[Wylug-discuss] Re: [Wylug-announce] WYLUG Social and micro-mapping

[Roy Mac]

Dave, 

Hopefully, this will help answer some of your questions:

The GPS satellites are in six orbit planes with an inclination of 55
degrees.  They are in a Medium Earth Orbit (MEO) forming a donut shaped area
about Earth's equator.  This provides greater "usable" Earth coverage.
Whereas satellites in a polar orbit (inclination = 90 degrees) spend a
significant amount of time over the poles which is of no real use to the
average GPS user.  

There are times when these will rise/set towards Greenland/Iceland and
Sweden/Finland when viewed from this area.  This appears like a broad
horseshoe.  

Now as you move further north, say north of 60 degrees latitude,
observations of satellites over the North Pole should occur.  At 60 degrees
and further north, the satellites will appear in a 360 degree band about the
horizon, but never overhead - in the donut.  These three dimensional dynamic
geometries can be quite confusing visualize - it is hard to explain as I
only have a working understanding of orbits (I am not an orbit expert).  

I have no idea why your GPS Rx would indicate a satellite to the north in
this area.

I hope this helps.

roym


-----Original Message-----
From: wylug-discuss-bounces at wylug.org.uk
[mailto:wylug-discuss-bounces at wylug.org.uk] On Behalf Of Dave Fisher
Sent: Saturday, June 21, 2008 1:14 AM
To: wylug-discuss at wylug.org.uk
Subject: Re: [Wylug-discuss] Re: [Wylug-announce] WYLUG Social and
micro-mapping

On Fri, Jun 20, 2008 at 08:50:22PM +0100, Roy Mac wrote:
> Keep in mind that at 54 degrees north, the GPS satellites are largely
towards the southern sky.  There will be times where they are straight
overhead or slightly to the northeast and northwest depending on your
longitude.  These satellites will never be to the north from this latitude.
Therefore, it will be helpful to avoid blockage from southern sky.  I am not
sure how much blockage the human body will add.  It might be good to try
some experiments ahead of time to determine how much your body blocks the
signals.  These satellites are in 12 hour orbits, so it would be good to try
it several times throughout the day as the geometries are constantly
changing.

Can you explain that in a little more detail?

How is it possible for the satellites ever to appear to our North East
or North West, and never to our North?

I can understand why geo-stationary orbits might always put them on an
arc to our South, but that would prevent them ever appearing to NE or
NW.

If they are not geostationary, I imagine they could have polar orbits
which were specifically designed to avoid the UK, but that would make
them somewhat less useful for our purposes here.

What am I missing?

And why have several GPS receivers which I've used in the past, *seemed*
to indicate that one or more of the locked satellites was actually
positioned to our North?

Dave

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