why did old (like 90s/2000s) GPS take up to 15 minutes to get a lock, in giant bulky units, but my running watch gets it in 15 seconds?
In: 3751
Wow, my old GPS was really doing its best with dial-up speeds while my running watch is on broadband!
In the 90s the GPS signal was degraded to keep our enemies from using the GPS system.
>[On May 2, 2000 “Selective Availability” was discontinued as a result of the 1996 executive order, allowing civilian users to receive a non-degraded signal globally.](https://en.wikipedia.org/wiki/Global_Positioning_System#Timeline_and_modernization)
After 2000 the signal was no longer garbled. We went form seeing yourself within about 20 meters, to seeing yourself within about 15 centimeters.
It’s a combination of factors.
Better technology, faster transmission time, more powerful computers that can process the data faster.
Also, and this is big one, more GPS satellites are available now.
When GPS was first developed, it was meant for military use. Gradually they introduced it in the civilian sector (a big one was due to some unfortunate incidents involving civilian airplanes and cold war era paranoia).
In the modern era, anywhere you switch on a GPS it will typically have access to at least a dozen satellites. As a result it has a lot more data to work with which means it can get a fix on its position faster and with more certainty.
Look Up.
In 1985 I started looking for Haley’s comet. This was before it went around the sun so you needed to know where to look (by knowing the stars) and be in a very dark sky.
Once in a great while someone I was with would see a satellite. You could tell it was a satellite because they were like not-very-bright-stars, but not as bright as airplanes and they moved at a pace faster than airplanes, but much slower than any other celestial body.
I think we saw 3 in total during all the months we tracked that comet.
I know a guy who knows nothing of satellites or stars but he has a bad back. He sits in his hot tub every night. He will sit there and say…Yep, there goes that one. Now watch! Three in a row are coming up.
There are vastly more satellites in the sky now.
One reason (there’s a few more) is improvements in computing and signal processing algorithms. Old GPS receivers only had a few “channels”, let’s say 12. The GPS satellites transmit a signal that looks random, unless you know it. But even if you do, it took many minutes to just find the signal (this process is called acquisition) from one satellite, and you need to track four for about 30 seconds to learn about where it is (50 bits per second, pretty slow). That’s why it took forever back then. Today, you often have a network connection of some sort giving you the information about where the satellites are in a fraction of a second. And time and rough position. Even if it doesn’t, frequency domain acquisition algorithms searches through the full search space in seconds instead of many minutes.
How does the gps system lock out opponent clients, by that I mean, us gps isn’t going to ‘aid’ or be available to, Russian cruise missiles to assist them in their mission accuracy? Do they just make the data or clock signal slightly inaccurate so it’s to be functionally worthless? I know that military GPS is far more accurate than consumer receivers, is that inaccuracy good enough to discourage use?
GPS signals are transmitted in such a way that to receive them, the GPS receiver needs to first accurately know the parameters of the signal.
These parameters are:
1. satellite id, which determines the pseudorandom sequence emitted by the satellite
2. the Doppler shift of the radio signal
3. the correct phase of the pseudo-random code transmitted by the satellite.
There are roughly 32x40x1024 possible combinations of these parameters, which the receiver has to search through. Without the correct parameters it simply does not see the signal from the satellite. Once the correct parameters are found, then the receiver can start getting the data from the satellite.
New digital chips are not only faster, but they are also built to try many combinations in parallel. This allows them to find satellite signals much faster than the old receivers could.
As had already been mentioned in other comments, if the receiver can get the data on the orbits of the satellites and the exact time and its own approximate location from other sources, this greatly simplifies the problem. In this case the correct signal parameters can be calculated with quite small error, and only a small amount of fine tuning is required, allowing the receiver to lock onto the signal almost instantaneously.
Here is a more detailed explanation from StackExchange: [Why do GPS receivers need so many correlators](https://electronics.stackexchange.com/a/11900).
Some missing details that I haven’t seen, so I’ll add to them:
First, in addition to using satellite positioning, phones (and watches if they also connect to your phone) can also use cell towers to triangulate position. This was an upgrade made to cell towers in the early 2010s as a response to slow emergency response times (in the US at least). The US government subsidized a large portion of the upgrades. Source: I worked in the cell industry when it was going on.
Second, your phone and watch have accelerometers so they can “cheat” by adding known distance traveled to last good position.
Part is that they cheat. Turn your phone off and fly to another country and turn it on and it will take a noticeably long time. But phones being always on means it can store and update your location and remember where you were. So it gets to start with the hypothesis you are still around where you were last time it checked.
[removed]
Latest Answers