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# GNSS Fix

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# GNSS Fix

14th Dec 2000, 18:03
Guest

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Do you mean how GNSS achieves a better fix than unaugmented GPS/GLONASS, or a general overview how GPS works in theory?

R

14th Dec 2000, 18:11
Cron
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Thanks SLF, I guess it's how the theory goes, If I may give an example: I'm looking for the explanation of a how say an a/c is fixed 200nm from a DME by GNSS - I have a broad understanding of the process but the intersecting circles business have got me cold

14th Dec 2000, 20:17
Cron
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GNSS Fix

Could someone explain (in detail if poss) how a GNSS system yields a 3D fix for an a/c? Thanks

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14th Dec 2000, 23:05
Capt Pit Bull
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The principle is reasonably straight forward. Your GPS receiver gets 'almanac data' from the constellation which defines where each satellite is in space.

A time distance speed of light calc yields a range from the satellite. So, you now know that you are somewhere on the surface of a sphere, of the calculated radius, from that known point in space that the satellite is currently at.

Fix another satellite, and this gives you another position sphere about another point is space.

The intersection of 2 spheres will yield a position circle, (think of 2 soap bubbles blown together).

Get another range from another (3rd) satellite. This gives you another sphere. Intersect this with our circle, and it will yield 2 points.

Normally, this is enough to give you a fix, because 1 of the 2 points will be unfeasable (say 500 miles underground, or 8,000 miles into space).

For best accuracy, another satellite (or an altimeter input, which provide a position sphere of its own) is required. The reasons for this are tricky to explain in words alone, and lend themselves to a diagram or 2. Suffice to say its because the clock inside your receiver is not accurate enough, but an extra fix provides a way of mathematically figuring out the clock inaccuracy.

So 4 sats = good fix.
3 sats = approx fix.
3 sats + 1 alt = better than 3, worse than 4.

Does that help?

CPB.

[This message has been edited by Capt Pit Bull (edited 14 December 2000).]

15th Dec 2000, 06:09
Guest

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It's all really elegant.

There's a set of satellites wandering the heavens, and each knows exactly where it is and what the time is. Each constantly transmits this information at the speed of light on certain radio frequencies.

You have a receiver tuned to those satellites. Assume your receiver also knows what the time is. When it picks up a signal from a satellite saying "I am at position Z and I sent this at time T", your receiver can say "I got this at X seconds after T, and since radio waves travel at the speed of light -- C miles per second -- that means the satellite's X times C miles away from me. But I've no idea in which direction, so I'm somewhere on a sphere X times C miles away from position Z."

That works with one satellite. If you repeat the trick for a second satellite, the receiver knows it's also somewhere on a second sphere centered on the second satellite. Imagine the two spheres colliding -- the receiver has to be somewhere along the circular line where they meet. Add a third satellite and a third sphere, and you're down to a couple of points.

How accurate all this is, is the primary concern of GNSS. If you add a ground-based transmitter that knows exactly where it is, it can monitor the satellite transmissions and spot when the timing's drifting a litte -- it can do this because of atmospheric conditions or small errors on the satellites themselves. This ground based transmitter can then be picked up by your receiver and used to correct the signals you pick up from the satellites.

There are other issues, like how you make sure your receiver has a clock good enough to do all this accurate timing and how you spot various errors that look like they're not errors. And how you get all that in a handheld unit that costs less than a hundred quid and runs for ages off batteries (that's the bit that really impresses me!)

R

15th Dec 2000, 07:48
EchoTango
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Cron,
If you wish I can Email you 11 pages from
"Aviators Guide to GPS" 2nd Edition by Bill Clarke 1996
TAB Books (McGraw Hill)
ET

15th Dec 2000, 08:03
411A
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Normally have 8 or 9 satelites "in view" in my aircraft (N. America) at all times which results in VERY accurate data. Works good, lasts a long time, invented in USA. Have no idea why the Europeans do not use it more as it is provided free of charge. The DOD have moved on to something much more accurate for military needs.

15th Dec 2000, 18:58
RATBOY
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If you really, really want to know how it really works check out <A HREF="http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html" TARGET="_blank">www.colorado.edu/geography/gcraft/notes/gps/gps_f.html</A> for a modular discussion of GPS, time and charting at as much of a level of detail as you care to take in.

The Wide Area Augmentation System in the US (still under test, not commissionable until they can prove 99.9999999 availability of accurate signals, which will take a few years) is a differential system that measures inaccuracies in GPS generated positions and provides those corrections over a wide area (Western Hemisphere) via satellite broadcast.

The "competing" system is GLONAS operated by the Russians, a satellite based but incompatible system.

Why Europeans have not taken to GPS, a few guesses:
1. they really have operationally, but can't make a big deal out of it for political reasons
2. not invented here
3. controlled by a foreign military agency

There was/is an animal called European GPS Navigation Overlay System (EGNOS) that would be an augmentation to at least GPS and possibly GLONAS designed to provide improved accuracy in Europe.

18th Dec 2000, 21:33
Cron
Guest

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Thanks everyone, now digesting.

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