Author Topic: GPS elevation changing during the day!  (Read 2631 times)

Kim

  • Timelord
Re: GPS elevation changing during the day!
« Reply #25 on: December 24, 2015, 04:11:04 pm »
The pressure sensor on my 705 always seemed to have thermal drift problems.  The first fix after a winter cafe stop usually showed a significant change in elevation which came back to 'normal' as the unit returned to ambient temperature.

The one and only time I've written code to read one of these pressure sensors, the first thing you had to do was work out the temperature so you could use it to apply a compensation factor.  Stands to reason that if the temperature and pressure sensing parts are heating up at a different rate then it's going to give a dodgy reading until it stabilises.
Careful, Kim. Your sarcasm's showing...

Re: GPS elevation changing during the day!
« Reply #26 on: December 24, 2015, 05:33:42 pm »
62m elevation different s in netherton tunnel  :o . I thought some of the potholes were deep but.......... 😀
the slower you go the more you see

Kim

  • Timelord
Re: GPS elevation changing during the day!
« Reply #27 on: December 24, 2015, 05:37:08 pm »
62m elevation different s in netherton tunnel  :o . I thought some of the potholes were deep but.......... 😀

The overhanging cliff on Strava's elevation profile is a bit interesting...
Careful, Kim. Your sarcasm's showing...

frankly frankie

  • I kid you not
    • Fuchsiaphile
Re: GPS elevation changing during the day!
« Reply #28 on: December 26, 2015, 11:37:53 am »
I think the Wikipedia page on GPS is a good place to start reading.  https://en.wikipedia.org/wiki/Global_Positioning_System
What always fascinates me is how incredibly weak the signals are - transmitters of about 50 Watts, over 20,000km away.

I have read quite a lot about how the GPS system works and I am confused by some of the statements made. The GPS signals are time signals and the position is determined from the time difference between the signals when they arrive at the GPS unit.

The distance (time delay) from a satellite puts the GPS somewhere on the surface of a notional sphere.
The distance from a 2nd satellite puts the GPS somewhere on the surface of a different notional sphere, which intersects the first to form a notional circle.
The distance from a 3rd satellite puts the GPS somewhere on the surface of a different notional sphere, which intersects the circle at 2 points.  The GPS is on one of those two points, and generally one is much more likely (ie close to the surface of the Earth) than the other which will be out in space somewhere.  Really that is enough.
The distance from a 4th satellite puts the GPS somewhere on the surface of a different notional sphere, which would confirm which of the two points is the correct one, ie you have your 3D fix.
It's not dark yet but it's getting there.


Kim

  • Timelord
Re: GPS elevation changing during the day!
« Reply #30 on: December 26, 2015, 01:33:29 pm »
What always fascinates me is how incredibly weak the signals are - transmitters of about 50 Watts, over 20,000km away.

The SNR of the GPS signal is negative, with receivers being able to work with an SNR of -30dB or so.  That's equivalent to understanding normal conversational speech while standing next to a revving engine.  Witchcraft!
Careful, Kim. Your sarcasm's showing...

Re: GPS elevation changing during the day!
« Reply #31 on: December 31, 2015, 11:17:50 am »
What always fascinates me is how incredibly weak the signals are - transmitters of about 50 Watts, over 20,000km away.

The SNR of the GPS signal is negative, with receivers being able to work with an SNR of -30dB or so.  That's equivalent to understanding normal conversational speech while standing next to a revving engine.  Witchcraft!

Engine testers communicate by facial expression. If the engine grenades, they communicate by sense of smell.

Re: GPS elevation changing during the day!
« Reply #32 on: December 31, 2015, 02:19:40 pm »
I have read quite a lot about how the GPS system works and I am confused by some of the statements made. The GPS signals are time signals and the position is determined from the time difference between the signals when they arrive at the GPS unit.

The distance (time delay) from a satellite puts the GPS somewhere on the surface of a notional sphere.
The flaw in that description is that the receiver doesn't have a good clock, so it has to get its time reference from the satellites. The receivers actually measure the time difference between the signals from the various satellites in view, and calculate the position from the satellites' positions and these time differences.

It may be that the calculation is actually done by guessing time and position from one signal, and then modifying that guess to align with signals from the other satellites. The result is the same, that the time delay is not what matters, it is the difference in time delays that gives the position.

The chess-board analogy shows this. Moving the receiver up and down means the signals arrive sooner or later, but that can't be detected if the satellites are largely overhead. Position can be detected because horizontal movement puts the receiver closer to some satellites and further from others.  To get altitude accurately, there need to be satellites that are low, nearer the horizon, visible to the receiver.

Also, on GPS receivers that have external aerials, the position calculated is that of the aerial, not the receiver, and the length of aerial cable makes no difference, unless you want a really good time reference and you are worried about being 0.000000005 seconds out for every metre of aerial cable.


Quote from: Kim
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