The downside of buying a used iPhone

[chscag]

All iPhone models from the 3G to the present have GPS capability. You would have to find an antique iPhone in order to find one without it. I guess that's what Patrick was referring to.

[MacInWin]

If you really want to know how GPS works, watch this video at YouTube: YouTube

It is accurate and pretty simple in the description of how it works. One interesting fact is that every day the satellite clocks lose 38 microseconds because the satellite is moving and as Einstein said, moving clocks run slower. So the USAF updates the clocks against the master clock to keep everything accurate. (As a trivia question, if you didn't adjust the clocks the system would still work pretty well because all of them should lose about the same time and the fixes would still be accurate. However, over time the small differences in orbital speeds would cause the clocks to fall out of sync and fix accuracy would degrade. Bear in mind that the speed of light is about 1 foot in a nanosecond, so it doesn't take many nanoseconds of drift to produce some major error.) In addition, the USAF tests the orbital data to see if the satellite has moved, and if so, updates the ephemeris appropriately. The bit in the video about the differential GPS and WAAS is there to make sure that aircraft using the system have the most accurate fixes. What they do is to survey a spot very accurately, then put a GPS receiver there and constantly compute where the GPS data thinks the receiver is. The difference between the very accurate survey data and the GPS calculated position is then broadcast to all the GPS receivers nearby to adjust their fix by that differential. In WAAS, the differential data from all over the US is transmitted by satellite to all receivers in the US and the receiver uses the data for the nearest differential spot as the correction.

I remember back in the early days when President Reagan released the accuracy limits on GPS how excited everybody was about suddenly being able to know where you are to within 5 m! Geocaching took off as a sport, road navigation suddenly became really accurate and GPS receivers for PDAs became a popular item. (Before smartphones for the young'uns.)

[MacInWin]

One more thing: The way that the cell service does the most to speed up the GPS fix is that your cell phone can get the almanac data faster over cell using a bit of your data allowance so that as soon as it has ephemeris data from 4 satellites, you have a fix. Normally that's in less than a minute, which can be disguised in the start up routine for the GPS application on the phone. To you, it looks practically instantaneous, but a lot of stuff is going on in the background to make it happen.

EDIT: One more one more thing. GPS (or SatNav) will be showing where you WERE, usually about a second ago, as it takes some time for the signals to arrive, the fix to be calculated and the map updated. The navigation software anticipates that delay and forecasts where you SHOULD BE based on speed and direction you travelled a second ago. Normally, that forecast is correct, but sometimes it's not. If you are directed to take a turn and do not do so, it takes a second before the GPS fix shows you not there, and two fixes to figure out what direction/speed you did travel. So the map software assumes you took the turn, projects where you SHOULD BE if you did turn and has to correct when it figures out you didn't turn. That delay is why you see the map jump a bit at turns you ignore.

[pm-r]

...navigation suddenly became really accurate and GPS receivers for PDAs became a popular item. (Before smartphones for the young'uns.)

That's some nice interesting info Jake.
Back in those days, and even when in Canada in the costal marine boating world after they had installed Coastal Navigation Differential or whatever it was called, some acquaintances had installed this new thing called GPS on their 45 foot+ boat and were over full of confidence as they navigated through some narrow channels in the thick fog.

Skipper was rather surprised when there was a horrendous crunch as the yacht hit the rocks.

What they hadn't made any adjustment for was the fact that they had mounted the GPS antenna at the stern of the yacht, plus the fact that GPS wasn't quite as accurate in those days as it is now, even with the Coastal Navigation Differential stations enabled.



- Patrick
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[MacInWin]

Patrick, I've heard other stories about GPS inaccuracy leading people into trouble. As an experiment, back in the mid-80's I set up a GPS in a fixed location and let it run for four hours, just to see how inaccurate it could be. At the time there were not as many satellites up, so when one of the birds "set" or got low in the sky, the atmospheric affects got pretty strange. Once I had 4 hours of data (fixes every second) I then plotted them all on a map. Most of the fixes were very close, within the 15 meter error then claimed. However, the worst fix was over a mile away. The fixes leading to that worst error showed the unit losing the position over about 15 seconds as one of the satellites set. Once the signal was lost, it snapped back to within the 15 meter circle. Today, with about 30 satellites up, there should always be 5 visible all over the surface of the earth, so the software can ignore the "Lone Ranger" that wants to wander off and use just the ones that agree pretty well. I haven't seen a wanderer in a long time.

I read the other day that Iran is blocking and inducing errors in the GPS signals in the strait of Hormuz to induce tankers to wander into their territorial waters so they can "arrest" the ship for violating their borders.

Lesson to be learned? That GPS, while really useful, needs to be checked with reality every once in a while.

[MikeFromMesa]

It is accurate and pretty simple in the description of how it works. One interesting fact is that every day the satellite clocks lose 38 microseconds because the satellite is moving and as Einstein said, moving clocks run slower.

Interesting. I had not even thought about that and the effect it would have. Thanks for posting it.