They could not put a special tax on receivers, let the manufacture and sale and commercial use of receivers create economic activity, and then tax that economic activity through, e.g., personal and corporate income tax, and use tht to fund the system.
Even if I do a reset + cold start in u-center or similar, I still don't have to wait 12 minutes. I wonder why.
Almanac gives coarse satellite position information (and some other stuff), good enough to know which ones are probably visible and therefore prioritize signal acquisition attempts which used to be very very costly in terms of signal processing. That's the message that takes up 12.5 minutes to piece together. Nowadays you can just brute force all possible satellite signals and there's no need to wait around for the almanac information. Each satellite signal broadcasts precise satellite location information (ephemeris), which takes maybe 30 seconds to get a few frames I believe. So that's basically the bottleneck for a modern chipset which starts with zero information and relies solely on the GPS signals to navigate.
Classical GPS receivers use the almanac (and a reasonably accurate local clock) to determine which satellites are probably in view, and with which Doppler shifts. I would not be surprised if modern GPS chips had enough compute power to simply correlate the received signal with all gold codes and at all reasonable Doppler shifts. The almanac is then no longer necessary.
While the general orbits of the satellites (each is uniquely identifiable) can be pre-shipped, orbits do fluctuate, so the data would still need to be updated / fine tuned.
> The receiver is missing or has inaccurate estimates of its position, velocity, the time, or the visibility of any of the GPS satellites. As such, the receiver must systematically search for all possible satellites. After acquiring a satellite signal, the receiver can begin to obtain approximate information on all the other satellites, called the almanac. This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days.
* https://en.wikipedia.org/wiki/Time_to_first_fix
But a general fix can do done without any initial data, and is sped up by being near the previous last fix and caching some data.
AFAIK originally the DoD weren't thinking of making GPS for public, just for military. Having these requirements would just make it extra complex, and unnecessary. First, it would require a lot of bandwidth, and then yeah, a bottleneck for how many receivers can be deployed, and it could take a lot of seconds until a unit finds a matching key to decrypt. Imagine a soldier needing to wait for her equipment to get the key to find her location while in a firefight...
GPS in combat debuted during the Gulf War, but mostly as a backup for navigation rather than targeting. We did not have 100% coverage back then, all of the satellites were not up yet.
IOC was defined as always being in sight of 3 or more satellites, plus maintenance capability. That was achieved in 1993.
Modern ICBMs use GNSS. Historically, they didn’t. Certainly not when the GPS constellation was deployed.
Do they? The US hasn't deployed a new generation of ICBMs since the GPS constellation became operational. I would also expect that plans for nuclear war wouldn't rely on vulnerable satellites remaining operational.
I thought that US weapons used GPS guidance as a low-cost alternative to other guidance methods in forgiving environments that don't have electronic warfare countermeasures (e.g. fighting the Taliban or armed forces equipped with old gear).
Thought two: I've never considered how bizarre it is that GPS is an enormously expensive satellite constellation launched and maintained by the United States military that we make available for free to the entire world. As a US citizen I take it for granted - but it must be strange for someone to grow up in (for instance) Lebanon and use a GPS device.
I still remember my fascination with it when I got to use it for the first time, using a Garmin app on my Nokia 5800.
For good 30 years the strategy generally worked so much that satnav got synonymous with GPS.
Big players launched their own constellations anyway. GLONASS, Beidou and fledgling but accurate Galileo. And there are still more systems. So if GPS wasn't free manufacturers would use something else.
>After Korean Air Lines Flight 007, a Boeing 747 carrying 269 people, was shot down by a Soviet interceptor aircraft after straying in prohibited airspace because of navigational errors,[42] in the vicinity of Sakhalin and Moneron Islands, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good.
That's the neutral phrasing on Wikipedia. The actual announcement: https://www.reaganlibrary.gov/archives/speech/statement-depu...
>In their recent statements on the Korean Air Lines tragedy, senior Soviet officials have shocked the world by their assertion of the right to shoot down innocent civilian airliners which accidentally intrude into Soviet airspace. Despite the murder of 269 innocent victims, the Soviet Union is not prepared to recognize its obligations under international law to refrain from the use of force against civilian airliners. World opinion is united in its determination that this awful tragedy must not be repeated. As a contribution to the achievement of this objective, the President has determined that the United States is prepared to make available to civilian aircraft the facilities of its Global Positioning System when it becomes operational in 1988. This system will provide civilian airliners three-dimensional positional information.
Also, it would be disaster to lose GPS accuracy worldwide since so many users depend on it. It would also be pointless because there are other constellations like Galileo and GLOSNAS. Plus, multiband receivers are getting more common to support the new L5 signal.
GPS jamming and spoofing are more useful because they can be targeted to area. As is happening in Ukraine.
Both of these techniques are still good for removing much of GPS's error, which mostly involve the uncertainty about the "speed of light". It changes locally as the total electron content of the ionosphere changes. Measure against a reference station or between two different radio frequencies (L1/L2C), and that error can be minimized. You don't need a reference station anymore, as things like WAAS try to transmit these corrections from a satellite.
If you want to play with high-precisions GPS, you can easily buy fancy u-blox modules from Sparkfun, connect them to a Windows computer running uCenter (from ublox), and get access to reference stations through your state to do full DGPS. With good sky conditions, you should be able to move your receiver a few centimeters and watch the lat/lon change accordingly. (New York State's service is here: https://cors.dot.ny.gov/. It's free! They use a software suite that's designed to charge customers money for using the data, but the prices are all set to $0.)
People say that the achievement of the 20th century was the Internet, but I think GPS was a lot harder. The Internet is just computers connected to each other with cables! This is atomic clocks in space sending you messages indistinguishable from noise that let you find your position in space within a centimeter. When the aliens invade, I think they'll be impressed! I am, anyway.
As for difficulty, both are achievements and don’t undersell the difficulty of networking. At the 500ft level it’s all just wires but the actual details of doing it are incredibly difficult. It’s just that we’ve had so much experience pushing it forward that there’s a lot we can just take for granted which removes the magic from it.
https://www.allaboutcircuits.com/news/ethernet-turns-50-look...
And that’s just local area networking. Networking broad area networks is even more complicated.
As for people calling it the achievement of the 20th century, it’s more because of how it made the world smaller and the network effects of the changes it’s wrought to society (and from that perspective it is bigger than GPS and the computer itself).
https://www.spaceforce.mil/About-Us/Fact-Sheets/Article/2197...
