When you receive a radio signal, you have a lock - you're locked on to the signal and can receive the message, whatever it is.
It's an old school piece of radio slang that picked up a formal meaning.
The signal that GPS satellites put out is a time code.
It's all about triangulation.
Suppose I put you between two speakers that put out a ping sound and you have magical ears - I don't know, like a bat or something ok.
If you hear both pings at exactly the same time, you would correctly conclude that you are midway between the two speakers.
Now suppose that you hear one before the other - you would correctly conclude which speaker you are closer to.
And if you knew their locations, you could do math with the speed of sound (a constant) and calculate how close you were to one and far from the other.
Now instead of a line, let's make it an area - and to do that, you need three speakers, making like the points of a triangle - and if you're inside the triangle, you can just do more of the same math to know where you are - triangulation.
Now instead of speakers, make them radio transmitters and use a radio receiver instead of ears.
Because, like sound, radio waves have a constant speed, you use the same math, different numbers, to get your location.
Your cell phone does that and it's called tower location.
But as you know from tuning your TV antenna, radio signals bounce around - a lot.
The higher the towers, so you get a better direct signal to do the math, the better.
And you can't get higher than outer space - welcome to GPS.
And the more signals you have from different directions, the more accurately you can calculate your position.
Now to just do that with simple outer space pings and have radios sensitive enough, your cell phone would only fit into the back of a pickup truck.
So instead of pings, you use an atomic clock, make all of the GPS satellites agree on precisely the same time and have them transmit time messages instead of just pings.
Still the same triangulation math but you get part of it done for you, with easier radios.
Now - you need to know where the satellites are. Turns out, in addition to time, they tell you that too.
But you have to have a starting point and you have to deal with the fact that they're not stationary - each one whips around the Earth twice a day.
To pull that off, you need an expensive, power hungry special processor, built in to car and boat GPS receivers.
But if you can find a way to cheat and just magically know the starting point, the whole thing gets simpler.
Enter a-GPS, assisted GPS, built in to your phone.
It gets assistance from the cell towers - and it gets assistance from the Internet to get the most recent GPS satellite positions to get started.
After that, GPS is GPS, just arithmetic.
The arithmetic ends up giving latitude and longitude numbers without knowing what they really mean.
Send those two numbers into a smart map that is all about latitude and longitude, boom, there's your position.
Except for a-GPS to work, your sensors, compass and starting knowledge of the satellite positions has to be good.
And often it just is.
Just as often, it gets all wonky bonky and needs to be whacked upside the head and get it right.
That's where the GPS Status & Toolbox app, along with the procedure I mentioned, comes in.
Hope this helps!
) to navigate as long as you have a map held on your phone. Google maps relies on downloading the map as you go - with no coverage this is obviously not possible (I know Gmaps can download "some" maps ahead of time but it's not proper off-line mapping). 
