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greengage22
1st Sep 2002, 18:38
Many stand-alone hand-held GPSs (ie, those used by mountain walkers, or in light GA) give you a left/right steer to a waypoint.

I understand how GPS works out its present position. And I don’t have any difficulty with the idea of computing a great circle track from the present position to a waypoint.

But how does a stand-alone GPS, with no connection to aircraft avionics/compasses/INS/IRS, know which way it’s pointing? How does it know whether to turn you left or right until you’re steering towards the target?

Genghis the Engineer
1st Sep 2002, 20:18
They don't, all they know is which way you are moving - which can be determined in a matter of seconds from the rate of change in the position signals.

G

greengage22
1st Sep 2002, 20:51
What, even for a mountain walker?

Do you have any sources for this?

I'm not suspicious: you obviously know what you're talking about.

I'd just like to research it further.

GoneWest
1st Sep 2002, 21:37
What "sources" would you like Genghis to quote??

You answered the question yourself when you said I understand how GPS works out its present position.

It keeps working out the present position. If you walk to the other side of the room - you are at a different lattitude/longitude co-ordinate.

It knows you have moved - however short a distance or however slow a speed - you have a new co-ordinate.

Clever, isn't it!!

RatherBeFlying
2nd Sep 2002, 04:02
According to the manual that comes with my basic hiker GPS, you must be moving at least 2 mph for the navigation pages to work

The GPS will display the track it has calculated and show the position of North relative to your calculated track.

If you point the top of my unit in the direction you are proceeding, then North will be shown by an arrow relative to the top of the unit.

Overindulgence at a pub may interfere with navigational preformance:D

compressor stall
2nd Sep 2002, 04:20
There is at least one Model designed for the outdoors by Silva which has a built in compass. If memory serves me correctly one can deselect the GPS functions and just use the compass option. It is electronic, and will show the direction in which the top of the unit is pointing without the unit requiring any movement.
As for exactly how they work, I don't know.

The electronic compass is separate to the GPS receiver in the XL1000. THe receiver is actually made by Rockwell.

Genghis the Engineer
2nd Sep 2002, 06:53
The chaps above seem to cover it pretty well, but my specific sources are:-

(1) Operators manual for my current and previous handheld GPS units (both Magellans)

(2) ETPS course on avionics assessment circa 1996

(3) Personal experience using my handhelds both flying and hillwalking since about 1995.

G

greengage22
3rd Sep 2002, 09:26
Thanks, Genghis (and everybody else), for your replies.

Very helpful.

Capt Pit Bull
3rd Sep 2002, 16:44
My hand held likewise requires you to be moving, and in practice at a reasonable pace, before showing your current track (it still doesn't know which way its heading - you could be walking sideways!).

However, it also has a useful feature that in some of its display modes, it will show the present azimuth position of the sun or the moon around the outside of a compass rose. That way you can orientate the unit correctly if you can see either of them. If you can't, get your compass out!

CPB

northwing
3rd Sep 2002, 22:02
Try walking backwards and see what it tells you to do.

twistedenginestarter
3rd Sep 2002, 22:53
How does it know whether to turn you left or right until you’re steering towards the target?

You have to look at it the opposite way. We are taught to navigate using, as one input to the puzzle, the direction we are pointing. The GPS, somewhat like a Martian, doesn't understand this cultural anomaly. It logically thinks you are really interested where you are actually going. So in common with the colour of the aircraft and the pressure in the tyres, it completely ignores orientation.

There are solid state chips nowadays that can detect the earth's magnetic field and others that can detect 'gravity' to give you orientation information. Unfortunately these seem to be always inferior to the traditional compass and gyro.

Genghis the Engineer
4th Sep 2002, 15:01
Just as an aside, I bought my first handheld GPS after inadvertently walking the wrong way down a mountain in Scotland. I was walking in cloud and didn't know (I do now !) that the mountain in question contained a lot of iron bearing rock, so my compass showed North (which happened to be correct) as I walked up it and South, whichever route I took down.

Mrs Genghis, who was with me at the time, was distinctly unimpressed and suggested quite strongly I should do something about my navigation, so I bought a GPS !

G

msmorley
4th Sep 2002, 21:48
There is at least one Model designed for the outdoors by Silva which has a built in compass. If memory serves me correctly one can deselect the GPS functions and just use the compass option. It is electronic, and will show the direction in which the top of the unit is pointing without the unit requiring any movement.

I believe they incorporate a fluxgate magnetometer for the static bearing derivation. There are several (handheld) models that have this functionality including Garmin's eTrex Summit and Vista models - which also have barometric altimeters. A sensible combination - adding the GPS to the compass means that local variations in magnetic declination can be automatically compensated for.

Ok... Enough waffling on subjects on which I know not nearly enough... ;)

m.

FlyingForFun
5th Sep 2002, 17:40
This isn't restricted just to hand-helds, either.

The Garmin 430 in my aircraft (well, it was in my aircraft until it broke and we sent it back for repair) doesn't know what way it's pointing when it's stationary, and that's most definitely not a hand-held. But as soon as you add power and start to taxi, it figures it out (and orientates the map properly if you've selected track-up).

FFF
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Notso Fantastic
6th Sep 2002, 14:24
Greengage, the handheld has no idea which way it is pointing, whether true or magnetic. All it knows is where it was when you set your route, where the destination is, the line between them, and where it is at that instant and the direction of motion in relation to the desired track. So whilst moving, twisting around will not alter the indication of turn left/right, but changing your direction of travel is the only thing that will do that.