minutes and get a GPS fix, even if in airplane mode, because as you say GPS is purely passive. Then you can follow along nicely where you are, at the resolution you want. (Sure, many airlines have moving maps, but they're not as good as Apple maps or Google maps.)
- therealmarv 5y ago
  I do that with OSMand on Android too. It's interesting if you fly on the window side.
  - terinjokes 5y ago
    Once I was flying in the middle section of economy, on a flight without in-flight WiFi, and accidentally opened Google Maps on my phone. I was shocked to find it managed to see enough GPS satellites to provide a lock, which matched the map in the seat back in front of me.
- kubanczyk 5y ago
  Tsk, tsk. Keep bragging and soon enough we all hear: All GPS devices MUST be switched off for the duration of the flight. Because terrorism. /s
  - kortilla 5y ago
    Terrorism isn’t the reason you can’t use cell networks during a flight. Early on it was ostensibly interference concerns but those are not a real problem (if they were they wouldn’t let you carry cell phones onto a plane). At this point it’s an explicit request from the cell phone companies to not have a plane with 200 people ripping through their cell networks with association attempts that are most likely going to fail and then immediately become stale.

beerandt 5y ago

There are way more (real-world) versions than two, with quite a few arrangements that can cancel out some of the systematic error, both with and without inputs requiring a data connection, and with some augmented solutions passively broadcast as "one-way" data. The funny thing is that the James Bond/ Tomorrow Never Dies plot device has turned out to be the most realistic, but doesn't actually require the theft of some encoding device, with various record and delay replay attacks. Other underused plot device: lots is made out of over reliance on GPS and what would happen in the event of an attack on the system. But most ignores the fact that the US NAVSTAR GPS constellation is multipurpose, and is also a (confirmed) primary component of worldwide nuke detection, with some (afaik unconfirmed) claims of a missile launch detection capability, as well.

Rebelgecko 5y ago

The original study by Woodford and Nakamura (which laid the foundations for what eventually became NAVSTAR/GPS) has a really fascinating slide where they consider the tradeoffs of alternative configurations. What if GPS receivers had transmitters? What if the mathy computation was offloaded to a nearby ground station? What if every GPS receiver had an atomic clock? Or just a cheap quartz clock? How does that impact the quality of the signal and the number of satellites you need to get a fix? I think we're really fortunate that they made the choices they did. If they hadn't take the route that was the most complicated technically, GPS wouldn't have become as ubiquitous as it is today.

jcrawfordor 5y ago
There's good reason for them to have considered these questions too, as a lot of the satellite-based positioning systems prior to GPS, such as the Navy's TRANSIT, involved both a more active receiver and offloading parts of the work to ground stations. This was very practical at the time, as is TRANSIT fixes were so complex that GE had to design a special computer with a cylindrical chassis so that it would fit through the porthole for installation in submarines. This replaced the previous situation of the submarine having to send its TRANSIT observations to a ground station for fix calculation. You can still do this with GPS if you want. In the surveying community, it's not unusual to collect an extended period of raw GPS observations (e.g. 48 hours) and then submit them to NOAA's offline GPS computation service OPUS which will return a fix by email a while later. This can result in a more accurate fix but perhaps more importantly a more consistent fix, since OPUS will apply the exact same sophisticated solver used for other government geodetics like survey benchmarks. The tradeoff is that OPUS is slow enough that it tends to run on a queue. In any case modern GPS involves surprisingly active receivers since smartphones commonly use AGPS over IP to accelerate receiving ephemera.

FabHK 5y ago

Related: When I fly, I like to cache a map of the region I fly over on my phone, particularly the airport region at the destination. Then, you can hold your phone to the window for a few minutes and get a GPS fix, even if in airplane mode, because as you say GPS is purely passive. Then you can follow along nicely where you are, at the resolution you want. (Sure, many airlines have moving maps, but they're not as good as Apple maps or Google maps.)

therealmarv 5y ago
I do that with OSMand on Android too. It's interesting if you fly on the window side.
- terinjokes 5y ago
  Once I was flying in the middle section of economy, on a flight without in-flight WiFi, and accidentally opened Google Maps on my phone. I was shocked to find it managed to see enough GPS satellites to provide a lock, which matched the map in the seat back in front of me.
kubanczyk 5y ago
Tsk, tsk. Keep bragging and soon enough we all hear: All GPS devices MUST be switched off for the duration of the flight. Because terrorism. /s
- kortilla 5y ago
  Terrorism isn’t the reason you can’t use cell networks during a flight. Early on it was ostensibly interference concerns but those are not a real problem (if they were they wouldn’t let you carry cell phones onto a plane). At this point it’s an explicit request from the cell phone companies to not have a plane with 200 people ripping through their cell networks with association attempts that are most likely going to fail and then immediately become stale.