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 Hóa 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 linear algebra 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 linear algebra, but just know that in this way, all GPS receivers receive very precise timing, as well as their position.

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, surveilling, 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 (I am omitting the reasoning I have always heard for this because I cannot cite a source). 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!

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