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4ndynorfolk Regular Visitor
Joined: Jul 02, 2008 Posts: 237 Location: Norfolk
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Posted: Sun Aug 23, 2009 4:08 pm Post subject: Satellite Channel 01 |
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I have noticed for a few weeks now that in the GPS Status screen, channel 01 does not turn red dispite being of good strength (above the second mark) and according to the diagram, almost ovehead.
As a consequence I often can't get better fix that 5m in open areas.
Has anyone else noticed this?
Is the GPS network operationg at less than full efficiency?
Exactly what does the diagram show - is up north? |
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stevendunn99 Occasional Visitor
Joined: Apr 22, 2009 Posts: 20
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Posted: Sun Aug 23, 2009 10:54 pm Post subject: |
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GPS status screen - I will check another time, but I think mine has been okay.
Accuracy - I sometime question it but recently I placed the device on a trig point and it was dead centre on the one on the map.
Map direction - it is North or your direction depending upon which you select, if that answers your question. I have no idea whether North is Grid north, true North or magnetic north but I am sure someone else here can confirm.
Steven |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Mon Aug 24, 2009 7:49 am Post subject: |
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This is a really simplistic explanation. It ain't this simple really.
True North. The earth rotates about two points. The North and South poles (True North). True north is along any of the lines of longitude that join the North and South Poles. A compas device (NOT a magnetic compas) such as GPS or a fibre optic gyroscope device will be aligned to True North, and as such will 'point' to the true North pole and calculate bearings etc. as degrees east or west of true north.
Magnetic North. The magnetic poles of the earth are not situated at the real poles, but some distance from them. A magnetic compass 'points' to these poles, so there is (almost always) a difference between True and magnetic North (Magnetic Variation or MagVar). In the UK the magnetic pole is about 8 degrees West of True North, although it depends exactly where you are. The magnetic poles also move with relation to the True poles over a long period of time, so the MagVar varies with time, although this movement is is very, very slow. (Measured in tens of years).
Grid North. This is a line vertically from the bottom to the top of a printed map (grid line). Because you can't fit a curved surface onto a flat piece of paper, map makers have to make a compromise (projection) and align their maps such that the top of the map is in a northerly direction, but because of the projection, this cannot always be exactly True North as it varies over the extent of the map, especially ones that covers large areas.
The differences between True, Magnetic and Grid Norths can normally be seen on a decent map (OS, Nautical etc.) in one of the corners. |
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stevendunn99 Occasional Visitor
Joined: Apr 22, 2009 Posts: 20
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Posted: Mon Aug 24, 2009 11:19 am Post subject: |
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M8TJT wrote: | A compas device (NOT a magnetic compas) such as GPS or a fibre optic gyroscope device will be aligned to True North |
That is what I assumed.
To the OP, I forgot to add: when you are moving the bottom left button has the map view option of either North, or the direction you are moving in. Though when you zoom in & out to check your surrounding area it annoyingly always goes back to North.
Steven |
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jonesbach Regular Visitor
Joined: Jun 07, 2008 Posts: 185
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Posted: Mon Aug 24, 2009 11:22 am Post subject: Re: Satellite Channel 01 |
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4ndynorfolk wrote: | I have noticed for a few weeks now that in the GPS Status screen, channel 01 does not turn red dispite being of good strength (above the second mark) and according to the diagram, almost ovehead.
As a consequence I often can't get better fix that 5m in open areas.
| By channel I assume you mean the satellite ID, not the first slot on the screen. At the times I've checked it has not been visible. Looking on Wikipedia it is reported that PRN 01 is unhealthy and so would be seen but not used, but I'm unsure if the ID on Satmap matches this PRN number.
Even if this is the case it would only affect you while the satellite was above the horizon. Are you seeing the satellite permanently overhead or was that just at one time?
As for fix I rarely do better than 15', but that is consistent and accurate within range. |
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4ndynorfolk Regular Visitor
Joined: Jul 02, 2008 Posts: 237 Location: Norfolk
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Posted: Mon Aug 24, 2009 4:34 pm Post subject: |
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Thank you for your contributions.
Yes, I was refering to th ID number of the satellite. It seems likely that the wikipedia entry explains the problem.
My second question resers to the gps status screen also. Is up north in this screen? Thanks for the comprehensive answer in context of the map screen. |
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jonesbach Regular Visitor
Joined: Jun 07, 2008 Posts: 185
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Posted: Mon Aug 24, 2009 5:57 pm Post subject: |
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I have assumed North is up but have not searched sufficiently to be sure. A quick look produced this site which gives a short description of the status screen on the Garmin. I doubt if Satmap is any different.
http://www.gpsreview.net/satellite-info-screen/ |
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davidor Regular Visitor
Joined: Nov 14, 2008 Posts: 117
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Posted: Wed Aug 26, 2009 9:00 pm Post subject: |
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I understood 5 metres (16ft) is as good as you will get with a stand-alone gps. To get better you either have to be a friend of the us military, or be linked directly to a fixed base station so that the induced drift can be negated. Which makes you wonder why the US military don't allow gps receivers to operate to much higher accuracy - surely their "enemies" have sussed out the system now. |
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philpugh Lifetime Member
Joined: Dec 28, 2005 Posts: 2003 Location: Antrobus, Cheshire
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Posted: Tue Sep 08, 2009 10:50 am Post subject: Re: Satellite Channel 01 |
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4ndynorfolk wrote: |
Is the GPS network operationg at less than full efficiency?
Exactly what does the diagram show - is up north? |
It should be working at 'full efficiency' as there are usually spare sats in orbit ready to take over from failing devices. 5m is a pretty good "error" value. (Don't forget this figure is a statistically derived measure and should only be used as a rough estimate of positional error).
I assume you mean the diagram on the Sat status screen? Don't forget this is a 2D representation of a 3D scenario where sats are distributed in the hemisphere above your head - but I believe North is towards the top.
When viewing an OS map - North Up refers to OSGB grid North, which is NOT the same as WSG84 grid north. I believe the GPS derived 'compass' directions (calculated from successive positions) is referenced to current map grid North - but I need to check this.
The in-built compass is based on magnetic North - just to confuse everyone _________________ Phil |
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FrequentFlyer Lifetime Member
Joined: Jun 12, 2006 Posts: 962 Location: London
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Posted: Sat Oct 24, 2009 12:14 pm Post subject: Appropriate settings ? |
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Following on from this topic...what is generally the best settings advice if out up in the hills, (the Lakes) for the day, with the Lakes map installed....Grid North, Magnetic North ?? |
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lucevans Frequent Visitor
Joined: Mar 21, 2007 Posts: 261 Location: Cambridgeshire, UK
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Posted: Sat Oct 24, 2009 4:32 pm Post subject: Re: Appropriate settings ? |
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FrequentFlyer wrote: | Following on from this topic...what is generally the best settings advice if out up in the hills, (the Lakes) for the day, with the Lakes map installed....Grid North, Magnetic North ?? |
I always set mine to magnetic north - that way, if I need to walk on a bearing using my old Silva type-4 for any reason (like dying A10 batteries!) I can just move the cursor in the planning screen to my destination and the bearing given is directly transferable to the compass.
Of course, this is assuming that the "Automatic" setting for magnetic variation correction in the A10's compass menu is accurate! |
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